Studies on bud development in the rhizome of Agvopyvon vepens. II. The effect of the nitrogen supply

GORDON I. MCINTYRE Regha Resemch Station, Canada Department of Agricultlrre, Regina, Saskatchervar~

Received July 29, 1971

MCINTYRE, G. I . 1972. Studies on bud development in the rhizome of Agropyron repens. 11. The effect of the nitrogen supply. Can. J. Bot. 50: 393401.

Bud growth on isolated rhizomes of Agropyron repens showed a basipetal gradient of decreasing activity and was strongly inhibited at the basal nodes. This evident polarity was correlated with a gradient of decreasing nitrogen content at successively older nodes and with an apparent translocation of nitrogen from the basal to the apical nodes. Isolating the buds from one another reduced growth of the apical buds and prolonged the growth of buds at the basal nodes so that the polarity of bud growth, although still apparent, was much reduced.

Supplying nitrogen as NH4N03 through the cut end of rhizomes still attached to the parent plant caused apical buds to develop as shoots instead of rhizomes. Increasing the nitrogen supply to the rooting medium extended this response to buds at older nodes, restricting rhizome production to basal buds whose growth was inhibited in low nitrogen rhizomes.

Buds developing as shoots had a considerably higher total nitrogen and moisture content and a lower dry weight than buds developing as rhizomes.

The results emphasized the importance of the nitrogen supply not only in determining the polarity of bud growth and the degree of correlative inhibition, but also as a morphogenetic factor controlling bud development.

MCINTYRE, G. I. 1972. Studies on bud development in the rhizome of Agropyrott repens. 11. The effect of the nitrogen supply. Can. J. Bot. 50: 393-401.

La croissance des bourgeons sur les rhizomes isolCs d'Agropyron repens a montrC un gradient basipkte d'activitk dkcroissante et a CtC fortement inhibCe au niveau des nceuds basilaires. Cette polarit6 Cvidente Ctait en corrtlation avec un gradient dtcroissant dans le contenu en azote au niveau des nceuds de plus en plus vieux, et avec une translocation apparente de l'azote i partir de la base vers les nceuds apicaux. L'isolement des bourgeons les uns des autres a rCduit la croissance des bourgeons apicaux et a prolongC celle des bourgeons au niveau des nceuds basilaires, de telle sorte que la polaritt de la croissance des bourgeons, bien qu'encore apparente, Ctait diminuee de beaucoup.

L'azote fourni sous forme de NH4N03 par l'intermediaire de la surface coupCe des rhizomes encore attaches a la plante-mtre, a amen6 les bourgeons apicaux i se developper comme des tiges plutbt que comme des rhizomes. En augmentant la quantitC d'azote dans le milieu d'enracinement, cette rCaction s'est Ctendue jusqu'aux bourgeons des nceuds plus vieux, restreignant la production des rhizomes aux bourgeons basilaires dont la croissance etait inhibCe dans les rhizomes pauvres en azote.

Les bourgeons qui se sont dCveloppCs en tiges avaient un contenu en azote total et en humiditt beau- coup plus ClevC et un poids sec plus faible que les bourgeons qui se sont developpts en rhizomes.

Les rtsultats soulignent l'importance de la provision d'azote non seulement en determinant la polaritt de la croissance des bourgeons et le degrC d'inhibition corrklative, mais aussi conlme facteur morpho- gCnCtique contr6lant le dkveloppement des bourgeons.

Introduction A preliminary investigation of bud develop-

ment in the rhizome of Agropyron repens (5 ) showed that when the lateral buds were released from apical dominance by removal of the rhi- zome apex there was a marked tendency for the bud nearest the cut end to develop as a shoot, those further from the apex as rhizomes, and for buds at the basal nodes to remain inhibited. Al- though the behavior of the individual buds was clearly related to their position on the rhizome, environmental factors were also involved. The light intensity at which the plants were grown was shown to be a factor of particular signifi- cance, determining to a marked extent the pat- tern of development.

