Apical dominance in the rhizome of Agropyro~t repens. Evidence of competition for carbohydrate as a factor in the mechanism of inhibition

GORDON I. MCINTYRE Regina Research Station, Canada Departmerzt of Agricultrrie, Regina, Saskatcl~ewan

Received February 24, 1969

MCINTYRE, G. I. 1969. Apical dominance in the rhizome of Agropyron repens. Evidence of competition for carbohydrate as a factor in the mechanism of inhibition. Can. J. Bot. 47: 1189-1197.

When plants of Agropyron repensL.Beauv. are grown at a high nitrogen level (210p.p.m. N)apicaldom- inance in the rhizome is sufficiently reduced to permit the continued growth of the lateral buds. If, however, the rhizome is isolated from the parent shoot the dominance of the apex is markedly increased and lateral bud growth is strongly inhibited.

Experiments with these isolated, high-nitrogen rhizomes showed that apical dominance could be significantly reduced either by increasing the length of the rhizome o r by retarding the growth of the rhizome apex by exposing it to light. The growth potential of the lateral buds declined rapidly as the duration of their attachment to the rhizome apex was increased. This effect was associated with the translocation of carbohydrate to the rhizome apex and could be overcome by providing the isolated buds with a 2% sucrose solution. When buds were isolated from the rhizome apex before their growth potential was exhausted a marked increase in their carbohydrate content was apparent after 48 h. This increase was associated with their resumption of growth. Buds still attached to the apex could he released from inhibition by supplying sucrose solutions to the cut end of the rhizome.

The results suggest that, under the experimental conditions, apical dominance was due primarily to competition for a limited carbohydrate supply.

Introduction In a previous paper on the influence of nu-

I trition on the growth and development of Agro- I pymz repetw L. Beauv. (10) it was reported that I apical dominance in the rhizome could be I 1

readily controlled by varying the nitrogen supply. At low nitrogen levels (e.g. 2-5 p.p.m.

1 N) the growth of the lateral buds was strongly inhibited by the rhizome apex and was com-

I pletely arrested when the buds were only 2-3 mm in length. This behavior contrasted markedly with that of the buds on plants supplied with standard Hoagland's solution. At this relatively high nitrogen level (210 p.p.m. N) the rhizome apex failed to suppress the growth of the buds, all of which grew out as lateral branches on the intact plants (Fig. 1).

Subsequent investigations have shown, how- ever, that when rhizomes from high-nitrogen plants are detached from the parent shoot the growth of the young buds close to the rhizome apex is quickly arrested. Since this inhibition of bud growth does not occur if the rhizome apex is also removed the effect of isolating the rhizome is to enhance the inhibiting influence of the rhizome apex. This suggested, as a working hypothesis, that the reduction of apical domin- ance in the rhizome of plants grown at a high nitrogen level is dependent on the continuous supply of some factor(s) from the parent shoot.

The object of the experiments described in the present report was to test this hypothesis and to elucidate the nature of the mechanism in- volved.

Materials and Method Plant Propagation anrl Etzviror~me~~tnl Con~litiows

The experimental plants were grown either fro111 seed or from one-bud pieces of rhizome, both types of material being obtained from the same location in Pullman, Washington. The seeds were surface-sterilized by a 20- minute treatment with 207, Javex bleach and germinated at a temperature alternation of 15 "C (night) and 25 "C (day) and a 16-h photoperiod. The plants were grown in vermiculite, either in plastic pots 73 in. in diameter with three plants per pot, o r in polyethylene trays (13 in. X 9 in. X 22 in.) with a row of drainage holes at each end. Each tray held either three or four plants.

