Apical dominance in the rhizome of Agropyron repens: the i~fluence of water stress on bud activity

GORDON I. MCINTYRE Agricrrltrrre Ccoltrdtr, Resecrrch Sttrtiot~, Bos 440, Rcgitlo, Snsk., Cot~nclrr S4P3A2

Received May 4, 1976

MCINTYRE. G. I. 1976. Apical dominance in the rhizome of Agropyr,otl r~eperr.s: the influence of water stress on bud activity. Can. J . Bot. 54: 2747-2754.

Experiments conducted under both field and growth-chamber conditions showed that buds on the rhizome of Agropyrotl reperrs L. Beauv. could be released from inhibition by a localized reduction of water stress, e.g. by enclosing the rhizomes in moist vermiculite. This response was obtained even at low N levels. a fact which may be due partly to the relatively low N requirement of buds developing as rhizomes as compared with those developing as shoots. The induced growth of the lateral buds was correlated with a reduction or complete inhibition of apicalgrowth of the parent rhizome or with its transition from rhizome to shoot development. Continuous root removal reduced the bud response to high humidity in N-deficient plants but had relatively little effect at a higherN level. In water-stressed rhizomes the apparent increase of bud inhibition with distance from the apex, a characteristic feature of apicaI dominance, was correlated with the water content of the rhizome, which was greatest at the apex and decreased basipetally. It is postulated that this gradient of decreasing rhizome water content may be causally related to the increasing inhibition of bud activity.

Introduction In a previous study of apical dominance in

isolated rhizomes of quackgrass (Agropyron repens L. Beauv.) it was observed that when the plants were grown under controlled conditions and well supplied with water and mineral nutrients, there was a marked tendency for the buds on the rhizome to escape from inhibition and to develop as lateral branches while the rhizome was still attached to the parent shoot (1 I). Although earlier investigations (8, 10) had shown that buds may be released from inhibition at high N levels, this apparent loss of apical dominance was also observed in plants receiving a reduced N supply and exhibiting quite severe deficiency symptoms. In attempting to account for these observations it was also noted that quackgrass plants growing in a fertile soil in the field and showing no symptoms of N deficiency were typically characterized by a high degree of apical dominance, the growth of the rhizome buds being arrested when only 3 to 4 mm in length. The apparent loss of apical dominance at low N levels in the growth chamber and the much more complete inhibition of bud growth shown by plants receiving a more abundant supply of N in the field suggested that bud growth in both environments was being con- trolled by another factor which was tending to override the effect of the N supply.

There were several reasons for suspecting that

the degree of water stress might be the overriding factor controlling bud activity under both growth-chamber and field conditions. This was suggested, firstly, by the results of previous experiments with isolated low N rhizomes in which the provision of water through the cut end of the rhizome consistently produced a marked stimulation of bud activity (1 1). The hypothesis was further supported by a com- parison of root growth on rhizomes of plants in the growth chamber and in the field. Under - growth-chamber conditions, several long and vigorously growing roots were normally pro- duced at each rhizome node. whereas observa- tions on several areas of quackgrass at the Regina Research Station showed that by mid- summer most of the roots ~roduced earlier in the season were either dead or moribund, apparently as a result of desiccation. Micro- scopic examination of these field rhizomes showed that although other roots had been initiated, their growth had been arrested at the rhizome surface. It also seemed probable that, during the summer, transpiration rates would be considerably higher in the field than under normal growth-chamber conditions (16) and that this would tend to increase the degree of water stress in the rhizome.

Thus, there appeared to be sufficient evidence to justify an experimental investigation designed to assess the importance of water stress as a

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2748 CAN. 1. ROT.

factor in the mechanism of bud inhibition. A preliminary experiment was therefore conducted in the field to determine the effect on bud and root growth of maintaining the rhizome in a constantly high humidity. Further investigations were then conducted in the growth chamber so that the response of the rhizome buds could be related more precisely to the environmental conditions and to other factors which might be involved.

