The correlative inhibition of bud and shoot growth in flax (Linum usitatissimum). Some factors affecting the pattern and degree of inhibition

Regina Research Station, Canada Department of Agriculture, Box 440, Regina, Saskatchewan

Received August 13,1974

MCINTYRE, G. I. 1975. The correlative inhibition of bud and shoot growth in flax (Linum usitatissimum). Some factors affecting the pattern and degree of inhibition. Can. J. Bot. 53: 390402.

When flax (Linum usitatissimum L.) seedlings were grown in sand culture under controlled conditions, growth of the lateral buds and their release from apical dominance showed a positive correlation with the N supply. The response of the buds was related to their position on the shoot and a well-defined gradient of bud growth potential was apparent at all N levels. Removal of the basal buds, which have the highest growth potential, induced the outgrowth of buds at higher nodes and increased the growth of the main shoot apex. Similar effects were produced by increasing the N supply to the intact plant.

Providing N only as nitrate inhibited apical growth and caused severe leaf necrosis. These effects were due to zinc deficiency and could be prevented or significantly reduced not only by increasing the zinc supply but also by reducing the N level, providing some NH,-N, reducing the light intensity, or increasing the depth of planting. They could also be prevented by removalof the lateral buds at the basal nodes, thus indicating that the shoot apex and lateral buds may compete for the limited zinc supply. The obviously complex relationship between zinc and nitrogen nutrition in this species requires further investigation.

MCINTYRE, G. I. 1975. The correlative inhibition of bud and shoot growth in flax (Linltm usitatissimum). Some factors affecting the pattern and degree of inhibition. Can. J. Bot. 53: 390402.

Si on cultive des plantules de lin sur sable et sous des conditions contrblees, la croissance des bourgeons latkraux et leur echappement a la dominance apicale montrent une corrClation positive avec I'apport d'azote. La reaction des bourgeons est relike B leur position sur la tige et ungradient du potentiel de croissance des bourgeons est evident B toutes les concentrations d'azote. L'ablation des bourgeons situes la base, ceux dont la croissance potentielle est la plus Clevee, induit I'excroissance des bourgeons situCs plus haut et augmente Cgalement la croissance de I'apex de la tige principale. Les mCmes effets sont produits en augrnentant I'azote fourni aux plants intacts.

L'apport d'azote seulement sous forme de nitrate inhibe la croissance apicale et cause de sevtres necroses foliaires. Ces effets sont dCs a une dificience en zinc et sont elimines ou reduits non seulement en augmentant le zinc, mais aussi en reduisant l'azote, en fournissant un peu de NH,-N, en reduisant l'intensite lumineuse, ou en augmentant la profondeur de plantation. Ils peuvent Cgalement Ctre empEches par ablation des bourgeons lateraux des noeuds de la base, ce qui montre que I'apex de la tige et les bourgeons lateraux peuvent entrer en competition pour une quantite limitee de zinc. La relation visiblement complexe qui existe entre la nutrition en zinc et en azote chez cette esptce exige des recherches plus poussees. [Traduit par le journal]

Introduction [their] results in the light of current theory"

The importance of nutritional factors in apical dominance was well illustrated by the work of Gregory and Veale (7), who showed, in experiments with flax, that the degree of inhibition of the lateral buds was largely determined by the nitrogen and carbohydrate status of the plants. Attempting to "interpret

'I am indebted to Dr. E. Spratt of Agriculture Canada, Research Station, Brandon, Manitoba, for suggesting the possibility that zinc deficiency might be involved.

these workers also postulated that auxin from the dominant apex may restrict the supply of nutrients to the buds by impeding the formation of their vascular connections.

Investigations by the writer, using the same species, agreed well with Gregory and Veale's conclusions regarding the importance of nutri- tion but failed to provide any evidence that the vascular connections were directly involved. It was found, for example (lo), that while the degree of correlative inhibition between the

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McINTYRE: BUD AND SHOl DT GROWTH INHIBITION 39 1

cotyledonary shoots of decapitated seedlings could be readily controlled by varying either the N or P supply, the shoot whose growth was arrested had well-developed vascular con- nections with the parent stele (unpublished observations). In a more recent investigation (1 I), it was shown that when lateral buds on the intact plant were released from inhibition by increasing the nitrogen supply, their growth response was initiated before the establishment of xylem connections with the parent shoot. Similar observations on the lack of any correla- tion between vascular connections and the release of buds from apical dominance have been reported by other investigators (1,6,15,18).

Thus, while there is considerable evidence that, at least in the flax seedling, internal com- petition for a limiting nutrient supply plays a major role in the correlative inhibition of bud activity, the details of the mechanism by which nutritional factors determine the pattern and degree of inhibition have yet to be fully eluci- dated. It was thus with the object of shedding some light on this aspect of the problem that the present investigation was initiated. In studying the influence of the nitrogen supply on the pattern of bud activity, however, it became apparent that other factors were involved and after further investigations it was found that under certain conditions, growth of the main shoot apex was inhibited by zinc deficiency. The influence of this additional factor is also described and its significance is discussed in relation to its apparent involvement in the cor- relative inhibition of apical growth.