In all experiments the plants were watered with Hoag- land's solution (6), in which iron was present in chelated form (Sequestrene). Each container received a consider- able excess of nutrient solution a t 2-day intervals, evaporation losses being replaced with distilled water on alternate days. In certain experiments (Nos. 1, 2, 6, and 7) the plants were grown under controlled conditions in a growth chamber at a constant temperature of 10°C. Illumination was provided by 24 VHO cool-whitefluores- cent lights plus 45 40-W incandescent lamps giving a light intensity of about 3200 ft-c and a photoperiod of 18 h. For the rest of the experiments the plants were grown in the greenhouse under a bank of 16 XHO Grolux fluorescent lights which extended the photoperiod to 18 h and provided an additional light intensity of about 1000 ft-c. The mean temperature was about 10°C at night and 20 "C during the day.

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1190 CANADIAN JOURNAL OF BOTANY. VOL. 47. 1969

The Seleciion of Rhizomes Rhizomes were removed from the plants for each

experiment when they were 15-25 cm in length. A typical rhizome is illustrated in Fig. 1. In all experiments the lateral bud whose growth was recorded and which pro- vided a measure of apical dominance was the one at the base of the youngest internode (A) which had reached a length of at least 20 mm. The length of the lateral bud when the rhizome was isolated ranged from about 2.0 to 3.5 mm and that of internode A from 20 to 50 mm. The scale leaf produced from the node at the distal end of internode A completely enclosed a series of successively younger internodes which, together with the apical meristem itself, were considered to make up the "apical bud" (AB). Except where the effect of rhizome length was being investigated (expt. I), the rhizome was severed at a distance of 2.0-20 mm behind the selected lateral bud, the exact distance varying with the experiment, and the older part of the rhizome was discarded. In those treat- ments in which the lateral bud was isolated from the apex the basal 20 mm of internode A was left attached to the isolated bud.

The initial length of the lateral bud, measured to the nearest 0.1 mm a t X 10 magnification, was the main criterion used in the final selection of rhizomes for each experiment. The importance of this factor became ap- parent in preliminary experiments which showed that the size of the bud markedly affected the extent to which its growth was inhibited by the rhizome apex. In general, buds less than ca. 2.0 rnm in length tended to be strongly inhibited even under conditions favoring a reduction in apical dominance; and conversely, buds greater than ca. 4.0 rnm frequently escaped from inhibition irrespective of the treatment applied. Rhizomes with buds exceeding these limits were therefore re.jected and the remaining variation in bud size was uniformly distributed between the treatments by first grouping the rhizomes according to initial bud size and then assigning them to the various treatments by random selection of an equal number of rhizomes from each group.

After the initial length of the lateral and apical buds had been measured the selected rhizomes were laid between sheets of wet filter paper in Pyrex dishes (12 in. X 74 in. X 2 in.), only the lateral bud being left exposed for convenience of observation and measurement. With the exception of expt. 2, the rhizomes were kept in a growth cabinet in continuous darkness at 22" + 1 "C. a temperature which lies within the reported optimuni range for bud growth in this species (13). In most experi-

ments the length of the lateral bud was measured to the nearest 0.5 mm at 2-day intervals with a ruler graduated in millimeters. The length of the apical bud was recorded only at the beginning and end of the experiment.

In testing the effect of an exogenous carbohydrate supply (expt. 7) the rhizomes were first reduced to the required length by cutting them below the surface of the test solution 5-10 mm behind the lateral bud. They were then placed in Pyrex dishes of the type used in the pre- vious experiments, the dishes being supported by wooden racks which held them inclined at about 30" to the horizontal. This enabled the cut end of the rhizome to be immersed in the so l~~ t ion in the lower end of the dish while leaving the lateral bud just above the surface. The apical portion of the rhizome was placed between sheets of filter paper moistened with distilled water. The dishes were covered and kept in the dark at 22°C throughout the experiment.

The sucrose solutions used were previously sterilized by autoclaving. To reduce the growth of contaminants the solutions were replaced with fresh, sterile solution a t 2-day intervals. On each of these occasions a slice ca. 0.5 mrn thick was cut from the end of the rhizome to promote continued uptake of the sucrose.

