IRPS 125 Source-sink relationships in crop plants

8/3/2019 IRPS 125 Source-sink relationships in crop plants

1/20


2/20

SOURCE-SINK RELATIONSHIPS IN CROP PLANTSB . Venkateswarlu and R.M. Visperas'

ABSTRACTIn crop plants, the physiological basis of dry matter production is dependent onthe source-sink concept, where the source is the potential capacity for photo-synthesis and the sink is the potential capacity to utilize the photosyntheticproducts. Ifthe sink is small, the yield cannot be high; and even if the sink is large,the yield cannot be high if the source capacity is limited. Several crop plantsincluding rice and wheat have shown changes in source and sink during thecourse of domestication and evolution into modern cultivars. The modern planttype concept is essentially a blend of improved source and sink where the leaf(source) IS short and erectofile to capture greater solar radiation and the sink ISexpanded by increased grain number and size and by profuse tillering.A developing leaf is a sink, but when fully grown it becomes a potential source.

Instances of a single organ playing the roles of both source and sink are discussed,as well as the nature of additional sinks and metabolic sinks. Sink size is thepotential capacity for maximum production of a crop. The intensity and capacityof sink dynamics, the mechanics of source and sink, and the factors influencingthem are reviewed with examples.

In tropical and subtropical climates the source appears to be the majorlimitation to higher yields in rice because of the wide gap between the number ofspike lets and the number of grains. In temperate climates, and particularly withjaponicas, the sink is the limiting factor, as nearly all spikelets become grains.Examples of crop plants are reviewed, focusing on the limitations of their

sources and sinks.

'V isit ing scientist and assistant scientist , Plant Physiology Depar tment, International Rice Research Ins titute, P.O. Box 933, Mani la, Phi lippines.


3/20

SOURCE-SINK RELATIONSHIPS IN CROP PLANTS

Plateaus in productivity have been reported in all cerealcrops as well as in cotton, sugarcane, tomato, and potato;and some oilseeds and pulse crops are approaching thatposition. The sustainability of modern agricultural break-throughs is being questioned, as further research seems notto enhance yield potentials. This situation calls for analysis.Crop Improvement programs must have the specificobjective of improving yield potentials.Generally speaking, the organ or site that synthesizes

food materials is called a source; in crop plants the matureleaf is always a source. The site of accumulation ofsynthesized food materials is known as sink. The source isthe potential capacity for photosynthesis (Po) and the sink isthe potential capacity to utilize the photosynthetic productsof the source.The major part of the starch in rice grains at harvest is the

photosynthetic product of the leaves, which is translocatedfrom the leaves directly to the growing grains after flowering.The ripening of rice grains can be considered a process ofaccumulation of contents in a container. The grain yieldmay be limited by either the size of the container or theamount of its contents (S2) . If the container issmall the yieldcannot be high. On the other hand, even if the container islarge, the yield cannot be high if the contents are limited.While the relationships of sink and source, and their

limitations, are not fully understood, their mechanics anddynamics appear to determine the nature of crop per-formance. Beevers (3) and Evans (IS) gave some broadindications of the nature of the source and the sink.Molecular level transformations involving volume flow ofassimilates, phloem loading and unloading, general assi-milate pools, and metabolic steps in multiple enzymesystems with coupled biochemical processes were covered ina very basic way by Gifford et al (17), Hendrix et al (21),Koch et al (28), Wyse and Saftner (73), and Lafitte andTravis (31). The scope of this review is limited to structuraland productivity levels of source and sink.The following lines of inquiry are relevant for clarifying

questions in crop improvement programs: the nature of sources and sinks, including their mech-anics and dynamics;

their interactions and interdependence; and research directions for crop improvement.These lines of work envisage an imaginative action plan of

intensive basic research and system studies in these areas. I tis hoped that this review of the vital systems in crop plantswill be useful to researchers in appraising current knowledgeand will clarify goals in plant improvement programs.

SOURCES AND SINKSThc continued growth of any plant depends primarily onphotosynthetic activity in the leaves and the transport oforganic compounds from the leaves to heterotrophic cells.The source supplies assimilates (C compounds) to the sink.The sink accepts and consumes the assimilates for its owngrowth or accumulates them for a certain period. The sink-source relationship resembles the demand-supply relation-ship in economics (S2) . In general. any plant part that hasphotosynthetic ability isconsidered a source, ar-d nongreencells, tissues. and organs arc considered sinks. Thus. leavesarc typical sources; and stems. roots, and tubers are typicalsinks. The sink and source functions of any plant part maychange depend ing on its developmental stage. For example,in rice. before flowering, the leaf sheath and culm accu-mulate sugars and starch, i.e., act as sinks (79). Afterflowering, most of the accumulated carbohydrates moveinto the spikclets (10).Developing organs, tubers, nongreen aerial plant parts,

buds. flowers, fruits. and most of the cells in stems andpetioles also constitute a drain on photosynthetic productsand are thus sinks. Even within leaves there are many cellswithout chloroplasts, and the autotrophic cells themselvesconsume photosynthate in their own growth and respira-tion.Developing buds and meristernatic regions in roots place

demands on the available assimilates and compete success-fullv as sinks with developing leaves (3). The onset offlowering and subsequent fruit development have a markedeffect on the redistribution of assimilates; fruits develop atthe expense of vegetative growth, and at this time the growthof roots may be restricted.In a vegetative plant the developing green leaves are sinks

not only for their own photosynthate but also for thatprod uced in the older leaves. The oldest leaves near the baseof the plant. provided they receive adequate illumination,export sugars to the roots. Wardlaw (70) reported that oncea leaf is mature. it is no longer a sink, even when it is madeheterotrophic by natural or experimental shading.

DIVISION OF FUNCTIONIn several crop plants, and particularly in rice, there is adivision of function among the leaves on a culm after theelongation of internodes. The sink for photosynthates fromthe upper two to three leaves is the panicle (Fig. I); that forthe lower leaves is the root system (Sl). Leaves in anIntermediate position may send assimilates in either or both


4/20

4 IRPSNo. 125,January 1987

I. Division of function. Upper three leaves translocate to panicleand lower leaves to root system, while intermediate leaves mobilizetoward both ends, but mainly to roots.

d ircct ions (75). I n maize, the source for ripening grains is theleaves above the ear, and the source for roots is the leavesbelow the ear (43). Except for several leaves at the base of thestem, which send their photosynthates to the roots, thetomato plant is composed of several units, each of which hasthree leaves, a truss, and a bud (54). This unit is the source-sink unit (Fig. 2), and the photosynthates of the leaves of aunit go preferentially to the sinks within the unit. However,this source-sink unit is not an absolute one. There is aninterunit translocation of photosynthates, the extent of whichdepends on the conditions of the plant. There are source-sink partnerships between certain leaves and organs, as intomato, where several trusses containing fruits are fed byspecific leaves positioned around them, e.g., for leaf 5, theroots: for leaf9. truss I; for leaf2, truss 2; for leaf 14, truss 3;and for leaf 16, truss 3 (56).Such a system is hardly seen in higher animals because

assimilation is organ-oriented. There are no special storagestructures, but fat deposits are concentrated at somevulnerable points like the stomach, chest, and hips. How-ever, animals like camels store some items like water for use

2. A source-sink unit in tomato. The young fruit can be a sourcebecause it remains green for some time.

in times of need. While in a developing plant embryo, theembryo is the sink and the endosperm is the source, in achicken egg, the growing chick is a sink. Sources and sinksare highly interdependent.

KINDS OF SINKS AND SOURCES

Several kinds of sources and sinks exist in crop plants. Someplant parts are always sources; some are both sources andsinks; others are always sinks. The following descriptionsclarify the types of sinks in crops:

Primarv sinks. These are fruits, grains, and sexualorgans (Fig. 3).

Secondarv sinks. These occur in plants where morethan one organ accumulates photosynthates and theother organ takes precedence in filling up before thesexual organs. I n most cases secondary sinks also serveas material for vegetative repr od uction such asrhizomes and tubers.

Alternate sink. Some plants mobilize their foodmaterials to other organs like the stems, petioles, andsheaths, after filling the main sinks (Fig. 3). Some fiberplants like hibiscus havc fiber development in the. stemas the primary sink, and the stem and seed as alternatesinks. In cotton, seed and oil are alternate sinks.

Additional sinks. Some organisms grow in the rhizo-sphere on root ex udates, or develop symbiosis or para-sitism. The extra burden constitutes an additional sink,which can sometimes be of lethal conseq uence (Fig. 4).It can be a drain on the growth and productivity of themain host plant. In legume rhizobia, additional sinkscan be beneficial to the host plant.

Metabolic sinks. In rapidly growing cells or rneristerns- whether in vegetative apices, cambial regions,storage organs, floral parts, or embryos- sugar ismainly converted to new cell materials. More than halfof the sugar may be diverted to protein, fats, and othercell wall constituents (3). Classical experiments with adrop of kinetin on a senescing leaf resulted in that area


5/20

IRPS No. 125, January 1987 5

3. Alternate sink: sheath in cereals.

staying green at the expense of surrounding tissues. Thekinetin induces a sink in certain cells and influencesadjacent tissues to translocate to the sink. Somethingsimilar must happen normally with ripening fruits andgrain. I n cereal plants, the growing and maturing grainforms an intensely active sink. The production of arange of secondary products or compounds having nostructural function or readily discernible role in meta-bolism represents a metabolic sink (3). In most plantsonly a small fraction of the photosynthate is diverted tothese compounds, but in some, e.g., those extrudingresins and latex, the drain is considerable.

Sinks, then, are cells not only with a high demand forcarbohydrate, but also with intensely active mechanisms formoving it across the limiting cell membranes.The establishment of new sinks during plant growth

depends on specific developmental processes whose onsetmay be controlled by environmental influences such as

Additionalsink

Sorghum./-t'V...Iim~ , -Bacteria--additionalsinks

4. Ad d it iona l sinks.


6/20

6 [RPS No. 125 January 1987

temperature, photoperiod, and light intensity, which in turndepend for their expression on internal regulatory com-pounds.Somc sources and sinks are dynamic; others are static.

