Apical dominance in the rhizome of Agvopyvon vepens. Some factors affecting the degree of dominance in isolated rhizomes

GORDON 1.-MCINTYRE Regina Research Station,l Regina, Saskarchewari

Received April 30, 1970

MCINTYRE, G. I. 1971. Apical dominance in the rhizome of Agropyron repens. Some factors affecting the degree of dominance in isolated rhizomes. Can. J. Bot. 49: 99-109.

A study of apical dominance in isolated rhizomes of Agropyron repens L. Beauv. showed that increas- ing the length of the rhizome significantly reduced the degree of dominance only in rhizomes from plants grown at a high nitrogen level (210 ppm). Exposing the rhizomes to light also reduced dominance more effectively in high-nitrogen rhizomes but the response of rhizomes from low-nitrogen plants was greatly increased by supplying water through the end of the rhizome. Further experiments with low- nitrogen rhizomes showed that buds could be released from apical dominance by treatment with kinetin. When isolated from the plant the buds showed a significant increase in length after 24 h and an associated increase in moisture and insoluble nitrogen content. Soluble nitrogen and carbohydrate increased concurrently during the next 24 h. Rhizomes kept in darkness showed a small reduction of bud inhibition when water was supplied through the cut end; NH4N03 solution had a greater effect, while solutions in which both nitrogen and carbohydrate (as sucrose) were supplied resulted in the almost complete elimination of apical dominance.

The results suggest that apical dominance in isolated, low-nitrogen rhizomes was due mainly to competition between the apex and the lateral buds for water, nitrogen, and carbohydrate.

Introduction In a previous investigation of apical dominance

in the rhizome of Agropyron repens L. Beauv. (6) it was shown that the rhizome apex failed to suppress the growth of the lateral buds when the plants were grown at a high nitrogen level. If, however, the rhizome was detached from the parent shoot apical dominance was rapidly estab- lished. Subsequent experiments provided evidence that dominance in these isolated, high-nitrogen rhizomes was due primarily to competition between the apex and the lateral buds for a limited carbohydrate supply (7).

The experiments described in the present re- port are also concerned with the factors affect- ing apical dominance in isolated rhizomes but with particular reference to rhizomes from plants grown under conditions of nitrogen deficiency. Such rhizomes, unlike those of high- nitrogen plants, typically show a high degree of apical dominance while still attached to the parent shoot, the growth of the lateral buds being completely arrested when only a few millimeters in length. In this respect their be- havior is similar to that of plants growing in the field. It was therefore hoped that a study of apical dominance in these low-nitrogen rhizomes would contribute to a better understanding of the factors controlling bud activity under natural conditions. In addition, a comparison of

the results with those previously obtained from similar experiments with high-nitrogen rhizomes seemed likely to reveal some significant differ- ences. As in the previous investigation, the work was conducted entirely with isolated rhizomes since this approach permitted a degree of precision and environmental control which would not have been possible in experiments with the intact plant.

Materials and Methods I . Pla~zt Propagation and Eriviro~~nzetzfal Conditiolis

In experiments 1 and 3 the plants were grown from one-bud pieces of rhizome dug up from an area of uncultivated land on the Research Station at Regina. In the other experiments they were grown from seed collected in the vicinity of Pullman, Washington. The growing methods used were the same as previously described (7) except that the Hoagland's solution was modified by equimolar substitution of CaC12 and K2S04 for Ca(N03)~ and KN03 respectively to give the required reductions in the nitrogen supply.

In experiments 2 and 4-7, the plants were grown in controlled environment rooms at a constant tem- perature of 15°C. Illumination was provided by a bank of 24 VHO cool-white fluorescent tubes plus forty- five 40-W incandescent lamps, giving a light intensity of ca. 3000 ft-c and a 16-h photoperiod. In expts. 1 and 3 the plants were grown in the greenhouse during the spring and fall, the daylength being extended by a bank of Grolux ("wide-spectrum") lamps in expt. 1 and VHO cool-white lamps in expt. 3. The intensity of the supple- mentary lighting at plant level was ca. 1500ft-c. The temperature was only partly controlled, its approximate range being 20-25OC during the day and 10-15°C at night.

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100 CANADIAN JOURNAL OF BOTANY. VOL. 49, 1971

2. Selection and Treattr~etlr of Rhizomes Rhizomes, ranging from 15 to 30 cm in length, were

removed from the plants for use in the experiments after a growing period of 8-10 weeks. By this time, the shoots of all plants receiving a reduced nitrogen supply had developed pronounced nitrogen-deficiency symptoms. The rhizomes also showed clear evidence of nitrogen deficiency. They were somewhat thinner and considerably less succulent than those from the high-nitrogen plants of the previous investigation, while their relatively small lateral buds, whose growth had been arrested at a length of 2-4 mm, were indicative of a high degree of apical dominance. The criteria used in the selection of rhizomes, the morphology and dimensions of the apical region isolated for experimental treatment (Fig. I), and the conditions under which the rhizomes were kept during the treatment period were as previously described (7).