Materials and Method Plant Culture Technique

The variety of flax used throughout the investigation was Redwood 65. The seeds were germinated on moist filter paper in petri dishes a t an alternating temperature of 8 h a t 30°C in the light (300ft-c from cool-white fluorescent lamps) and 16 h at 10°C in the dark. After 48 h, germinating seeds with their primary root about 1 cm long were planted in 12- to 30-mesh silica sand in square 10-cm plastic pots, except in experiment 3 in which round 12.5-cm-diam pots were used. Four to eight seedlings were planted initially in each pot. They were later selected for uniformity in the height of the epicotyl and the size of the buds a t the cotyledonary node and were thinned to one per pot in experiments 1 to 3, two per pot in experiments 4 and 5. and four per pot in experiment 6.

The plants were watered with Hoagland's solution (9) in which the iron was supplied in chelated form (Seques- trene). The nitrogen concentration was varied experi-

mentally by equimolar substitution of and CaCIZ for KNO, and Ca(N03)*, respectively. The highest nitrogen level used, i.e. 420ppm, was obtained by addition of NH4N0, to the standard Hoagland's solution. All solutions were adjusted to pH 6 with KOH. Each pot received a considerable excess of nutrient solution (about 200 ml) at 2-day intervals. Water lost by evaporation was replaced with distilled water on alternate days. All of the experiments were conducted in growth chambers under controlled con- ditions. Details of the environmental conditions are given below in the description of each experiment.

Bud and Shoot Measurements At the end of each experimental period, measurements

were recorded of the length of the main shoot and of the axillary buds and shoots at each node. The nodes were numbered acropetally from the base of the epicotyl. The length of each of the two shoots at the cotyledonary node (node I), which tend to develop equally, was separately recorded, but only their combined lengths are given in the presentation of the results. The lengths of the two shoots at node 2 were also combined since the two leaves a t this position are borne, like the cotyle- dons, on opposite sides of the stem, apparently a t the same level and usually less than 1 mm above the cotyle- donary node. Since the second and third internodes normally undergo considerably greater but rather variable elongation, the leaves a t nodes 3 and 4 are usually some distance apart and thus the length of the axillary buds and shoots a t these nodes is separately recorded. In the few instances where no difference in the level of insertion of leaves 3 and 4 could be detected, their node numbers were randomly assigned. At higher nodes the leaves are borne in a spiral, but their phyllotaxis is not clearly defined and is reported to vary at different levels on the shoot (4).

Axillary shoots 5 1 cm were measured to the nearest millimetre with a ruler. In measuring buds less than 1 mm in length the associated leaf was stripped off and the measurement was made under a dissecting microscope fitted with an ocular micrometer at a 16 x magnification. This close scrutiny permitted nodes bearing very small buds, i.e. <O.l mm, to be readily distinguished from those a t which no bud was present. In some experiments the main shoot and all lateral branches 5 1 cm in length, after having been measured, were separated and dried a t 75°C for 24 h and their dry weights were determined.

All the data were analyzed statistically by analysis of variance, and Duncan's Multiple Range test was used to determine the significance of the differences between the mean values for each treatment. With the exception of experiment I all experiments were repeated a t least once. The results in every case were in good agreement with those which are described below.

Experiments and Results The Pattern of Lateral Bud Growth in Relation to

the Nitrogen Supply Experiment 1 The object of the first experiment was to

investigate the effect of the nitrogen supply on

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392 CAN. J . BOT. VOL. 53, 1975

TABLE 1

Effect of the nitrogen supply on the growth of axillary buds in relation to their position on the shoot

No. of next node Apical nodest Bud and shoot length at basal nodes,* cm above node 4

Nitrogen - with axillary bud No. No. buds level, ppm 1 2 3* 4* externally visible with bud > I mm

NOTE: All data are mean values based o n 10 plants per treatment. Means in the same column followed by different letters are significantly different a t the 5z level.

*The basal nodes are numbered acropelally from the cotyledonary node (node 1). A t nodes 3 and 4 the percentage with the bud externally visible is shown in parentheses.

t T h e apical nodes in each treatment extend from the node shown in the preceding column to node 80, which was immediately below the a ~ ~ c a l bud.

the growth of the lateral buds and, in particular, to relate the growth of the buds to their position on the shoot. The environmental conditions were as follows: temperature, 15 $. 1°C; day length, 16 h ; light intensity, 3000 ft-c; relative humidity, 60 $. 2%. Initially, plants were grown at four nitrogen levels, 2.1, 21.0, 210, and 420 ppm, with 10 plants per treatment. After about 3 weeks the plants receiving the 210 pprn nitrogen solution started to show signs of injury. The symptoms were characterized by the arrested growth of the main apex and by the marked chlorosis of the young apical leaves, the basal regions of which developed small necrotic areas on either side of the midrib. The severity of the symptoms and the fact that they could not be detected at the other three nitrogen levels suggested that an error might have been made in the preparation of the solu- tion. This treatment was, therefore, discon- tinued, leaving only three nitrogen levels in the experiment. Evidence as to the true nature of the injury to the plants was provided by subsequent investigations (experiment 6).

