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Canadian Journal Journal canadien of Botany de botanique Published by Publie' par le THE NATIONAL RESEARCH COUNCIL OF CANADA CONSEIL NATIONAL DE RECHERCHES DU CANADA
Volume 51 Number 2 February 1973 Volume 51 numCro 2 fevrier 1973
Environmental control of apical dominance in Phaseolus vulgaris
GORDON I. MCINTYRE Regina Research Station, Box 440, Regina, Saskatcl~e~van
Received August 24, 1972
MCINTYRE, G. I. 1973. Environmental control of apical dominance in Phnseolrcs vulgaris. Can. J . Bot. 51: 293-299.
When seedlings of Phaseolus vulgaris were grown under controlled conditions a t a light intensity of 3200 ft-c, 60% relative humidity, and at nitrogen levels of 5.25, 52.5, and 210 ppm, growth of the buds at the cotyledonary node, which served as a measure of apical dominance, showed a positive correlation with the nitrogen supply and with the soluble nitrogen content of the hypocotyl. Increasing the nitrogen supply to 420 ppm caused a proportionate increase in soluble nitrogen content but no additional bud growth response. That the growth response was limited by water supply was shown by growing plants at 420 ppm nitrogen and relative humidities of 30, 60, and goy0. Each reduction in water stress, as measured by leaf relative turgidity, caused a highly significant increase in growth of the cotyledonary buds. Under high nitrogen, low water stress conditions, bud growth was markedly inhibited by reduction of the light intensity from 3200 to 700 ft-c.
These results support the concept of nutrient competition as a major factor in the mechanism of apical dominance and also suggest that conflicting reports on the effect of externally applied growth-regulating substances on lateral bud inhibition may be due partly to environmentally induced diferences in nutri- tional status of the experimental plants.
MCINTYRE, G. I. 1973. Environmental control of apical dominance in Plraseolus vulgaris. Can. J . Bot. 51: 293-299.
Lorsque des plantules de Phaseolus vlrlgnris ont Cte cultivCes sous des conditions contrblkes, a une intensite lumineuse de 3200 pieds-c, une humiditt relative de 60% et des niveaux d'azote de 5.25,52.5 et 210 ppm, la croissance des bourgeons axillaires des cotylCdons, servant comrne mesure de la dominance apicale, ont montre une correlation positive avec I'approvisionnement en azote et avec le contenu en azote soluble de l'hypocotyle. Une augmentation de la source d'azote a la concentration de 420 ppm a produit une augmentation proportionnelle dans le contenu en azote soluble mais aucune croissance addi- tionnelle des bourgeons. En faisant croitre des plantes a 420 ppm d'azote et a des humidites relatives de 30,60 et 90%, il a CtC possible de montrer que la reaction de croissance Ctait limitee par la disponibilitk de l'eau. Chaque reduction dans la privation d'eau mesurke par la turgescence foliaire relative, a cause une augmentation importante dans la croissance des bourgeons 8 I'aisselle des cotyles. Sous les condi- tions ou I'azote etait Blevt et la privation en eau faible, la croissance des bourgeons etait fortement in- hibte par une reduction de I'intensitC lumineuse de 3200 pieds-c a 700 pieds-c.
Ces resultats supportent le concept que la competition pour la nourriture est un facteur majeur dans le mecanisme de la dominance apicale; ils suggkrent Cgalement que les rksultats contradictoires obtenus, a la suite d'applications externes de substances rigulatrices de la croissance, sur I'inhibition des bourgeons latkraux, pourraient i t re dus en partie des differences dans 1'Ctat de nutrition des plantes, induites par les conditions du milieu. [Traduit par le journal]
Introduction and carbohydrate (10). More conclusive evidence A study of apical dominance in isolated that internal competition for water may be a
rhizomes of Agropyron repens showed that the significant factor in the mechanism of apical lateral buds could be released from inhibition dominance was later obtained in experiments only under experimental conditions which with peas (1 1). These experiments also showed provided an adequate supply of water, nitrogen, that when water stress was sufficiently reduced,
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294 CAN. J. BOT. VOL. 51. 1973
either nitrogen or carbohydrate could act as a limiting factor and determine the degree of inhibition.