In considering these observations in relation to results obtained in previous experiments it was suggested, as a working hypothesis, that the nitrogen supply to the buds might be a factor of major importance and that the observed polarity of bud behavior inight reflect the existence within the rhizome of a basipetal gradient of decreasing nitrogen concentration. Further experiments, designed to test this hypothesis, are described in the present report.

Materials and Methods The area from which the seed was collected and the

methods used in growing the plants were the same as in the previous investigation (5). As before, the required variations in the nitrogen supply were obtained by ap- propriate modification of Hoagland's solution (1). In

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3 94 CANADIAN JOURNAL OF BOTANY. VOL. 50. 1972

experiments 1 and 3 the plants were grown in 21-cm- diameter plastic pots, three per pot, while in experiment 2 they were transplanted from 12-cm pots into plastic trays shortly after rhizome initiation to avoid any restriction of rhizome development. All the experiments were conducted in controlled environment rooms at a constant tempera- ture of 15C and a 16-h photoperiod. Except for expt. 2B (see Fig. 3), the light intensity, measured at the base of the shoots, was 3000 ft-c.

As in the previous investigation, the buds on the rhi- zome were numbered basipetally, the youngest bud of the series (bud 1) being selected as the one at the base of the youngest internode which had reached a length of at least 20 mni.

In expt. 2A, in which rhizomes were fed with a solution of NH4N03 through the cut end, the rhizomes were cut obliquely below the surface of the test solution at a point about 4 cm distal to bud 2, the bud and roots at node 1 having first been excised. The cut end of the rhizome was then immersed in the NH4N03 solution contained in a 70 X 12 mm glass vial, the rhizome being inserted through a hole bored in the plastic cap. To promote continued uptake of the solution, a slice 2 rnm thick was cut from the end of the rhizomes at 2-day intervals.

Satlipling and Analysis In studying the relation of rhizome nitrogen content to

the polarity of bud activity (expt. l), 20 five-node pieces of rhizome were taken for bud growth measurement and nitrogen determinations at various times after their isola- tion from the plant. The nodal regions taken for analysis included the lateral bud (or the shoot, where growth had occurred) together with 1.5 cm of rhizome on either side of the node. The intervening pieces of rliizo~ne internode were discarded. The scale leaves and roots at the nodes were stripped off at the time that the rhizomes were iso- lated from the parent shoot but any new roots subse- quently produced were included in the samples. The corresponding nodes of the 20 rhizomes were combined to give five composite samples on each sampling occasion. The samples were dried in a forced draft oven at 80C for 24 h and their dry weight determined. They were then ground up in a Wiley mill to pass an 80-mesh screen and the total nitrogen content of duplicate aliquots deter- mined using a modification (6) of the rnicroKjeldahl procedure (2).

In expt. G the bud samples analyzed for total nitrogen were not ground up, the buds being transferred directly to the Kjeldahl flasks. Where the sample was too large it was divided into two approximately equal parts and each part analyzed separately.

Experiments and Results

Espt. I . The Relation of Bud Activity to t11e Dis- tributio~z o f Nitrogen and Dry Weigllt in Isolated Rhizonres

It was suggested in the previous paper (5) that the basipetal gradient of decreasing bud activity in decapitated rhizomes might be due to an as- sociated gradient of decreasing nitrogen concen- tration. An experiment was designed to test this

hypothesis and to follow changes in nitrogen and dry weight distribution in isolated rhizomes in relation to the polarity of bud activity.

Rhizomes, ranging from 20 to 40 cm in length, were taken from plants which had been grown at a nitrogen level of 10.5 ppm for 8 weeks. At this low nitrogen level the plants showed severe de- ficiency symptoms and the growth of the lateral buds on the rhizomes was completely inhibited. The apical and basal regions of the rhizomes were cut off and the intervening portion, com- prising buds 1 to 5 (as defined above), was used for the experiment. After stripping off the roots and scale leaves, the length of the lateral buds was measured to the nearest 0.1 mm and the rhi- zomes were sampled for dry weight and nitrogen determinations as previously described. The re- maining rhizomes were divided into two groups. Those in one group were left intact while those in the other were cut up into one-bud pieces, leaving 1.5 cm of rhizome on either side of the isolated buds. The intact rhizomes and the iso- lated nodes were planted in vermiculite moist- ened with distilled water and were kept in darkness at 20C. Bud and shoot lengths were recorded at intervals of 3, 6, and 9 days and on each of these occasions 20 of the intact rhizome cuttings were sampled for dry weight and nitro- gen determinations.

