Effect of gibberellic acid on the growth of main shoot and ...

Prne. Indian Acad. Sci., Vol. 87 B (Plant Sciences-4), No. 11, November 1978, pp. 255-270, �9 printed in India.

Effect of gibberellic acid on the growth of main shoot and axillary branches in Calendula ofl cinalis

H Y MOHAN RAM and USHA MEHTA Department of Botany, University of Delhi, Delhi 110 007

MS received 24November 1977

Abstract. Calendula officinalis L. bears a short main axis and branches of several orders. The upper axillary branches grow vigorously whereas the lower branches remain arrested at different levels. Foliar application of gibberellic acid (GA3) stimulated the elongation of the main shoot and of the upper few primary axillary branches (PAB's). The length of the main axis nearly doubled 10 days after application. The endogenous level of GA3 is probably sub-optimal, since external application enhances stern growth. However, treatment caused varying degrees of stimulation of the arrested branches. Whereas all the concentration of GA3 were equally effective in promoting the growth of the first six primary axillary branches after 60 days of application, in plants treated with 500 ppm, the lower primary axillary branches failed to develop. Since the lower axillaries remained inhibited, it is presumed that 500 ppm of GAa is supra-optimal. At 100 and 250 ppm, however, the growth of the arrested branches at the base was significantly increased.

Thirty days after application, GAs promoted growth and augmented the number of secondary axillary branches. Curiously at 100 ppm GA a markedly inhibited the length of the secondary axillary branches of PAB a to PAIl 6.

These studies show that a complex mechanism of apical dominance operates in Calendula officinalis.

Keywords. Calendula offtcinalis; Scotch marigold; branching pattern; gibberellic acid (GA3); axillary branches.

1. Introduction

GibbereUins control many important physiological processes in plants such as shoot elongation, flowering, and seed germination (Lang 1956, 1970; Lang et al 1957; Stant 1959; Brian 1961, 1966; Brian and Hemming 1955; Paleg 1965; Sachs 1965; Nanda et al 1967; Jones 1973; Krishnamoorthy 1975). A preliminary observation in the field suggested that in a few of the ornamental Compositae, elongation of the main axis and branching of the shoot were in some way regulated by the initiation of the capitulum. It was presumed that a key hormone implicated in these activities could be gibberellin. S6ding (1926) had demonstrated that flower buds contribute substances necessary for the development of the scape (leafless inflorescence stalk) in Bellis perennis. Removal of the inflorescence stopped the elongation of the scape. Sachs (1968) has also observed stunting of the scape in Gerbera plants in response to decapitation. On the basis of surgical experiments followed by the application of hormones, Sachs has gathered evidence that involucral bracts and receptacle may be synthesising substances that control the activity of the intercalary meristem, which is responsible for the elongation of the scape.

Prec. B.--I 255

256 H Y Mohan Ram and Usha Mehta

Calendula officinalis L., commonly called Scotch marigold, shows a unique pattern of branching. The plant has a short main axis which terminates in a capitulum. Soon after the appearance of the latter, axillary branches arise basipetally and bear terminal capitula (figure 1). The main axis usually has 13 or 14 leaves. Although each leaf axil bears a lateral bud, the extent to which the branches develop differs. The most vigorous are those situated at nodes 1-5, stunted ones occupy positions 7-12 and the one located at node No. 6 is intermediate. Lateral buds remain totally arrested at nodes 13 and or 14. This system represents a gradient of correlative inhibition, the degree being minimum closest to the apex and maximum with in- creasing distance from the main shoot ape~ (figure 1). This trend is also reflected in the production of secondary, tertiary and quaternary branches borne by each primary branch. In spite of the large number of papers that have appeared on the problem of apical dominance, very few deal with the details concerning the effect of the intact apical bud on the growth of the lateral buds situated at various positions on the main shoot. The present investigation was therefore taken up with the objective of filling up this gap and also to study the effects of exogenously applied GA3 on the pattern of shoot development in this plant. A quantitative account of the growth responses of the main axis, primary and secondary axiUary branches has been given and the results are discussed in relevance to contemporary literature.