Atralyiicnl Metlzods In determining the carbohydrate content of the

rhizomes (expt. 5) the selected rhizome samples were chopped into thin slices and dried in a forced draft oven at 100 OC for 15 minutes and then at 70 OC for 24 h. The dried material was ground in a Wiley-mill to pass a 60-mesh screen and aliquots of ca. 30 mg were extracted for 12 h in a micro-Soxhlet apparatus with 80% alcohol. The concentration of the condensed alcohol in contact with the sample was presumed to be about 90% and thus, according to the work of Smith and Grotelueschen (17), sufficiently high to achieve a satis- factory separation of the simple sugars from the re- latively short chain fructosan polymers characteristic of this species. The extracts, which had a slight brown coloration, were clarified by treatment with charcoal and after appropriate dilution the total carbohydrate content of duplicate samples was determined by the anthrone method (3) as modified by Fairbairn (4). The dried residue from the alcohol extraction was extracted with water at 95 "C for 1 h. No clarification was necessary and after suitable dilution the fructosan content was determined on duplicate aliquots by McRary and Slattery's (12) modification of the method by Roe (16).

FIG. 1. A rhizome of Agropyron repens from a plant grown in vermiculite and supplied with full strength Hoagland's solution. At this relatively high nitrogen level (210 p.p.m.) apical dominance is sufficiently re- duced to permit the continued growth of the lateral buds. In all experiments only the apical portion of the rhizome (from vertical line to apex) was used. The morphology of this region is described in the text. B = lateral bud, A = internode, AB = apical bud. ( X 8)

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MclNTYRE: RHIZOME OF AGROPYRON REPENS 1191

Bud samples from expt. 6, each containing 10 buds of the mean. Examination of the data showed and having a total dry weight of 2.5-3.0 mg, were killed that there was a highly negative and dried at the same temperature as used for the rhizome samples and were then ground up in a glass homogenizer ( r = -0'773; < O.O1) between with 0.5 ml of 50% alcohol. After centrifugation. the the final length of the lateral bud and the growth residue was resuspended twice more in 2-ml aliquots of of the rhizome apex. This correlation. illustrated alcohol. The extracts were combined and diluted to in Fig. 2, suggests that the variation in lateral either 25 or 50 ml and the carbohydrate content of dupli- bud inhibition was a consequence of differences cate 2-ml aliquots determined by the anthrone method referred to above. The use of 50% alcohol for the bud in growth. the factors respOn- samples ensured complete extraction not only of simple sible for the variable growth of the apex were sugars but also of any fructosan which may have been not determined, it may well have been caused by present (17). In this analysis no attempt was made to initial differences in the nutritional status of the estimate these fractions separately. rhizomes.

Experiments and Results

A. FACTORS AFFECTING THE DEGREE

OF DOMINANCE

Experiment I . Effect of Rhizome Length The first experiment, which was designed to

compare the degree of apical dominance in rhizomes of different lengths, showed (Table I) that increasing the length of the rhizome markedly reduced the inhibiting influence of the rhizome apex. Although the growth of the isolated buds was almost doubled in the long rhizome treatment there was a fourfold increase in the growth of the buds left attached to the rhizome apex. Thus, increasing the length of the rhizome reduced the degree of apical dominance by more than 50y0.

There was, however, considerable variation within the long rhizome treatment in the extent to which lateral bud growth was inhibited, the standard error (+ 8.61) being more rhan 25%

20 40 60 80 100 120 140

GROWTH OF RHIZOME APICAL BUD

( rnrn/8 DAYS)

FIG. 2. The relationship between the growth of the rhizome apex and that of the lateral bud. Note the ap- parent negative correlation.

TABLE I Effect of rhizome length on apical dominance

Ratio Length? of bud Growth? of of lateral length in rhizome bud after treatments a ~ i c a l bud.

Treatment* 8 days, mm A and B mm/8 days

1. Short rhizome (A) Bud isolated 38.5k4.67

C 7 J . 1

(B) Bud attached to rhizome apex 6 .722.30 26.6k4.10

2. Long rhizome (A) Bud isolated

(B) Bud attached to rhizome apex 28.6k8.61

'The lensth of rhizome from the cut end to the lateral bud in the short and long rhizome treat- ments was 0.2 cm and 10.0 cm respeclively.

tValues are the means (+S.E.) of 10 rhizomes per treatment.