The main objective of the present investiga- tion was to test further the general validity of these conclusions by conducting similar experi- ments with beans (Phaseolus vulgaris), another species which has been widely used in the study i f apical dominance. It was also hoped that further information on the extent to which bud inhibition is affected by the plant's nutritional status might help to explain some of the appar- ently conflicting results reported from other studies on apical dominance in which the effect of various growth-regulating substances has been investigated.
Materials and Methods The beans used (Phaseol~u vulgaris L., var. Red
Kidney) were selected for uniformity in size, chipped a t each end to promote water absorption and uniform germination, and planted in plastic trays of vermiculite moistened with distilled water. They were kept in a germinator at 25 + 1°C and a light intensity of about 350 ft-c supplied by cool-white fluorescent lamps. After 5 days the seed coats were removed from the expanding cotyledons and the trays were transferred to a growth cabinet kept at 15 + 1°C and 60y0 relative humidity. A combination of fluorescent (cool-white) and incandescent lamps provided a day length of 16 h and a light intensity of about 3200 ft-c. After 7 days plants whose primary leaves had just completely unfolded were selected for uniformity and transplanted into 12.5-crn plastic pots, one plant per pot. The plants were then transferred to the various environmental conditions described below for each experiment.
The plants were watered with Hoagland's solution (5) modified to provide various nitrogen levels ranging from 21.0 to 420 ppm. Nitrogen concentrations lower than the amount present in the standard solution, i.e. 210 ppm, were obtained by appropriate eqililllolar sub- stitution of KzS04 and CaClz for K N 0 3 and Ca(N03)2, respectively, while the 420-ppm level was obtained by addition of NH4N03. Each pot received an excess of nutrient solution (about 200ml) at 2-day intervals, evaporation losses being replaced by distilled water on alternate days.
Bud Growth Measure~r~ents The effect of the various experimental conditions
on apical dominance was evaluated by recording the length and dry weight of the buds in the axils of the cotyledons. The buds were measured with a ruler to the nearest 0.5 mm, generally at 2-day intervals. The measure- ment was from the base of the bud to the tip; young leaves which had started to unfold from the bud apex were not included. Thus, it was mainly elongation of the stem that was measured, leaf expansion contributiilg to only a small but undetermined extent to the measure- ments recorded. In order that valid comparisons could
be made between different treatments, all final measure- ments were recorded when the plants had reached the same stage of main shoot development, i.e. when the leaf at node 6 had just separated from the apical bud (Fig. 3). The buds were then removed, dried a t 70°C for 24 h, and their dry weight determined.
Relative Turgidity Measrrremerrts Water stress was measured by determining the relative
turgidity of the leaves as described by Weatherly (17). The plants used were the same ones that provided the bud growth data and were sampled for turgidity measure- ments immediately before the final bud lengths were recorded. Duplicate samples, each comprising 20 leaf
DAYS TO FINAL M E A S U R E M E N T
FIG. 1. Effect of the nitrogen supply on the growth of buds at the cotyledonary node. Standard errors of the means, which are based on 10 plants per treatment, are shown for the final measurements.
DAYS TO FINAL M E A S U R E M E N T
FIG. 2. Growth of buds a t the cotyledonary node under high (A), medium (B), and low (C) water stress conditions. The environmental conditions are fully described in the text. Standard errors of the means, which are based on 10 plants per treatment, are shown in Table 2.
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FIG. 3. A seedling of Pl~aseol~rs vulgaris illustrating the region of the hypocotyl taken for analysis (S), the leaf used for relative turgidity measurenlents (arrowed), and the stage of development of the plant when the sanlples were taken and final bud measurements recorded. For clarity of illustration both the primary leaves (node 2) and one of the axillary shoots at the primary node (on far side) were removed. Note that both buds at the cotyledonary node (immediately above sampled region of hypocotyl) have escaped from apical dominance and developed as vigorous shoots 1-2 cm in length. All other axillary buds are also in active growth. Environnlental conditions: day length, 16 h ; light intensity, 3200 ft-c; temperature, 15 + 1°C; nitrogen level, 420 ppnl; relative humidity, 90 f 5%. X 0.9.