2. Material and methods

Seedlings of Calendula officinalis L. (double yellow) were raised in a nursery bed in

Figure I. Diagrammatic representation of the branching pattern of the shoot artd the sequence of development of the capitula in the control plant. /VIA, main axis; 1-14, primary axillary branches; SAB, secondary axillary brancll; TAB, tertiary axillary branch; O/Q, open capitulum, capitulum at the bud stage; the short lateral lines, emerging from different branches indicate leaves.

Effect of GA 3 on C. officinalis 257

mid-October, and were transplanted to earthen pots (25 em diameter) containing garden soil when two to three weeks old.

Three foliar sprays of GAa (100, 250 and 500 ppm) were given to young plants bearing 10-12 leaves (just after floral evocation) at 10-day intervals using Triton X-114 (0"01 ~) as the wetting agent. Fifteen plants were chosen for each treatment. Control plants received only Triton X-114. The plants were maintained under natural conditions prevailing in the Botanical Garden of the Department.

The results of the experiments were subjected to standard analysis of variance. The findings were further analysed by using S. E. difference test (S. E. (diff.)). The (S.E. (diff.)) was calculated by using the formula (2o2/n) 1/2 where a 2 is the error mean sum of squares and n is the number of replicates (Snedecor 1946; Federer 1955).

To test the significance of a particular treatment, the value obtained by (S.E. (diff.)) was multiplied by the corresponding value of t at 1 or 5 ~o level. The difference of the means between any two treatments was compared with the value obtained in (S.E. (diff.)) and this has been represented by a bar (between any two means) in the tables.

3. Observations

3.1. General effects

Plants treated with GA a were light green and spindly, and flowered one week earlier than the controls. They showed a significant increase in the height of the main axis and in the number, length, and degree of branching of the axillary branches. Treatment with GA3 caused different degrees of stimulation of the arrested axillary branches depending on the concentration.

3.2. Main axis

The influence of GAa on the elongation of the main axis (MA) is presented in table 1. In all the GA 3 treatments height was nearly doubled. Figure 2 illustrates that a sudden rise in elongation occurs in MA 10 days after treatment even at 100 ppm. The response curve remained unchanged at 250 and 500 ppm. The phase of elongation of the MA in controls and in treated plants was, therefore restricted to the first 30 days. The effects of the three concentrations were not statistically different among themselves (table 1).

3.3. Primary axillary branches

GA3 treatment elicited different types of response in the length of the primary axillary branches 30 and 60 days after treatment (table 2, figures 2, 3). Thirty days after treatment all concentrations of GA~ were equally effective in. causing elongation o f PAB 1 (table 2). No further elongation was observed thereafter. The response curves plotted for PABx and MA had the same mode (figure 2). In PAB~ the effects elicited by 100 and 250 ppm or 250 and 500 ppm were not statistically different from one another, but the differences at 100 and 500 ppm GA3 were highly significant


Table 1. Influence of GA~ on the elongation of the main axis* Height (cm)

i

Days after Concentration of GAa (ppm) treatment 0 (control) 100 250 500 F value

- - i i ~ - r I " l . . . . r T i - - - " �9 ~ . . . . i �9 ~ ~ i

I0 21 "45 42"30 43.26 45"26 98"73**

30 26"02 47"28 48'23 49"35 94.86**

60 26"02 47.28 48.23 49"35 94.86**

*Mean values from 15 plants; **Highly significant at 1% level; underlining denotes differences not significant

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60 . . . . 10 Days

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40

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100 250

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500 100 250 500 ~; Concentration of G.A 3 (ppm)

Figure 2. Effect of GAs on the length of main axis (MA) and the first five primary axillary branches (PAIl I to PABs)

(table 2). Some elongation in PAB= was noted with 100 ppm (figure 2). With increase in the concentration o f GA3 from 100 to 500 ppm, the length of PAB 4 increased substantially 30 days after treatment (figure 2). However, the data for 60 days showed that even at 100 ppm, GAa caused marked elongation of PAB t. Fur- ther increase in concentration was not o f much consecmence (figure 2, table 2).

