Planta (Berl.) 90, 222--229 (1970)
The Hormonal Regulation of Bud Outgrowth in Phaseolus vulgaris L. J o ~ I-LrLLMAN Botany Department, University College of Wales, Aberystwyth, U.K. Received October 9, 1969 S~mmary. Aqueous solutions of indole acetic acid, kinetin, gibberellic acid and abscisic acid were applied singly and in combination to the decapitated stem stump of Phaseolus seedlings. Application of indole acetic acid will not completely replace the intact stem apex with regard to the inhibition of lateral bud extension. The greatest inhibition of bud growth is obtained when indole acetic acid is applied in combination with both kinetin and abscisic acid. Treatment with gibberellic acid causes massive bud growth even in the presence of indole acetic acid, kinetin and abscisic acid. Although both abscisie acid and kinetin have only a slight promoting effect on bud outgrowth when applied singly, these hormones will modify the effects of indole acetic acid and gibberellic acid.
Introduction Apical dominance in the shoots of certain herbaceous plants is manifest by almost complete correlative inhibition of axillary bud extension by the presence of an intact apex. The hormonal physiology of this phenomenon has been the subject of much work during the past five decades and has been reviewed by Phillips (1969). Most attention has been directed towards the controlling influence of auxin which, in certain instances, can substitute for the intact apex when applied to the decapitated stem. However, there is evidence for the involvement of other plant growth hormones. Thus, cytokinins m a y be involved in this phenomenon, treatment with kinetin often releasing axillary buds from correlative inhibition (Sachs and Thimann, 1967). In addition, gibberellin treatment has been found to lead to enhanced apical dominance in intact plants (Brian et al., 1959). Furthermore, axillary buds subject to correlative inhibition have been shown to contain higher concentrations of endogenous growth inhibitors than those released from apical dominance (DSrffling, 1966). Both the special correlative growth inhibitor proposed by Snow (1936) and the neutral inhibitor which m a y control apical dominance in potato tubers (Goodwin and Cansfield, 1966) have not been isolated. I n reviewing the available literature on correlative inhibition it is apparent t h a t four important factors must be taken into account when considering the role of the known plant growth regulators. Firstly, m a n y reports tend to concentrate on the effects of just one or two substances
Hormonal Regulation of Bud Outgrowth
223
applied singly or in c o m b i n a t i o n to i n t a c t p l a n t s or isolated stem segments. Secondly, little a t t e n t i o n has been paid to the role of abseisic acid, the r e c e n t l y discovered p l a n t growth regulator. Thirdly, no m e n t i o n has been made of complex h o r m o n e interactions. F o u r t h l y , in m a n y instances e x t r e m e l y high concentrations of the h o r m o n e u n d e r s t u d y have been applied, often i n l a n o l i n pastes. Scott et al. (1967) reported t h a t the p u r s u i t of a classical approach to the p r o b l e m of h o r m o n a l regulation of axillary b u d outgrowth t h r o u g h the use of material i n which the apex has been r e m o v e d is the best compromise between i n t a c t p l a n t s a n d isolated segments. I t was therefore decided to use this approach i n i n v e s t i g a t i n g the h o r m o n a l regulat i o n of apical d o m i n a n c e i n Phaseolus, a species i n which the i n t a c t apex exerts strong d o m i n a n c e over the axillary buds s u b t e n d i n g the opposite p r i m a r y leaves d u r i n g the first eight weeks. I n c o n t r a s t to previous studies i n this species low c o n c e n t r a t i o n s of four growth substances viz. : indole-3-acetic acid (IAA), k i n e t i n (K), gibberellic acid (GA) a n d abscisic acid (AbA), were applied i n a q u e o u s solution to the cut stem s t u m p . These hormones were applied singly a n d i n c o m b i n a t i o n .
Materials and Methods Plants of Phaseolus vutgaris L. c.v. Canadian Wonder were grown in 10 cm pots containing John Innes No. 1 Compost and maintained under long-day conditions of 16 hours photoperiods at 18~ C for 6 weeks. Uniform plants with intact leaves were then selected and the main stem was decapitated 4 cm above the primary leaves. A rubber tube, 3 cm in length, was pushed onto the cut stump for a distance of 5 mm and sealed with hydrous lanolin. The following treatments with reference numbers were employed (Table): Table. Treatments employed with re/erence numbers 1. 2. 3. 4. 5. 6. 7. 8.
Intact Water IAA GA AbA K IAA + IAA +
Control Control
GA AbA
9. 10. 11. 12. 13. 14. 15. 16.
IAA + K GA ~- AbA GA § K AbA A-K IAA ~- GA § K IAA + AbA -~-K GA + AbA -[~K IAA + GA + AbA + K
All growth regulators were applied in aqueous solution at a final concentration of 25 ppm. There were 10 replicates per treatment The application tube was topped up with solution twice daily one hour after the plants were watered. Every two days a 2 mm section was removed from the cut stump and the treatments reapplied. Each day the length of each axillary shoot was determined to the nearest mm with a pair of calipers and ruler. Initial bud lengths were assumed to be no less than
224
J. ttillman:
0.5 mm. Separate measurements were taken for the initially shorter bud (SB) and longer bud (LB). At the end of an experiment measurements were taken of the stem diameter 3 cm above the point of insertion of the primary leaves. The experiments reported here have been repeated at least two times with similar results.