The effect of increasing the nitrogen supply to the parent plants before isolation of the rhizome was also investigated. The plants in this treat- ment were grown initially at the same low nitro- gen level as the others but after 6 weeks growth the solution was leached out with distilled water and replaced with standard Hoagland's solution (i.e., 210 ppm N). The plants continued to re- ceive this high nitrogen solution at Zday inter- vals for the next 10 davs. When the rhizomes were then collected it was found that many of the lateral buds had started to grow out, presum- ably in response to the increased nitrogen supply. Sufficient rhizomes were found, however, in which no stimulation of bud growth was ap- parent. From these, 10 rhizomes whose buds had a similar length to those in the other treatments were selected and the portions bearing buds 1 to 5 were isolated as before.

Analysis of the initial samples (Table 1) re- vealed a well-defined basipetal gradient of de- creasing nitrogen content, the value for node 1 being about twice as great as for the basal node.

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MclNTYRE: BUD DEVELOPMENT IN RHIZOMES 395

After 3 days the nitrogen content had increased at all five nodes, presumably as a result of uptake from adjacent (unsampled) parts of the rhizome internodes, but the rate of increase was greatest at node 1 and decreased progressively at the older nodes. After 6 days the tendency for nitrogen to be translocated preferentially to the apical end of the rhizome was readily apparent for while the nitrogen content of nodes 1 and 2 had continued to increase, that of nodes 3 to 5 had markedly declined. Continuation of this trend increased the steepness of the gradient and after 9 days the nitrogen content at node 1 was more than 4 times greater than at node 5.

Changes in dry weight followed a somewhat similar pattern but the initial weights increased basipetally with the age of the node. The incre- ments recorded after 3 days were closely cor- related with the initial weight of the samples but the increase in weight at node 1 was delayed as compared with the early increase in nitrogen content at this node. There was, however, a sharp increase in the weight of the apical node after 6 days and a still greater increase in the final sample. These changes were accompanied by a loss of weight at the basal nodes.

The data on bud length (Fig. 1) revealed some additional features of interest. The increase in length after 3 days was closely similar at all five nodes; nor did the growth of the isolated buds differ significantly from those on the intact cut- tings. Thus, at this stage, there was no evidence of any correlative inhibition of bud develop- ment. After 6 days, however, buds 1 and 2 on the intact rhizome had grown considerably more than the buds at the older nodes. By 9 days the dominance of bud 1 was clearly established and growth at nodes 3 to 5 had been completely ar- rested. The isolated buds also showed greatest

growth at nodes 1 and 2 in the 6- and 9-day Sam- ples but because of the more persistent growth of the basal buds in this treatment the degree of polarity was reduced. The behavior of the iso- lated buds also differed from those on the intact rhizomes in that their greatest growth, in both the 6- and 9-day samples, occurred at node 2. This difference is also of interest in considering the response of the buds from the treatment in which the nitrogen supply was increased. Not only did the additional nitrogen increase bud growth at all nodes as compared with the intact rhizomes of the low nitrogen plants; it also re- sembled the effect of bud isolation in shifting the maximum growth response from node 1 to the bud at node 2.

Expt. 2. Effect of Iizcreasing the Nitrogen Supply It was also suggested in the previous paper

that the nitrogen supply might be of importance not only in the control of bud activity but also as a inorphogenetic factor affecting the path of bud development. This suggestion was well supported by the following experiments.

