Apical dominance in the rhizome of Agropyron repens: the influence of humidity and light on the regenerative growth of isolated rhizomes

GORDON I. MCINTYRE Agriculture Canada, Research Station, 5000 Wascana Parkway, Regina, Sask., Canada S4P 3A2

Received December 4, 1979

MCINTYRE, G. I. 1981. Apical dominance in the rhizome of Agropyron repens: the influence of humidity and light on the regenerative growth of isolated rhizomes. Can. J . Bot. 59: 549-555.

The influence of humidity and light on the regenerative growth of isolated five-node segments from rhizomes of Agropyron repens was investigated under controlled conditions. When the rhizomes were incubated in the dark at 20 t- 1°C shoot growth at the apical node was significantly reduced by each 0.5% reduction in relative humidity between 100 and 98.5% and at 98.0% growth at all nodes was completely inhibited. The restriction of these effects to the apical end of the rhizome (nodes 1 and 2) was attributed to their interaction with the basipetal gradient of decreasing N concentration previously identified as one of the causal factors in the polarity of bud activity. Bud inhibition at arelative humidity of 98% was eliminated by supplying water through the basal end of the rhizome. This treatment also released the buds from the inhibition induced by the exposure of the rhizomes to light. Since the uptake of water by the rhizomes was also greater in the light than in the dark it was postulated that the light-induced inhibition of bud growth was due to a reduction in rhizome water potential mediated by an increase in the rate of transpiration.

MCINTYRE, G. I. 1981. Apical dominance in the rhizome of Agropyron repens: the influence of humidity and light on the regenerative growth of isolated rhizomes. Can. J . Bot. 59: 549-555.

On a Ctudie, dans des conditions contr8lees, l'influence de I'humiditC et de la lumikre sur la croissance rCg6nCrative de segments de 5 nceuds isolCs de rhizomes d'Agropyron repens. Lorsque les rhizomes sont incubCs 1'obscuritC a 20 t- l0C, la croissance de la pousse au nceud apical est significativement rCduite par chaque diminution de 0,5% de l'humiditk relative entre 100 et 98,5%; i une humidit6 relative de 98%, la croissance est complktement inhibCe a chaque nceud. Le fait que ces effets soient restreints a 1'extremitC apicale du rhizome (nceuds 1 et 2) est attribue a une interaction avec le gradient basipkte de la concentration decroissante d'azote; ce gradient a CtC antkrieurement identifie comme une des causes de la polarit6 dans 1'activitC des bourgeons. L'inhibition des bourgeons a une humidit6 relative de 98% est levee si I'on foumit de l'eau par 1'extrCmitC basale du segment de rhizome. Ce traitement libere aussi les bourgeons de l'inhibition induite par l'exposition des rhizomes a la lumikre. Puisque l'absorption d'eau par les rhizomes est Cgalement plus forte a la lumikre qu'a l'obscuritC, on emet I'hypothkse que l'inhibition de la croissance du bourgeon par la lumikre est due a une rkduction du potentiel hydrique du rhizome, provoquCe par I'augmentation du taux de transpiration.

[Traduit par le journal]

Introduction Previous experiments on the mechanism of apical

dominance in the rhizome of Agropyron repens showed that when the rhizomes were permitted to develop in a constantly moist but well-aerated environment (e.g. in moist vermiculite) the lateral buds were protected from correlative inhibition and grew out as lateral branches on the intact plant (13). Even when bud growth had already been arrested by apical dominance increasing the humidity around the rhizome induced a rapid resump- tion of bud activity and, within 24 h, had significantly increased the uptake of 14C-labelled assimilates from the parent shoot (1 6).