Results After 6 days all measurements were terminated when cotyledonary and auxiliary bud outgrowth had occurred in those plants treated with GA. Except in the GA treatment (treatment 4) the growth of the LB b y the end of the experiment was significantly greater than t h a t of the SB (Fig. 1). However, on transposing the data to percentage increments above the initial length then the growth increment of the LB never exceeded t h a t of the SB (Fig. 2). Indeed, in certain instances the growth increment of the SB exceeded t h a t of the LB. Thus, treatment with GA promoted a significant differential between the growth of the LB and SB and the GA effect ~was not inhibited b y the presence of K (treatment 11), AbA (treatment 10), IAA (treatment 7) and A b A ~ - K (treatment 15). The GA effect was nullified in the presence of I A A - [ - K (treatments 13 and 16). Although a differential existed between the lengths of the LB and SB with AbA application, this effect was not shown in the presence of IAA (treatment 8), K (treatment 12), I A A Jr K (treatment 14), I A A Jr GA ~ - K (treatment 16). Preliminary experiments have shown that no such differential exists when a lower concentration of 10 p p m AbA is used. IAA application (treatment 3) did not replace the intact stem apex (treatment l) with respect to the inhibition of lateral bud extension. I n terms of the difference between the final and initial bud lengths there were no significant differences between the IAA treatment and the water control (treatment 2), although the growth of the laterals was significantly lower than the K (treatment 6), AbA (treatment 5), and GA (treatment 4) treatments (Fig. 1). Only on the basis of a comparison between percentage increments, however, was there a significant reduction in growth increment below t h a t of the water control. When used in combination with other growth substances I A A retarded their effect on the growth of one or both buds except in the case of I A A - k GA application. The bud outgrowth obtained in the I A A ~-AbA q - K treatment was less than t h a t of any other hormone combination (Fig. 1). However, the percentage increment in growth of the I A A - k A b A - k K treatment did not differ significantly from t h a t of the IAA and I A A q-AbA treatments (Fig. 2). No synergistic effects on bud growth were noted between IAA and GA. The data show that GA promotes lateral bud extension even when in combination with any or all the other plant growth substances (Fig. 1 and 2). The greatest inhibition of the GA effect was obtained with I A A - k
Hormonal Regulation of Bud Outgrowth
225
600 500 E E c_ hO0 x 300-
-g
= 200-
"~:
The Hormonal Regulation of Bud Outgrowth in Phaseolus vulgaris L. J o ~ I-LrLLMAN Botany Department, University College of Wales, Aberystwyth, U.K. Received October 9, 1969 S~mmary. Aqueous solutions of indole acetic acid, kinetin, gibberellic acid and abscisic acid were applied singly and in combination to the decapitated stem stump of Phaseolus seedlings. Application of indole acetic acid will not completely replace the intact stem apex with regard to the inhibition of lateral bud extension. The greatest inhibition of bud growth is obtained when indole acetic acid is applied in combination with both kinetin and abscisic acid. Treatment with gibberellic acid causes massive bud growth even in the presence of indole acetic acid, kinetin and abscisic acid. Although both abscisie acid and kinetin have only a slight promoting effect on bud outgrowth when applied singly, these hormones will modify the effects of indole acetic acid and gibberellic acid.
Introduction Apical dominance in the shoots of certain herbaceous plants is manifest by almost complete correlative inhibition of axillary bud extension by the presence of an intact apex. The hormonal physiology of this phenomenon has been the subject of much work during the past five decades and has been reviewed by Phillips (1969). Most attention has been directed towards the controlling influence of auxin which, in certain instances, can substitute for the intact apex when applied to the decapitated stem. However, there is evidence for the involvement of other plant growth hormones. Thus, cytokinins m a y be involved in this phenomenon, treatment with kinetin often releasing axillary buds from correlative inhibition (Sachs and Thimann, 1967). In addition, gibberellin treatment has been found to lead to enhanced apical dominance in intact plants (Brian et al., 1959). Furthermore, axillary buds subject to correlative inhibition have been shown to contain higher concentrations of endogenous growth inhibitors than those released from apical dominance (DSrffling, 1966). Both the special correlative growth inhibitor proposed by Snow (1936) and the neutral inhibitor which m a y control apical dominance in potato tubers (Goodwin and Cansfield, 1966) have not been isolated. I n reviewing the available literature on correlative inhibition it is apparent t h a t four important factors must be taken into account when considering the role of the known plant growth regulators. Firstly, m a n y reports tend to concentrate on the effects of just one or two substances
Hormonal Regulation of Bud Outgrowth
223
applied singly or in c o m b i n a t i o n to i n t a c t p l a n t s or isolated stem segments. Secondly, little a t t e n t i o n has been paid to the role of abseisic acid, the r e c e n t l y discovered p l a n t growth regulator. Thirdly, no m e n t i o n has been made of complex h o r m o n e interactions. F o u r t h l y , in m a n y instances e x t r e m e l y high concentrations of the h o r m o n e u n d e r s t u d y have been applied, often i n l a n o l i n pastes. Scott et al. (1967) reported t h a t the p u r s u i t of a classical approach to the p r o b l e m of h o r m o n a l regulation of axillary b u d outgrowth t h r o u g h the use of material i n which the apex has been r e m o v e d is the best compromise between i n t a c t p l a n t s a n d isolated segments. I t was therefore decided to use this approach i n i n v e s t i g a t i n g the h o r m o n a l regulat i o n of apical d o m i n a n c e i n Phaseolus, a species i n which the i n t a c t apex exerts strong d o m i n a n c e over the axillary buds s u b t e n d i n g the opposite p r i m a r y leaves d u r i n g the first eight weeks. I n c o n t r a s t to previous studies i n this species low c o n c e n t r a t i o n s of four growth substances viz. : indole-3-acetic acid (IAA), k i n e t i n (K), gibberellic acid (GA) a n d abscisic acid (AbA), were applied i n a q u e o u s solution to the cut stem s t u m p . These hormones were applied singly a n d i n c o m b i n a t i o n .