(a) Nitrogen Supplied through the Cut End of the Rhizome

In a treatment designed to increase the nitro- gen content of the rhizome, nitrogen was sup- plied as a solution of NH4N03 through the cut end of decapitated rhizomes using the method described above. The plants were grown at a nitrogen level of 5.25 ppm under the conditions previously described and which were shown to favor the production of rhizomes rather than shoots at the apical nodes.

The results, recorded after 12 days (Fig. 2), showed that the nitrogen treatment markedly increased shoot development from the bud nearest the cut end (node 2), the effect increasing

TABLE 1

Changes in the distribution of dry weight (D.W.) and total nitrogen in isolated, decapitated rhizomes (all data expressed as nlill~grams per 20 nodes*)

-.

Node position (apex to base of rhizonle)

Days from 1 - 7 3 4 5 isolation

of rhizome D.W. N D.W. N D.W. N D.W. N D.W. N

*Thc nodal regions taken for analysis comprised the lateral bud and 1.5 crn of rhizome on either side of the node.

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396 CANADIAN JOURNAL OF BOTANY. VOL. 50, 1972

with the concentration of the NH4N03 solution. The much reduced response at nodes 3 and 4 may have been due to insufficient uptake and (or) translocation of the nitrogen solution, or may perhaps reflect the increasing effectiveness at the older-nodes of other factors (as yet unknown) conducive to rhizome development.

This experiment was later repeated with es- sentially the same result.

(b) Effect of Supplyii?g the Additiorlal Nitrogen to the Rooting Medium

A further experiment was conducted to deter- mine whether a greater effect could be produced by supplying thgsupplementary nitrogen via the rooting medium, thus enabling it to be taken up at all nodes of the rhizome. The experiment was also designed to compare the effect of increasing the nitrogen supply at high and low light inten- sities. This treatment was included in view of the results obtained in the previous investigatioil (5) in which it was shown for plants grown at a low nitrogen level that reducing the light intensity not only increased the proportion of buds developing

as shoots rather than as rhizomes but also greatly increased the number of buds whose growth was inhibited. These observations suggested that buds developing as shoots might have a higher nitro- gen requirement than those developing as rhi- zomes and that the greater number of shoots produced at the lower light intensity, by increas- ing competition for nitrogen within the rhizome, might be directly responsible for the associated increase in the number of inhibited buds. If this were so, bud inhibition at the lower light intensity should be eliminated or significantly reduced by increasing the nitrogen supply.

All of the plants were grown initially at a low nitrogen level of 5.25 ppm and a high light inten- sity of 3000 ft-c. When the rhizomes had reached the required stage of development the plants were divided into four groups, two of which were transferred to another growth room where the light intensity was reduced to about 350 ft-c. At the same time one group of plants at each light intensity had its nitrogen supply increased. This was done by leaching out the low nitrogen solu-

TIME. DAYS

FIG. 1. The growth of buds on rhizome cuttings after the isolation of the rhizome from the parent shoot. The buds in each sample are numbered basipetally (1-5) from the apical end of the rhizome. Solid bar: buds on intact rhizome cuttings. Open bar: buds isolated from one another, each borne on a 3-cnl piece of rhizome, i.e., 1.5 cm on either side of the node. Vertically lined bar: nitrogen supply to parent shoot increased from 10.5 ppm to 210 ppm 10 days before isolation of the rhizomes. The initial length of these buds was not sig- nificantly different from that of the buds in the other two treatments. Data are mean values based on 20 rhizomes in treatments 1 and 2 and on 10 rhizomes in treatment 3.

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McINTYRE: BUD DEVELOPMENT 1N RHIZOMES 397

tion with distilled water and replacing it with standard Hoagland's solution (i.e., 210 ppm N). Two days later the rhizomes in all four treat- ments were decapitated to release the buds from apical dominance. Final observations on bud development were recorded 14 days after decap- itation of Ihe rhizomes.

In the high-intensity light treatment (Figs. 3A and B, 4) increasing the nitrogen supply had two main effects. Firstly, it caused most of the buds at nodes 2 to 5, which had developed as rhizomes at the lower nitrogen level, to develop as shoots; and secondly, it almost completely eliminated inhibition of the buds at the older nodes. Com- parison of the data at the two nitrogen levels in- dicated that most of the older buds released from inhibition had developed as rhizomes. There was thus both a morphogenetic effect and a simple growth response.