All of the plants were harvested at the same stage of development, i.e. when they had an average of 83 or 84 leaves on the main shoot (including the youngest leaf, which had just started to separate from the apical bud). The number of days to harvest for the 420-, 21.0-, and 2.1-ppm N levels were 38, 47, and 60 days, respectively.

The growth of the lateral buds showed a positive correlation with the nitrogen supply and varied significantly with their position on the shoot (Table 1). The buds at the cotyledonary

node, which have the greatest capacity for growth (7), were strongly inhibited a t the lowest nitrogen level, but at 2 1.0 pprn N they produced vigorous lateral shoots. Increasing the nitrogen concentration to 420 ppm, however, produced no additional response, the length of the cotyle- donary shoots in this treatment being the same as at the intermediate nitrogen level. This was not the case, however, a t the other basal nodes (2 to 4), where growth of the axillary buds de- creased with their distance from the base of the shoot and responded significantly to each in- crease in the nitrogen supply. The number of plants with a bud externally visible at nodes 3 and 4 was also related to the nitrogen level. Thus, at 21.0 pprn N only 2 of the 10 plants had a bud at node 3, while none had a bud at node 4; whereas all but one of the plants had produced a bud at nodes 3 and 4 at the 420 pprn N level. Since the plants were not subjected to an anatomical study, it is not known whether bud meristems were pres- ent at those nodes where no bud was visible and thus whether the nitrogen supply was controlling the initiation of buds, i.e. the inception of the meristem, as well as their subsequent growth.

Above node 4 there is a region consisting of a variable number of nodes at which no buds are present. The number of nodes included in this region was significantly greater in the 2.1-ppm N treatment than at either of the other nitrogen levels. Effects of the nitrogen supply at the apical nodes were less pronounced but were readily apparent. Raising the nitrogen level from 2.1 to 21.0 pprn did not affect the number of buds externally visible but consider- ably increased their subsequent growth, while

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TABLE 2

Effect of the nitrogen supply and the removal of the basal axillary buds on the growth of the main shoot and lateral branches. Length (centimetres) and dry weight (grams) of main shoot and lateral branches F 1 cm

Main shoot Basal laterals* Apical laterals't

Treatment Length Dry wt. Length Dry wt. Length Dry wt.

(1) 5.25 ppm N 38.90 0 .420 13.50 0 .060 - (2) 21.0 ppm N,

shoot intact 15.8 b 0.44 b 64.2 b 1 .14b - - (3) 21.0 ppm N,

basal buds removed 37.60 1.51 b - - 18.1 cr 0.09

(4) 420 ppm N 39.1 cr 2 . 1 3 ~ 98.7 c $ 18 .60

NOTE: Al l data are mean values based on 10 plants per treatment. Means in the same column followed by different letters are significantly different at the 5% level.

'The basal laterals are those borne at node 1 (cotyledonary node) to node 4 inclusive. ?The mean node numbers ( + SE) of the basal and apical limits of the zone of apical laterals are 20 .1 k 2 . 0 3 lo 3 7 . 6 k 1.82 for treatment 3

and 16 9 + 1.27 to 26 9 + 0 . 7 2 for treatment 4. The apical limit values are significantly different at the 5% level. $The individual weights o f these samples were not determined. Their combined dry weight was 2.87 g.

at 420 ppm N both the number and growth of the buds were significantly increased.

The Inhibiting Influence of the Basal Laterals Experiment 2 It was noted in the first experiment that the

cotyledonary shoots showed equal growth in the 21.0- and 420-ppm N treatments, but that the growth of buds at the apical nodes was significantly greater at the higher nitrogen level. These observations, together with the evidence of intershoot competition reported by Gregory and Veale (7), suggested that the basal shoots may tend to inhibit the growth of buds at higher nodes and that the degree of inhibition may be dependent on the nitrogen supply.

In an experiment designed to test this hypo- thesis, plants were grown at nitrogen levels of 5.25,21.0, and 420 ppm. Shortly after emergence the plants in the 21.0-ppm N treatment were divided into two equal groups. Those in one group were left intact while those in the other had the basal buds (at nodes 1 to 4) removed. The buds at the cotyledonary node (node 1) were excised 12 days after the seedlings were planted. At this time their length was 2.1 & 0.16 mm and that of the parent shoot ranged from 30 to 40mm. The buds at node 2 were removed 5 days later when they were 2.91 + 0.31 mm long and those at nodes 3 and 4 after a further 8 days at a length of 7.5 + 1.8 and 4.7 + 2.0, respectively. All of the plants were grown under the following conditions: tempera- ture, 20 + 1°C; day length, 16 h ; light intensity, 3600 ft-c; relative humidity, 55 + 2%.