Many organs in plants shift from one position to the otherbased on their location and demands for function. A/I1'avs sources: Chlorophyll-containing cells and leavesof green plants are always considered sources.

A/waFS sinks: Tubers, roots, underground stems, develop-ing buds, flowers, some aerial stems, and some fruits actas sinks throughout their existence.

Sources and sinks: Fruits of okra, papaya, tomato,banana, mango, and grape can synthesize food materialsby themselves, besides acting simultaneously as powerfulsinks. Developing leaves and fruits that are green act assinks, but when they attain full size they also photo-synthesize. They originate as sinks and later function assources. The tomato fruit might synthesize 15% of itsneeds and procure the rest from other sources. There is athin line between source and sink in certain plant organs.The role as source or sink depends on the utility, growth

phase, primary and secondary functions, and simultaneousroles of an organ, The growth of a plant itself is highlysink-oriented. In the early stages all the leaves of a stemwork for the growing points of the stem, leaves, and roots(5 I), Later, when the flower primordia initiate, they startspecializing; the upper leaves work for the growing sink,while the lower ones work for the root system.

MEASURES OF THE SINKSinks are measured in terms of carbohydrates, proteins, oils,biological energy, or fresh or dried plant materials - basedon the type of crop. In several crops, the sink is measured interms of number, volume, or weight. But the number offruits or grains per plant per unit land area does not allowcomparison of crops or varieties, because number is in-fluenced by size. When the grain or fruit is small, the numberbecomes larger, and vice versa. Nonetheless, number offruits or grains is a useful measure for comparison within ahomogeneous group of cultivars, e.g., those having the samegrowth duration, tillering habit, and grain size. Volume is abetter measure than number but suffers from other limita-tions such as cumbersomeness in determination. Moreover,at a given volume, the packing of starch granules could bedifferent (Fig. 5). Therefore, weight is the most dependablemeasure of the sink. However, a combination of numberand weight helps in making comparisons and evaluations.Quantifying the sink suggests the potential yield of a crop orvariety.

Sink sizeSink size is expressed as:

sink size = sink capacity x sink activity

Volume of 1,000 groins (cm3)30

I ,OOO-groin weight (g)30

Specific gravity5. Volume and weight of 1,000 grains ofBinato at different specificgravities.

Sink capacity is the maximum space available for theaccumulation of photosynthetic products, In grain crops itis expressed as number and size of grains. The sink size ofrice can be determined easily, as the hull size ofthe spikelet isthe determinant of kernel size. Sink capacity in grain crops isexpressed on a unit of land area basis, e.g.,sink capacity = panicles per unit area x spikelets per

panicle x individual grain weightIn fruit trees, sink capacity can be expressed in terms offruits per tree or fruits per hectare:sink capacity = number of fruits per tree x individual

fruit weightSink activity is the inherent capacity of the sink to create a

translocation grad ient from the source to the center ofaccumulation. In the reproductive structures, the sinkbecomes more active and shifts to a higher potential onlyafter fertilization. The process may involve stimulation,hydrolyt ic conversion, and consequent creation of a movinggradient. The power of stimulation varies and could serve asa starter to determine the dimensions of accumulation,Unfortunately, few investigators have inquired into thisarea.Unless both sink capacity and sink activity are fully

investigated, we cannot draw complete conclusions aboutsink size.


7/20

In te nsity a nd capacityThe SilC of an organ that functions as a sink depends on twofactors: I) capucitv, which can be expressed by dry weight,and 2) intcnsit v, which can be expressed by specific activity.Young growing organs such as very young trusses and thestern can have very high intensity but small capacity (56).The intensity of very young leaves as sinks for the photo-synt hates of ot her leaves is not very high, because, mostprobably. they start to photosynthesize from an early stageof development. The intensity becomes extremely low whenthe leaves arc old. However. their intensity as sinks for theirown photosynthates is maintained for a long period. Theintensity of fruits or reproductive organs as sinks is high atvery early stages of their development and becomes relativelylow with growth. However, since capacity increases withgrowth. the sink size of fruits becomes large.

MEASURES OF THE SOlJRCF

The source is generally measured on an area basis despitethc interference of factors like thickness. life duration.nutrition. and growth stage. In rice. source activity has beenmcusu red as percent N in leaves (62). The leaf area per plantor rcr square meter of land - called leaf area index (LAI)

is the usual measure. Functional ability and potentialcapacity depend upon the crop and cultivar. Thickness.functional ability. and life duration arc highly influenced bylight. temperature. relative humidity, wind velocity, andnutritional factors. There is a compensatory mechanismbet wccn leaf size and leaf weight: the thickness of a leaf is thedominant function at a given leaf area. Thickness isexpressed as weight per unit area of leaf and is known asspecific leaf weight (SLW). In some situations LAI will notgivc the proper relationship with sink capacity or pro-ductivity. Hence, other leaf measures like leaf weight pertiller or per plant indicate the proper relationship. Leaf arearcr weight of t iller or per plant. which is known as leaf arearat io (LA R). is a good measure. Unlike the sink. the sourceis more vulnerable to several biotic and abiotic stresses.The source is more sensitive to cultural, nutritional. and

climatic factors. Nutritional and cultural practices oftenst imulate the source. making it more responsive. The sourceis the first organ to respond to management. Therefore. thesource may suffer if stress affects its balance with the sink.Many workers (57. 63. 75) have related LAI to productivityand sink capacity (Fig. 6). The source not only producesassimilates but may exert pressure and create a trans-location stream to the sink. Although the sink is the starter,it is rnainly the source that pushes the assimilates through itscapillary mechanism. It is not clear whether the pushingforce of t he source or the pulling force of the sink determinesthe flow, but the complementary role of both is apparent.Another view regarding the movement of the assimilates

to the respective sinks is that in the primary sink. somehydrolytic activity begins after fertilization and creates a

IRPS No, 125, January 1987 7

translocation gradient (67). A third view is that somesubstance or functional units produced in the sinks migrateto the source and enhance its functional activity. alsodetermining its duration of activity. The production ofcertain hormones has been projected as a possible mecha-nism. It is possible that a combination of these processesoccurs.Although chlorophyllous tissue is present in several

organs, the nature of its function and its effectivity vary withorgan. location, intrinsic life span, purpose. and nutritional,genotypic, and environmental factors. The senescence of aleaf, its pattern of senescence. and the senescence of thesink-bearing structures are all influencing factors. Thechlorophyll of culms and panicles is short-lived, but thechlorophyll of panicles, spikelets, and awns is more effectiveand efficient, perhaps because of its nearness to the sink.Yoshida (75) felt that slow senescence is an advantage forprolonged source-sink efficiency. In rice, however. this hasnot been clearly established.

Soikelets and grOins (103)58 o

48 yo778+427" ,-00385 ,2ro09664""

38o

28 o Grains~ .8yo-O 186+3.943**-0 0187*",2ro 09568""

LAI6. Relationship between leaf area index (LA!) and sink size (innumbers) (66),

7, Movement of assimilates in maize toward the sink.


8/20

8 IRPS No. 125, January 1987

The source is sink-dependent, since the assim ilates movefrom the source to the sink irrespective of its location. inm aize. the leaves around the cob supply the cob, them ovem ent being both downward and upward (Fig . 7). Thesame phenomenon is reported in tom ato, pea cotton, andother axil-borne fruits and vegetables. In tuberous crops likepotato , carrot. radish , and sugar beet, the flow is invariablydownward. w ith specific feeding channels for differenttubers. Even in cereals like rice, a leaf and a branch of ap anic le are p artne rs, alth ou gh tiller-to -tille r an d lea f-to -lea fm ovements are known. Compensation am ong the leaves infeed ing particular branches of a panicle is also observed (I).

TRANSI.OCATION FROM THE SOURCEThe source-sink relationsh ip between a leaf and otherorgans is complex; it is a function of the grow th stage of theplant and the position of the leaf. In the vegetative phase, thelower leaves export more C up than down, and the upperleaves export a larger proportion downward (25, 26). In theyoung fruiting tom ato plant, a ll leaves supply all the trusses,but as the number of trusses increases, groups of leavescom e to have primary but not absolute responsibility fors up ply in g sin gle tru sse s.

K h an and Sagar (26) suggested that photosynthates of t heleaves move down to the roots first. and then retrunslocaiefrom the roots to the grow ing organs of the shoots. Thesequence source - roots - sink is not necessarily the onlypossibility . A direct course source - sink does exist, andis probably the major course (Fig. 8) when fruits or primarysinks are developing (56).

A sim ilar course is known in cereals like rice and w heatam ong the plants exclusively depending on current photo-synthetic contribution to grain y ield. In others the sequenceis sOllrce - /l'lIIporarr sinks (like culrns and sheaths) -primary sinks under certa in stresses (Fig . 8).

Panicle Ianicle

Roots

A 8

8. Translocation in grain crops. A: assimilates move to roots andthen to panicle, 13: a ssimilates move directly to panicle.

Most of the metabolic sinks in plants are connected w iththe source by ph loem clements in the vascular strands. Inconsidering the fate of photosynthate at the sinks. it ISimportant to consider what has been established about thematerial moving in the ph loem and the direction of itsmovement. The experiments of Mason and Phillis (32) andSw anson (48) h ave revealed several in teresting ph enom ena.It was considered that sugar m ovem ent in to plant cells

was stric tly a dow nhill diffusion process. Concentrationgradients between leaves and sinks were given strongem phasis. The concentration of sugars in leaves, where theywere produced, w as h igher than that in the sinks: theconsumption of sugars in roots. m eristerns. e tc . was con-sidered to give direction, if not the driving force. Of course.the rates of m ovement --- computed from information oncross-sectional area of the ph loem connection and theincreasing weigh t of tubers and fruits (12) showed thatthese were enorm ously greater than could be expected Irornsim ple diffusion. and direct m easurem ents of rate ofmovement have shown that rates of >50 em ! h are com -monlyachieved.It thus seems clear that the introduction of sucrose into

the ph loem cells of leaves is an active loading process. onerequiring an expenditure of cellular energy (30.70).