The hypothesis mentioned above, i.e., that increased bud inhibition at lower light intensities

= RHIZOME

might be due to increased competition for nitro- gen, was not supported by the results (Fig. 3C and D). Although there was in fact an increase in the number of shoots produced at the higher nitrogen level and a consequent reduction of bud inhibition, the effect was small and a high pro- portion of inhibited buds was still present in the high nitrogen treatment. Clearly, their inhibition could not be attributed to nitrogen deficiency.

Expt. 3. A Conlparisotz of the Dry Weight, Total Nitrogen, and Moisture Content of Buds Developing as Shoots and as Rkizonles

In view of the results obtained in the previous experiment it might reasonably be expected that buds developing as shoots would differ signifi- cantly in their nitrogen content from those devel- oping as rhizomes. A preliminary experiment, as preparation for a more detailed analysis, was therefore designed to induce buds to develop

:SHOOT = DORMANT

BUD POSITIONS

(APEX TO BASE OF RHIZOME)

FIG. 2. Effect on bud development of supplying a solution of NH4N03 through the cut end of the decap- itated rhizome. Bud 1 (as defined in the text) was excised. A, control, supplied with water only; B, 0.01 M solution of NH4N03 (280 ppm N); C, 0.02 M solution of NH4N03 (560 ppm N). Percentages are based on I0 plants per treatment.

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398 CANADIAN JOURNAL OF BOTANY. VOL. 50, 1972

along these alternative paths and to follow and compare the changes in their relative dry weight, total nitrogen, and moisture content.

The plants used were from the same batch which provided the material for expt. 1 and were therefore grown under the same conditions. They were, however, grown for an additional 2 weeks to ensure that a sufficient number of suitable rhizomes would be available. By the end of this period it was possible to remove the plants from their pots with little dist~irbance of the vermicu- lite which was firmly bound together by the well-

developed root system. The rhizomes, most of which were growing around the inside of the pot, could then be treated as required and the plants returned to their pots.

Buds developing as shoots were obtained by decapitating the selected rhizomes about 1 cm from the distal side of bud 1 and removing the portion of rhizome bearing buds 1 to 3. These isolated fragments were then planted in trays of vermiculite and watered with the same nutrient solution and kept under the same conditions as the rhizomes remaining in the pots. To induce

=RHIZOME a]] : SHOOT : DORMANT

loor n

BUD POSITIONS (APEX TO BASE OF RHIZOME )

FIG. 3. Effect of the nitrogen supply on bud development on decapitated rhizomes at high and low light intensities. A, B. High light intensity (3000 ft-c). A, 5.25 ppm N; B, 210 ppnl N. C, D. Low light intensity (350 ft-c). C, 5.25 ppm N; D, 210 ppm N. Treatments are described more fully in the text. Percentages are based on 20 rhizomes per treatment.

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MclNTYRE: BUD DEVELOPMENT IN RHIZOMES 399

rhizome development selected rhizomes were again decapitated but were left attached to their parent shoot and the plants were returned to their pots.

A sample comprising 60 buds (i.e., 20 rhi- zomes) was taken initially for analysis and similar samples were collected from each treatment after 48 and 96 h. All of the buds on the isolated rhi- zomes in the 96-h sample had started to turn upwards at the tip and were clearly developing as shoots while the majority of the buds left at-

tached to the parent shoot had turned down- wards and were apparently developing as rhi- zomes. Some of the buds in the latter treatment, however, had not grown sufficiently for their path of development to be determined while a few of the buds at the node 1 position showed a slight upward curvature suggestive of shoot develop- ment. In view of the results of the previous ex- periment it was to be expected that some of these buds would indeed develop as shoots. For this reason the results of the analysis may be assumed

FIG. 4. Seedlings of Agropyron repens, about 8 weeks old, illustrating the effects of the nitrogen supply on bud development on the decapitated rhizomes. A, nitrogen level 5.25 ppm; B, nitrogen supply increased from 5.25 ppm to 210 ppm 2 days before removal of the rhizome apex. The photographs were taken 14 days after decapitation of the rhizomes. The plants are from the corresponding treatments (A, B) illustrated in Fig. 3. Magnification : X 112.

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400 CANADIAN JOURNAL OF BOTANY. VOL. 50. 1972

to slightly underestimate the characteristic dif- to the rooting medium released the buds at the ferences between shoot and rhizome develop- ment.

The dry weight of buds developing as rhizomes was considerably greater than that of buds de- veloping as shoots in both the 48- and 96-h sam- ples (Table 2). The total nitrogen content per sample, on the other hand, was markedly lower for rhizomes than shoots and when expressed as percentage dry weight this difference was further increased. Also of interest was the considerably greater uptake of water by buds developing as shoots. On a fresh weight basis the moisture con- tent of the young rhizomes actually decreased during the first 48 h and returned only to its initial value in the final sample; whereas buds developing as shoots showed a relatively rapid and continuous increase in moisture content throughout the sampling period.

Discussion It is evident from these results that, under the

experimental conditions, both the activity and the path of development of the buds on the rhi- zome were largely determined by the nitrogen supply. There was also considerable evidence that the distribution of nitrogen and its subse- quent translocation within the rhizome was a major factor in the mechanism underlying the polarity of regenerative growth.

Considering first the effects on bud activity the result of expt. 1 showed that the polarity of bud growth was closely correlated with changes in the distribution of nitrogen in the isolated rhizomes. The causal nature of this relationship was indi- cated by the results of the second experiment, which showed that increasing the nitrogen supply

basal nodes from inhibition, thereby significantly reducing the polarity of bud activity. The fact that bud growth in expt. 1 showed an appreciable degree of polarity even when the buds were iso- lated from one another may be attributed to the nitrogen gradient which was present initially while the considerably greater polarity shown by the buds on the intact cuttings probably resulted from the subsequent translocation of nitrogen from the basal to the apical nodes. This view is supported by the results of the previous investi- gation ( 5 ) , which showed that the activity and path of development of the lateral buds was determined partly by their position on the rhi- zcme and partly by the influence of the younger buds. That this influence was produced by com- petition for nitrogen is suggested by the results of the present investigation. It was noted, for exam- ple, in expt. 1, that the largest shoot on the intact low-nitrogen cuttings was produced by the bud at the apical node but that its growth was ex- ceeded by the shoot at node 2 not only when the buds were isolated from one another but also on intact rhizomes from plants which had received an increased nitrogen supply. These observations suggest that bud 2, perhaps by virtue of its larger initial size, had the greater growth poten- tial but that this could only be fully expressed when competition for nitrogen with the younger bud was sufficiently reduced.

The hypothesis that the increase in bud inhibi- tion produced by reducing the light intensity may also have been due to competition for nitrogen was not supported by the results. Although in- creasing the nitrogen supply at the lower light intensity did cause a slight reduction of bud in-

TABLE 2 Con~parison of changes in dry weight, total nitrogen, and water content of buds developing as shoots

and as rhizomes

Hours from Total nitrogen Moisture content treat- Bud Dry weight, ment development mg/60 buds mg/60 buds % D.W.* mg/60 buds % F.W.

0 - 23.1 0.62 2.68 110 82.7

48 As shoot 37.8 1.21 3.20 244 85.5 As rhizome 44.1 0.93 2.12 177 80.1

96 As shoot 63.9 2.30 3.59 48 1 88.3 As rhizome 76.1 2.07 2.72 368 82.7

'D.W. = dry weight; F.W. = fresh weight.

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McINTYRE: BUD DEVEI ;OPMENT IN RHIZOMES 40 1

hibition, most of the buds at the basal nodes showed no response. A simpler and perhaps a more likely explanation is that the inhibition of bud growth was due to competition for carbo- hydrate. This possibility is not only consistent with previous evidence of competition for carbo- hydrate in isolated low-nitrogen rhizomes (4) but is also supported by the changes in dry weight distribution recorded in expt. 1 (Table 1) and which seemed indicative of a considerable drain of carbohydrate from the basal nodes of the rhizome to the more active apical buds.