The formulation used in the preparation of the kinetin solution (expt. 3) was based on the one used for the same purpose by Sachs and Thimann (9) and shown by these workers to be particularly effective. The kinetin was dissolved in 30% ethyl alcohol containing 1% Carbowax 1500 to give a 500-pprn solution. The same solvent preparation was then used to prepare the required series of kinetin concentrations by appropriate dilution of aliquots of the stock solution. The lateral buds to be treated were ringed with lanolin and a 10-p1 drop of the kinetin solution was then applied, directly on top of the bud with a micropipette.

In a study of the changes in the nutritional status of the isolated buds (expt. 6) the buds selected for analysis were taken only from the basal region of each rhizome; the apical portion, consisting of bud B (Fig. 1) and the bud at the next older node, was discarded. This was done to ensure that buds whose growth had not yet been completely inhibited were excluded from the samples. Any rhizomes in which the basal buds showed signs of re- newed growth were also rejected. Eighty buds, which com- prised the initial sample, were measured to the nearest 0.1 mm at X 12.5 magnification using a dissecting micro- scope fitted with an eyepiece micronleter and were then immediately killed and dried for analysis. On the next day a further 240 buds were isolated, leaving 1 cm of rhizome internode on either side of the bud, placed between sheets of wet blotting paper, and kept in darkness a t 21" + 1°C. Samples of 80 buds were taken at intervals of 24, 48, and 72 h. The buds in each sample were measured and dried for analysis. They were later divided into two groups, each of 40 buds, one group being used for carbohydrate determinations and the other for nitro-

FIG. 1. Diagrammatic illustration of the typical morphology of the isolated rhizollle apex used for all of the experinlental treatments. B, lateral bud; A, youngest internode 5 2.0 cm; AB, rhizon~e "apical bud." Except in expt. 2, in which the effect of rhizome length was in- vestigated, the cut end of the rhizonle was about 1.0 cm from the lateral bud.

gen analysis. No attempt was made to include "control" buds (i.e. buds left on the plant until taken for analysis) since all such buds would have to have been selected initially and marked for later identification, a procedure which was considered impracticable in view of the large number of buds involved and which might have resulted in some stimulation of bud activity. However, in view of the care taken to ensure that all the selected buds were fully inhibited, there is little doubt that the subsequently recorded changes in their length and nutritional status were caused by their isolation from the plant.

The effect of various nutrient solutions supplied through the cut end of the rhizome was tested by the technique illustrated in Fig. 5. Plastic syringes served as reservoirs for the test solutions and were connected to the end of the rhizome with latex rubber tubing 0.D. and wall each ca. 1.5 mm). Solutions containing sucrose were sterilized by autoclaving at 15 Ib for 20 min and were changed at 2-day intervals to reduce the growth of contaminants.

3. Analytical Methods The methods used for carbohydrate and nitrogen

determinations were as previously described (7) except in the following respects. In estimating the soluble- nitrogen content of the rhizomes (expt. 2) the dried tissue was ground up in a Wiley mill to pass an 80-mesh screen and duplicate aliquots of about 150 mg were transferred to a parchment extraction thimble. Forty milliliters of 80% ethyl alcohol was dripped onto the plant material from a modified separatory funnel, the rate being adjusted so that the total volume percolated through the sample in about 1 h. Preliminary tests had shown that this method extracted all but a negligible amount of the alcohol-soluble nitrogen. The leachate, collected in a 100-ml beaker, was reduced in volume in a forced draft oven at 30°C, transferred to a 30-ml micro- Kjeldahl flask, and evaporated to dryness under reduced pressure in a rotary evaporator at room temperature. The same method was used in determining the nitrogen content of the lateral buds (expt. 6 ) except that the sample of dried buds was ground up with 0.5 ml 80% ethyl alcohol in a glass homogenizer to form a fine suspension before being transferred to the extraction thimble. The precision of the micro-Kjeldahl procedure was also increased by determining the end point of the titration directly by use of a p H meter instead of the Conway indicator, the color change of which is not sharply defined.

Experiments and Results

Expt. 1. The Persistence of Apicul Donzinaizce in Isolated Rhizomes

In the previous investigation (7) the use of isolated rhizomes proved a useful means of assessing the influence of various factors on apical dominance. Before adopting the same approach in the present investigation, however, it was necessarv to determine whether the rhi- zomes of low-iitrogen plants would show the

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McINTYRE: RHIZOME OF AGROPYRON REPENS 101

required persistence and degree of apical domi- nance after they had been isolated from the parent shoot.

An experiment designed to answer this ques- tion showed (Fig. 2) that the apex of the isolated rhizome exerted a strong inhibition of lateral bud activity and that this inhibitory effect per- sisted for the 21-day period of the experiment with only a slight reduction occurring during the last 7 days.

In the previous investigation with high- nitrogen rhizomes it was found that the lateral bud completely lost its capacity for regenerative growth if left attached to the apex of the isolated rhizome for 7 days. This was attributed to the low carbohydrate content of the high-nitrogen rhizomes and to the rapid and severe depletion of these reserves when the rhizome was isolated from the supply of fresh assimilate. In the present experiment, however, in which the plants were grown at a low nitrogen level of 5.25 ppm, isolation of the lateral bud from the rhizome

I

I apex after 7 days (treatment B) showed that it had retained a capacity for prolonged and

vigorous growth. This difference almost certainly reflects the considerably greater amount of carbohydrate in the low-nitrogen rhizomes. Analysis of rhizomes from plants grown at the same low nitrogen level (5.25 ppm) showed that the "fructosan" content (i.e. the 90% alcohol- insoluble fraction extractable with water) was ca. 25y0 of the dry weight (expressed as fructose). This contrasts markedly with the corresponding value of 2.5y0 previously reported for high- nitrogen rhizomes (7). Similar high levels of carbohydrate normally occur in the rhizomes of this species when the plant is growing in the field (2) and undoubtedly contribute to the weed's remarkable capacity for regenerative growth (5).

Expt. 2. Effect of Rhizome Length In the previous investigation (7) increasing the

length of isolated, high-nitrogen rhizomes caused a marked reduction of apical dominance. In repeating this experiment with low-nitrogen rhizomes it was found that although an increase in rhizome length greatly promoted the growth of the rhizome apex, it failed to release the lateral bud from inhibition. The two experi- ments, however, were not strictly comparable for the initial length of the lateral bud-a factor previously found to have a significant effect on the degree of inhibition-was considerably smaller in the low-nitrogen rhizomes. A further experiment was therefore designed to compare the effect of rhizome length on apical dominance in rhizomes from plants grown at a wide range of nitrogen levels, care being taken to achieve a high degree of uniformity in the initial length of the lateral bud. The length of the bud at all nitrogen levels varied only from 2.5 to 2.7 mm.

There was no significant effect of rhizome length on apical dominance at the lowest nitrogen level (Table 1). Although the increase in nitrogen supply from 5.25 to 105 ppm was correlated with a progressive increase in the growth of the lateral bud where the bud was attached to the apex in the long-rhizome treat- ment (D) this response was relatively slight and it was evident that lateral bud activity was

FIG. 2. The occurrence and persistence of apical strongly inhibited at all of these reduced nitrogen

dominance in isolated, low-nitrogen rhizomes. Note that levels. Measurements of apical growth in these the lateral bud was still capable of growth when isolated same treatments showed that increasing the from the rhizome apex after 7 days. A, bud isolated from rhizome apex initially; B, bud isolated after days length of the rhizome resulted in a 2.5-3.5 times (arrowed); C, bud left attached to apex (control). increase in the growth of the rhizome apical bud.

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1 02 CANADIAN JOURNAL OF BOTANY. VOL. 49, 1971

TABLE 1 Effect of rhizome length on apical dominance in rhizomes from plants grown at a range of nitrogen levels

pp - Length of lateral bud after 10 days, mm*'

Nitrogen level, pprn

Treatment.! 5.25 21 .O 52.5 105 210

1. Short rhizome A. Bud isolated 15.7(6)+ 3 . 6 39.2(9)? 5 .9 25.5!81 + 5.1 1 8 . 1 + 5 . 2 22.8(4) + 4 .0 B. Bud attached to

rhizome apex 3.2(0) + 0.20 3.8(1) 2 0.62 3.8(0) + 0.18 3 . 8(01 + 0.29 5.3(1) + 1.32

2. Long rhizome C. Bud isolated 34.1(71 ? 5 .8 50.9(s) + 10.2 65.8(10) + 6.8 92.9!10) + 8 .4 69.7(8) + 9 . 9 D. Bud attached to

rhizome apex 3.5(0) + 0.18 4.7(1) + 1 .0 5.0(1) 2 1 . 3 6.0(2' + 1 .5 21.3(4) + 8 .5 Bud length ratio A/B 4.9 10.3 6 .7 4 .8 4 .3

C / D 9.7 10.8 13.2 15.5 3.4 A I D 4 .5 8.3 5 .1 3 .O 1 .1

'Mean values (+. S.E.) based on 10 rhizomes per treatment, except for 210 ppm N treatment where values are the average (and average S.E.) of the mean values for three replicate experiments. Figures in parentheses are the number of buds still growing after 8 days.

?The length of rhizome from the cut end to the lateral bud in the short and Ions rhizome treatments was 0.2 c n ~ and 10.0 cm respectively.

At 210 pprn N, however, the increase in rhizome length caused a marked reduction in apical dominance, the magnitude of this effect being in good agreement with the results of a similar treatment in the previous investigation (7). If the growth of the bud when attached to the

I apex of the long rhizome is compared with its I growth when isolated from the apex in the short- I rhizome treatment and the resulting ratio (AID)

is taken as a measure of the degree of apical I , dominance it may be concluded that increasing I the length of the rhizome at the highest nitrogen

level significantly reduced the inhibiting effect of the rhizome apex. The fact that lateral bud growth in treatment D was still considerably less than the growth of the isolated bud when the latter was borne on a long piece of rhizome (i.e. C/D) may be attributed to the capacity of the

TABLE 2 Effect of the nitrogen supply on the nitrogen

content of the rhizomes

Nitrogen supply (pprn)

Nitrogen fractions* 21.0 105 210

Nitrogen content of rhizome ('3% D.W.>i

Soluble N 0.15 1.57 2.37 Insoluble N 0.37 0.56 0.66 Total N 0.42 2.13 3.03

*The extracting solvent was 80% ethyl alcohol. ?The values shown are the means of duplicate determinations.

rhizome apex to monopolize a considerable pro- portion of the additional nutrients provided by the increased length of rhizome, this additional supply becoming fully available to the lateral bud only when the bud is isolated from the competitive influence of the rhizome apex.

The effect of the nitrogen supply on the nitro- gen content of the rhizome is shown in Table 2. Notable features of these data are the very large increase in soluble-nitrogen content as the nitrogen supply is increased from 21.0 to 210 pprn and the associated reversal in the relative amounts of soluble and insoluble nitrogen.

Expt. 3. Effect of Kinetin It was reported by Sachs and Thimann (9)

from experiments with peas and several other species that lateral buds on the intact plant were released from apical dominance when treated with kinetin (6-furfurylaminopurine). An experi- ment was therefore designed to test the effect of this substance on apical dominance in isolated rhizomes. Preliminary tests, using the formula- tion and procedure described above, showed that a minimal concentration of ca. 20 pprn was re- quired to produce a measurable growth response in the lateral bud while a concentration of 1000 pprn caused distinctly injurious effects. On the basis of this information a more critical experi- ment was conducted to test the effect of a range of concentrations lying between these two extremes.

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McINTYRE: RHIZOME OF AGROPYRON REPENS 103

The results (Fig. 3) showed a well-defined dosage-response relationship and clearly estab- lished the ability of kinetin to release the lateral buds from inhibition. Examination of the data for the individual buds showed, however, that only a few of the buds in each treatment had

Kinetin ( P P ~ )

DAYS AFTER TREATMENT

FIG. 3. Effect of kinetin on apical dominance. The treatment in which the lateral bud was isolated from the rhizome apex is labeled. In the others the bud re- mained attached to the apex and received a 10-p1 drop of solution containing kinetin at the stated concentra- tions. The rhizomes were from plants grown at a low- nitrogen level of 5.25 ppm.

escaped completely from inhibition and were still growing at the end of the experiment. Although most of the hnetin-treated buds showed a phase of rapid elongation, occurring between 2 and 5 days after treatment, their growth subsequently declined and in most cases was almost completely arrested when the buds were still only 5-9 mm in length. Somewhat similar evidence of a limitation in the growth response of kinetin-treated buds was described by Sachs and Thimann (9) in their experiments with peas.

Expt. 4. Efect of Light It was shown in the previous investigation (7),

that when the apical bud of an isolated, high- nitrogen rhizome was exposed to light its growth was greatly reduced and the lateral bud was completely released from inhibition. In the present investigation, however, the result of a preliminary experiment suggested that, in the case of low-nitrogen rhizomes, exposure to light was considerably less effective in reducing the degree of apical dominance. This difference received confirmation from a more critical com- parison in which two sets of plants were grown under the same growth room conditions, one receiving standard Hoagland's solution contain- ing 210 ppm nitrogen and the other a modified solution with a reduced nitrogen concentration of 5.25 ppm. Selected rhizomes from each nitro- gen level were placed on wet filter paper in Pyrex dishes and covered with a sheet of poly- ethylene to prevent desiccation. The dishes were covered with sheets of glass. In one treatment they were wrapped in black cloth to exclude the

TABLE 3 Effect of light on apical dominance in rhizomes from plants grown at high- and low-nitrogen levels

1. High-nitrogen plants 2. Low-nitrogen plants N-level: 210 ppm N-level: 5.25 ppm

Length of lateral budl"atio, Length of lateral bud* Ratio, Treatments after 10 days, mm A/B after 10 days, mm A/B

1. Rhizome in dark A. Bud isolated 34.2k8.4 25.6k4.1

7.4 5.4 B. Bud attached to rhizome apex 4.6k1.1 4.7k1.3

2. Rhizome in light A. Bud isolated 45.2k5.0 38.3k3.2

1.4 2.6 B. Bud attached to rhizome apex 32.6k8.9 14.725.0

*Means (and S.E.) are based on 10 rhizomes per treatment.

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104 CANADIAN JOURNAL OF BOTANY. VOL. 49. 1971

light and in the other, they were left uncovered, exposing the rhizomes to a light intensity of about 2500 ft-c for a 16-h photoperiod.

Exposing the rhizomes to light increased the growth of the isolated buds to a similar extent at both nitrogen levels but while this treatment almost eliminated apical dominance in the high- nitrogen rhizomes it had considerably less effect at the lower nitrogen level (Table 3). Data on rhizome apical growth showed that during the 10-day experimental period the mean growth of the apical bud in the dark and light treatments respectively was 18.0 k 2.5 and 10.3 +. 2.8 for the high-nitrogen rhizomes and 26.5 L- 6.3 and 4.0 k 1.8 in the low-nitrogen treatment. Thus, the smaller effect of light on apical dominance at the lower nitrogen level could not be attributed to a reduced effect on apical growth which was, in fact, more strongly inhibited in the low nitro- gen rhizomes.

-- - 0 2 4 4 8 7 2

TIME (HOURS)

Expt. 5 . Efect of Water and NH4N03 Solutiotz Supplied to Low-Nitrogen Rhizomes Kept itz the Light

In seeking an explanation for the result of the previous experiment it seemed that the growth of the lateral buds of the low-nitrogen rhizomes might have been restricted by their inability to compete with the apex for a limited nitrogen supply. A further experiment was therefore designed to determine whether apical dominance in illuminated low-nitrogen rhizomes could be further red~iced by increasing the nitrogen supply. The plants were grown at the same low- nitrogen level and under the same conditions as

FIG. 4. Changes in the level of various constituents in FIG. 5. The technique ~ ~ s e d in supplying water and the lateral buds after their isolation from the plant. nutrient solutions to the cut end of the isolated rhizome. Standard errors are shown for the bud length data. The materials are described in the text. Approx. X 113.

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McINTYRE: RHIZOME OF AGROPYRON REPENS 105

in expt. 4. The increased nitrogen supply to the nitrogen rhizomes have a considerably higher isolated rhizomes was provided as a 0.02 M moisture content than those from plants grown solution of NH4N03, using the technique at lower nitrogen levels. Data from an earlier illustrated in Fig. 5. During the treatment period experiment gave moisture content values (yo the rhizomes were provided with the same con- F.W.) of 84 0.34% and 72 t- 0.76% for rhi- ditions of light and temperature as in the previous zomes from plants grown at 210 ppm and 2.6 experiment. ppm nitrogen respectively. In view of these data,

Measurements after 10 days (Table 4) showed it might be expected that internal competition that the buds in treatments A and B had made for water, and its postulated effect on the degree considerably more growth than in the corre- sponding treatments in expt. 4. This difference was attributed to the fact that limited growth- room facilities had made it necessary to reduce the growing period of the plants from 8 to 6 weeks, a change which prevented the onset of nitrogen-deficiency symptoms of the usual severity and probably resulted in the rhizomes having a higher nitrogen content than in the previous experiment. However, since the growth of the isolated buds and of those attached to the apex was similarly affected, the degree of apical dominance (A/B) was of the same order of magnitude as in expt. 4. Of greater interest, how- ever, were the results of the other two treat- ments. These showed not only that apical dom- inance was completely eliminated by the NH4- NO3 solution but also that the control treatment (C), in which only water was supplied, was almost equally effective in releasing the lateral bud from-inhibition.

This result suggested that, under the experi- mental conditions, competition between the apex and the lateral bud for a limited water supply was a major factor in the mechanism of inhibi- tion. This conclusion also provides a satisfactory explanation for the difference in the effect of light on apical dominance in high- and low-nitrogen rhizomes for investigation showed that high-

of apical dominance, would tend to be somewhat greater in the low-nitrogen rhizomes.

Expt. 6. Clzanges in the Nutritional Status of the Lateral Buds after Their. Release from In- hibition

To test the hypothesis that, under the experi- mental conditions, competition for water and other nutrients may determine to some extent the degree of apical dominance, a study was made of changes in the nutritional status of the lateral buds after their isolation from the plant.

In one such experiment the plants were grown at a nitrogen level of 5.25 ppm until the three- to four-leaf stage. Then, to promote the continued production and growth of rhizomes, the nitro- gen supply was increased to 10.5 ppm and was kept at this level until the rhizome buds were taken for analysis. The methods adopted in the sampling, measurement, and analysis of the buds were as described in the previous section.

A significant increase in the length of the isolated buds was apparent after only 24 h (Fig. 4). This early growth response was ac- companied by an increase in moisture content and in the total- and insoluble-nitrogen fractions. The absence of any significant increase in soluble nitrogen during this period suggests a balance between its rate of uptake and its use in protein

TABLE 4

Effect on apical dominance of supplying water and arnnlonium nitrate solution to the cut end of isolated low-nitrogen rhizomes which are exposed to light

- -

Treatment (Rhizome in light)

Ratio of bud length in treatments indicated

Length of lateral bud" after 10 days, mm A/B A/C A/D

A. Bud isolated 58.6k4.0 B. Bud attached to rhizome apex 27.5k9.1 2.1 C. Bud attached to rhizome apex, f water 54 .6k9 .0 1 . I D. Bud attached to rhizome a ~ e x . i- NH4NO1 60.2k10.3 1 .O

'Means (and S.E.) based on 10 rhizomes per treatment. NOTE: The NH4N03 solution in treatment D was used at a concentration of 0.02 M and pH 6.5.

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106 CANADIAN JOURNAL OF BOTANY. VOL. 49, 1971

synthesis. During the next 24-h period, however, there was a rapid rise in the soluble-nitrogen level and this was associated with an increase in dry weight, most of which could be accounted for by the concomitant increase in soluble carbohydrate. The samples taken after 72 h showed a marked decline in both soluble nitro- gen and carbohydrate, with a proportionate reduction in total nitrogen and dry weight. This apparent diversion of nutrients from the develop- ing bud, the continued growth of which was not appreciably affected, probably resulted from the initiation and early growth of root primordia, which are normally produced at each rhizome node.

Similar experiments involving changes in dry weight and in nitrogen and moisture content were conducted on three other occasions. Al- though a comparison of the results of all four experiments revealed some differences in the degree and precise timing of the changes re- corded, certain characteristic features of con- stant occurrence were readily apparent. In every case, there was a marked rise in the moisture content of the bud during the first 24 h, the in- crease ranging from 18% to as high as 97% in one instance. This change was invariably ac- companied by an increase in the amount of insoluble nitrogen, suggesting an early stimula- tion of protein synthesis. Changes in the level of soluble nitrogen during this initial period, however, were much more variable. In two experiments this fraction decreased markedly, presumably because of its rapid use in protein synthesis without any equivalent uptake of nitrogen by the bud. In another experiment, it increased slightly, while in the experiment described above, its level remained relatively constant. This variation seemed to be related to differences in the severity of the nitrogen- deficiency symptoms shown by the parent plant, while this in turn was attributable to differences either in the nitrogen supply or in the duration of the growing period. In general, where the deficiency symptoms were less pronounced there was an earlier and more rapid increase in the soluble-nitrogen content of the isolated bud.

Another highly characteristic feature of the results was the close correlation between the in- crease in soluble nitrogen and carbohydrate (assumed in three experiments to be represented with sufficient accuracy by changes in dry

weight). In one experiment both showed a marked increase during the first 24 h ; in the others a significant increase in the soluble- nitrogen content was not recorded until either 48 or 72 h after isolation of the bud, and in these instances the increase in carbohydrate content showed a corresponding delay. Although ex- amination of all the data suggested that the increase in nitrogen may slightly precede the rise in carbohydrate content, reliable informa- tion on this point would require more critical methods of analysis.

Expt. 7. E&ct of Water and Nutrient Solutiorzs Supplied to Low-Nitrogen Rhizonzes Kept in the Dark

I t was shown in expt. 5 that the effect of light in reducing apical dominance could be markedly increased and the lateral bud released from in- hibition by supplying either water or NH4N03 through the cut end of the rhizome. Similar treatments proved much less effective, however, when the rhizomes were kept in the dark. The results of two such experiments are shown in Table 5. Although the NH4N03 solutions used in these and other similar experiments in- variably caused some reduction in apical dom- inance, there was considerable variation in the response of individual buds within each treat- ment. The slight response to water only was equally variable. A few of the buds were com- pletely released from inhibition by the NH4N03 treatment; others showed only a short-lived stimulation of growth or failed to respond. The results, in fact, were very similar to those ob- tained from treatments with kinetin (expt. 3).

The limited nature of the response to NH4N03 solutions suggested that the continued growth of the buds might require an increased supply of some other factor. Evidence that carbohydrate was the limiting factor involved was obtained in subsequent experiments in which sucrose at concentrations of either 6% or 8% was in- cluded in the NH4N03 solution. The results of three such experiments (Table 6) showed that this combination of nutrients was extremely effective in promoting the growth of the lateral buds and almost completely eliminated apical dominance. These experiments also provided further evidence that water by itself, when supplied in this way, can appreciably reduce the degree of inhibition.

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McINTYRE: RHIZOME O F AGROPYRON REPENS

TABLE 5 Effect on apical dominance of supplying water and NH4N03 solution to the

cut end of low-nitrogen rhizomes kept in the dark --

Length of lateral bud (mm) after 12 dayst

Treatments* Exut. 1 Exut. 2

A. Bud isolated 22 .2k5 .41 23.7k6.50 B. Bud attached to apex, control 3 .4k0 .13 3 .4k0 .13 C. Bud attached to apex, + water 3 .5k0 .18 6 .4k2.63 D. Bud attached to apex, + NH4N03 soln. 7 .5k1 .63 8 .9k4 .28 Bud length ratio A/B 6.5 7 .O

A/D 3 .O 2.7 *The NH~NOJ solution in treatment D was used at a concentration of 0.02 M and pH 6.5. ?Values are the means (2 S.E.) of 10 rhizomes per treatment.

TABLE 6 Effect on apical dominance of water and nutrient solution (NH4N03 + sucrose)

supplied through the cut end of the rhizome

Length of lateral bud (mm) after 10 dayst

Treatments* E x ~ t . 1 E x ~ t . 2 E x ~ t . 3

A. Bud isolated 23.6k6.26 40.0k4.82 37.6k5.26 B. Bud attached to apex, control 5.2k1.78 3 .4k0 .21 5 .9k1.73 C. Bud attached to apex, + water 7 .6k3 .31 12.5k5.15 13.2k4.54 D. Bud attached to apex, + nutrient soln. 20.7k7.10 22.7k4.93 30.6k7.06 Bud length ratio A/B

3; *The nutrient solution in treatment D was 0.02 M NH4N03 with the addition of sucrose at a concentration of 8%

in expt. 1 and 6% in expts. 2 and 3. tBud lensths are the means (f S.E.) of 10 rhizomes per treatment.

Discussion The results of this investigation are in good

agreement with those previously reported from similar experiments with high-nitrogen rhizomes (7) and provide further evidence that competition for nutrients is a major factor in the mechanism of apical dominance. More specifically, they suggest that, under the experimental conditions, the degree of dominance was mainly determined by the capacity of the lateral buds to compete effectively with the rhizome apex for three basic nutritional requirements, namely water, nitro- gen, and carbohydrate.

Considering first the evidence that com- petition for water was one of the factors in- volved, it was shown in expt. 5 that the reduction of dominance produced by exposing low-nitrogen rhizomes to light could be markedly increased and the buds almost completely released from inhibition by supplying water through the cut end of the rhizome. The simplest interpretation of this result is that the light-induced inhibition of rhizome apical growth made nutrients more

readily available to the lateral bud but that the apex continued to exert a competitive influence on the limited water supply. That the high- nitrogen rhizomes showed no evidence of com- petition for water can be attributed, as previously suggested, to their higher moisture content. It was also shown (expt. 6) that when inhibited buds were isolated from the parent plant there was a marked and consistent increase in their moisture content during the next 24 h. This change was invariably associated with an in- crease in the insoluble-nitrogen fraction. The possibility that these two effects may be causally related seems worth considering, in view of the well-established influence of changes in moisture content on protein synthesis in meristematic tissues (4). Finally, there is the observation (expt. 7) that even when the rhizomes were kept in the dark, supplying water through the end of the rhizome produced a small but quite apparent stimulation of lateral bud activity.

Although it has been clearly demonstrated that stem apices (10) and other active meristems

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108 CANADIAN JOURNAL OF BOTANY. VOL. 49, 1971

(see, for example, ref. 1) are capable of maintain- ing an adequate water supply at the expense of other parts of the plant which are more fully mature, the significance of such observations in relation to apical dominance does not appear to have received any critical experimental assess- ment. That internal competition for water, at least under certain conditions, may indeed play a significant part in the mechanism of inhibition, is suggested, not only by the present observa- tions, but also by some recent experiments on apical dominance in peas (8) in which treat- ments designed to alter the distribution of water in the plant and the degree of moisture stress were shown to have very pronounced effects on the pattern and degree of inhibition of the lateral buds.

It was apparent from the present results that the com~lete release of lateral buds from in- hibition was dependent on an adequate supply of both nitrogen and carbohydrate. In the previous investigation with high-nitrogen rhi- zomes (7) it was shown that apical dominance could be almost completely eliminated by im- mersing the cut end of the isolated rhizome in an 8%-sucrose solution. These rhizomes had a total nitrogen content varying from about 3.0 to 3.5% and in this respect were comparable with the high-nitrogen rhizomes used in the present investigation. It was also found, however (un- published data), that if the plants were grown for a longer period the nitrogen content fell to about 2.0-2.5% and that when sucrose was supplied to these relatively low-nitrogen rhi- zomes, there was no significant effect on apical dominance. This observation is in agreement with the results of the present investigation (expt. 2) which showed that when low-nitrogen rhizomes were used, illcreasing the length of the isolated rhizome-a treatment resumed to in- crease the supply of carbohydrate-considerably promoted the growth of the rhizome apex but failed to release the lateral bud from inhibition. Only when the rhizomes were from plants grown at a high-nitrogen level did the increased length of rhizome significantly reduce the degree of apical dominance. These results suggest, as a working hypothesis, that the lateral buds of the high-nitrogen rhizomes, perhaps by virtue of their higher nitrogen content, were able to compete more effectively with the rhizome apex for the increased carbohydrate supply. The need