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1192 CANADIAN JOURNAL OF BOTANY. VOL. 47, 1969

Experiment 2. Effect of Light Similar evidence of a correlation between

the growth of the rhizome apex and the in- hibition of the lateral bud was obtained in an experiment designed to investigate the effect of exposing the apex to light.

The rhizomes selected for treatment were isolated a t a distance of 5 mm behind the lateral bud and were placed between sheets of filter paper in a lightproof plastic sandwich box (5 in. X 5 in. X 14 in.) with the apical bud projecting through a hole in the side of the box. There were two such boxes per treatment, each containing five rhizomes. Each pair of boxes was placed in a glass dish lined with wet filter paper. The dishes were wrapped in transparent plastic for the treatment in which the apical bud was to be illuminated and in black plastic for the dark, control treatment. Buds isolated from the apex were also placed in darkness to provide a measure of growth in the absence of apical dominance. The experiment was con- ducted in a growth chamber under the conditions

I

of light and temperature previously described.

Measurements recorded after 8 days (Table II) showed that exposing the rhizome apical bud to light completely eliminated apical dom- inance. This effect was associated with a marked reduction of apical growth.

B. THE REDUCTION IN GROWTH POTENTIAL

OF INHIBITED BUDS

Experiment 3 The capacity of the apex to deplete the rhizome

of certain nutrients was clearly indicated from a comparison of the growth of isolated buds which had been left attached to the apex for periods of increasing duration. Data from one such experiment (Table 111) showed that when the bud was left attached to the apex for 3 days its growth when isolated was reduced by 35% as compared with buds which were isolated . immediately after the rhizome was detached from the parent shoot. Buds left attached to the apex for 7 days made no significant growth when the apex was then removed. In other experiments, in which the period of attachment was increased to 10 or 14 days, several of the buds were dead

TABLE I1 Effect on apical dominance of exposing the rhizome apex to light

Length* of Ratio of Growth* of lateral bud bud length

rhizome apex after 8 in treatments Treatments (1nm/8 days) days (mm) indicated

A. Bud isolated, in dark - 30.1+2.8 A/B = 0.96

B. Bud attached to rhizome apex Rhizome apex in light 5.7+ 1.9 31.357.6

A/C = 7.92 C. Bud attached to rhizome apex

Rhizome apex in dark 39.755.0 3 . 8 5 0 . 3

*Mean values (?S.E.) based on 10 rhizomes per treatment.

TABLE I11 The growth potential of the lateral bud on isolated rhizomes, as affected by the

duration of its attachment to the rhizome apex

Bud length, mm*

Treatment Initial length1 Final length?

1. Bud left attached to rhizome apex 1 .7k0.12 2 .3k0.64 2. Rhizome apex removed:

(0) when rhizome isolated 2 .2k0.15 12.8k1.86 (b) 3 days after isolation of rhizome 2.1 + 0.64 8 .0k1.40 (c) 7 days after isolation of rhizome 1.7 k 0.12 2 .4k0.64

'Means (+S.E.) of 10 rhizomes per treatment. tlnitial length reeorded when rhizome isolated from parent plant. Final length recorded 7 days

after isolation of rhizome in treatment 1 and 7 days arter removal of the rhizome apex in treat- ment 2, i.e. 0, 10, and 14 days after isolation of the rhizome in a, b and c respectively.

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McINTYRE: RHIZOME OF AGROPYRON REPENS 1193

when examined 7 days after isolation from the apex while the adjacent rhizome tissue appeared to be in a moribund condition.