Australian investigators studied sugar accum ulation insugarcane internodes (4,5,19.20,47). Sugar concentration(sucrose) was rough ly 20% in the m ature internode. 4 - I O I Y c ,in the young internode, and 2-3% in the leaf. No starchappeared in the storage tissue. There w as no apparentdownward gradient between source and sink.

SOURCr. AND SINK DYNAMICS

Nosberger and Humphries (41) discussed the evidence for acausal relationsh ip between the assim ilation rate of photo-synthetic sy stem s and sink activity . Som e outstandingexam ples w ere as follow s:

In detached rooted leaves of dwarf bean, the photo-synthetic rate (Pr) is related to the activity of the rootsy stem (22, 23).

The nature of the root stock controls the net assim ila-tion rate (NAR) in spinach beet-sugar beet grafts (61).

The assim ilation rate of potato is controlled by tuberactivity at least during the bulking period (2, 36.41).

The evidence allow s the conclusion that assim ilation bythese plants is linked w ith the size and activity of their sinks.

Lafitte and Travis (31) exam ined rice genotypes differingin sink-source ratio . L ines w ith a h igh sink-source ratioexh ibited h igher rates of C exchange per unit area andaccum ulated low er am ounts of nonstruetural carboh ydratesin vegetative storage tissues than other lines. The line w iththe lowest sink-source ratio accumulated the m ost carbo-hydrates in storage tissue. The results suggest that thephotosynthetic potentia l of rice was not fully realized, andthat an increase in the sink-source ratio m ight result in


9/20

higher viclds. On the other hand, the low source lines hadhigh SL W, which may have been largely responsible forperceived increases in PI'. These observations infer that suchmanipulations can affect plant yields.I n wheat, when the ear is removed, the Pr of the flag leal',

which is the major source of the ear, decreases rapidlybecause the major sink is impaired. Whcn the lower leaves ofa plant from which the car has been removed are darkened,the Pr of the flag leaf recovers considerably because thoseleaves and roots become the sink of the flag leaf (27).However, an important question arises, viz., How far in

the normal growth and development of a whole plant do thesinks determine I) assimilation rates and 2) the now ofphotosynt het ic prod ucts from photosynthetic areas')Information on the effect of sinks on assimilation rate

during the normal growth and development ora whole plantis available from work on cereals (42, 44, 59), where there ismore than circumstantial evidence that excluding light fromparts of the photosynthetic system does not depress grainyield in proportion to the darkened-leaf area. Compensa-tion by the remaining photosynthetic area is believed toaccount for the disagreement between observed andexpected values. Hartt et al (18) postulated in sugarcane thatsome control of the amounts translocated is exerted by theblade itself. Unless compensatory effects merely reflectchanged patterns of distribution, observations of this natureimply that leaves often contribute less than their fullpotential of photosynthates in the normal intact plant.There is limited evidence on the effect of the sinks on the

flow of photosynthetic products from photosynthetic areasduring the normal growth and development of a wholeplant. The best support comes from the sugar beet-spinachbeet grafts of Thorne and Evans (61), where it appears thatthe NA R of spinach beet leaves is increased by grafting themonto a sugar beet rootstock. Sweet and Wareing (49) alsoreported increased Pr in pine needles by reducing thephotosynthetic area.In fluence of the sin kIn tomato the fruits ofa truss develop and ripen by receivingphotosynthates from the leaves of the source-sink unit towhich the truss belongs. However, if the source exceeds thesink within a unit. the excess photosynthates may trans-locate to another unit. In this way the photosynthates from aunit go to the other units to a certain extent. Such interunittranslocation becomes greater when the leaves or the truss ofa unit is removed. Thus, there is a mutual gap-filling deviceamong the units (54).The pot ent ial si7e of a fruit is larger in several situations

than under ordinary conditions. However, the flexibility islimited: a fruit can be about 1.5 times larger than normalwhen the source is extraordinarily large. For example, whenT-I (T = truss) was removed in tomato, the photosynthates01 the leaves of the unit to which T-I belonged translocatedto T-I L and the fruits of T-II became larger. but the weight

IRPS No, 125,January 1987 9

of the fruits olT-llunder these conditions was much smallerthan the total weight offruits ofT-1 and T-Ilunder ordinaryconditions.When the major sink of a leaf, i.c., the fruit, was removed,

the phorosynthares could not leave the leaf, and trans-location became as low as IOCIr(54). Some of the photo-synihatcs that left the leafaccumulated again in the leaves orin thc stem, which do not act as sinks under ordinaryconditions, and the weight of these organs became heavier.Under such conditions, the leaves became curled andpigmented, and died early.PI' responds to the demand for assimilates. In wheat,

during grain filling, when most of the assimilates from thenag leal arc translocated to t he ear, removal of the ear leadsto an accumulation olassimilates in the Ilag leaf and to a fallin its Pr by about half within hours. If the lower leaves areshaded so that the nag leaf has to support the rest of theplant, the assimilates are exported at a high rate, and the PI'rises to its original level (27). Such pronounced feedbackeffects have not always been found, probably becausealternate sinks Cor assimilates, such as young tillers, werepresent.Demand for assimilates can also influence the rate,

velocity, and translocation pattern in wheat (45, (9) andpresumably in other plants. Another subtle relationship thathas been recognized lor many years (27, 40) is that the size ofactivity in the sink may have an influence on Pr in the sourceleaves. If assimilates are not transported to sinks, Pr isdepressed, and if new sinks are provided, Pr is increased.Neales and Incoll (40) summarized the effects of changingstatus of sinks, which suggest control of Po such asI) decreased rate on detachment, 2) fatigue effects andmidday depression of Pr, 3) increased Pr in the remainingleaves following partial defoliation, 4) decreased Pr fol-lowing interference with translocation or removing sink, 5)decreasing Pr at temperatures suboptimal for growth, and6) influence of grafting different sinks. However, otherpossibilities cannot be ruled out, such as feedback from sinkto source, including hormonal signals. Wareing et el (71)showed that the stimulatory effect of partial defoliation onremaining leaves caused increased levels of protein andparticularly of carboxylating enzymes. They suggested thata corresponding increase in photosynthetic capacity, ratherthan increased relative demand by the sink, was responsiblefor the higher Pr of surviving leaves, and they favored adynamic hormonal interaction between sink and source.However, King et al (27) showed that in wheat, 2 wk afteranthesis, 45% of the nag leaf assimilates were transferred tothe developing ear, which was itself photosynthesizing.Removal of the ear resulted in a 50% reduction in Pr of thenag leaf within 15 h. Darkening of the other leaves resultedin recovery of Pr of the nag leaf, with the assimilates beingdiverted to roots and shoot (Fig. 9). Under some conditionsinhibition of Po in the ear brought about an increased Pr inthe flag leaf. In this system, then, there are relatively rapid


10/20

10 IRPSNo 125 January 1987

Photosynthesrs (0/0 of control)110

Control

\\,,,,,,\\\\

\,\

Darkened

. . . .~ . . . .. , . . . . . ; . . . .",.' Ear removed ..............

Lower leaves darkened

24 72 968Time after ear removal (h)

9 The rate of net photosynthesis of wheat flag leaves after varioustreatments (27).

interactions, and the Pr of the source appears to be closelyregulated by the demands of the sink.Source-sink graftsThe tomato experiments of Khan and Sagar (26) showedthat the proportion of current photosynthetic productionthat is exported in a 24-h period may be increased byred ucing the size of the source or by increasing the activity ofthe sink through growth-regulating compounds. They con-ducted grafting experiments with potato and tomato. Aplant with potato scion on tomato stock produced neithertubers nor fruits, and export from the leaves was very low.Yet when the graft was reversed and both tubers and fruitswere formed, the amounts exported were doubled. There-fore, the authors contended that for both tomato and potatoleaves, the presence and activity of sinks play a role indetermining the rate of export of currently producedassimilates.Insuch systems where the sinks control distribution of the

photosynthates, competition between sources will occur (18,25), perhaps leading to accumulation of material in or closeto the channels of transport. Nosberger and Humphries (41)reported increases in sugars, starch, and protein in potatostems and leaves following the removal of tubers. Nosbergerand Thorne (42) observed similar diversions where sink sizewas reduced in barley. More research is needed to determinehow far the growth and development of plants is controlledby sink activity on the one hand, and by direct and indirecteffects of environmental factors on the other.In an intact plant with numerous sinks, cause and effect

cannot always be distinguished (22): it may not be obviouswhether increased Po is a consequence of greater sinkcapacity or whether increased growth of the sink is a resultof greater assimilation. To clarify this situation, rooted

detached leaves of dwarf bean Phaseolus vulgaris were used.This simple system has a photosynthetic surface thatchanges little in time, and a single sink that can be varied insize. Carbohydrate moves from the lamina in one directiononly, because no bud arises to initiate stem growth. Rootgrowth rate controls the rate at which photosyntheticproducts move from the lamina, and any treatment thatincreases root growth rate and accelerates translocation ratealso increased NAR. Photosynthates accumulate steadily inthe leaf, while NAR does not vary; so it is not accumulationof products that inhibits Po but the rate of movement ofphotosynthates from the source that isthe important factor.May (34) discussed the possibility that Po in pasture

plants is limited by rate of translocation, causing assimilatesto accumulate in the leaf. Brenwer (8) found that after peashad begun to flower and fruit, the same assimilating areadelivered fewer assimilates per week than before, and hethought that inhibition of growth caused assimilationproducts to accumulate and to decrease assimilation. Went(72) postulated that in some circumstances Pr might belimited by growth rate; he suggested that translocation ofphotosynthates out of the leaf cells also controlled Pr.