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McINTYRE: APICAL DOMINANCE IN PHASEOLUS VULGARIS 295
disks, were obtained by punching out 8 disks from a single leaf of five plants in each treatment. The disks, 8 mm in diameter, were punched from the basal third of the leaf at node 4 (Fig. 3), one from each side of the midrib of the middle leaflet and three from each of the lateral leaflets, care being taken to avoid the inclusion of major veins.
Ctlernical Analysis When final bud measurements had been recorded a
sample was taken from the hypocotyl of each plant for chemical analysis. The sampled portion (Fig. 3) was 20 nlm long and extended from 5 to 25 mm below the coty- ledonary node. The 10 samples from each treatment were combined, heated in a forced-draft oven at 100°C for 20 min, then dried at 70°C for 24 h. The dried material was ground up to 80 mesh in a Wiley mill and stored over CaS04 until analyzed.
For the nitrogen determinations four aliquots, ranging from 25 to 45 mg, were taken for analysis from each treatment. Two of these were analyzed for total nitrogen by a standard micro-Kjeldahl procedure (6). The other two were weighed into Soxhlet extraction thimbles, immersed in 15 mi of 80% ethyl alcohol in a 50-ml beaker, and extracted on a water bath for 2 h at 80°C. The thimbles were then transferred to a Soxhlet extractor with 80% alcohol in the flask and extraction was con- tinued for a further 12 h. Preliminary work showed that this procedure removed all but a negligible amount of the soluble nitrogen. The alcohol-insoluble nitrogen content was determined by micro-Kjeldahl analysis of the extracted residue, and the soluble nitrogen, by sub- tracting this value from that obtained for total nitrogen.
T o determine the soluble carbohydrate content the alcohol extracts from the nater bath and Soxhlet ex- tractions were combined and made up to 50 ml. Duplicate aliquots were then analyzed for total soluble carbohydrate by the anthrone method (2), as nlodified by Fairbairn (3), using dextrose as the standard.
Experiments and Results
were grown at nitrogen levels of 21.0, 52.5, 210, and 420 ppm. All other environmental conditions were kept constant. These conditions were as follows: day length, 16 h ; light intensity (at the level of the primary leaves), 3200 ft-c; tem- perature, 15 f 1°C; relative humidity, 60 f 5y0.
The results (Table 1, Fig. 1) showed that growth of the cotyledonary buds was very res- ponsive to the nitrogen supply. At the lowest nitrogen level the buds on all of the plants had their growth completely arrested when they were only 2-3.5 inm in length (Table 1 shows only the total length of both buds). At the 52.5 ppm N level growth was considerably increased but was again of limited duration and by the end of the experiment the buds on all but three of the plants had ceased to grow. Increasing the nitro- gen supply to 210ppm resulted in a further increase in bud growth and when the final measurements were recorded buds on eight of the plants had their first leaf partly expanded and appeared to have developed the capacity for continued growth. It is noteworthy, however, that under the experimental conditions, maxi- mum bud response was attained at this nitro- gen level. Increasing the nitrogen supply from 210 to 420 ppm produced no further increase in either the length or dry weight of the cotyledon- ary buds.
The total dry weight of the samples from the hypocotyl was greatest at the lowest nitrogen level. I t decreased by about 20% in the 52.5 ppm N treatment and to a relatively small extent with increasing nitrogen supply. These
Effect of the Nitrogerz Supply changes presumably reflect the hecreasing To determine the effect of the nitrogen supply amount of carbohydrate accumulated in the
on the growth of the cotyledonary buds, plants hypocotyl as storage or structural material.