Experiment 4 That the limited growth response of the

isolated buds in the previous experiment was due to a deficiency of carbohydrate was shown by the experiment described in Table IV. Buds whose growth potential had been exhausted by attachment to the rhizome apex for 7 days (treatment 2) showed no significant response when supplied either with water or with KN03, but when floated on a 2y0 sucrose solution they sprouted almost as vigorously as those which had been isolated from the rhizome apex at the beginning of the experiment (treat- ment I).

C. CHANGES IN THE CARBOHYDRATE CONTENT

OF THE RHIZOME AND LATERAL BUD

been removed (treatment B) reduced the con- centration of alcohol-soluble carbohydrate by about 607,. This reduction was presumed to be due to respiration. Allowing for a similar loss by respiration in the other treatments and as- suming a similar initial carbohydrate content as in A it may be deduced from the data in the table that the growth of the lateral bud (treat- ment C ) reduced the carbohydrate concentration by 26.9y0 while in treatment D, in which lateral bud growth was completely inhibited, 36y0 of the carbohydrate initially present in the basal 4 cm of rhizome was translocated acropetally, pre- sumably to the rhizome apex. Only in the initial

TABLE V Effect of respiration, lateral bud growth, and apical

growth on the carbohydrate content of the rhizome

Ethanol- solublec

carbohvdrate Fructosand Experiment 5 (as dextrose) (as fructose)

Samplea Treatmentsb O/o D.W. O/o D.W. More direct evidence of the tendencv for the

growth of the rhizome apex to exhaust the Initial sample A 15.4 2.61 supply of carbohydrate in the basal region of 7-day samples

B 6.10 0.45 C 1.96 0.0

the rhizome was obtained bv the analvsis of D 0.60 0.0 rhizome samples. The analytical procedure has a Samples made up the basal 4.0 cm of the isolated rhizome, i.e. already been described. ~h~ treatments and 2.0cm on either side of the lateral bud. The bud itself was not included.

b Treatments: A, sample taken for analysis immediately after data recorded are summarized in Table V (see isolation of rhizome.

B, rhizome apex and lateral bud removed. foot~lote for treatments). C, rhizome apex removed.

D , bud left attached to rhizome apex. A comparison of treatments A and B showed After treatment the rhizomes of B, C , and D were kept in darkness

at 22 "C for 7 days before beins sampled for analysis. that keeping the rhizomes at 22 O C for 7 days c ~ a t a are the means of ~ \ V O composite samples, each of four

after both the apical and lateral meristems had '"?gZhre from one composite sample offour rhizomL.s.

TABLE IV Effects of supplying nutrients to lateral buds whose growth potential has

been exhausted by a period of attachment to the rhizome apex

Initial length of lateral bud, mm: 2.9k 0.14 2.050.12

Treatments 1. Bud isolated from 2. Bud left attached rhizome apex to rhizome apex

Bud growth,* mm 18.Ok2.12 13.55 1.96

Buds from treatment 2 then isolated from apex and treated as follows

Treatments 3. + water 4. + K N 0 3 5. + sucrose (control) (1 X M ) (2%)

Bud growth,* mm 0.1 k0.11 0.2k0.09 12.2k 2.33 0.250.07 0.250.11 10.7k3.35

'Bud growth was recorded 8 days after each treatment in expt. 1 (upper set of data) and after 7 days in expt. 2 (lower data). NOTE: Therc were eight biids in treatment 1 and 24 in treatment 2. Those in treatment 2

wcre divided into three equal groups for the subsequent treatments (i.e. 3, 4, 5).

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I 1191 CANADIAN JOURNAL O F BOTANY. VOL. 47, 1969

sample was there any significant amount of carbohydrate remaining after alcohol extraction. This residue, the fructosan fraction, was much reduced by respiration and was completely absent in those treatments in which growth had occurred. The extreme depletion of the carbo- hydrate level in treatment D is a notable feature and would seem to account for the previous observation that buds which are isolated after a 10- to 14-day period of attachment to the rhizome apex are frequently in a moribund condition.