After about 4 weeks growth it was noticed that some of the plants receiving the 420-ppm N solution were wilting. Since the plants failed to regain full turgor after watering, they were removed from their pots and examined. In contrast to the extremely vigorous growth of the shoot, the root system was very poorly developed and some of the roots were either dead or in a moribund condition. These ob- servations suggested that the growth of the numerous lateral branches on the shoot may have restricted root development, probably by competition for carbohydrate, thereby limiting the supply of water to the shoot. This hypothesis was supported by the rather high value of 3.19 f 0.21 obtained for the mean shoot/root ratio of the 10 plants.

To remedy this situation the treatment was repeated, but to promote greater root growth the plants were grown at a reduced nitrogen level of 21.0 ppm for the first 10 days. The nitrogen supply was then increased to 420 ppm and kept at this level for the rest of the experi- ment. With this modified treatment, branching and shoot growth were not appreciably reduced, but no wilting occurred. The plants in all of the treatments were harvested and bud and shoot lengths were recorded 45 or 46 days after planting (Table 2).

The plants grown at the lowest nitrogen level showed well-marked nitrogen deficiency symp- toms. Presumably as a result of the high degree of apical dominance the few lateral branches produced were relatively short and were entirely restricted to the cotyledonary node. At the

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394 CAN. J. BOT. VOL. 53. 1975

TABLE 3

Comparative effects of nitrogen source and basal-bud removal on growth of main shoot and lateral branches. Length (centimeters) and dry weight (grams) of main shoot and lateral branches 5 1 cm

Ratio main shoot / Main shoot Basal laterals* Apical laterals-) basal laterals

Treatment Length Dry wt. Length Dry wt. Length Dry wt. Length Dry wt.

(1) N supplied as NO, only 9.55 a 0 . 2 7 ~ 9 7 . 5 ~ 2 . 0 0 ~ - 0 . 1 0 ~ 0 . 1 3 ~

(2) N supplied as NO3 only, basal buds removed 33.2 b 2.72 b - 56.7 0.30 -

(3) N supplied as NH4NO3 4 5 . 6 ~ 1 . 8 5 ~ 162.9b 2 .97b - 0 .30b 0 .65 b

NOTE: All data are mcan values based on 10 plants per treatment. Means followed by different letters are significantly different at the 5% level. 'The basal laterals are those borne at node 1 (cotyledonary node) to node 4 inclusive. tThe mean node numbers ( 2 SE) for the basal and apical limits o f the zone o f apical laterals are 19.3 + 2.94 and 6 2 . 1 2 3.34, respectively.

21.0-ppm N level basal branching by the intact plants was greatly increased. This response was associated with a marked reduction of growth of the main shoot, which was less than half as tall as at the lower N level. This apparent inhibition of apical growth was accompanied by some injury to the young leaves, the symptoms being similar to but less severe than these described for the 210-ppm N plants in experi- ment 1. That these effects were not detected in the plants receiving the 21 .O-ppm N solution in the first experiment can probably be attributed to differences in the environmental conditions (vide infra). Removal of the buds at the basal nodes (treatment 3) had two main effects. Firstly, it significantly increased the growth of the main shoot, eliminating all symptoms of injury, and secondly, it induced the develop- ment of lateral branches at higher nodes. That both of these responses to bud removal were nutritionally induced was suggested from a comparison with the intact plants of treatment 4, which showed a closely similar pattern of development when grown at a higher N level. It should be noted that the amount of shoot growth at the upper nodes at 420 ppm N was the same as that induced by bud removal in treatment 3 in spite of the associated and very vigorous growth of laterals at the basal nodes at the higher N level.

Factors Affecting the Inhibition of Apical Growth Experiment 3. Effect of Nitrogen Source When the 210-ppm N treatment used in the

first experiment was later repeated, the plants showed the same inhibition of apical growth

and leaf injury as before, indicating that these symptoms were not due to an error in the preparation of the solution. It was also noted, however, that in experiment 2, the plants grown at 420 ppm N showed no sign of injury. This fact suggested that injury may be prevented by the provision of NH,-N, some of which was provided as NH4N03 in the 420-ppm N solution.

In an experiment designed to test this hypo- thesis one group of plants was watered with standard Hoagland's solution, and thus re- ceived 210 ppm N all in the form of K and Ca nitrates. Half of these plants were left intact to serve as controls while the other half had the basal buds removed as described in the previous experiment. Another group of plants was supplied with a modified Hoagland's solution, which also had a total N concentration of 210 ppm but with all of the KNO, and Ca(N03), replaced by K,S04 and CaCl,, respectively, and with the appropriate amount of nitrogen added as NH4N03 only. The plants were grown under the same conditions as in experiment 2 and for the same length of time.

The results (Table 3 and Fig. 1) agreed well with those of the previous experiment. The control plants (Fig. lA), supplied with NO3-N only, produced vigorous lateral branches at the basal nodes while the main shoot showed a similar degree of growth inhibition and leaf injury to that produced by the same solution in experiment 2. Removal of the basal buds again resulted in a significant increase in the growth of the main shoot, the leaves of which showed no sign of injury, and also induced branching at the higher nodes (Fig. 1B). Where

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the plants were left intact but were supplied with nitrogen as NH4N0, only (Fig. lC), branching at the basal nodes was increased by more than 50% as compared with the corre- sponding NO3-N treatment; yet the main shoot showed no sign of injury and its length was about 30% greater than that of the plants whose basal buds had been removed.