It was concluded from these observations that the degree of water stress to which the plant is normally subjected in the field is likely to play a major role in the mechanism of bud inhibition (1 3). In the field, however, the rhizomes are frequently broken up by cultivation, a treatment which promotes the growth of the lateral buds by isolating them from the inhibiting influence of the

rhizome apex and the parent shoot. Studies on the regenerative behaviour of such multinode rhizome fragments (2 ,7 , 12) showed that correlative inhibition is later reestablished, the growth of the younger bud at the apical end of the rhizome maintaining the inhibition of bud growth at the basal nodes. The resulting polarity of bud activity is closely correlated with a gradient in the total N (12) and amino acid (15) @ontent of the rhizome and is significantly reduced by increasing the N supply (2, 15). However, in view of the marked effect of water stress on bud growth in the intact plant (13) and the disruption of the water supply when the rhizomes are isolated from the parent shoot, it might be expected that, in addition to the N supply, factors affecting the water potential of isolated rhizome fragments will also play an important part in determining the pattern and vigour of regenerative growth. The object of the present investiga- tion was to assess, under controlled conditions, the influence of the two factors, i.e. humidity and exposure to light, which seemed likely to be of particular

0008-402618 11040549-07$01 .OO/O 01981 National Research Council of CanadaIConseil national de recherches du Canada

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significance. Experiments were also designed to test the hypothesis that the reported inhibition of bud growth by light (8) is essentially a water stress effect.

Materials and methods Plant culture technique

The plants from which the rhizomes were obtained were grown under controlled conditions and were propagated either from one-bud rhizome segments (experiments 1 and 3) or from seed (experiments 2 and 4). Both materials were derived initially from a stand of quackgrass established at the Regina Research Station from seed collected at Pullman, WA. The seeds were germinated on moist filter paper at a temperature alternation of 15°C (dark) and 30°C (light), a 16-h photo- period, and a light intensity of 2000 lx. Both the seedlings and the rhizome cuttings were grown in 12- to 30-mesh silica sand in plastic trays (32 X 21 X 7.5 cm) for about 3 weeks. They were then transplanted into 21 cm diameter plastic pots containing a 1: 1 volume mixture of peat-perlite (experiments 1 and 2) or peat and 12- to 30-mesh silica sand (experiments 3 and 4), with three plants per pot. While in the trays the plants received an excess of %-strength Hoagland's solution (5), which contains 105 ppm N, at 2-day intervals and distilled water on alternate days. After they had been transplanted into pots all of the plants continued to receive the %-strength solution, i.e. 105 ppm N, for 4 weeks. The solution was then increased to full-strength, but to prevent the outgrowth of the rhizome buds until the rhizomes were isolated for experimental treatment the N level was reduced by means of the substi- tutions previously described (10). The level was reduced to 21.0 ppm for 4 weeks in experiment 1 and for 7 to 8 weeks in experiments 2 and 3. In experiment 4, to promote more vigorous bud growth following rhizome isolation, the N level was reduced to only 52.5 ppm for 7 weeks and was increased to 210 ppm, i.e. the level in the standard solution, for 2 days immediately prior to isolation of the rhizome.

All of the plants were grown in controlled environment rooms at a temperature of 15 + 1°C and 50 + 15% relative humidity (RH). The light intensity, measured in all of the experiments with a Lambda L1-185 light meter using the L1-190s quantum sensor, ranged from approximately 500 to 600 ~ E . m - ~ - s - l and was provided by a bank of 24 cool-white fluorescent tubes plus forty-five, 40-W incandescent lamps. The photoperiod was 16 h.

Experimental treatments Rhizomes were collected for treatment after growing

periods of 11 weeks in experiment 1 and 15 weeks in experiments 2 to 4. After removing the roots and scale leaves the rhizomes were selected for uniformity in bud and internode length and were cut into five-node segments. Buds were numbered as in previous investigations (12), i.e. in basipetal sequences, with bud 1 located at the base of the youngest internode 2 2 cm. Since most internodes were about 3 cm long the total length of the cuttings, including a 2 cm length of internode which was left at either end, ranged from approxi- mately 15 to 20 cm.