TABLE 1 Effect of the nitrogen supply on bud growth at the cotyledonary node and correlated effects
on the nitrogen content of the hypocotyl
Hypocotyl samples;
Total Cotyledonary bud growth" dry wt./ Insoluble N Soluble N
Nitrogen 10 samples, supply, PPlll Length, mm Dry wt., mg nlg Ojg dry wt. mg/sample % dry wt. mg/sample
*Mean values (i- S.E.), based on 10 plants per treatment, are for the total length of the two buds at the cotyledonary node. Means preceded by the same letter are not significantly direrent at the 57 , level.
?Nitrogen data are means of duplicate determinations. NOTE: All data were recorded and samples taken when the plants had reached the stage ol'development illustrated in Fig. 3.
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296 CAN. J. BOT. VOL. 51, 1973
The insoluble nitrogen content of the samples increased with the nitrogen level, reaching its maxiinal value in the 210 ppm N treatment. The soluble nitrogen content, expressed on either a dry weight or per sample basis, showed a close positive correlation with the external nitrogen supply over the entire range of nitrogen levels. Its increment when the nitrogen level was raised from 210 to 420 ppm was almost exactly pro- portional to the increased nitrogen supply.
Effect of Water Stress An interesting feature of the results described
above was that, under the experimental con- ditions, bud growth was correlated with the nitrogen supply only up to the 210ppm N level. That there was no bud growth response when the nitrogen supply was raised to 420 ppm, in spite of the associated increase in the soluble nitrogen content of the hypocotyl, pointed to the probable intervention of some limiting factor. Since the previous experiments with peas (11) had shown that the response to nitrogen may be limited by the water supply an experiment was designed to determine whether water stress might be the factor involved.
All of the plants were grown at the highest nitrogen level used in the previous experiment, i.e. 420 ppm. The other environmental conditions were varied to produce three levels of water stress. Since the facilities available did not permit a sufficient range of relative humidity values to be obtained at constant temperature the lowest humidity (30%) was achieved by in- creasing the temperature to 25OC. For the other two humidity levels the temperature was kept at 15°C and only the humidity was varied. As in the previous experiment the day length was 16 h and the light intensity was 3200 ft-c.
The results (Table 2, Fig. 2) showed that the outgrowth of the cotyledonary buds was cor- related with the degree of water stress as measur- ed by the relative turgidity of the leaves. Under the conditions of greatest stress bud growth was strongly inhibited. This inhibition was marked.1~ reduced at the 6OY0 humidity level; the bud growth data from this treatment showed good agreement with those obtained under the same conditions in the previous experiment (Table 1, treatment D). A further reduction in water stress, which raised the relative turgidity of the leaves from 84.4 to 88.0%, produced a highly
TABLE 2
Relation of water stress to growth of the cotyledonary buds at high levels of light intensity and nitrogen supply
Environmental conditions Leaf Growth of cotyledonary buds*
Rel. relative Temp., OC humidity, % turgidity, % Length, mm Dry wt., mg
*Mean values (f S.E.), based on 10 plants per treatment, are for the total length of the two buds at the cotyle- donary node. All means are significantly dilfercnt at the 1% level.
TABLE 3
Effect of light intensity on growth of the cotyledonary buds under conditions of low water stress and high nitrogen supply
Hypocotyl samples
Total 80% alcohol-soluble Cotyledonary bud growth dry wt. / carbohydrate?
10 samples, Light intensity* Length, mm Dry wt., mg mg % dry wt. mg/sample
High 24.311.43 15.9k1.55 526 2.97 15.31 Low 12.9k0.53 3.1k0.36 353 2.91 10.27
*High light, 3200 ft-c; low light, 700 ft-c. Mean values (?S.E.), based on 10 plants per treatment, are for total length of the two buds at the cotyledonary node. Bud lengths and dry weights are significantly different (P < 0.01) at each light intensity.
tMeans of duplicate determinations, expressed as dextrose.