Experiment 6 Changes occurring in the carbohydrate con-

tent of the lateral bud after its isolation from the rhizome apex were also investigated. The results, illustrated in Fig. 3, showed firstly that the growth of the control buds, i.e. those left attached to the rhizome apex, whether measured by changes in dry weight or in length (Fig. 3,

c and d), continued for 48 hours after isolation of the rhizome from the parent shoot, before being arrested by the influence of the rhizome apex. This initial persistence of growth, which was also observed in all the previous experi- ments, was associated during the first 24 hours with a sharp decline in the carbohydrate content of the bud (Fig. 3a). That this reduction was still more pronounced when expressed on a dry weight basis (Fig. 3b) presumably reflects the synthesis in the bud of other constituents. This initial fall in carbohydrate content oc- curred to a similar extent in both the isolated and control buds and can probably be attributed to a disruption of the translocation mechanism resulting from the cutting of the rhizome. After 24 h, however, the carbohydrate content of the isolated buds increased rapidly while that of the controls showed a gradual, continuous decline. During the same 24- to 48-h interval the growth of the isolated bud showed a slight

3 -30 d

3 x - Y t- - 20 2 n > 1 0

Bud isolated - 10 -=f

L o = : U Control A

: 0::

1 1 - - - 10 - c d - 12 -

v, n 3

8 - - 10 0 -

E E d

1 t-

t- w I Z

W

- 6 A n 3 m

4

/ control

,/e- 0

' 2

0 24 48 96 0 24 48 96

HOURS AFTER TREATMENT

FIG. 3. Changes in the carbohydrate content (a,b) and associated changes in growth (c,d) of lateral rhizome buds after their isolation from the rhizome apex.

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McINTYRE: RHIZOME OF : AGROPYRON REPENS 1195

increase over that of the control, but owing to the lack of replication of the samples the sig- nificance of this difference could not be evaluated statistically. It may be concluded, however, that the increased carbohydrate supply to the bud either preceded or was very closely associ- ated with the increase in growth.

D. EFFECT OF AN EXOGENOUS CARBOHYDRATE SUPPLY

Experiment 7 As a further, more critical test of the carbo-

hydrate competition hypothesis, experiments were conducted to determine the effcct of supplying carbohydrate exogenously through the cut end of the rhizome. Using the technique de- scribed above, sucrose solutions were supplied at various concentrations. A 4% solution, the lowest level tested, produced only a small and variable response. At 10yo the reduction of apical dominance was much more pronounced

, but this treatment also caused a marked re- duction in the growth of the isolated bud and of the rhizome apex, effects which seemed to be caused by water stress, induced by the high

, osmotic pressure of the solution. This osmotic I effect was avoided by reducing the sucrose

concentration to 8y0. As indicated in Fig. 4, the provision of an 8Gj, solution not only caused a marked increase in the growth of the isolated buds but also was highly effective in reducing apical dominance. In treatment C, 7 of the 10 lateral buds appeared to have been completely released from apical dominance and were still growing when the final measurements were recorded. That the variation within this treat- ment (as indicated by the standard error shown in Fig. 4) was relatively high can be attributed to the fact that the other three buds ceased to grow appreciably when only 4-6 mm long. It was noted, however, that these buds also differed markedly from the others in their mode of development. Whereas all of the buds which escaped from inhibition turned up and grew erect as typical orthotropic shoots the tips of the other three buds turned sharply downwards. This difference, together with their somewhat greater increase in diameter and slower initial growth, strongly suggested that these buds were developing not as shoots but as rhizomes. In this event, their restricted period of growth may

perhaps have been due to a failure of the ex- perimental conditions to provide the specific requirements for continued rhizome develop- ment. Additional evidence of this morphogenetic effect of an exogenous sucrose supply has been obtained in a further preliminary experiment and the possibility of producing a more-uniform and persistent response is currently being in- vestigated.