Effect of humidity (experiments 1 to 3 ) In the experiments on the effect of humidity each rhizome

VOL. 59, 1981

was attached to a piece of plastic (Plexiglas) (25 x 2.5 x 0.3 cm) with strips of masking tape. These were laid on two cross pieces of plastic which were supported by No. 5 rubber stoppers attached with adhesive. The whole assembly, which included five rhizomes, was placed in a plastic tray (32.5 x 21.0 X 7.5 cm) with a tightly fitting lid sealed with silicone grease. In experiment 1 the humidity within the tray was controlled by use of saturated salt solutions (17). The saltsused and the corresponding relative humidities (at 20°C) were as follows: Pb(N03)?, 98%; Na2HP04. 12H20, 95%;NH4H2P04, 93.1%; ZnS04.7H20, 90%; KC1 , 86.4%; and (NH4)?S04, 81%. A 750-mL aliquot of the appropriate solutions was placed in each tray. The buds on the selected rhizomes were measured to the nearest 0.1 mm under a binocular microscope at 16x magnification and five rhizomes were assigned to each treatment. The trays were kept in an incubator in the dark at 20 + 1°C. The length of the buds or shoots at each node was recorded after 14 days. Those <10 mm were measured to the nearest 0.1 mm at 16 X magnification and those s 10 mm were measured to the nearest millimetre with a ruler. A similar technique was employed in experiment 2 except that aqueous solutions of sulphuric acid were used to vary the humidity between 100 and 98% by steps of 0.5%, the data by Walter (see reference 17) being used in preparing the solutions. Since no means were available for measuring the humidity within the trays with sufficient precision it had to be assumed that the values were not significantly different from those reported in the literature (17).

Experiment 3 was designed to determine whether the inhibiting influence of low humidity on bud growth could be overcome by supplying water through the end of the rhizome. The technique was similar to that in experiment 1. Two trays of rhizomes were maintained at 98% humidity by use of saturated solutions of Pb(N03)2 but the rhizomes in one of the trays were supplied with distilled water by means of latex rubber tubes (1.5 mm inside diameter) which passed through holes in the end of the tray and were attached to the basal ends of the rhizomes. The rhizomes were cut below the surface of the water just before the tubes were attached. The water was supplied from plastic (Plastipac) graduated syringes supported on a rack (see reference 11, Fig. 5). The level of the water in the syringes was 18 cm above the rhizomes. In a third treatment the rhizomes were maintained at 100% RH and were not supplied with water.

Effect of light (experiment 4 ) In experiments designed to investigate the effect of light on

the growth of the rhizome buds the rhizomes were taped to pieces of plastic as in the previous experiments and were placed in Pyrex dishes (18 X 13 X 5 cm) lined with wet blotting paper and covered with a sheet of thin transparent plastic (Saran wrap) sealed around the edges of the dish with adhesive tape. A film of condensed moisture was present on the undersurface of the plastic throughout the experiment and it was assumed that the humidity within the dish remained at 100%. Each experimental treatment comprised two dishes, each with four rhizomes. Two sets of dishes were kept in a growth chamber where the rhizomes received continuous illumination from a bank of 18 cool-white fluorescent tubes. The light intensity at the level of the rhizomes was 400

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pE.m-2.s-1. The rhizomes in two of the four dishes were supplied continuously with distilled water using the same technique as in experiment 3 . The temperature in the chamber was 15 f 1°C but owing to the heating effect of the lights the temperature inside the culture dishes was 20 * 1°C. Two other sets of dishes were kept in an incubator in darkness at 20 * 1°C and were exposed to light of low intensity only briefly for daily observations. The rhizomes in one set were supplied with water in the same manner as those in the light. Bud and shoot measurements were recorded at the end of the experiment which was terminated after 10 days.