LIMITATIONS IN SOURCE AND SINKSome studies show that the sink is a limiting factor forachieving higher yields. Other studies suggest that the sourcelimits yields. The source is the powerful system that createsthe sink and sustains it. It is therefore natural that the sourcedetermines sink capacity and any alternate, secondary, andmetabolic sinks. In considering improvements in cropplants, it is necessary to identify whether the yield-limitingfactor is the source or the sink.Source as a limiting factorIn several cereals, including traditional unimproved ricevarieties, at high N levels, the yield-limiting factor is thesource. An improvement of the source through an improve-ment in plant type can remarkably improve grain yield.The leaf is the major source in crop plants, although other

organs can assimilate C. Leaf clipping and organ shadingstudies (14, 29, 63) have indicated that the leaf is a majordeterminant of the nature of grain filling.In rice, the leaves contribute about 51% of the grain yield

(63), while nonstructural carbohydrate contributes 15%,thestem up to 18%, and the panicles up to 16%. In rice the earcontributes less - only 8-23% (14) - while in wheat the earcontributes 10-49% (7,29). There ishardly any contributionof reserves in wheat under normal conditions, while understress conditions accumulated reserves are utilized to varieddegrees. Thus, the leaf emerges as a major contributingfactor that is unfortunately vulnerable to various stresses inthe field, like moisture stress, low light intensity, low andhigh temperatures, mutual shading, insect pests, anddiseases. In a shading study (63), stem weight, total dry


11/20

0 F NF F NF F NF F NF0 2 4 6 8 10 0 95 5 95 5 93 7 90 10LAI 50 92 8 92 ~ 92 8 92 8

lO. Relationship between source and sink in rice (63). 100 90 10 94 6 90 10 93 7

matter, tillers, grain yield, and grain filling were severelyaffected, although change in leaf weight was insignificant(Table I). Although the leaf is endowed with capabilities, itsfunction IS subject to vanous stresses operating undercompetitive and isolated systems.When the source and the sink were varied by adopting

different population densities and N levels, the increase inLA I (source) was not reflected in increased grain number.Grain number was far lower than spikelet number (Fig. 10),indicating the possibility of obtaining higher yields withproper support from the source (63, 65). The increased LAIleads to considerable production of spikelets, but a sizablenumber do not fill. Similar data from other importantcenters in India (viz., Cuttack, Pattambi, Mandya, andCalcutta) also confirmed that, despite increased LAI, thegap between spikelet number and grain number remainsvery wide. This is now recognized as a general phenomenonin several modern rice cultivars.A path coefficient analysis made with LAI, total dry

weight (source), spikelet number (sink), and spikelet sterility(39) revealed LAI is the major source component exertingthe largest direct influence (negative) on sterility, followed

Table I. Effect of continuous shading (40-50% of natural l ight) on drymat ter distribut ion. tillers. fill ed grains, and grain yield of rice varietySona, 1973 dry season (63).

Character Control ShadeLeaf weight (gi hill)Stem weight (g! hi ll)Total dry weight (g/ hill)Tillers (no/hill)Grain yield (gim')Filled grain ( o / n )Sterilitv (o /n)

2.513.145.018.0820.077.023.0

2.46.522.310.0275.059.041.0

80 I Sprkelets 100 kgN yo-5717+15053x-0.9626x22 Grains 100 kgN yo-2 709 + 10.970x-0.767853 Spikelets 200kgN yo-16831+17.39Ix-0.7485x4 Grains 200kg N yo-5113+9.844x-0.522x2 3

o_ _ _ _ _ _ _ _ . s ; > I

30

o;,';' r--", 0 _---~_..:,.-- .." -r _ -~---2,';;, . . . - - . . _ _ . . . .., ' ~ , . e . . - " " " " ' "" , , _ . .. - -/~ 0 ./ .

//, .,/'//. yo, .',,20

IRPS No. 125, January 1987 II

by sink or spikelet number (positive) (Table 2, 3). Sourcecapacity exerted the largest influence on sterili ty and henceshould be enhanced to realize higher filled grain number.In several studies in rice in tropical and subtropical

climates, the proportion of partially filled grains and chaffwas higher, and the gap between spikelets and grains wasseen at all LAI levels. The gap was wider at higher LAIlevels. Such a situation raises the question of whethersterility isa prefertilization or postfertilization problem. Ifitisa prefertilization problem, the source isout of the picture,and the factors concerning failure of fertilization are to beblamed. Observations of fertilized and nonfertil ized spike-lets (Table 4) decisively show there is no apparent constraintoperating at the fertilization level irrespective of the N statusof the crop (65). In addition, the data on chaff, partiallyfilled grains, and filled grains even at different N levels inferthat the former two fractions are high and suggest thatphotosynthates do not adequately reach the spikelets toconvert them to grains. The source IS probably unable tomeet the demands of the sink in competitive communities.Table 2. Correlation coefficients of source and sink capacity (39).Variable" r-value t,YX I -0.42*YX 2 0.56**YX , 0.57**X,X, 0.44**X,X, 0.08X,X, -0.15"X, = total dry weight at flowering, X, = leaf area index at flowering.X, = spikelet number. Y = spikelet steril ity. "* = Significant at the 0.05level, ** = significant at the 0.0 I level.

Table 3. Direct (diagonal) and indirect effects of source and sink onsterility (39)."

Source Sink TotalCharacter correlationTDWat LAI at Spikelet with steri-flowering flowering no. lity (%)"

TDW at flowering(source) -0.28 -016 0.02 -0.42

LAI at flowering(source) -0.13 -0.38 -0.05 -0.56"

Spikelet no.(sink) -0.02 0.25 0.34 0.57

"Residual factors = 0.74, coefficient of deterrninauon = 0.4524.TDW = total dry weight , LAI = leaf area index. "* = Significant at the0.05 level.

Table 4. Fertilized (F) and nonfertilized (NF) spikelets (%) in short-duration and medium-duration varieties, 1974 wet season (65).

Short-duration varieties Medium-duration varietiesSonalevel(kg/ha) Cauvery IEI"I444 RP79-14 .Iaya RF4-14--F NF F NF

96 4 93 7% 4 93 795 5 94 6


12/20

12 IRPS No 125, January 1987

Parameterfable 5 Effect of sbading (45-50% of natural light) during flowering to maturity on sterility and yield components of rice, 1974 dry season (65).

Sona Vijaya RP4-14Control Shaded Control Shaded Control Shaded20 52 27 41 17 4720 31 17 21 12 1460 17 56 3R 71 39486 440 421 358 595 57330.0 11.1 43.7 18.1 48.1 2H9.9 3.8 9.8 4.6 10.1 4.2

Chalf ( ' Y r . )Partia lly fil led grains (%)Filled grains (%)Panicles (no.. 111 ')Grains (no. x 10\/1112)Grain yield (t/ha)

I ,OOO-grain weight (g)Rika

adger

30OL_~--~--~~--~~

45

40

b =0.56:t 0.126 b=028:t0078 4 5 63 4 5 oGrain weight (t Iha)

I I . Regressions of 1,000-grain weight on grain yield for 5 barleyvarieties, introduced between 1951 and 1963, and for Kenia, intro-duced in 1934 (Data from trials of National Institute of AgriculturalBotany) (60).

Th tS raises anot her question - whether the supply or thetranslocation of phorosynthates is a limitation in grainfilling in several cereal crops. To clarify this point, data onchaff. part iallv filled, and filled grains, and grain yield werecollected from a crop shaded from flowering to maturity.The chaff and partially filled grain percentages were veryhigh in the shaded crop, the chaff being 52% in Sona and4 1 (( in Vijaya and Prakash (65). The partially filled grain

percentage in the shaded crop was 31 % in Soria, 21 % inVijaya, and 14% in Prakash; the partially filled grainpercentages were 20% in Sona, 17 % in Yijaya, and 12 % inPrakash (Table 5). Thus, inadeq uate leaf support or supplycan cause a high proportion of unfilled grains and, therefore,assimilation or support could be more limiting than trans-location. Under competitive conditions, mutual shadingmight still reduce the supply of the photosynthates. How-ever, the environment, particularly a combination ofmoderate temperatures and moderate light intensity, mightenhance the activity of the source and narrow the gapbetween spikelets and grains .Thorne (60) showed that the increased grain yield of

modern barley varieties is closely correlated with increasedgrain weight. Obviously at the present level of grain yieldsand climate in England, yield capacity does not limit thegrain yield of these barley varieties, but the activity of thesource, perhaps controlled by light intensity during ripening,limits yield (Fig. II).Sink capacity limits yieldEvans (15) listed evidence that photosynthetic capacity doesnot limit yield. Leaf size has increased more than Po hasfallen in the course of evolution, but since ear size hasincreased even more than leaf size, the supply of assimilatescould not have been limiting.The balance sheets of supply and demand for assimilates

throughout grain development for several productive wheatvarieties grown under controlled conditions show that evenduring peak demand, ample amounts of assimilates areavailable for grain filling (16).In the field and under controlled conditions, shading or

leaf removal have only small effects on grain growth andyield, implying that the supply of assimilates is not limiting.Rawson and Evans (46) reported that increases in flag leafPr and in the mobilization of stem reserves compensate forinhibition of ear Po. Another indication of surplus supply ofassimilates is the almost linear increase in grain weight perear during the middle period of grain filling under controlledtemperature despite substantial variations in incident dailyradiation found in Triple Disk wheat (45) and maize (13).Treatments involving sterilization of the most advanced

florets in ears of Triple Disk wheat at anthesis increased thetotal grain set and yield per ear by 20 % (45), implying thatassimilate supply does not limit grain yield.