Materials and Methods Field I~~vestigation

Experir~ier~t I The effect of increasing the humidity around the rhi-

zome was investigated in a field experiment in the summer of 1974. The quackgrass used had been planted the pre- vious year in an area of cultivated land at the Regina Research Station and was from the same seed collection as in previous investigations (9). The fertile soil (Regina heavy clay) and the absence of competition resulted in the rapid establishment of a dense stand and when plants were selected for treatment in the next spring, they appeared extremely vigorous and showed no sign of N deficiency. On June 13 the soil was removed from around the base of selected plants located around the edge of the patch and a young rhizome on each plant, ranging from 3 to 10 cm long, was inserted through a hole in the end of a narrow plastic tray (38 x 7.5 x 5 cm) (Fig. 2), the lower half of which was buried in the soil. The trays were filled with vermiculite and covered with aluminum foil. In one treatment the vermiculite was kept dry and in the other it was kept thoroughly moist by daily additions of distilled water. The use of a coarse grade of vermiculite and the avoidance of overwatering insured good aeration, a necessary condition for optimum growth of the rhizome buds (6). The third set of rhizomes served as controls and were allowed to continue growing under natural condi- tions. There were eight rhizomes per treatment. The length of the rhizomes was recorded at intervals of several days and each rhizome was harvested for bud length measurements and other observations when it had reached the end of the tray.

Experin~ents rmder. Controlled Conclitions Experiment 2 The influence of humidity on bud growth was also

investigated in an experiment conducted in a growth room under controlled conditions. The plants were propagated from one-bud pieces of rhizomes obtained from the same stand of quackgrass used in experiment I . They were grown initially in trays of vermiculite and were watered at 2-day intervals with an excess of half- strength Hoaglands solution (3) until they had reached the third leaf stage. The N level was then reduced to 10.5 ppm as previously described (9). Two weeks later plants with rhizomes 3.0 to 8.0 cm long were selected and transplanted into 10-cm-square plastic pots, which were attached to the ends of plastic trays similar to those used in experiment 1 (Fig. 2). The rhizomes were inserted through holes bored in the side of the pot and in the tray. A short piece of rubber tubing which fitted tightly

through the two holes served to keep the pots and trays in proper alignment and to prevent injury to the young rhizome.

In the low-humidity treatment no rooting medium was provided so that the rhizomes were exposed to the approx- imate relative humidity of the growth room, which was 60 2 5%. In the high-humidity treatment the rhizomes were covered with coarse vermiculite kept thoroughly moist with distilled water as in experiment 1. The trays in both treatments were covered with aluminum foil. The nutrient solution (10.5 ppm N) was supplied only to the pot, the space between the rhizome and the inside of the rubber tubing being sealed with lanolin to prevent leakage of the solution into the tray. Each pot received 200 ml of nutrient solution at 2-day intervals and suffi- cient water to resaturate the vermiculite on alternate days. Illumination was provided by a bank of 24 cool- white V.H.O. fluorescent tubes and 45 40-W incandescent lamps providing an intensity of 2800-3200 ft-c at pot level and a 16-h photoperiod. The temperature was held constant at 15 2 1 "C.

The length of the rhizomes was measured at intervals of 2-4 days until they had reached the end of the tray. They were then removed and the length of the buds or lateral branches at each node was recorded. To conform with the terminology previously used (13) the buds were numbered in order of increasing age starting from the one at the basal end of the youngest internode 5 2 cm (bud 1). Measurements were also recorded of the next two younger buds (A, and A*) which were revealed by removing the scale leaves enclosing the rhizome apex. All buds 7 1 0 mm were measured under a dissecting microscope at x 16 magnification; those > 10 mm were measured to the nearest millimetre with a ruler.

T o examine the relationship between bud growth and the water content of the rhizome, sanlples were taken for moisture determinations. The samples consisted of the apical 2 cm (measured from the tip of the apical meristem after all leaves had been removed) and a 3.0-cm piece of rhizome (1.5 cm on either side of the node) from nodes 1 to 7 inclusive. The lateral buds, scale leaves, and roots were removed and excluded from thesample. After having been weighed, the samples were dried in the oven for 24 h at 80 "C and their dry weight and moisture content were determined.

Experirner~t 3 Since the release of the lateral buds from inhibition

a t high humidity was consistently associated with the growth of roots at the nodes, a further experiment was designed to investigate the possibility that the bud growth response may be dependent on some factor supplied by the roots. In addition the effect of N nutrition was also investigated by comparing the effect of root removal at high and low N levels.

The method and conditions were the same as in ex- periment 1 except in the following respects. The rhizomes of all of the plants were maintained in a high humidity, but half of the plants had the roots at the nodes removed at 2-day intervals while the rest were left intact. Half of the plants in each of these treatments were grown at a high N level of 105 ppm N and the other half received a low N concentration of 5.25 ppm. As in experiment 2 the nutrient solutions were supplied only to the pots, the vermiculite in the trays receiving distilled water only.

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McINTYRE 2749

FIG. 1. Rhizomes of Agropyron repens illustrating the effect of humidity on bud and root growth. The rhizomes were grown in trays (see Fig. 2) containing (A) dry vern~iculite and (B) vermiculite kept moist with distilled water. They were left attached to the parent plants which were growing under natural conditions in the field. x 0.45.

The length of the rhizomes was measured at 2-day intervals. Final measurements, including the length of the lateral buds and branches, were recorded either when the parent rhizome had stopped growing or when it had reached the end of the tray.

Results Experiment 1

In the field experiment the provision of a high humidity around the rhizome proved ex- tremely effective in promoting growth of the buds and roots a t the rhizome nodes (Fig. 1). Measurements showed that the buds increased progressively in size at successively older nodes and by the time the parent rhizome had reached the end of the tray some of the lateral branches at the basal nodes were more than 20 cm in length. Root growth was also extremely vigorous, resulting in the development of an extensive root system.

In marked contrast with this behaviour the rhi- zomes growing either in soil or in the dry vermi- culite had remained completely unbranched, growth of the lateral buds having been arrested when they were only 3 to 6 mm in length. Al- though root growth under these conditions had also been entirely suppressed, examination of the rhizomes showed that several roots had been initiated at each node but that their growth had

apparently been arrested a t the surface of the rhizome. The absence of root growth, however, did not reduce the rate of growth of the rhizome. Indeed, a comparison of the number of days required for the rhizomes to reach the end of the tray showed that the rhizomes in the dry vermiculite had grown more rapidly than those receiving the high humidity treatment. However, owing to the small number of plants in each treatment and the considerable variation in the rate of rhizome growth this difference was not statistically significant.

Experiment 2 The results of the field experiment were well

substantiated by those obtained under controlled conditions. The same striking difference in bud and root growth between the high- and low- humidity treatments was again apparent (Fig. 3B). In addition, the difference in the rate of rhizome growth between the two treatments was more pronounced, the outgrowth of the lateral buds in response to the increased humidity being associated with a marked retardation in growth of the parent rhizome (Fig. 3A). Bud measurements also showed that in the low- humidity treatment the buds grew rapidly during their early development but that their

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CAN. J. BOT. VOL. 54, 1976

FIG. 2. Plants of Agr.opyr.012 repens illustrating the method used in studying the effect of humidit on rhizome bud activity. Nutrient solutions were supplied only to the parent plants (in the pots); ti- rhizomes in the trays were either left exposed to air of low humidity or were covered in vermiculif kept moist with distilled water. The plants shown are from later experiments not described in th text. x 0.25.

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DAYS T

L 1 2 3 4 5 6 7

BUD POSITION APEX TO BASE OF RHIZOME

FIG. 3. The effect of humidity on growth of (A) the rhizome and (B) the lateral buds. LH, low humidity; HH, high humidity. The experimental treatments are fully described in the text. All data are mean values. The standard errors of the means are indicated by vertical lines.

growth was progressively inhibited as their distance from the apex increased. This gradient of decreasing bud activity was correlated with the water content of the rhizome which was greatest in the apical region and decreased basipetally (Table 1).

Experitnent 3 When the rhizomes were maintained in a

high humidity, continuous removal of the roots did not prevent the outgrowth of the lateral buds, but the amount of bud growth was con- siderably reduced. This effect of root removal

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2752 CAN. J. BOT. VOL. 54. 1976

B U D P O S I T I O N

FIG. 4. The effect of root removal on lateral bud growth in relation to the N supply. (A) 5.25 ppm N; (B) 210 PPm N.

was evident in both N treatments but was con- siderably greater at the lower N level (Fig. 4).

As in the previous experiments the release of the lateral buds from inhibition was associated with a reduction in growth of the parent rhi- zome. In this experiment, however, the effect was much more pronounced and about half of the rhizomes in each treatment had their growth completely arrested before they had reached the end of the tray. Of the rest, several showed a transition from rhizome to shoot development. The reason for this difference is not known but the fact that low temperatures were found to promote the initiation and growth of rhizomes in this species (9) suggests that the reduction in the niglit temperature to 10 OC may have increased the growth of the lateral buds, thereby increasing their inhibitory effect on the rhizome apex.