Results Effect of humidity

Experiment 1 In the experiment in which the humidity was con-

trolled by the use of saturated salt solutions reducing the humidity from 100 to 98% (i.e., a vapour pressure deficit of approximately 0.47 mbar (1 bar = 105Pa) caused complete inhibition of bud and root growth at all of the rhizome nodes (Table 1). As mentioned above, the basipetal gradient of decreasing shoot length along the rhizome is a characteristic feature of the regenerative growth of isolated rhizomes in this species and is related to the N content of the rhizome nodes. At 98% humidity there was evidence of shrinkage of the buds but the differences between their initial and final lengths were not statistically significant. Some loss of water from the rhizome was also evident from a slight wrinkling of the surface, particularly in the apical internode, which was still immature. Several roots, ranging from approxi- mately 0.5 to 2.0 cm long, were present at each node at 100% humidity but at 98% only the root apices were visible at the surface of the rhizome, their growth having apparently been arrested. No bud or root growth occurred at any of the lower humidities.

Experiment 2 The use of sulphuric acid solutions for more precise

control of humidity provided further evidence of the importance of water as a limiting factor in the regenera- tive growth of the rhizome buds (Fig. 1). At node 1 each 0.5% reduction in humidity between 100 and 98.5% caused a significant reduction (P < 0.05) in shoot growth and at 98% growth was completely inhibited. Similar differences were present at node 2 but owing to the greater variation in bud growth within each treatment only the difference between the 100 and 99.5% humidi- ties was significant at the 5% level. The longer period for which the parent plants had been growing at a reduced N level as compared with the previous experi- ment can account for the steeper basipetal gradient of decreasing bud growth along the rhizome (15) and the absence of any response to humidity at the basal nodes. The effect on root growth showed some variation within each treatment but was roughly correlated with the

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RELATIVE HUMlDlTlY (O/o)

100

99.5

99.0

n 50

E 98.5 E u

40 98.0 I t- u z W 30 1

0 3 m 20

10

NODE POSITION

(APEX TO BASE OF RHIZOME)

FIG. 1. Effect of relative humidity on bud growth on isolated rhizomes kept in the dark at 20 ? 1°C. Values for water potential at each percent RH are as follows: loo%, 1.0; 99.5%, -6.78; 99.0%, -13.6; 98.5%, -20.4; 98.0%, -27.2. Data were recorded after 14 days and are mean values based on five rhizomes per treatment. Vertical bars indicate standard errors > 1 mm. At nodes 1 and 2 bud lengths labelled with the same letter are not significantly different at the 5% level, as determined by Student-Neuman-Keuls multiple range test. There are no significant effects of humidity at nodes 3 to 5.

response of the buds. The mean number of roots per rhizome at each humidity was 10 + 1.7, 100%; 5 + 0.5, 99.5%; 7 + 1.2, 99.0%; 0.4 + 0.2, 98.5%; and 0.0, 98%.

Experiment 3 The provision of water through the cut end of the

rhizome was highly effective in preventing the inhibi- tion of bud growth by reduced humidity (Table 2). Indeed, the growth of the shoots in response to this treatment was significantly greater ( P < 0.5) than where the rhizomes were kept at a humidity of 100%.

Effect of light Experiment 4 Exposure of the rhizomes to light strongly inhibited

the growth of the rhizome buds (Fig. 2). This light- induced inhibition, however, was effectively counter- acted by the provision of water to the end of the rhizome. The growth promoting effect of this treatment was evident at all of the rhizome nodes and at nodes 1 to 3 was significant at the 5% level. Only at node 2 was the growth of the shoots in the dark significantly greater than that of the shoots on rhizomes supplied with water in the light. There was also evidence that the greater length of the shoots in the dark could be attributed at least partly to etiolation for their dry weight was significantly less ( P < 0.05) than that of the illuminated shoots to which water was supplied. Records of water uptake by the rhizomes during the first 4 days of treatment showed that the

average absorption in millilitres per day (+SE) was 0.14 + 0.08 for the rhizomes in the dark and 0.97 + 0.1 1 for those in the light. Also noteworthy is the fact that, except for node 2, the provision of an external water supply increased shoot growth both in the light and in the dark. Although the dark response was not statistically significant its occurrence at all but one node suggested that, even in the dark, water may have been the limiting factor in shoot extension.