Effect of GAa on C. officinalis 259

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The growth of PAB s and PAB 5 was not influenced by 100 ppm GA s 30 days after treatment. However, 250 and 500 ppm GA s (though not different statistically in their effects f rom one another) brought about elongation of these branches more or less to the same extent (table 2). The response curve of PAB~ was, however, different from that o f PABa and PAB 5 (figure 2).

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Figure 3. Effect at" GAs on the length of the lower primary axillary branches (PAB6 to PAB1J

Effect of GAa on C. o~cinalis 261

The increase in length of PAB 6 brought about by GAa at 100 ppm over that of the control after 30 days was negligible. GAa at 250 ppm stimulated it but further increase in concentration caused a depression (table 2). The growth in length as measured at 60 days was different because 100 ppm showed a delayed stimulatory action (figure 3). However, the extent of elongation gradually dropped at higher concentrations (figure 3).

An appreciable degree of stimulation in elongation of the primary axillary branches (7 to !0 and 12) was observed between days 30 and 60. Whereas, in general the branches responded better to 250 ppm, after 30 days treatment there was either no increase or a marked decrease at 500 ppm. This trend was common to the branches both at 30 and 60 days (figure 3).

One feature common to PAB's 7 to 10 and 12 after 30 days treatment was a marked stimulation at 250 ppm followed by a sharp decrease at 500 ppm (figure 3). Each of these PAB's, however, responded differently at 60 days. In PAB 8 maximum stimulation was registered at 100 ppm followed by a steep decline at 250 and 500 ppm of GA3 (table 2, figure 3).

In PAB's 9 and 10 the intermediate concentration of GA3 (250 ppm) was found to be more effective than 100 or 500 ppm. Interestingly, PABll exhibited the same trend at 30 and 60 days (figure 3). A decrease in the effectiveness of GAz at 100 ppm was seen in PABx~. A poor response to GA 3 was shown by PAB~3 and PAB~4. As PAB's 9 to 14 remain generally inhibited in nature, it is highly significant that GA 3 can stimulate elongation. This is especially so in PAB's 8-12. GA3 at 500 ppm was unable to promote the growth of the lower primary axillary branches. Curiously 250 ppm enhanced the growth of these axillary branches.

It is noteworthy that all the concentrations of GAz were equally effective in promoting the growth of the first six primary axillary branches after 60 days treatment (table 2). The mean differences in the length of the first six PAB's observed were not found to be different statistically (see bar diagrams in table 2). Even by the 60th day 500 ppm GA 3 was unable to elicit any elongation in the basal axillaries (PAB 7 to PAB14), whereas 100 and 250 ppm were equally effective in enhancing the length of these axillary branches (table 2).

3.4. Secondary axillary branches (30 days after treatment)

In general, GAa treatment stimulated the growth and enhanced the number of SAB's. However, the responses elicited by the three concentrations of GAa were different in magnitude. GA a at 100 ppm significantly inhibited the length of the SAB's of PABa to PAB 6 (table 3). The SAB's of PAB~ and PAB s were not affected by 100 ppm GAa (tables 3, 4). But those belonging to PAB a to PAB12 were stimulated in their growth (table 4).

The average length of SAB 1 and SAB2 of PABa and PAB~ was not influenced by 250 ppm GA~ (table 3). In general, the length of the first four or five SAB's of PAB 5 and PAB6 was stimulated by 250 ppm GAa over those of the controls (table 3). Interestingly, GA3 (250 ppm) enhanced the number of SAB's (tables 3, 4) and highly promoted the growth and development of the basal axillary branches (PAB7 to PABI~.).

GAa at 500 ppm caused greater elongation of the SAB's of PABa and PAB 4 over


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Effect of GA~ on C. officinalis 263

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Effect o f GA 3 on C. offieinalis 265

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those of the controls (table 3). The length of SAB's of PAB 5 and PAB e was found to be greater than that of the SAB's of the other two concentrations of GA 3 and controls. The growth of SAB's of PAB 7 to PAB u was stimulated. No SAB's were observed on PABx~ (tables 3, 4).