Experiment 4. Effect of Light Intensity A comparison of experiments 1 and 2 showed

that whereas the 21 .O-ppm N solution caused no apparent injury to the young leaves of the main shoot in experiment 1, such injury, although not severe, was clearly recognizable in the plants receiving the same solution in experiment 2. It was noted also that in experiment 1 the light intensity and the temperature were both lower and the relative humidity was somewhat higher than in experiment 2. To determine whether these differences in the environmental conditions might account for the apparent discrepancy in plant response, two sets of plants were grown in separate growth cabinets at high and low light intensities of 4000 and 2000 ft-c, respec- tively. Facilities did not permit the same relative humidity to be maintained in both cabinets. Its value was 60 f 2% in the high light treatment and 70 ) 2% at the lower light intensity. The temperature in both growth cabinets was kept at 20 f 1°C. All of the plants were watered with standard Hoagland's solution and thus received 210 ppm N as nitrate only. Half of the plants at each light intensity were left intact to serve as controls and the other half had the basal buds removed in the same manner as in experiments 2 and 3. The plants were harvested and shoot lengths were recorded after 28 days growth.

It was apparent from the results (Table 4) that the inhibiting influence of the basal laterals on the growth of the main shoot was very sensitive to the environmental conditions. At the higher light intensity the main shoot apex of all the intact control plants showed severe injury to the young leaves and apical growth was strongly inhibited. A relatively slight degree of injury was also shown by the apices of most of the debudded plants in this treatment, indicating that, under those environmental conditions, the suppression of the basal laterals, although greatly reducing apical injury, failed to provide complete protection. At the lower light intensity none of the plants showed any

sign of leaf injury and although bud removal did cause a significant increase in apical growth, this effect, as indicated by the shoot length ratios shown in Table 4, was much reduced as compared with the high light treatment. This reduction of apical inhibition could not be attributed to an effect on the growth of the laterals at the basal nodes since reducing the illumination actually increased the length of the basal laterals by more than 50%.

Experiment 5. Effect of Depth of Planting Although the leaf necrosis and shoot growth

inhibition observed in the previous experiments invariablv occurred under certain environ- mental conditions, there was considerable variation both within treatments and between experiments with respect to the severity of these symptoms and the stage of development at which they could first be detected. This variation is reflected in the difference in mean length of the main shoot of plants grown under similar conditions in experiments 3 and 4, i.e. 9.5 cm in treatment 1 (Table 3) and 15.2 cm in treat- ment A (Table 4). A critical comparison of the conditions in these two experiments sug- gested that the only difference which might account for this apparent discrepancy was in the depth of planting. In experiment 3 the germinating seeds were planted at a depth of only 1.0 cm, but in experiment 4, to reduce the tendency for the seeds to be washed out of the sand during watering, the depth of planting was increased to 2.0 cm. To determine whether this difference could account for the observed difference in the degree of shoot growth in- hibition an experiment was designed to in- vestigate the effect of depth of planting. In one treatment the germinating seeds were planted at a depth of 1.0 cm and in the other at 2.5 cm. Both were watered with Hoagland's solution in which all nitrogen was present as K and Ca nitrates. A third set of seeds was planted at a depth of 1.0 cm, but the plants were watered with the modified solution used in treatment 3 of experiment 3, in which NH4N03 was the N source. As in the previous experiments the germinating seeds were planted when the primary root was 1-2 cm long. The environ- mental conditions were the same as those described for experiment 2, except that the relative humidity was about 50%.

The results (Table 5) showed that increasing the depth of planting caused a significant

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TABLE 4

Effect of light intensity on the inhibiting influence of the basal laterals on the growth of the parent shoot

Length of Shoot length Length of Treatment main shoot, cm ratio basal laterals, cm

(A) High light, control 15.20 5 1 . 2 a A/B = 0.58 a

(B) High light, basal buds removed 26 .4b

(C) Low light, control 32.0 c 87.7 b CID = 0.82 b

(D) Low light, basal buds removed 39.2 d

NOTE: All data are mean values based on 20 plants per trentment. Means in the samecolumn followed by different letters are significantly ditTerent at the 5Z level.

reduction in the degree of inhibition of apical growth and markedly delayed the onset of leaf injury, the severity of which was also considerably reduced. Supplying the nitrogen in the form of NH4N03 proved effective in preventing the adverse effects of shallow planting. As in the previous experiments, the plants receiving this solution showed no sign of growth inhibition or leaf necrosis.