13/20

Lowland rice lends itself to a different, but less conclusive,approach to the question of whether photosynthetic capacitylimits yield. Murata (37) found a good correlation betweenthe Pr of the flag leaves of six rice varieties and their cropgrowth rate, but little correlation between their Pr and grainyield at a given level of N, again suggesting that the supply ofassimilates did not limit yield. Yin et al (74) and Murata (38)reported that yield was closely related to spikelet numberper square meter, suggesting that storage capacity limitedyieldThe C02 concentration in the atmosphere is also likely to

limit grain yield. Yoshida (76) studied the effects of C02enrichment on grain yield before and after heading. Enrich-ment before heading increased grain yield 29%, and afterheading, 21% (Table 6). The yield increase from C02enrichment before heading was caused by increased grainnumber and grain weight. Enrichment after heading did notchange grain number but increased grain weight and filledgrain percentage. Hence, if the yield capacity can beincreased by some means, apparently neither photosyntheticcapacity of the plant, light, nor C02 concentration is likelyto limit grain yield, at least in Los Banos (80).Yoshida (78) further reported a close correlation between

gram yield and gram number per square meter. The grain


weight and filled grain percentage were practically constant,suggesting that total grain number limits the yield of I R8 inLos Banos (Fig. 12). Sink size is the limiting factor for grainyield.Tanaka et al (55) found that the photosynthetic potential

of tomato leaves seems to exceed the requirements ofgrowing organs, and the leaves generally continue to groweven when the fruits are developing rapidly. Thcy alsoreported a continuous accumulation of carbohydrates in thevegetative organs during rapid fruit growth. In the riceplant, on the other hand, there is apparent retranslocation ofaccumulated carbohydrates from the vegetative organs tothe grains during ripening (51), and a similar phenomenon isobserved in maize, although the amount is limited (58).There arc also reports that some leaves of tomato can beremoved without impairing yield (II). The young developingvegetative organs, such as t he apex of the stem or the lateralbuds, do not consume a large amount of photosynthate,because these can photosynthesize from early stages ofdevelopment, unlike in cereals, where the growing points arealways hidden by the leaf sheaths (54).Murata (38) gave three examples for the relative

importance of yield capacity and assimilate supply to grainyield: I) yield capacity (sink) is limiting; 2) assimilate supply

Table 6. Effect of CO, enrichment before and after flowering on grain yield components of IR8. IRRI , 1972 dry season (76)."Time of CO2 enrichment Yield Grain no. Filled Grain weight'In relat ion to Developmental ( t/ hal (lOJ/m2) grains (%) (mg)flowering (d) stage"

Control (no CO2 enrichment) 10.2 c 40.4 c 85.4 b 27.5 dJJ to 24 I 11.3 b 45.0 ab 84.2 b 27.7 d24 to 14 II 11.4 b 43.4 bc 86.2 b 28.7 c14 to 0 III 10.2 c 38.9 c 85.8 b 29.1 beJJ to 0 I-III 13.3 a 48.2 a 87.6 b 29.9 abo to 30 IV 11.5 b 38.9 c 92.4 a 30.2 a

"Means followed by the same Ictter arc not significantly different at the 5% level by Duncan's multiple range test. hI = neck node di fferentiat ion todif ferentiat ion of secondary rachis-branch, II = dif ferentiat ion of spikelets, I II = differentiation of pollen mother cell, reduction division stage toflowering, IV = flowering to harvest. 'Panicles were dried at 75C for 48 h , a nd threshed, then grains were kept at 50C for 12 h and weighed.

Grain Yield (tlho)10

Fil led grains (%)100

10 20 30 40 10Grains (XI03/m2)

(g)

12. Relationships between total numberof grains per square meter and grainyield, filled grain percentage, and 1,000-grain weight (0 = dry season, = wetseason, \7. = di rect seeding) (78).

20 40 500


14/20

14 [RPS No. 125, January 1987

Yield It/ha) /96/ I

/962

5.6 5.2 . . 48 t#o T I32 36 40Spikelets 1103/m2)

60Filled grains 1% )

Yield It/ha)8

7

Early culturedV Slightly earlyX Ordinary seasan

6O'----'------__j-----__j3 4

Spikelets I 104/m3 )5

13. Relationships of brown rice yield to spikelet number and percentage of completely filled grains of rice plantscultured under various combinations of plowing depth, planting density, and N level (38).

(source) is limiting; and :1) yield capacity and assimilatesupply are well balanced (Fig. 13). Such instances are duemainly to the cultivar. It is possible to have all thesecom binations in other crops. Identifying such varieties helpsgreatly In formulating cultural and nutrient management formaximizing productivity.

SOURcr-SINK MECHANICS

Both source and sink could grow independently, or theirgrowth could be based on some ratio. In crop plants, threedifferent sit uations can be visualized. I) the source could begreater than the sink in physical area; 2) the sink size couldbe greater than the source; and 3) both could be in dynamicequilibrium.

Source greater than sinkTraditional varieties in several crop plants possess greatersources than Sinks because they are leafy. In the course ofcrop improvement, leafiness is reduced and sink sizegraduallv mcrcaxe s. Greater source capacity leads to poorcrop pcrlormuncc as fertilization and other cultural practicesresult 11 1 greater foliage and poor productivity. The bestexamples are the cereals, pulses, oilseeds, and a variety oftrees. shrubs. and annuals.E\ en within a crop or cult ivar, there are shifts in source-

Sink balance based on nutrient availability, environmentalcond it ions. and the soil-water system. Among cereals,

particularly in rice, maize, and wheat, at low N levels, theplant produces enough leaves but poor panicles withunproductive tillers. Nutrient deficiencies (e.g., P, Zn, Mn),toxicities (e.g., Fe, AI), salinity, or alkalinity normally leadto adequate leaves but very poor returns of grain. In all thesesituations the source is greater than the sink size.Sink greater than sourceIn some fruit trees (e.g., mango, citrus, guava), vegetables(e.g., cucurbits, tomato), pulses (e.g. pigeon pea, gram),oilseeds (e.g. ground nut, gingelly, sunflower), and cotton,the sinks in terms of flower buds, fruit set, pegs, pods, andbolls arc greater than the sources. But they prematurelydrop due to inadequate assimilate supply, the formation ofan abscission layer, or both. In mango, citrus, and guava, anincrease of 1-2% in fruit set may lead to a 50-100% increasein crop yield. Hence, they call for appropriate nutrient andcrop management, besides genetic improvement in thecapacity of the source.Even in cereals and other grain crops, breeding has

resulted in greater sink size. In several situations, althoughthe sink size is adequate, grain yield is poor. Even in rice,increased LAI is not associated with increased grainproduction but reaches a plateau (63). The gap that existsbetween spikelets and grain increases with increased LAI.These cultivars and crops require proper treatment toenhance the functional capabilities of their sources. In somesituations, their physical leaf area is adequate and even morethan required, but the functional efficiencies are far lower.


15/20

Source and sink in equilibriumThe sink In rice, particularly the number ofspikelets formedper square centimeter of leaf. was projected to elucidate theucnd-; between source and sink in physical terms. Thenumber of spikelets formed per square centimeter of leaf atdifferent I.AI values was worked out (65); based on thosecalculations. the possible spikelet production at differentlAl levcls was determined. The actual spikelet number att hc respect ive LA I levels served as checks. There werevarietal differences by duration and genotype. In Jaya(medium-duration type), at 0 N level. the spikelet numberper I LA I was 8,500. which meant there could be 34,000spikelets per around 4 LAI; but actually only 23,000spikclets could be realized, suggesting that at lower N levels.sink size was smaller than the source, But at 50 and 100 kg Nlevels. spikelet production increased proportionately withhigher LAI values (around 4-5), suggesting a tendency forequilibrium between source and sink. Interestingly, whenthc same analogy was applied for RPW6-17 (late type),source and sink were in equilibrium at all N levels.Obviously. in several irrigated rice varieties grown innonproblcrn soil. source and sink are in equilibrium exceptin situations where nutrients, particularly N, are at lowerlevels.Apparently, either a larger source or an increased sink

si/c results in lower productivity; despite full expression ofthc sink. the realization of yield is lower due to smaller sinksize in the former. and to partial realization of sink in thelatter. When both are in equilibrium. there is the risk oflower and unstable yields, because the source is moresensitive to climatic. water. soil. nutritional, and bioticfactors. However. when the environment and eli mate arefavorable, high spikelet number is associated with high ormatching filled grain number. Therefore. instead of largephysical dimensions of the source, greater and more stablefunctional efficiency at moderate source size are moreadvantageous to realize the potential sink size under fieldconditions. Thus. based on crop and variety. differentsituations exist among source-sink combinations. Suchanalysis can clarify the goals of varietal and crop manage-ment programs.

INCRFASI;\iC SI:\K SIZE fOR ENHANCED YIELDPOTE:\TIAI.S

Higher sink size could be realized through appropriateblend ing of fertilization and cultural management in a givenenvironmcnt. In cucurbits. fruits like mango and citrus.fiber crops Iike cotton, and pulse and oilseed crops belongingto the Legurninosae. there is considerable flower. prematurebud. and fruit drop. The potential sink size is always high.but the realized sink is lower. Hence, in these cropsrealization of higher sink or yield depends mainly onsustaining thc existing sinks. Cultural practices aimed toretain more buds would give higher yields. Increasing thesi/e of fruits and vegetables also increases yields eon-

IRPS No 125 January 1987 15

xiderably: in some the sizes could be increased 50%.Moreover. the large proportion of fruits and vegetablesbelow normal size indicates score for enhancing theirpossi ble yields.In cereals, particularly in rice and wheat. sink size is

increased through appropriate combinations of cultural,nutritional, and water management factors. However, thepotential has a limit beyond which it is almost impossible toincrease sink size in terms of number. The differences aregenerally attributed to variety. In cereals like rice, potentialsink size is based largely on spikelet number, while therealized sink in terms of filled grains is invariably lower.The development of inflorescence of maize, wheat, rye,

barley, and oats is well documented by Bonnet (6). For rice,Matsushima's work CD ) is most extensive and informative.Differentiation of panicle neck node in rice starts about 32 dbefore flowering. The differcntiation of spikelets proceedsfrom about 23 to about 15 d before flowering, during whichtime the maximum number of spikelets IS determined.Spikelet degeneration occurs afterwards, during the reduc-tion division stage, which occurs about 11-13 d beforeflowering and is most sensitive to degeneration. The size ofhull is determined I wk before flowering. Thus, the potentialsize of the rice panicle is determined largely during theperiod from 32 to 15 d before flowering. After this periodinsufficient nutrition or adverse climate can cause spikeletdegeneration.