These conclusions were fully substantiated when the experiment was repeated. Again the provision of water through the end of the rhizome released the buds from light-induced inhibition and in the dark, again with the exception of node 2, the length of the shoots on rhizomes supplied with water was markedly increased, ranging from ca. 50 to 60% greater than those of the dark controls. While the exceptional behaviour of the bud at node 2 in both experiments may well be fortuitous and requires further confirmation, there was evidence in previous investigations (12) that the competitive interac- tion between the buds at nodes 1 and 2 is particularly sensitive to the influence of nutritional factors.

Discussion It is evident from these results that the humidity to

which isolated rhizomes are exposed is a major factor in determining their capacity for regenerative growth. When the rhizomes were kept in the dark reducing the humidity from 100 to 99.5% (equivalent, at 20°C, to a

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McINTYRE

TABLE 2. Effect of humidity and water supply on bud growth on isolated rhizomes

Bud length, mmt

Treatment* Node 11: Node 2 Node 3 Node 4 Node 5

100% RH 37.025.86 26 .5k4.4~6 15.3k5.8n 15.0?5.7a 1 2 . 3 2 4 . 6 ~ 98% RH 4 . 3 k 0 . 2 ~ 4.0k0.26 4.520.3n 4 . 3 k 0 . 3 ~ 4 . 0 k 0 . 3 ~ 98%RH + water 7 6 . 3 k 7 . 0 ~ 4 8 . 0 2 3 . 3 ~ 2 7 . 3 2 1 . 5 ~ 22 .523 .0~ 10 .522 .2~

'The rhizomes were incubated in the dark at 20 + 1°C. In the 98% RH + water treatment water was supplied to the basal end of the rhizome as described in the text.

tAll data are mean values (f SE) based on four rhizomes per treatment. Means in the same column followed by the same letter are not significantly different at the 5% level, as determined by the Student-Neuman-Keuls multiple range test.

$Nodes are numbered from the apex to the base of the rhizome.

I T BUD L E N G T H

60 L I G H T + WATER

r. 50 DARK + WATER E u

r 40

0 Z

3 0

n 3 m

20

10

1 2 3 4 5 NODE POSITION

CAPEX TO BASE OF RHIZOME)

FIG. 2. Effect of light and water supply on bud growth on isolated, five-node rhizome segments. Experimental treatments and the environmental conditions are fully described in the text. Data were recorded after 10 days and are mean values based on eight rhizomes per treatment. Dry weight data are for total bud and shoot weight per rhizome. Vertical bars indicate standard errors >1 mm. Bud lengths labelled with the same letter are not significantly different at the 5% level, as determined by Student-Neuman-Keuls multiple range test.

reduction in water potential of -6.8 bar) caused a significant reduction in the growth of the lateral buds while at 98% (i.e., -27.2 bar) bud and root growth were completely arrested. The response of the shoots to water supplied through the end of the rhizome suggested that, even at 100% humidity, both in the light and in the dark, the water status of the tissues was still the limiting factor. This conclusion is consistent with the increasing recognition of the importance of tissue water potential, and more specifically of the degree of cell turgor, in the regulation of cell and shoot extension (e.g., 3, 6).

In contrast to the behaviour of buds on isolated rhizomes the growth of buds on rhizomes which are decapitated but are left attached to the parent shoot is not inhibited by exposure to growth-room humidities as low as 40 to 50% (G. McIntyre, unpublished observations). The relative insensitivity of such buds to low humidity can probably be attributed mainly to their continued

access to a supply of water from the roots of the parent shoot. Their growth may also be favoured, however, by the fact that most of them normally develop as lateral rhizome branches and thus have a lower water (and N) requirement than buds on isolzted rhizomes which normally develop as leafy shoots (12). In addition, the supply of sugar from the attached shoot, which may be one of the morphogenetic factors promoting rhizome development (see 9, 14), may also reduce the osmotic potential of the buds, thereby increasing their ability to compete for a limiting water supply.