Experiment 6. Effect oj'the Zinc S~rpply In studying the literature relating to the

growth and nutrition of flax it was noted that - the leaf necrosis and inhibition of apical growth shown by the plants in the present investigation closely resembled the effects described by Millikan (12) as characteristic symptoms of zinc deficiency.' A further experiment was therefore designed to determine the effect of varying the zinc supply. Four zinc levels were compared. The control plants received the same zinc concentration as in all the previous experi- ments, i.e. 0.05 ppm, which is the amount present in the standard Hoagland's solution, while in the other treatments concentrations of 0.1, 0.25, and 0.5 ppm were provided by addition of zinc sulfate to the standard solution.

Another set of plants was watered with the same solution as was used for treatment 3 in experiments 3 and 5, i.e. in which the Zn con- centration was 0.05 ppm, but with the N supplied as NH4N03 only. All other environ- mental conditions were the same as in experi- ment 5.

It was evident from the results (Fig. 2 and Table 6) that the observed inhibition of shoot growth and the necrotic lesions which developed on the leaves were caused by zinc deficiency. At the four zinc levels in which N was provided as nitrates only shoot growth was closely cor- related with the zinc supply. At the 0.05-ppm Zn concentration the symptoins of growth inhibition and leaf necrosis were similar to those observed under the same conditions in the previous experiment. Increasing the Zn supply to 0.1 ppm caused a significant increase in shoot length and dry weight, and although most of the plants showed a slight chlorosis of the apical leaves, the incidence of necrotic lesions was greatly reduced. At the 0.25-ppm concentration there was a further marked in- crease in shoot growth and no symptoms of leaf injury could be detected. Increasing the Zn level to 0.5 ppm produced a further growth

FIG. 1. Seedlings of flax (Litrum ttsitatissimutn L.) illustrating the effects of nutritional and debudding treatments on growth of the main shoot and lateral branches. All plants were watered with a modified Hoag- land's solution that had a total nitrogen concentration of 210 ppm. Only the relative amounts of NOB- and NH4-N were varied. The plants were photographed immediately before harvest, i.e. after a growing period of 45 days. About x 113. (A) Nitrogen supplied as K and Ca nitrates. Vigorous lateral branches have been pro- duced from buds at the basal nodes while growth of the main shoot (arrowed) is completely arrested. (B) Nitrogen supplied as in A, but the buds at the basal nodes (1-4) were removed. Note the promotion of lateral shoot growth at higher nodes and the continued growth of the main shoot. (C) Nitrogen supplied as N H L N O ~ only. With this nutrient supply the vigorous branches produced at the basal nodes failed to inhibit the growth of the main shoot apex.

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McINTYRE: BUD AND SHOOT GROWTH INHIBITION

TABLE 5

Effect of depth of planting and nitrogen source on the inhibition of apical growth

Treatments Main shoot length, cm

Depth of After After planting. cm Nitrogen source 14 days 24 days Increase

1 .O Nitrates only 4.8 a 5 . 6 a 0 .8 a (65) ( 1 00)

2.5 Nitrates only 7 . 3 b 9.9 b 2.6 b (40) (100)

1 .O Ammonium nitrate 7.1 b 16.7 c 9 .7 c (0) (0)

NOTE: All data are mean values based on 12 plants per treatment. Means in the same column followed by a dif- ferent letter are significantly different at the 5'7 level.

*The percentage of plants with necrotic lesi:ns on the leaves is shown in parentheses.

TABLE 6

Effect of the zinc supply on growth of the main shoot and lateral branches at the cotyledonary node

Treatment Main shoot Length ratio, main

Zinc level, Cotyledonary shoot shoot/cotyledonary PPm Nitrogen source Length, cm Dry wt., g length, cm shoot

0.05 Nitrates only 6 . 2 a 0.487 a 6 .45 a 0.95 0 .10 Nitrates only 10.8 b 0.788 b 9.60 b 1.1 0.25 Nitrates only 1 4 . 6 c d 0.941 b c 13.3 c d 1.1 0.50 Nitrates only 15 .5d I .09 c 14.9 d I .O 0.05 Ammonium nitrate 1 2 . 6 b c 1 . 0 6 ~ 1 1 . 7 b c 1.1

NOTE: All data are mean values based on 16 plants per treatment. Means in the same column followed by different letters are significantly different at the 59, level.

response, but since this effect was relatively slight and not statistically significant, it seemed probable that, under the experimental condi- tions, the 0.5-ppm concentration was close to the optimum level. To determine whether the absence of Zn deficiency symptoms in plants receiving the NH4N03 solution might be due to a change in pH, the leachate from pots- in this treatment was collected and its pH was compared with that of the leachate from pots receiving the standard Hoagland's solution. The solutions were displaced from the pots by application of 200 ml of fresh solution ac- cording to the normal watering procedure and had thus been in the pots and in contact with the plants for about 2 days. Both solutions had been adjusted initially to pH 6 . Measurements showed that the pH of the standard solution has risen to 7.0 while that of the NH4N03 solution had fallen to 5.1. Presumably, the increase in pH of the standard solution was

due to uptake of nitrate while the reduction in pH of the NH4N03 solution reflected the preferential absorption of the NH,+ ion.