From thc physiological point olvicw, panicle size may beincreased if more assimilates go to developing panicles. Thiscan be achieved in two ways: by increasing total Po, or bymcreaxmg partition of more axsimilatcs into dcvelopingpanicles. Nutritional factors increase photosynthesis.Another way of increasing Po is through C02 enrichment.Yoshida (76) showed that C02 enrichment fOJ"33 d beforeflowering increased grain Yield 3 (VIc . largely attributable toincreased spikelet number and grain weight, i.e . . increasedsink size.The combination of light and temperature not only

controls spikelet production but conditions the degree ofgrain filling. Moderate light and temperature are ideal forrealizing higher sink levels. High temperatures in associa-tion with either low or high light intensities are detrimental,as arc low temperatures with low light intensities. Yoshida(77) reported that in a controlled environment and within atemperature range of 22-,1 le. spikelet number per plantincreased with lower temperature (Fig. 14). However. forvegetative growth. higher temperature was more favorablewithin the same temperature range. Relatively low tempera-turc around panicle initiation produced largcr sink sizes inrice in tropical and xuhtrnpical conditions.Characterization of sink potential goes a long way in

formulating action plans for their realization. In onc studydifferent panicle numbers were created by adopting variousplant densities and N levels. Beyond 400 panicles m '.spikelet number either was quadratic or plateaud. Grainnumber lollowcd suit. with a gap at all levels and particularly