3.5. Secondary axillary branches (60 days after treatment)

The secondary axillary branches of PABz showed a varied response (table 3). SAB1 and SAB a belonging to PABs were not influenced by 100 and 250 ppm of GA 3 but responded favourably to 500 ppm (table 3). SABs to SABs were stimulated in their growth and the degree of elongation was more or less the same at all concentrations of GA s tested (table 3).

The amount of growth occurring in the SAB's of PAB3 in response to 100 ppm GA3 was consistently high between days 30 and 60 (table 3). In PAB 4, SAB6 responded favourably to GA3 at 100 ppm. The next higher concentration of GAs was most effective in promoting the elongation of SABa to SAB7 (table 3). SAB 1 to SAB 4 'showed significant elongation at 500 ppm of GAa. The extent of growth that occurred in the other SAB's was not statistically significant. Growth ensuing in SABI to SABs of PAB 4 between the 30th and the 60th day in plants treated with 100 ppm GAs was much greater than that in controls and in plants treated with 250 and 500 ppm (table 3).

Although a trend similar to that described above was seen in SAB's of PAB 6, all the concentrations of GA 3 highly stimulated the growth of the SAB's (table 3). Stimulation in length was greater in the lower secondary axillary branches than in the upper ones. However, GA 3 at 100 ppm did not elicit any change in the growth of the lower SAB's (table 3).

All the concentrations of GA3 significantly increased the length of SAB1 to SAB 4 in PAB e (table 3). SAil's 5 to 8 failed to elongate in the controls as well as in plants treated with 100 ppm GA3. Surprisingly, a slight elongation of these branches occurred in response to 250 ppm. Unexpectedly, GA~ at 500 ppm, instead of further promoting their elongation, permitted only a few of the SAB's to grow out. This response was not, however, statistically significant (table 3).

In the ease of PAB6, as fat' as controls were concerned, no growth in elongation occurred in SAB 1 to SAB 4 between 30 and 60 days. However, the magnitude of stimulation caused by 100 ppm GA3 far surpassed that noted for the other concentrations (table 3).

Whereas the SAB's, particularly 1 and 2 of PAB7 and PABs, remained rather suppressed in their growth during the first 30 days, their inhibition became consi- derably released in the subsequent 30 days. The SAB's 1 to 3 responded rather strikingly to 100 ppm GAs (tables 3, 4).

The lower concentrations of GA 3 (100 and 250 ppm) greatly stimulated the growth of SAB's borne by PAB 8 to PABtt which generally remain inhibited in nature (table 4). The SAB's belonging to PABls responded in the same manner to GAs at 100 and 250 ppm as did the SAB's of PAB7 to PABll (table 4).

4. Disenssio-

Gibberellins have been implicated in many phases of plant growth and development.

Effect of GA 3 on C. officinalis 26"/

The most striking effect of gibberellins is shoot elongation, especially in the geneti- cally dwarf plants (Brian and Hemming 1955; Phinney 1957).

The young plants of Calendula offieinalis begin with a rosette-like habit with a telescoped axis. After producing 8-10 leaves, the vegetative shoot meristem enters transition to become the reproductive meristem. With the initiation of floret primordia, the pith rib meristem undergoes a rapid phase of mitotic activity causing intercalary growth and lifting the capitulum above. This is accompanied by inter- nodal elongation.

GAs brought about elongation of the main axis in Calendula at all the three concentrations tested but nearly to the same extent. Most of the growth occurred with- in the first ten days. Elongation of the peduncle in this plant seems to be under the control of the activities occurring in the capitulum. Sachs (1968) has investigated the control of intercalary growth in the scape of Gerbera. He has shown that elongation is caused by the activity of the intercalary meristem located below the receptacle. Sachs noted that growth of the scape ceased if it was decapitated. Ap- plied GAa and IAA enhanced the growth of decapitated scapes, but even when com- bined, these substances were unable to induce meristematic activity equivalent to that occurring in intact scapes.

It may be conjectured that the endogenous level of GAa in Calendula plants is probably sub-optimai, since external supply enhances stem growth. As the degree of elongation induced by the three concentrations does not vary much, it may be inferred that the optimal level is reached at 100 ppm. Higher concentrations fall on the saturation plateau.