Discussion It is evident from these results that water

stress plays an important role in the regulation of bud activity in the rhizome of quackgrass. Although the observed effect of humidity on bud growth does not appear to have been previously recorded for this species, Gardner (2) reported that rhizomes of the closely related Agiopyron

smitllii remained unbranched when growing in a dry soil but branched freely when sufficient moisture was available.

While the present investigation provides no evidence as to the nature of the mechanism in- volved, it seems probable that the promotion of bud and root growth by high humidity is a direct response to increased hydration of the tissues resulting from a reduction in the loss of water by evaporation. That this increase in the availability of water should have a growth- promoting effect is not surprising for numerous investigations (reviewed by Hsiao (4)) have shown that such processes as cell division and extension, cell wall synthesis, and protein syn- thesis are all extremely responsive to changes in water potential. Of particular relevance to the present observations is the work of Nir et al. (15), who showed that exposure of the root tips of maize to a low humidity, either when isolated or still attached to the plant, markedly reduced the uptake of amino acids and their incorpora- tion into proteins. A reduction in tissue water content of as little as 1 1 % (fresh weight basis) was sufficient to produce a marked effect. It would clearly be of interest, in view of the present observations, to conduct a similar study of the effect of water stress on N metabolism in the buds and roots of the quackgrass rhizome.

It may be assumed, however, that the moisture content of the lateral buds, and hence their ability to escape from inhibition, will be deter- mined not only by the rate at which water is lost by evaporation but also by the rate of supply. Thus, competition for water within the plant seems likely to play a part in determining the degree of inhibition. Evidence of such competi- tion was obtained in previous experiments with isolated rhizomes (1 1) in which apical dominance was markedly reduced when water was supplied through the cut end of the rhizome. It would seem, however, that competition with the parent shoot is likely to have a greater effect, at least under field conditions. This view is supported by the data of Baker and Moorby ( I ) , who showed that in the potato, the fresh weight of developing tubers decreased sharply when the plants were illuminated and increased again in the dark. Water competition, controlled by transpiration, would seem to be the factor in- volved. Thus, when quackgrass is growing in the field under conditions conducive to rapid transpiration and when the soil is sufficiently

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dry to inhibit the growth of roots at the rhizome nodes, it seems probable that competition for water between the rhizome and the parent shoot will play a major role in determining the degree of inhibition of the rhizome buds.

A notable feature of the present investigation was that the buds on the rhizome could be released from inhibition by the provision of a high humidity even when the plants were grown at a low N level and were showing quite severe symptoms of N deficiency. This fact is in marked contrast with the results previously obtained in experiments with various other species in which it was shown that the release of the axillary buds from apical dominance by a reduction of water stress could only be achieved when the plants were also grown at a high N level (12, 14). In these latter investigations, however, the bud released from inhibition developed as leafy shoots, whereas in the present investigation they developed as rhizomes. The probable significance of this difference is suggested by the fact that, at least in quackgrass, buds developing as rhi- zomes have a considerably smaller requirement for both N and water than those developing as shoots (13). Thus, when competition for water had been sufficiently reduced by providing the bud with a high humidity, the bud would then be capable of competing with the parent shoots for the relatively small amount of N required for its development as a rhizome. A similar explanation may account for the results reported by Kumar and Wareing (7), who also showed that in the wild potato (Solanurn andigena), a localized high-humidity treatment was sufficient to release the axillary buds from inhibition. As in quackgrass these buds developed not as shoots but as stolons (i.e. aboveground rhizomes), and it seems probable that in this case also, the relatively small amount of N required for stolon development effectively minimized the competi- tive influence of the parent shoot, enabling the bud to escape from inhibition.

In the present investigation the effect of root removal in reducing the growth of the rhizome buds varied with the N supply, being relatively slight in the high IV treatment but considerably greater at the lower N level. There are several hypotheses which could be advanced to account for this result. It could be postulated, for ex- ample, that bud growth tends to be limited by the supply of cytokinins or some related nitro- genous metabolites which are synthesized in the

roots. Since the concentration of such metabo- lites in the rhizome is likely to be reduced by N deficiency, it might be expected that removing the source of supply would cause a greater re- duction in bud growth at the lower N level. Alternatively, it could be postulated that bud growth was promoted by water supplied from the roots and since the water content of the rhi- zomes is significantly reduced by N deficiency (1 I ) , any reduction in water uptake would have a greater effect in the low-N plants. A third possibility, which seems equally probable, is that the continuous removal of the emerging roots may have induced the initiation and growth of new root primordia which may have competed for N with the buds (11). The effect of such competition would, of course, be greater at the lower N level.