Discussion The results of this investigation serve to

substantiate several of the observations pre- viously reported by Gregory and Veale (7). These are (a) that, under favorable conditions of illumination, the degree of apical dominance in the flax seedling, as measured by the amount of lateral bud growth on the intact plant, is largely dependent on the nitrogen supply, (6) that the lateral branches produced at the base of the shoot inhibit the growth of buds at higher nodes, and (c) that the differences in the capacity of the lateral buds to escape from inhibition are related to their position on the shoot.

Of these three features, the acropetal in- hibition exerted by the basal laterals is of particular interest because of its significance in

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400 CAN. J. BOT. VOL. 53, 1975

relation to the mechanism of apical dominance. As already mentioned, it was postulated by Gregory and Veale (7) that auxin, transported basipetally from the dominant shoot apex, inhibited the growth of the lateral buds by impeding the formation of their vascular connections with the parent stele. However, since auxin transport in shoots is known to be predominantly basipetal, it is unlikely that the acropetal inhibition exerted by the basal laterals can be similarly explained. On the other hand, it seems equally improbable that two distinct mechanisms are involved, partic- ularly since the inhibiting effect of the basal laterals and of the main shoot apex is similarly controlled by nutritional factors. Thus, the existence of the acropetal inhibition of bud growth would seem to detract from the credibility of Gregory and Veale's hypothesis and indeed from that of any other hypothesis in which the transport of auxin from the dominant apex to the inhibited bud is suggested as a major factor in the mechanism involved.

The correlation between growth of the lateral buds and their relative positions on the shoot, which was described by Gregory and Veale, was also apparent in the present in- vestigation. A similar pattern of lateral bud activity was described by Champagnat (2) in his study of growth correlations in Cicer arieturn. In that species, as in flax, the buds at the basal nodes and those borne on the middle region of the stem have the greatest growth potential and their degree of inhibition varies with the nutrient supply. Buds borne on the intervening region of the shoot have the lowest growth potential. These buds are relatively small, strongly inhibited, and unresponsive to nutritional factors. Champagnat suggests that the gradient of bud activity and the observed nutritional effects may be ascribed to competi- tion between the buds for a limited nutrient supply.

The results of the present investigation provided similar evidence that the gradient of bud growth could be modified by varying the nutrition of the plant. For example, at the lowest N level the region of minimal growth potential, i.e. where buds were not externally visible, was initiated at a lower node (node 4) and extended to a significantly higher node than at the other two nitrogen levels. Also, in the

apical portion of the shoot, there were more than twice as many nodes with a visible bud at the highest nitrogen level than in either of the other treatments.

In seeking an explanation for the gradient of bud growth consistent with the observed nutritional effects it would seem that such a gradient might result from differences in the time of initiation of the lateral buds both in relation to one another and to ontogenetic changes in the nutritional status of the main shoot apex. Support for this suggestion is provided by Crook's observation (4) that in the early development of the epicotyl of the flax seedling, leaf primordia are so close to- gether that a length of 0.1-0.15 mm of the growing point includes 10-20 nodes and inter- nodes. The intense morphogenetic activity suggested by this observation, occurring at a stage of ontogeny when the nutritional status of the shoot apex may still be at a relatively low level, could well result in a high degree of nutrient competition and a consequent in- hibition of the initiation and growth of the axillary buds. The apparent reduction of apical dominance during the later stages of develop- ment may result from a slower rate of leaf production and (or) the increasing nutritional status of the shoot apex.

Turning now to consider the observed in- hibition of apical growth and the associated leaf necrosis it was evident from the results of experiment 6 that these were symptoms of zinc deficiency. It was also apparent that the occur- rence and severity of these symptoms was extremely sensitive to environmental conditions. In this respect the present observations agree with those of previous investigators, who also reported that zinc deficiency symptoms were increased by N fertilization (13) and by in- creased light intensity (8, 14, 17). Particularly striking, however, was the influence of the N source and the observation that Zn deficiency effects were eliminated by inclusion of some NH4-N in the nutrient solution. Although the mechanism involved requires further study, it is known that the availability of Zn is reduced under alkaline conditions (16) and it, therefore, seems probable that the reduction in pH that occurred in the NH4N0, solution, and which presumably resulted from the preferential ab- sorption of the NH4+ ion, promoted zinc uptake

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McINTYRE: BUD AND SHOOT GROWTH INHIBITION 40 1

and was thus indirectly responsible for preven- tion of the deficiency effects.