Since all of the rhizomes used in the present investiga- tion were taken from plants which had been grown at a low N level their regenerative growth was characterised by the basipetal gradient of bud activity which is typical of N deficient rhizomes and is causally related to a gradient in the N content of the rhizome nodes (12, 15). The existence of this gradient had the advantage that it

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revealed the interaction between the effects of N and humidity. Thus, only at the higher end of the N gradient, i.e., at nodes 1 and 2, did the differences in humidity have any significant effect; at the basal nodes (4 and 5) growth was arrested at all humidities, due presumably to the limiting N supply. Conversely, the response to N, i.e., the effect of node position, was apparent only at humidities of 90 to 100%; at the lower humidities bud growth was equally inhibited at all of the rhizome nodes. In view of the obvious importance of both of these factors in the regulation of bud activity further experi- ments will be conducted to examine their interaction more critically by comparing the response to humidity of rhizomes from plants which have been grown at a range of N levels.

The present observations on the inhibition of bud growth by light are similar to those reported by Leakey et al. (8). They are also consistent with the hypothesis that the light-induced inhibition is due primarily to a reduction in the water potential of the tissues resulting from an incease in transpiration. This hypothesis is supported not only by the observed sensitivity of bud growth to reduced humidity but also by the ability of water, supplied through the cut end of the rhizome, to release the buds on illuminated rhizomes from inhibi- tion. Also consistent with this hypothesis was the greater uptake of water by the rhizomes exposed to light as compared with those in the dark. Since bud growth was similar in both treatments the greater absorption of water in the light was presumably due mainly to the increased rate of transpiration. An inhibition which is mediated indirectly by transpiration might also explain the re- ported effect of light quality on the inhibition of the rhizome buds (8) for it might be expected that the rate of transpiration will vary with the spectral composition of the light. Possible correlations between the nature of the light source (quantity and quality), the transpiration rate, and the degree of bud inhibition could be readily evaluated by the methods used in the present investiga- tions.

The effect of light on transpiration could also be at least partly responsible for certain observations on the regenerative behaviour of rhizome fragments in the field. It was noted, for example (see reference 9) that when several shoots emerge into the light before the dominance of a single shoot has been fully established all of the emergent shoots continue to grow but inhibit the growth of other buds which are still below the surface of the soil. It was suggested (8) that since the expanded shoots are contributing assimilates to the rhizome and the buds also have access to soil N from the roots at the nodes the inhibition of the buds is difficult to explain in terms of competition for a limited nutrient supply. The results of the present investigation suggest, however, that the increased transpiration of the emer-

gent shoots, resulting from their exposure to light, may reduce the water potential of the rhizome below some critical level required for the resumption of bud activity. This suggestion is well supported by Boyer's observa- tion (1) that, in sunflowers, a light-induced reduction in leaf water potential of only about - 1.5 bar (i.e., the difference between day and night) strongly inhibited leaf expansion. In quackgrass such an effect of water potential could well have an overriding influence on the response to other nutritional factors. The apparent resistance of all of the emergent shoots to correlative inhibition may be similarly explained, for their rapid transpiration, combined perhaps with their capacity of osmoregulation by photosynthesis, may enable them to compete effectively for an adequate water supply.

From a practical standpoint the results of this study agree with the general recommendation that the cultiva- tion of quackgrass during periods of drought is one of the most effective methods of control (4). While exposure of the rhizomes to light and low humidity will greatly reduce their capacity of regenerative growth it is clearly important that the surface of the soil should also be thoroughly dry. The present observations suggest that if the rhizome is in contact with a moist substrate this may induce the rapid outgrowth of roots from preformed meristems and that the resulting uptake of water may release the lateral buds from inhibition. On the other hand, it is likely that the effectiveness of foliar-applied herbicides, as an alternative to cultivation, will be significantly enhanced by conditions which stimulate the activity of the rhizome buds (see reference 16). Plant water status must therefore be regarded as an important factor in relation to both cultural and chemical treat- ments. Thus, while the immediate objective of the investigations now in progress is to evaluate the role of water competition in the mechanism of bud inhibition it is hoped that the results obtained may also point the way to more effective methods of control.

Acknowledgements I wish to thank Mrs. Marilyp Loydl for excellent

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

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