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MCINTYRE: APICAL DOMINANCE IN PHASEOLUS VULGARIS 297
significant increase in both the length and dry weight of the buds. When the final measurements were recorded, all of the plants in this treatment had one leaf partly expanded and appeared to have attained the capacity for sustained and vigorous growth (Fig. 3).
Nitrogen analysis of hypocotyl samples from plants in treatment C gave values, expressed as percentage dry weight and per sample, respec- tively, of 1.35% and 7.10 mg for insoluble nitrogen, and 2.58y0 and 13.6 mg for soluble nitrogen. A comparison of these data with those froin plants grown at the same nitrogen level but under a higher degree of water stress (Table 1, treatment D) showed that the reduction of water stress was associated with an increase in the amount of insoluble nitrogen and a decrease in soluble nitrogen content. These changes probably reflect the stimulation of growth and protein synthesis produced by the increased water supply.
Effect of Light Intensity To determine the effect of light intensity on
growth of the lateral buds a group of plants was placed inside a wooden frame covered with a double layer of unbleached muslin which reduced the light intensity at the level of the primary leaves to 700 ft-c. The relative humidity under the shade was about 80yo. The group of plants in the minimal water stress treatment in the previous experiment (Table 2, treatment C) was grown concurrently in the adjacent high humidity growth room and served as the high light controls. Apart from the intended dif- ference in light intensity and the small difference in humidity, environmental conditions were the same for both groups.
Growth of the cotyledonary buds was con- siderably reduced by the reduction in light intensity (Table 3). This effect was associated with a 30% reduction in the total alcohol- soluble carbohydrate content of the samples. Since the dry weight of the samples was pro- portionately reduced, no change in carbohydrate content was apparent on a dry weight basis. Relative turgidity measurements gave a mean value of 88.6% for the shaded plants. That this value was slightly greater than that recorded for the high light plants which were grown at a somewhat higher relative humidity can probably be attributed to a reduction in transpiration rate at the lower light intensity.
Discussion The results of this investigation agree well
with those previously reported from similar experiments with peas (11) and provide further evidence of the importance of nutritional factors in the correlative inhibition of bud activity.
The apparent influence of the water supply is of particular interest for although much is known about the effects of water stress on plant growth, the possible significance of this factor in the phenomenon of apical dominance has re- ceived relatively little attention. The few relevant investieations found in the literature were cited " and briefly summarized in the previous paper (11). There is evidence, however, that the im- portance of the water supply in controlling lateral bud activity may vary greatly from one species to another. The results of the previous (11) and present investigations suggest that for peas and beans it is a factor of considerable significance. A marked effect of humidity on lateral bud growth has also been noted in Chrysanthemum (8) while preliminary (McIntyre, unpublished) experiments with sunflowers (Heli- anthus annuus var. Peredovik), a species which is characterized by a high degree of apical domi- nance, showed that the buds at the basal nodes could be released from inhibition when the plants were grown at a high N level if water stress was sufficiently reduced. On the other hand, experiments with flax (McIntyre, un- published) and leafy spurge (Euphorbia esula L.) (12) showed that in these species the lateral buds could be released from apical dominance even under dry greenhouse conditions provided that the plants were well illuminated and nitrogen was abundantly supplied. Such com- parisons suggest, as a working hypothesis, that the importance of water stress in the mechanism of apical dominance may tend to be greater for large-leaved species than for those of a more xerophytic character whose relatively small leaf area reduces the amount of water lost by transpiration. This generalization, however, needs to be tested more critically with a wider range of species.
The influence of nutrition on apical domi- nance in beans (P. vulgaris) was also investigated by McCallum (9), who concluded that the in- hibition of bud growth at the cotyledonary node could not be attributed to nutrient competition. He based this conclusion mainly on the observa-
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tion that even when the plants were subjected to conditions of extreme nutrient deficiency the buds still grew out when isolated from the parent plant. It seems probable, however, that the growth of the buds occurred at the expense of a limited supply of nutrients present in the stem tissue to which the buds were left attached. In the intact plant these nutrients may have been moving under the influence of other competing illeristems and for this reason would be unavailable to the inhibited buds. Moreland (13), also working with P. vulgaris, showed that the inhibiting effect of growing leaves on the cotyledonary buds was much greater than that produced by the apical bud itself and that the leaves ceased to inhibit as their growth rate declined. He also showed that bud growth on decapitated plants was very sensitive to experi- mentally induced differences in the nitrogen and carbohydrate supply. He considered that his results, although somewhat inconclusive, tended to favor a nutrient competition hypothesis.