16/20


increasing at higher levels (66), inferring that total spikeletnumber did not vary significantly despite higher number ofpanicles per square meter. Forty or fifty thousand spikeletsarc accommodated in 400-800 panicles. Thus, the content islimited and does not respond beyond a limit set by thevariety. I n such situations nutrient and cultural operationsshould aim at bridging the gap between spikelets and grains.St ud ies on sink potential would clarify future goals con-cerning not only increased grain filling but also enhancedpotential sink size.Spikelets (no/panicle)300

~~~----------~------------~------------~o 22 25 28Mean temperature (OC)

14. Spikelets per panicle vs daily mean temperature, 25 d beforeflowering (78).

Groins (1031m2 )45

40

CRMIO-4660-19

35

30

25 Mohsun Ponkcj

ET4086201 5

10

16I,OOO-groin weight

Some studies have indicated that in rice, grain number iscompensatory with grain size beyond a I,OOO-grain weightof 39 g (24). In another effort (68), there was no compensa-tion in short- and long-duration rice varieties (Fig. 15); itwas felt that grain number could be increased at a 1,000-grain weight of 22-24 g. Obviously, increased spikeletnumber at a given size and enhanced size at a given numberwould ultimately increase sink size. This situation calls forassessment of genetic materials for number and size.Growth-regulating substances are used to enhance sink

size and stimulate sink activity. Foliar spray of gibberellicacid (GA) at the reproductive stage increased spikeletnumber and hence grain yield (50). In wheat, injectingchlormequat chloride (Ccq into the stem at anthesisincreased grain number but decreased the weight of singlegrain. I n barley, cytokinin activity in caryopses decreasedrapidly after pollination during ripening. De-awning orexcessive moisture decreased cytokinin activity, which wasassociated with larger grain size (35). These studies suggestthat spikelet number or grain setting can be increased bycontrolling hormonal activity without any increase in Po.

SOURCE POTENTIAL

31 In nature, chlorophyll is abundant. The leaf is several timeslarger than the seed, fruit, vegetable, or tuber. Probably thisevolved to make the source-sink system a risk-free enterprise.In several crops, improved cultivars possessed greater leafarea, which was sensitive and responsive to climatic,

15. Relationship between grain weight andgrain number in rice (68).


17/20

cultural, and nutrient factors. Scissonng of such leaf areahrought the revolution 111 production potentials in modernvarieties, particularly 111 cereals.Leaf number, size, thickness, position, angle, rate of

development, and functional abilities determine the level ofcrop performance. In modern rice and wheat varieties,excessive leaf area does not lead to lodging or lowerproductivity as in traditional varieties. However, since itdoes not enhance productivity or grain filling, efficiency is amore Important consideration.In one study. the leaf area was manipulated to the level of

II l.AI, but the spikelet and grain numbers did not change;Increased source size did not help in increasing grain number(66). This means that the source (leaf), even in modern types,is still more responsive than the sink (panicles). The leaf isbecoming thinner and probably losing its functional effi-ciency, as it was compensating through thickness. Promot-ing functional ability in stress conditions and controlling theresponsive and reactive nature of the source would enhancepotential yield. The question of whether we should prefer athicker or thinner leaf has not been conclusively answered,although thinner ones appear to be better in diffused lightand thicker ones in bright light. The reflection and absorp-tion of rad iation and the sustenance of the leaf in relation todifferent degrees of thickness have to be worked out beforeany valid conclusions on the desirability of the nature of thesource can be drawn.Finally, it boils down to determining what is limiting in

furt hering yield potential. Is it the sink or the source?Experience has shown that in japonica rice, all spikelets arefilled 111 several favorable situations, and therefore manybelieve that further improvement in the sink would enhanceyield potential. In indicas, a clear gap exists betweenspikelets and grains in several tropical and subtropicalclimates, suggesting tha the source IS the hmitation.Identification of the yield-limiting factor is the key to theconsideration of a rational method to further improve grainyield

AGRONOMIC MANIPULATIONS OF SOURCE AND SINKAgriculture is the manipulation of the source and the sinkfor reali/.ing higher productivity. Nutrients are addedmainly to enrich and efficiently manage the source. Nutrientapplication enriches the chlorophyll system and favorsfunctional efficiency, which results in luxuriant growth.Nutrients are also applied around flowering to early grainfilling in some cereals, fruits, and vegetables to catalyze thegram- or fruit-filling process, leading to higher productivity.Cultural manipulations involving population density, roworientation. time of sowing, and monocropping all facilitatethe efficient functioning of the source. Even the plant typeconcept mainly concerns the greater utilization of solarenergy and nutrients through the functional efficiency of theleaves.


The principles of fertilization in agricultural crops areoriented not only to source efficiency but also to sinkproductivity. Spraying growth-regulating compounds ismeant not only to stop preharvest fruit drop, but also t 'enhance the size and quality of \I g ab = > , :" ! l sad fib-rs.The uses of GA in grapes and guava; naphthalene acetic acid(NAA) in mango, guava, and citrus; and CCC and NAA mcotton are good commercial practices Sink size is mani-pulated for higher sink realization. Cultural practices likepruning 111 grapes and tailoring of cucurbits are efforts toincrease sink size. Even selection pressure 111 crop plants ISsink-oriented. Modern technology is concentrating on finermanipulations of sources and sinks for enhancing potentialproductivity.Sink size is apparently limiting in cereals because of the

existing wide gap between theoretically possible yields (20-24 tjha in rice) and actual maximum Yields with modernplant types (8-16 t!ha). However, one can also argue that atthe present level of evolution, both source and sink arelimiting; if one is increased the other is likely to decrease, andvice versa. But the rnam question is whether in contemporaryvarieties the source or the sink size is limiting. This answerwould clarify the level of manipulation required.Tanaka (53) discussed the following historical sequence

in rice improvement:I. improvement of the s.nk by making the grain sizelarger,

2. improvement of the source through an increase in leafsize or improvement of the sink through an increase inspikelet number per panicle,

3. expansion of the sink size through an increase inpanicle number by making tillering ability morevigorous, and4 improvement of th ource by making the leavessmaller and more erect to minimize mutual shadingamong leaves.

A shorter stem was an important change, because thisincreased resistance to lodging. Thus, improvement meanttailoring of both source and sink. The source (leaf) in its size,thickness, color, life duration, nature of senescence, arrangemerit, and capacity ments 'r'p ensi e study Fieldstudies (63, 65) have conclusively shown a wide gap betweenspikelets and grains, which increases with increased LA!.Therefore, improving the capacity of the source goes a longway toward increasing the realization of potential yields,even by 20-40%. Hence, the capacity of the source deservespriority.But one should also consider the importance of the

capability of the sink, i.e., the functional ability to stimulatethe production of photosynthetic products and their accep-tance, in several crop plants and particularly in a majorcereal like rice. I t was shown that in wheat (7, 29) thecontribution of the panicle to yield (10-49%) is greater thanin rice (8-23%) (14, 63). But the sink capacity is moregoverned by certain enzymes like phosphorylases and


18/20


amvlascs besides the endogenous hormonal level, aboutwhich little is known.It \., important to investigate assimilate production during

ripening and its partitioning into grain, including studies onthe balance sheet, rates of flow, and diversion toward otherorgans. Obviously, sink components interact differently,calling for inquiry into their interrelationships. As Tanaka(53) stated, improvement of source and sink follows a zigzagpath: we move in one direction, get stuck, and divert ourroute based on the conditions. At every phase we have toassess the organ to be improved in our efforts to attainhigher yields.The information assembled in this review clarifies two

unportant issues: The source deserves priority in enhancing the realiza-tion of the sink in crops and their modern cultivars.lhis situation calls for proper evaluation of greenenergy resources with appropriate characterization.

Sink capacity and activity deserve equally seriousconsideration in producing greater sink size. Variouscomponents of the sink in different crop plants must becharacterized, and the possible ways of improving themdeserve priority.

Naturally, strategies to improve both sink and sourcesimultaneously would help to achieve good dividends incrop plants. Recently, Choi (9) found that all sink charactersexcept grain yield are regulated mainly by additive geneticcomponents. Potential kernel size and sink capacity perpanicle also showed significant dominance effects andnonallelic genic interaction for sink capacity per tiller. Theseare promising indications of possible manipulations forfurthcring yield potentials through source-sink concept.Thus, the source-sink system deserves priority in researchpursuits in crop plants.

RFFERFNCES CITEDAsacia. K., S Konishi, Y. Kawashima, and Z. Kasai. 1960. Transloca-tion of photosynthetic products assimilated by the top leaf to the ear ofr ice and wheat plants. Mcm. Res. lnst. Food Sci. Kyoto Univ. 22: I -II.

2. Hart, R.t.. 1964. Carbohydrate utilization as a factor in plant growth.Ausl..I. BioI. Sci. 17:867-877.

3. Beevers, H. 1969. Metabolic sinks. Pages 169-184 in Physiologicalaspects of crop yield. American Society of Agronomy and CropScience Society of America, Madison, Wisconsin, USA.

4. Hiclcski , R.L. 1960. Characteristics of sugar uptake in slices of matureand immature storage tissue. Aust J. BioI. Sci. 13:204-220.

5 Hiclevki. R.I .. 1962. The physiology of sugarcane. V. Kinetics of sugaraccumulation. Ausl. . I. BioI. Sci. 15:429-444.

6. Honncu. 0.1' 1966. Inflorescences of maize, wheat . rye, barley andoats: Iheir initiation and development. University of Ill inois, College ofAgr icul ture. Agric. Exp . Stn. Bull . 721.

7. Boonstra. A. E.H.R. 1929. Meded. Landbouwhogesch, Wageningen.H3-21

K. Hrcnwcr. R. 1959. The inf luence of temperature on the development ofpea" . .Iaarb. I.B.S. 17.

I). Choi, H.e 19K5. Studies on the inheritance and selection of somecharacter- related to source and sink in rice (OrFza saliva L.) [Englishsummurv]. Res. Rep. Rural Dev. Admin. (Korea) Crops 27 (I ):50-82.

10. Cock, J.H., and S. Yoshida. 1973. Changing sink-source relations inrice (On'zll sativa L.). Soil Sci. Plant Nutr. 19:229-234.

II Cooper, A ..I.. .I.G. Large, P. Proctor, and J.B. Rothwell. 1964. De-leafing of glasshouse tomatoes. Exp, Hortic . 11.1-6.

12. Crafts, A.S. 1961 Translocation in plants. Holt. Rinehart andWinston, New York. I g2 fl.

13. Duncan, W.G.,A.l.. Hatfield,and.l.L. Ragland.1965.Thegrowthandyield of corn. II. Daily growth of corn kernels. Agron. J. 57:221-223.

14. Enyi, B.A.C. 1962.The contribution ofd iflcrent organs to grain weightin upland and swamp rice. Ann. Bot. N.S. 26:529-531.15. Evans, L.T. 1972. Storage capacity as a limitation on grain yield . Pages

499-511 in Rice breeding. International Rice Research Insti tute , P.O.Box 933, Manila, Philippines.

16. Evans, L.T .. and H.M. Rawson. 1970. Photosynthesis and respirationhy the flag leaf and componcnts of the year during grain developmentin wheal. Aust. .I. BioI. Sci. 23:245-254.

17. Gifford, R.M., G.H. Thorne, W.O. Hitz, and R.P. Giaquinta. 1984.Crop productivity and photoassirnilate partitioning. Science225801-808 .

IX. Hartt, C.F .. H.P. Kortschuk , and G.O. Burr. 1964. Effects ofdefoliation. dcradiation. and darkening the blade upon translocation ofC" in sugarcane. Plant Physiol. 39: 15-22.