An interesting feature of the zinc deficiency response was that the leaf injury and inhibition of apical growth could be prevented or signifi- cantly reduced by removal of the lateral buds at the basal nodes, thus indicating that a correlative inhibition was involved. The most obvious explanation of this result is that the basal buds, when released from apical dominance by the provision of a sufficiently high N level, tend to compete with the main shoot apex for the limited zinc supply. This interpretation is supported by the results of experiment 6, which showed that the growth response of the main shoot was accompanied by a proportionate increase in the growth of the basal laterals. It is also consistent with the work of Crafts and Yamaguchi (3), whose autoradiographic studies of the translocation of 6 5 ~ n showed that the tracer was freely mobile in the symplast, being readily retranslocated from mature tissues

' to young growing leaves at the shoot apex and : to other active meristems. 1

In experiment 2 the removal of the buds at 1 the basal nodes of plants grown at the 21 .O-ppm

N level not only eliminated the symptoms of 1 leaf injury at the shoot apex but also signifi-

cantly increased the rate of gowth of the shoot and induced the outgrowth of lateral buds at the apical nodes. The fact that these additional growth responses were also produced in the intact plant by growing the plants at a higher N level suggested that they were due to an increased supply of nitrogen made available by removal of the competitive influence of the basal laterals. However, the high N concentra- tion (420ppm) used in this experiment was obtained by addition of NH,NO, to the standard solution and since subsequent experi- ments showed that NH,-N eliminated zinc deficiency effects, it may be argued that the growth response of the shoot and lateral buds could have been caused at least partly by an increase in the zinc supply. In view of the obviously complex interaction of zinc and nitrogen nutrition in this species further in- vestigation will be required before their relative effects on growth and development can be

, critically assessed. The acropetal inhibition exerted by the basal

, laterals would seem to be comparable with

the phenomenon of apical abortion, which is a characteristic feature of the normal develop- ment of Syringa vulgaris and which was &- vestigated experimentally by Garrison ana Wetrnore (5). As noted by these workers this type of behavior is found in a wide range of species, but the factors responsible for the inhibition of apical growth are still obscure. If, however, the two phenomena are indeed of a similar physiological nature, then the results of the present investigation suggest that in- ternal competition for a limiting nutrient supply may be a major factor in the mechanism in- volved.

Acknowledgments 1 thank Miss Shirley Larmour and Mr.

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

1. ALI, A., and R. A. FLETCHER. 1970. Xylemdifferenti- ation in inhibited cotyledonary buds of soybeans. Can. J. Bot. 48: 1139-1 148.

2. CHAMPAGNAT, P. 1965. Physiologic de la croissance et de l'inhibition des bourgeons: dominance apical et phenomenes analogues. In Handbuch der Pflanzenphysiologie. Vol. 15. Part 1. Edited by W. Ruhland. Springer, Berlin. pp. 1106-1 164.

3. CRAFTS, A. S., and S. YAMAGUCHI. 1964. The au- toradiography of plant materials. Chap. VIII. Calif. Agric. Exp. Sta. Ext. Serv. Manual 35. pp. 79-81.

4. CROOKS, D. M. 1933. Histological and regenerative studies on the flax seedling. Bot. Gaz. 95: 209-239.

5. GARRISON, R., and R. H. WETMORE. 1961. Studies in shoot-tip abortion: Syringa vulgaris. Am. J . Bot. 48: 789-795.

6. GOODWIN, P. B., and R. E. CANSFIELD. 1%7. The control of branch growth of potato tubers. 111. The basis of correlative inhibitibn. J. Exp. Bot. 18: 297-307.

7. GREGORY, F. G., and J. A. VEALE. 1957. A reassess- ment of the problem of apical dominance. Symp. Soc. Exp. Biol. 11: 1-20.

8. HOAGLAND, D. R. 1944. Inorganic plant nutrition. Prather lectures. Chronica Botanica Co., Waltham, Massachusetts.

9. HOAGLAND, D. R., and D. I. ARNON. 1939. The water culture method for growing plants without soil. Circ. Agric. Exp. Sta. No. 347.

10. MCINTYRE, G. 1. 1%8. Nutritional control of the cor- relative inhibition between lateral shoots in the flax seedling (Linum usiratissirnum L.). Can. J . Bot. 46: 147-155.

11. MCINTYRE, G. I., and S. D. LARMOUR. 1974. The correlative inhibition of bud and shoot growth in flax. Anatomical changes associated with the release of lateral buds from inhibition. Can. J. Bot. 52: 2269-2275.

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12. MILLIKAN, C. R. 1951. Diseases of flax and linseed. Victoria (Australia) Dep. Agric. Tech. Bull. No. 9.

13. OZANNE, P. G. 1955. The effect of nitrogen on zinc deficiency in subterranean clover. Aust. J. Biol. Sci. 8: 47-55.

14. OZANNE, P. G. 1955. The effect of light on zinc deficiency in subterranean clover (Trifolium subter- ranelrm L.). Aust. J. Biol. Sci. 8: 344-353.

15. PETERSON, C. A., and R. A. FLETCHER. 1973. Apical dominance is not due to a lack of functional xylem and phloem in inhibited buds. J. Exp. Bot. 24: 97-103.

VOL. 53, 1975

16. SAUCHELLI, V. 1969. Trace elements in agriculture. Chap. 9. Van Nostrand Reinhold Co. pp. 167-232.

17. SKOOG, F . 1940. Relationships between zinc and auxin in the growth of higher plants. Am. J. Bot. 27: 939-95 1.

18. WARDLAW, I . F., and D. C. MORTIMER. 1970. Car- bohydrate movement in pea plants in relation to axil- lary bud growth and vascular development. Can. J. Bot. 48: 229-237.

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