Experiments by Phillips (14) with P. vulgaris var. Canadian Wonder were designed to test the hypothesis that hormone-directed transport of nutrients is a major factor in the mechanism of apical dominance. This hypothesis, originally proposed by Went (18, 19) has received support from several investigations which Phillips him- self has reviewed (15). Phillips showed that the application of indole acetic acid (IAA) in lanolin to the decapitated epicotyl, while inhibiting growth of the axillary buds, also resulted in the accuinulation of nitrogen in the decapitated in- ternode and, moreover, that this nutrient accu- mulation was maintained throughout the whole period of inhibition. However, he also found that the total N, P, and K content of the decapitated epicotyl and the axillary buds, combined in one sample, was less for the auxin-treated plants than for the decapitated controls and he considered this result to be inconsistent with a nutrient com- petition hypothesis. As he pointed out, however, this apparent discrepancy might well have been due to an increased uptake of nutrients by the plants whose buds had been released from inhib- ition. Analysis of the apical 5 mm of buds which had escaped from inhibition showed that the total N, P, and K content, when expressed on a dry weight basis, was less than that of the inhib- ited buds. Phillips, therefore, suggested that the inhibition of bud growth could not be attributed
to nutrient deficiency. However, since his data also showed that the growing buds reached a length of almost 30 mm during the 5-day treat- ment period, it seems probable that their uptake of nutrients was considerably greater than that of the inhibited buds and that this would have been apparent had the data been expressed as changes in total nutrient content per bud.
The results of the present investigation, by demonstrating the extent to which apical dom- inance is dependent on nutritional factors, may also help to explain the lack of agreement be- tween results of experiments in which the effect of exogenously applied growth-regulating sub- stances has been investigated. Recent papers dealing with the hormonal control of apical dom- inance have drawn attention to these apparently conflicting results (1, 7). A survey of the cultural methods used shows that in some investigations the plants were supplied with water only, while in others they were grown in soil or were watered with full-strength Hoagland's solution. It is evident from the present results that, at least so far as beans are concerned, such differences in nutrient supply could significantly affect the degree of bud inhibition and by doing so could account for reported differences in the effects produced by similar hormone treatments. This view is well supported by the work of Thimann et al. (16) on the effects of IAA and kinetin on apical dominance in Coleus. In these investiga- tions lateral bud inhibition, whether induced by the dominant apex or by auxin applied to decap- itated shoots, was significantly reduced by in- creasing either the light intensity or the mineral nutrient supply. It was post~~lated that both of these environmental effects, which were previ- ously reported by Gregory and Veale (4), might be due to the increased synthesis of cytokinin. However, it would seem that the enhanced in- hibition of the lateral buds at the lower light intensity, which was reduced to only 270 lux, could be attributed more simply to increased competition for carbohydrate.
It is sometimes argued that significant effects of nutrition on apical dominance can only be achieved by subjecting the plant to a highly un- natural environment; and certainly, this ob- servation is applicable to the present investiga- tion. It must be remembered, however, that the mechanism of apical dominance, which undoubt- edly has a high survival value, evolved under
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MCINTYRE: APICAL DOMINANCE IN PHASEOLUS VULGARIS 299
natural conditions and thus in an environment where some degree of water stress is normally present and where nitrogen is frequently a limit- ing factor. It is therefore to be expected that a somewhat unnatural nutritional environment must be provided before bud inhibition can be significantly reduced.
Acknowledgment I thank Miss Miriam Rosin and Mr. William
Fleming for technical assistance and Dr. J. R. Hay for his critical reading of the manuscript.
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