With the exception of PAB~ all the three concentrations of GAa increased the length of the first six primary axillary branches to the same degree (60 days after treatment). The lower PAB's which generally remain arrested in their growth in nature grow to the same length as the upper PAB's at 100 and 250 ppm. These branches do not respond to GAa at 500 ppm probably because the concentration is supra-optimal.

Although the responses to the applied GA3 by the secondary axillary branches located at various positions have been recorded in detail, it has not been possible to explain the rather erratic responses shown by them. As no estimation of the endogenous hormone/inhibitor levels has been carried out in the present study, any explanation of the observed responses will be conjectural. Generally in the controls those SAB's which show a tendency to grow are further stimulated, where as those which are normally inhibited respond only to certain concentrations of GA3 (100 and 250 ppm). As with the lower PAB's, certain inhibited secondary axiUary branches also do not respond to 500 ppm of GA a.

The observation that some of the primary and secondary axillary branches located at certain positions (especially PAB7 to PABI~ and SAB's of PAB~ to PAB12 as shown in figures 2, 3, tables 2-4) do not elongate in the control plant suggests that a complicated system of apical dominance is in operation.

There are conflicting reports in the literature on the effects of gibberellins in relation to apical dominance. According to Jacobs and Case (1965) and Scott et al (1967) gibbereUin when applied with auxin to decapitated plant stumps enhances the auxin effect, whereas the results obtained by Kato (1958), Wickson and Thhnann (1958) and Nakumura (1965) indicate that GA3 antagonises the inhibitory effect of [AA in correlative inhibition of lateral buds.


Phillips. (1969) stated that gibbereUin treatment generally promoted axillary bud or shoot growth only when correlative inhibition had already been decreased by some other means, and represented the usual response of a growing shoot to exogenous gibberellin. Jones and Phillips (1966) showed that growing apical buds are sites of active gibberellin synthesis. Thomas (1972) reported that lateral buds of intact Brussel sprouts contained less auxin and gibberellin than the main apex. Direct application of either auxins or gibberellins to totally inhibited lateral buds, however, does not induce their development into shoots (Catalano and Hill 1969; Phillips 1969), indicating that any deficiency of these hormones in a bud is not the primary cause of its inhibition.

According to Phillips (1975) cytokinins and ABA become accumulated in the arrested branches. The inhibited vegetative buds of Xanthium strumarium have been shown to accumulate 50-250 times the amount of ABA present in all the other parts of the plant (Tucker 1976; Tucker and Mansfield 1972, 1973). According to these authors it is the high amount of the ABA present in these buds which prevents them from responding to the high levels of endogenous cytokinins. Whether or not such a situation occurs in Calendula needs to be understood by extracting hormones and inhibitors present in such branches.

Whereas intact plants frequently show enhanced apical dominance in response to gibberellin treatment (Phillips 1969; Pharis et al 1970; Narasimhan 1972). Cutter (1972) believes that the interacting effects of auxins and GA3 may differ according to the stage of development of the buds at the time of treatment.

Ifieracium aurantiacum and H'. pratense, the two species which respond to gibberellins, exhibit incomplete apical dominance in the vegetative stage. In both the species, buds present in the axils of the rosette leaves grow out before flowering com- mences. In If. floribundum, a species which does not respond to GA s, however, apical dominance is complete and axillary buds do not grow until long after inflorescence formation (Peterson and Yeung 1972). Another explanation for the difference in response to applied gibberellins shown by species of Hieracium may be that I f . aurantiacum and If. pratense may have larger number of GA a receptor sites (Musgrave et al 1969) in the subapical region of the stem than If. floribundum and therefore respond by stem elongation.

The effects of applied gibberellins on the shoot apex of Hieracium species which respond to gibberellins are similar to those described for other long day rosette plants (Sachs et al 1959; Bernier et al 1967) with the greatest increase in mitotic activity occurring in cells of the subapical region. A similar observation has been made in Calendula ofl~cinalis.

Acknowledgements

We thank Dr N C Khandekar, Director, Computer Centre, University of Delhi, for his help in the analysis of the data presented in this paper. The award of a fellow- ship by the CSIR, New Delhi, to one of us (UM) is gratefully acknowledged.

R ~ o n c e ~

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