Discussion

Taken as a whole, these results suggest that, under the experimental conditions, apical dom- inance in the isolated rhizome was due to the inability of the lateral bud to compete effectively with the rhizome apex for a limited carbohydrate supply. Several factors contributed to this situation. The abundant nitrogen supply, by reducing apical dominance in the intact plant, promoted the continued growth of the lateral buds, thereby creating a demand for assimilates sufficient to prevent any significant accumulation of carbohydrate reserves. Because of the absence

45 5"

40 - A- BUD ATTACHED + WATER B- BUD ISOLATED + WATER C- BUD ATTACHED + SUCROSE D- BUD ISOLATED + SUCROSE

35 -

I 1 I I I

0 2 4 6 8

DAYS FROM TREATMENT

FIG. 4. A comparison of apical dominance in isolated rhizomes with their basal cut end immersed in water (A,B) and in an 8% sucrose solution (C,D).

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1196 CANADIAN JOURNAL OF BOTANY. VOL. 47, 1969

of adequate reserves the subsequent isolation of the rhizome from the parent shoot resulted in an immediate and drastic limitation of the carbo- hydrate supply. That the apical "bud" should be the more s~~ccessful competitor is not sur- prising in view of its much greater size. Yet, as previously mentioned, small differences in the initial size of the lateral bud were apparently capable of affecting the balance of competition and of determining to some extent the degree of inhibition.

The present results contribute to an increasing body of experimental evidence supporting the concept of nutrient competition as a major factor in the mechanism of apical dominance. Much of this evidence is based on studies of the effects (1, 5, 10, 11) or traiislocation (8, 14, 15) of mineral nutrients but the influence of the carbohydrate supply has also received some attention. For example, Gregory and Veale (5), in experiments with flax, coiltrolled the carbo- hydrate level by varying the light intensity and showed that when the plants were grown at a high nitrogen level reducing the light intensity significantly increased the degree of apical dominance. Plants receiving a deficient nitrogen supply showed no such effect, probably because, for these plants, nitrogen and not carbohydrate was the limiting factor. More direct evidence of the involvement of the carbohydrate supply in apical dominance was reported by Ballard and Wildman (2), who showed that decapitation of the epicotyl of sunflower seedlings stimulated cell division in the cotyledonary buds if the plants had previously been illuminated but had no effect on plants kept in darkness for 3 days before treatment. Bud activity in the darkened plants with their apex intact could be induced, however, by supplying sucrose through the cut end of the hypocotyl.

The hypothesis that apical dominaiice is due to the diversion of nutrients from the lateral buds to the region of highest auxin content, i.e. the shoot apex, was first proposed by Went (18, 19) and has attracted considerable attention. Experimental work relating to this hypothesis was reviewed and discussed in a recent paper by Phillips (15), who regarded his own results as not entirely consistent with an explanation of this kind. In the present investigation no attempt was made to determine the relation of nutrient translocation to endogenous auxin

levels. As previously emphasized, however (g), the fact that the dominant apex is normally initiated before the lateral meristems in the ontogeny of the shoot (or rhizome) might reasonably be expected to provide it with the opportunity to develop a superior capacity for nutrient accumulation. Whether this capacity is dependent on the synthesis of mobilizing hor- mones or simply on the use of nutrients and the consequent maintenance of a metabolic sink is a auestioii which would seem to be more rele- vaht to the problem of the mechanism of nutrient transport than to the phenomenon of correlative inhibition per se. Nevertheless, a better under- standing of the mechanism by which the flow of nutrients is directed to active meristems would obviously be helpful in permitting a more com- plete and satisfactory explanation of apical dominance in terms of nutrient competition.

Acknowledgments

The technical assistance of Mr. William Flem- ming is gratefully acknowledged.

I . ASPINALL, D. 1961. The control of tillering in the barley plant. 1. The pattern of tillering and its re- lation to nutrient supply. Aust. J. Biol. Sci. 14: 493- 5n5

2. BAYLARD, L. A. T. and WILDMAN, S. G. 1964. In- duction of mitosis by sucrose in excised and attached dormant buds of sunflower (Helianthus annuus L.) Aust. J. Biol. Sci. 17: 36-43.

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