Turning now to consider the effects on bud development it was shown that supplying nitro- gen as NH4N03 solution through the cut end of the rhizome was highly effective in promoting shoot development and thus in counteracting the effect of environmental conditions more condu- cive to rhizome production. This result raises interesting questions concerning the nature of the biochemical mechanism involved and the use of this technique may prove helpful as a means of assessing the relative effectikeness of various other substances, including both nutritional and hormonal factors, and thus of identifying those substances more specifically concerned with the

I control of bud development.

Increasing the nitrogen supply to the rooting medium had a still greater effect on bud develop- ment and produced a well-marked response at all nodes of the rhizome. A comparison of the behavior of the buds at corresponding nodes at each nitrogen level (Figs. 3A and B, 4) showed that increasing the nitrogen supply resulted in a basipetal displacement of the pattern of develop- ment. Thus, in the high-nitrogen treatment, shoots were produced at a greater distance from the rhizome apex and rhizome production was restricted to the basal buds whose growth was inhibited at the lower nitrogen level. It is interest- ing to note that the pattern of development of the axillary buds on the seedling shoot shows a sim- ilar basipetal displacement in response to an in- crease in the nitrogen supply (3). Here also, the effect was to induce shoot production at nodes further from the main shoot apex and to restrict rhizome development to the bud at the basal (coleoptile) node, the growth of which is nor- mally inhibited under conditions of nitrogen defi- ciency. This similarity in the response to the nitrogen supply lends further support to the pre- vious assertion (5) that the mechanism respon-

sible for the polarity of bud development in the rhizome and in the seedling shoot is likely to be of a similar nature.

The present investigation, however, while establishing the importance of the nitrogen sup- ply, provides no evidence as to the other factors which must undoubtedly be involved in the con- trol of bud development. The preliminary anal- ysis of buds developing as shoots and as rhizomes revealed considerable differences not only in total nitrogen but also in moisture content and dry weight but it is not yet known whether these other differences are of any morphogenetic sig- nificance. It is also evident from the observed effects of light intensity and of isolation of the rhizomes that the behavior of the buds on the rhizome is controlled to a considerable extent by the regulating influence of the parent shoot. While this influence might well be due partly tc a competitive effect on nitrogen distribution-a possibility which is suggested by the present investigation-it may also reflect the role of the shoot in supplying the rhizome with carbohy- drate and other nutritional or hormonal factors. Further experiments are therefore being con- ducted to determine the extent to which the activity and development of the rhizome buds is controlled by the parent shoot and to investi- gate the mechanism by which such control is achieved.

Acknowledgments

I thank Mr. William Fleming for technical assistance and Dr. J. R. Hay for his critical read- ing of the manuscript.

1. HOAGLAND, D. R., and D. I. ARNON. 1939. The water culture method for growing plants without soil. Univ. Calif. Coll. Agric. Exp. Stn. Circ. 347.

2. HUMPHRIES, E. C. 1956. Mineral components and ash analysis. 111 Modern methods of plant analysis. Vol. 1, Edited by K. Paech and M. V. Tracey. Springer-Verlag. Berlin. pp. 468-502.

3. MCINTYRE, G. I. 1967. Environn~ental co~ltrol of bud and rhizome development in the seedling of Agropyron repens L. Beauv. Can. J. Bot. 45: 1315-1326.

4. MCINTYRE, G. I. 1969. Apical dominance in the rhi- zome of Apro~vron reoetls. Evidence of conl~etition for carbohidraie as a lactor in the mechanisA of in- hibition. Can. J. Bot. 47: 1189-1197.

5. MCINTYRE, G. I. 1970. Studies on bud development in the rhizome of Apronvrotz renetls. 1. The influence of temperature, lighi intensity, and bud position on the pattern of development. Can. J. Bot. 48: 1903-1909.

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