for an adequate supply of both nitrogen and carbohydrate was also indicated by the early and concurrent increase in the level of both these basic nutrients in the isolated buds (expt. 6), and, more directly, by the marked extent to which apical dominance was reduced by the addition of sucrose to the nutrient solution (expt. 7). This apparent requirement for ad- ditional carbohydrate suggests that the carbo- hydrate for which competition occurred was in the form of fresh assimilate and that the con- siderable quantity of storage carbohydrate (fructosan) accumulated by the low-nitrogen rhizomes was of little or no significance in this respect.

It is also of interest to compare the results obtained with high- and low-nitrogen rhizomes with respect to the regenerative capacity of the lateral buds. It was shown in the previous in- vestigation (7) that if the bud was left attached to the apex of an isolated high-nitrogen rhizome for a period of 7 days, it was no longer capable of regenerative growth. However, when buds on low-nitrogen rhizomes were subjected to a similar treatment in the present investigation (expt. 1) their growth potential was not appreci- ably reduced. As suggested above, this difference is almost certainly due to the considerably greater amount of carbohydrate which is characteristically present in low-nitrogen rhi- zomes. In a recent study of bud development in rhizomes of Johnson grass (3) Beasley reported that when buds were left attached to the apex of isolated rhizomes for 6 days, they showed no reduction in their capacity for regenerative growth and he remarked on the apparent dis- crepancy between this result and those previously reported by the writer (7). However, since the rhizomes used in Beasley's experiments were obtained from plants growing in the field, it seems probable that their nutritional status, and in particular their carbohydrate content, would be more closely comparable with the low- nitrogen rhizomes used in the present investiga- tion. If this assumption is correct, then their persistent capacity for regenerative growth can be attributed, as in the present experiments, to their abundant reserves of carbohydrate.

In conclusion, it must be emphasized that while the use of isolated rhizomes in this in- vestigation was advantageous from an experi- mental viewpoint, caution should be exercised

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McINTYRE: RHIZOME O F AGROPYRON REPENS 109

when considering the significance of the results in relation to the factors controlling bud activity in the intact plant. Although the general concept of nutrient competition as the factor of major importance seems likely to be equally valid in both cases, the details of the mechanism and the relative importance of the nutritional factors involved may well differ in some important respects. Nevertheless, the present results are of value in suggesting lines along which subsequent investigations should proceed.

Acknowledgments Part of this work was carried out in the unit

of Environmental Physiology at the Plant Re- search Institute in Ottawa. I thank Mr. V. A.

1. ANDERSON, D. B., and T. KERR. 1943. A note on the growth behaviour of cotton bolls. Plant Physiol. 18: 261-269.

2. ARNY, A. C. 1932. Variations in the organic reserves in underground parts of five perennial weeds from late April to November. Minn. Agr. Exp. Sta. Tech. Bull. 84.

3. BEASLEY, C. A. 1970. Development of axillary buds from Johnson grass rhizomes. Weed Sci. 18: 218-222.

4. GATES, C. T. 1968. Water deficits and growth of herbaceous plants. 111 Water deficits and plant growth. Edited by T. T. Kozlowski. Academic Press. pp. 135-190.

5. LEBARON: H. M., and S. N. FERTIG. 1961. The effects of chemical and cultural treatments on the food reserves of quack grass rhizomes. Proc. N.E. Weed Control Conf. 15: 319-328.

6. MCINTYRE, G. I. 1965. Some effects of the nitrogen supply on the growth and development of Agropyron repetzs L. Beauv. Weed Res. 5: 1-12.

7. MCINTYRE, G. I. 1969. Apical dominance in the rhizome of Agropyron reperzs. Evidence of competi- tion for carbohvdrate as a factor in the mechanism

Helson and his colleagues for their help in of inhibition. can. J. Bot. 47: 1189-1197.

providing the necessary facilities and Mrs. S. 8. REMY, M. 1968. Effects d'une modification du mouve- ment de l'eau dans les Cpicotyles de pois sur la

Groundwater and Miss P. Bonn for technical croissance des bourgeons axillaires et leurs corrkla- assistance. For the part of the work tions. C.R. Acad. Sci. Paris., Ser. D, Sci. Natur.

266: 676679. at Regina my thanks are due Mr. 9. SACHS, T., and K. V. THIMANN. 1964. Release of Fleming for technical assistance. I am grateful lateral buds from apical dominance. Nature, 201:

939-940. to J' for his reading of 10. WnsoN, C. C . 1948, Diurnal fluctuations of growth

the manuscript. in length of tomato stem. Plant Physiol. 23: 156157.

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