Also of interest is the observed correlation between the water content of the rhizome and the gradient of bud activity. The tendency for lateral buds to grow rapidly during their early development in the apical region of the shoot and for their growth to be more strongly in- hibited as their distance from the apex increases is a characteristic feature of apical dominance. There is no evidence, however, as to the nature of the mechanism involved. Snow (18) postulated that this 'distance effect' might result from an interaction between auxin and a hypothetical growth inhibitor, but the evidence for the participation of specific growth-inhibiting sub- stances in the correlative inhibition of buds is by no means conclusive (17). On the other hand, if apical dominance is due primarily to nutrient competition, a view for which there is con- siderable experimental support, it is possible that the existence of nutritional gradients within the shoot may have an important regulating influence on the pattern of bud development. In the present experiment, in which water was the limiting nutritional factor, the observed correlation between the respective gradients of bud growth and rhizome water content may well be indicative of a causal relationship. It was also noted in previous experiments (13) in which N was the limiting factor that both the gradient of bud growth along the rhizome and the charac- teristic polarity of bud development were corre- lated with a basipetal gradient of decreasing N content and could be quite precisely controlled by varying the N supply. Unfortunately, while the existence of N and water gradients has been

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reported for other species, no attempt has been made to study the relation of such gradients to the growth or degree of inhibition of the lateral buds. The evidence suggests, however, that this is an aspect of the apical dominance problem which merits a more critical investigation.

From a practical standpoint the present results are also of interest because of the extent to which the activity of the rhizome buds may influence the translocation and effect of foliar-applied herbicides. The im~ortance of this factor was evident from previous experiments with leafy spurge (Euphorbia esuln L.) (5) which showed that when the growth of the root buds was " promoted, either by decapitation of the shoot or by increasing the N supply, the uptake of 14C from a foliar application of 14C-labelled 2,4-dichlorophenoxyacetic acid (2,4-D) was markedly increased. Preliminary experiments with quackgrass (Qureshi and McIntyre, un- ~ublishedl have also shown that the stimulation of rhizoie bud activity by high humidity, as described in the present report, greatly increases the u ~ t a k e of 14C-labelled assimilates from the paren; shoot. In view of the well established correlation between the movement of assimilates and of growth regulating herbicides it seems probable that the translocation of foliar-applied herbicides will be similarly affected.

Acknowledgments I thank Mrs. Marjorie Richardson and Mr.

William Fleming for technical assistance and Dr. J . R. Hay for his critical reading of the manuscript.

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2. GARDNER, J. L. 1942. Studies on tillering. Ecology, 23: 162-174.

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

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6. JOHNSON, B. G.. and K . P. BUCHHOLTZ. 1961. An in vitro method of evaluating the activity of buds on the rhizomes of quackgrass (Ag~.opy~.o~z ~.c,pc.~ls). Weeds, 9: 600-606.

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12. MCINTYRE. G. 1. 1971. Water stress and apical domi- nance in Pisrr~n strti,.rr~n. Nat. New Biol. 230: 87-88.

13. MCINTYRE. G . 1. 1972. Studies on bud development in the rhizome of Agropp1.011 repetls. 11. The effect of the nitrogen supply. Can. J. Bot. 50: 393-401.

14. MCINTYRE. G. I . 1973. Environmental control of npi- cal dominance in P11nseol1r.s r~rr1gnri.s. Can. J. Bot. 51: 293-299.

15. N I R , I . , A. POLJAKOFF-MAYBER. and S. KLEIN. 1970. The effect of water stress on the polysome pop~llation and the ability to incolpornre amino acids in maize root tips. Isr. J. Bot. 19: 451-462.

16. PARSONS. L . R., W. T . DAVIES. D. T . PATTERSON. and P. J . KRAMER. 1975. Comparisons of leaf water potentials of soybean and cotton grown in phytotron and field. Plant Physiol. 56(Suppl.): 12.

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18. SNOW, R. 1937. On the nature of correlative inhibi- tion. New Phytol. 36: 283-300.

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