19. Hatch. M.D .. and K.T. Glasziou. 1963. Sugar accumulation insugarcane. II. Relat ionship of invertase act ivity to sugar content andgrowth rate in storage tissue of plants grown in controlled environ-mcnts. Plant Physiol. _\g:344-348.20. Hatch. M.D ... I.A. Sacher. and KT Glasziou. 1963. Sugar accumula-tion cycle in sugarcane. I. Studies on enzymes of the cycle. PlantPhysiol. 38:338-343.

21. Hendrix. J.E., P.R. Larson, R.F. Evert , R.T. Graquinta, .I.M. Ferries,and S. Sovonick-Dernford. 1979. Source to sink cont rol of assimi lateparti tioning. Pages 6-9 ill Partit ioning of assimilates. summary reportol a workshop by the ASPP.

22. Humphries. E.C. 1963. Dependence of net assimilation rate on rootgrowth of isolated leaves. Ann. 1301. N.S. 27(15):175-IX3

23. Humphries, E.e, and G.N. Thorne. 1964. The effect of root formationon photosynthesis of detached leaves. Ann. Bot. 28:391-400.

24. International Rice Research lnxtitute. 1977. Annual report for 1976.PO Box 933. Manila. Philippines. p. 222-223.

25. Khan, A.A., and G.R. Sagar. 1966. Distribution of C" labelledproducts of photosynthesis during the commercial li fe of the tomatocrop. Ann. Bol. 30:727-743.

26. Khan. A.A., and G.R. Sagar. 1969. Alteration of the pattern ofdistr ibution of photosynthetic products inthe tomato by manipulation01the plant. Ann. Bot. 33:753-762.

27. King, R.W .. I.F. Wardlaw, and LT. Evans. 1967. Effect of assimilateuti lization on photosynthesis in wheat. Planta 77:261-276.

28. Koch, K.E., CO. Tsui, LT. Schrader, and O.E. Nelson. 1982.Source-sink relations in maize mutants with starch deficient endosperrns. PlantPhvsiol. 70:322-345.

29. Kricdernann, P. 1966. The photosynthetic activity of the wheat car.Ann. 1301. N.S. 30:349-363.

30. Kursanov, A.L. 1963. Metabolism and the transport of organic solutes.Adv. Bol. Res. 1:209-278.

31. l.afiue. H.R .. and R.L. Travis. 1984. Photosynthesis and assimilatepar tit ioning and closely related lines of rice exhibit ing different sink-source relationships. Crop Sci. 24:447-452.

32. Mason, T.G., and E. Phillis. 1957. The migration of solutes Bol. Rev.3:47-71

33. Matsushima, S 1970. Crop science in rice. Fuji Publishing Co., Ltd.lokyo. 379 p

34. May. L.H. 1960. The utilization of carbohydrate reserves in pastureplants after defoliation. Herb. Abstr . 30:239.

35. Michael. G. . and Scilcr-Kclbitsch 1972. Cytokinin content and kernelsize of harley grain as affected by environmental and generic factors.Crop Sci. 12162-165.

36. Milthorpe. F.L. 196.' Some aspects of plant growth. Paged-16inThegrowth of the potato . . I .D.lvinsand F.I . Milthorpc.cds. BuuerworthsScientific Publications. London.

37. Murata, Y. 1965. Photosynthesis, respirat ion, and nit rogen response.Pages 385-400 in The mineral nutri tion of the rice plant . Proceedings ofa symposium at the International Rice Research Institute, February,1964.10hns Hopkins Press, Baltimore, Maryland.


19/20

38. Murata . Y 1969. Phy sio log ical responses to nitrogen in p lants. Pages2 .15 -26 .1 ill Physio log ical aspects o f crop y ield . .J . D . Eastin, F .A .Haskins. C Y. Sullvan, and C H . M . Van Bavel, eds. American Societyo f Agronomy . M adison. W iscon .n

.W . M urthv . I'.SS .. ;111(1 K S M urthy . 1983 . Path coefficrent analy sis o fsourcesink Oil sp ike le t ste rility in rice i Orv:a saliva 1..). A ndh ra A grie ..1 . 1( )( I )67-68.

40 . N calcs . T F . and I .. D . l nco ll. 1968 . The control of leaf pho tosy nth esisra te bv level of assim ila te concentration ill the leaf: a review of thehvpothcsis . B o t. R ev . .1 4: 1 07 -1 25 .41 Nosbcrgcr . .I. and LC. H um phries. 1965. The influence o f r em o vi ngtubers on dry m ailer production and net assim ila tion rate of potatopl.m tx Ann B o t 2 9. 57 9- 5/ (d

4 2 N os be rg er .l and C i N Thorne 1965 The e ffe ct o f T m ov in g florets orsh;ld ing the car of barley on production and distribution of dry m atterA I lI l. B o t. 2 96 .1 5- 64 4.

4.1. Palmer A .F E 1969 Translocation pattern of C 14 -la be lle d ph oto -svnt h atc in th e co rn plant during the car filing stagc. Ph D thesis.Cornell U niversity . N ew York.

44. Quin lan . .I.D .. and C .R . Sagar 1965. G rain y ield in two contrastingvarie ties of spring w heat. A nn . H ot. 29:68.1-697 .

45. R awson . H .H .. and I..T Evans . 1970. The pattern of grain g row thw ith in thc car of w heat Aust . I. B io i S ci. 2 3: 75 3- 76 4.

46. Raw son. H H .. and L .T. Evans. 1971 The cont nbuuon of s temre se rv es to g ra in development in a ran ge of w heat varie ties o f differentheight . A ust , . I. A g ric . R cs . 2 2. 85 1- 86 3.

47 . Sacher . .I.A .. M .D . Hatch . and K .T Glasziou. 1 96 3. S ug ar accumula-t ion cy cle in sugarcane. III. Phy sical and m etabolic aspects o f cycle inim mature sto ragc tissues . Plant Ph ysiol. .1R :348-354 .

4X . Sw anson. C A. 1959. T ranslocation of o rganic so lutes. Pages 481-545ill Plant physiology. vol. II. F.e. Steward . ed. Academ ic P ress . NewYork .

49. Sw cet.(j.B .. and P.F. W areing 1966 . Role o fp lant g row th substancesin reg ulatin g ph oto sy nthes is. N ature (Lo ndon ) 210:77-79.

50 Tak ahaxlu . K . K . Sato. and T. Fuuno 19 72 . E ffe ct o f g ib be re llic a cidtreatm en t on th e grow th a nd v ie ld of n ee plant. Rep Tohoku B r. CropSC I S OC .I pn. 14:43-46 .

51 Tanaka.A . 1961 S tudies on th e nutrio-physiology of leaves of ricep lan t. .I. Fac. A gric . H ok kaido U niv . 51 :449-550 .

52. Tanaka. A . 1972. The rela tive importance o f th e source and th e sink asth e y ield lim iting factors of rice . ASPAC FFTC Tech B ull. No .6 .

5 .1. Tanaka A . 1980. Source-sink rela tionsh ip in crop production. Tech .B ull. AS PAC Food Fert. Technol. Ccn t. 52 : 1-18.

54. Tanaka. A .. and K . Fujita . 1974 . Nutrio -physio log ical studies on thetom ato plan t. I V. Source-sink rela tionship and structure of th e source-s in k un it. S oil S Ci. P la nt N utr. 20 (3 ):3 05 -3 15 .

5 5. T a na ka . i\.. K. Fujita, and K . K ikuch i 1974 . N utrio-phy siolo gicalstudies on th e tom ato plant I. Outlne o f grow th and nutrien tabso rption. So il SCi. Plan t N utr 20( I ):57-68.

56. Tanaka. A .. K Fujita. and M . Shio ya. 1974 . N utrio -phy sio log icalstudies on the tom ato plan t. II. Translo cation ofphotosy nth ates . So ilSci. Pia III Nun 20(2):16.1-171.

57 . Tanaka. A . . K . Kaw ano. and Y . Yamaguch i. 1966. Pho tosyn thesis .resp iration. and plan t ty pe of th e tropical rice plan t. In ternational R iceResearch Institu tc Tech . B ull. 7.46 p .

'iX . Tanaka. A . . and .I. Yamaguch i. 1972 . D ry matter production . y ie ldcomponcntx and grain vicld of the maize plant. J. Fac. Agric.H o k k a ido U niv. 5 7: 71 -1 .1 2.

59 . Thorne. G . N . 1965. Pho tosynthesis of ears and nag leaves o f w heat andb arlc v. A nn . B ot. 2 9:3 17 -.1 29.

60 . Thorne. G .N . 1966. Physiological aspects of g rain y ield in cereals.P ag es 8 8- 10 5 in The grow th o f cereals and grasses . F . I . . . M ilth orpe an d.I.D . Ivin s. eds . B utterw orth s Scientific Publication s, L on do n.

6 1 T IH lr l1 e. G .0 i . . " nd A .F F va ns .1 96 4. ln fl uc n ce of to ps an dr oo ts on n etassim ila tion rate o f sugar beet and spinach beet and grails betw eenthem. Ann Bo t. 2 B :4 9 9- 50 8 .

62 . Txunoda. S . 1960. A developm ental analy s is o f y ie lding ability inv .u ic tic -, o f f ie ld c ro ps . III. The depth of g reen co lor and the nitrogenconten t of leaves . .lpn . .I. B re ed . 1 0: 10 7- 11 1.

6.1. V cnkurcsw .ulu . B . 1976 . Source-sin k in terre la tion sh ips in low lan d rice.P la nt S o il 4 4( .1 ): 57 5- 58 6.

64 . V cnk atcxw .n lu. B . 1977. In lluence of low ligh t intensity on grow th andpm ductiv itv o f rice t Ori :a sativo 1...). P la nt S oil 4 7:7 1J -7 19 .

IRPS No. 125,January 1987 19

65. V enkatesw arlu , B ., and 1 '. Y. M addulety . 1976. Chang ing source andsink rela tionsh ips in rice (Orvz sativa 1..) th ro ug h differen t can opyp ro file s un de r fie ld c on ditio ns . ln dia n J Plant Physio l 19(2) 194 20 I.

66 . Venkatesw arlu , 13 , S Padmaj Ra.: and P.I. Achary ulu . 1984Studies on Y ield poten tial and the in ter-relauonships of Sink cern-ponen ts In nee. lndian J. Plant Physiol. 27(4):313-321

67 . Venkateswarlu. B . . F .T Parao, an d B S. V ergara . 1986 . O ccurrence ofgood and qualny grains in pan icles o f nee t Orvzu saliva 1...). Submittedto Crop Sci.

68. Venkateswarlu, B .. (I.S .V . Prasad. and A .S.R Prasad. 1981. Stud ieson th e nature o f rela tionsh ips betw een grain size . spikelet number.g rain y ield and sp ikelet fil ling of la te duration varreucs o f rice t Orv:asaliva 1..) P la n t S I )] 16 0 :1 2 3- 1J 0

69. W ardlaw . I.F . 1965 The velocity and pattern o f a ss im ila te tra ns lo ca -tion In wheat p lants during gr.un development Aux. J. B io I. Sc18269-281

7(). W ard law . I.F . 1968 . The control and pattern o f movem en t ofcarboh ydrates in p lants . B ot. R ev . 34:79-105 .

7 1. W a re in g, P F. M . M . Khalifa , and K..I. Trchearr-e. 1 96 8. R ate lim itin gprocesses In pho tosyn thesis at saturating ligh t in tensities . Nature220:453-457.

72 . W ent. F . W 1959. The physiology o f photosyn thesis in p lants. Prestia30'225.

T), W yse RY. and R.A Saft ncr 1982. Reduction and sink mobhzinga bih ty lo llo wm g pe rio ds o f 1 1Ig hca rbo n n ux. P la nt P hY SIo l 69 '2 26 -2 28 .

74 . Y in . H C . Y K . Shen . and K .M . Shell. 1958. T ranslocation o fassim ila tes betw een tillers an d leaves in the rice plant during ripen ing .Acta Biol. Exp. S in . 6 : 1 05 -1 10 .

75. Yoshida. S 1972. Phy siological aspects o f g ra in yield. Annu. R ev .P la nt P hy sio l. 2 3:4 37 -4 64 .

76. Yoshida. S . 1973. Effect of CO~ enrichm ent at d ifferen t stages o fpanicle developmen t on y ield components and y ield of rice (Orrcasativa 1...). S oil S C!. P la nt Nutr. 19.111-316.

77. Yoshida. S . 197.\. E ffect o f temperature on grow th o f th e ri e pla nt(O""~(I sauva L) in a contro lled environm en t So' I ScI. Plan t Nutr19299-110

7~ . Yoshida. S. 197:\. S ink and source re la tions In crop plan ts Paperpresen ted at th e D epartm ent of B otany . U nivers ity of th e Philippines, 5Scptcrn bcr 197.1

79. Yosh ida. S ., and S.B . Ahn. 19 68 . T he a cc um ula tio n process ofcarbohy drate in rice varie ties to th eir r es po ns e to nitrogen in th etropics. Soil Sci. Plan t N utr 14:15.1-161

80. Yosh ida. S . . .J H Cock. and F.T Parae. 1972 . Physio log ical aspects ofh igh vield. Pages 455-469 in R ice breeding. In ternational R iceR esearch in stitu te . P .O . B ox 933. M an ila . P hilippin es .


20/20

T he Intern atio nal R ice R esearch Institu teP.o. B ox 933, M anila, P hilippines Stamp

Other papers in this series ISSN 0115-3862TITLES OF NUMBERS 1-45ARE LISTED ON THE LAST PAGE OF NO. 46;THOSE OF NUMBERS 46-70 ARE ON THE LAST PAGE OF NO. 71-80;AND THOSE OF NUMBERS 71-100ARE ONTHE LAST PAGE OF NO. 101-122.

No. 101No. 102No. 103No. 104No. 105No. lObNo. 107No. 108, )0. 1091"0 110No. IIINo. 112

The economics of hybrid rice production in ChinaRice ratooningGrowth and development of the deep water rice plantFaridpur: a computer-assisted instruction model for rainfed lowlandneeA reading and listening comprehension test in English for nonnative.pcakers applying for training at IRRIRice grassy stunt virus 2: a new strain of rice grassy stunt in thePhilippinesPhysical losses and quali ty deter ioration in r ice postproductionsystemsCopublication of IRRI matenals: a survey of translators andpublishersClassif ication of Philippine rainfall patternsContribut ions of modern r ice varieties to nutr ition in AsiaChanges in rice breeding in 10 Asian countr ies: 1965-84Design parameters affecting the performance of the IRRI-designedaxial-flow pump

No. 113 Boron toxicity in riceNo. 114 Energy analysis, rice production systems , and rice researchNo, 115 Production risk and optimal fertilizer rates: an application of the

random coefficient modelNo. 116 Consumer demand for rice grain quality inThailand, Indonesia, and

the PhilippinesNo. 117 Morphological changes in rice panicle development: a review o

hteratureNo, 118 IRRI-Korea collaborative project for the development of coldtolerant lines through anther cultureNo. 119 Problem soils as potential areas for adverse soils-tolerant ric

varieties in South and Southeast AsiaNo. 120 Changes in smal l-farm rice threshing technology inThai land and the

PhilippinesNo. 121 Landforms and modern rice varietiesNo. 122 Yield stabili ty and modern rice technologyNo. 123 Upland rice insect pests: their ecology, importance, and controlNo. 124 A review of wheat research at the International Rice Research

Institute

Documents

IRPS 125 Source-sink relationships in crop plants