Materials and Methods Plants of Phaseolus vutgaris L. c.v. Canadian Wonder were grown in 10 cm pots containing John Innes No. 1 Compost and maintained under long-day conditions of 16 hours photoperiods at 18~ C for 6 weeks. Uniform plants with intact leaves were then selected and the main stem was decapitated 4 cm above the primary leaves. A rubber tube, 3 cm in length, was pushed onto the cut stump for a distance of 5 mm and sealed with hydrous lanolin. The following treatments with reference numbers were employed (Table): Table. Treatments employed with re/erence numbers 1. 2. 3. 4. 5. 6. 7. 8.
Intact Water IAA GA AbA K IAA + IAA +
Control Control
GA AbA
9. 10. 11. 12. 13. 14. 15. 16.
IAA + K GA ~- AbA GA § K AbA A-K IAA ~- GA § K IAA + AbA -~-K GA + AbA -[~K IAA + GA + AbA + K
All growth regulators were applied in aqueous solution at a final concentration of 25 ppm. There were 10 replicates per treatment The application tube was topped up with solution twice daily one hour after the plants were watered. Every two days a 2 mm section was removed from the cut stump and the treatments reapplied. Each day the length of each axillary shoot was determined to the nearest mm with a pair of calipers and ruler. Initial bud lengths were assumed to be no less than
224
J. ttillman:
0.5 mm. Separate measurements were taken for the initially shorter bud (SB) and longer bud (LB). At the end of an experiment measurements were taken of the stem diameter 3 cm above the point of insertion of the primary leaves. The experiments reported here have been repeated at least two times with similar results.
Results After 6 days all measurements were terminated when cotyledonary and auxiliary bud outgrowth had occurred in those plants treated with GA. Except in the GA treatment (treatment 4) the growth of the LB b y the end of the experiment was significantly greater than t h a t of the SB (Fig. 1). However, on transposing the data to percentage increments above the initial length then the growth increment of the LB never exceeded t h a t of the SB (Fig. 2). Indeed, in certain instances the growth increment of the SB exceeded t h a t of the LB. Thus, treatment with GA promoted a significant differential between the growth of the LB and SB and the GA effect ~was not inhibited b y the presence of K (treatment 11), AbA (treatment 10), IAA (treatment 7) and A b A ~ - K (treatment 15). The GA effect was nullified in the presence of I A A - [ - K (treatments 13 and 16). Although a differential existed between the lengths of the LB and SB with AbA application, this effect was not shown in the presence of IAA (treatment 8), K (treatment 12), I A A Jr K (treatment 14), I A A Jr GA ~ - K (treatment 16). Preliminary experiments have shown that no such differential exists when a lower concentration of 10 p p m AbA is used. IAA application (treatment 3) did not replace the intact stem apex (treatment l) with respect to the inhibition of lateral bud extension. I n terms of the difference between the final and initial bud lengths there were no significant differences between the IAA treatment and the water control (treatment 2), although the growth of the laterals was significantly lower than the K (treatment 6), AbA (treatment 5), and GA (treatment 4) treatments (Fig. 1). Only on the basis of a comparison between percentage increments, however, was there a significant reduction in growth increment below t h a t of the water control. When used in combination with other growth substances I A A retarded their effect on the growth of one or both buds except in the case of I A A - k GA application. The bud outgrowth obtained in the I A A ~-AbA q - K treatment was less than t h a t of any other hormone combination (Fig. 1). However, the percentage increment in growth of the I A A - k A b A - k K treatment did not differ significantly from t h a t of the IAA and I A A q-AbA treatments (Fig. 2). No synergistic effects on bud growth were noted between IAA and GA. The data show that GA promotes lateral bud extension even when in combination with any or all the other plant growth substances (Fig. 1 and 2). The greatest inhibition of the GA effect was obtained with I A A - k
Hormonal Regulation of Bud Outgrowth
225
600 500 E E c_ hO0 x 300-
-g
= 200-
"~:
