Planta (BEE.) 86, 315--323 (1969)

Auxin-Gibberellin Interaction in Apical Dominance: Experiments with Tall and Dwarf Varieties of Pea and Bean I. D. J. PHILLIPS Department of Botany, University College of Wales, Aberystwyth, Cardiganshirc, United Kingdom Received March 31, 1969

Summary. Seedlings of dwarf and tall varieties of pea and bean, growing in John Innes Compost No. 2, were studied in relation to the effects of decapitation, indole-3-aeetic acid (IAA), and gibberellic acid (GA3) on axillary bud growth. In all varieties, GA~ antagonized the inhibitory influence of IAA on bud growth when both hormones were applied to the upper cut end of the stem. Thus, GA8 caused a reduction in IAA-induced correlative bud inhibition in tall, as well as in dwarf, plants. These results agree with those obtained by several workers, but contrast with some recent reports of increased apical dominance in a tall pea variety when seedlings were treated with GA3 in addition to IAA. An attempt was made to identify the cause of opposite results being obtained by different workers, and it is considered that possibly the most important factor is mineral nutrition. Introduction Axillary buds are subject to correlative inhibition b y the apical bud, although the degree of dominance of the apical bud over the axillaries varies from species to species, with prevailing environmental conditions, and with physiological age of each individual. Where apical dominance is clearly manifest, excision of the apical bud ("decapitation") normally results in the outgrowth of one or more axillary shoots. I t has been k n o w n for m a n y years t h a t auxins can substitute for the apical bud in maintaining correlative inhibition of axillary buds in decapitated plants (THIMANN and SKOOG, 1933, 1934). B y contrast, t r e a t m e n t of the stem top of decapitated plants with gibberellins promotes growth of axillary buds (KATo, 1953; BRIAN et al., ]955; NAI~A~UaA, 1965). I t is k n o w n t h a t the y o u n g leaves of the apical bud serve as the principal source of both endogenous auxins (JAcoBs, 1962) and gibberellins (JoNa~s and PHILLIPS, 1966) which are concerned in the control of internode elongation. I t may, therefore, be expected t h a t apically-synthesized auxin and gibberellin interact antagonistically in axillary bud g r o w t h ; auxin suppressing and gibberellin promoting. However, conflicting results have been obtained in experiments on the effects of simultaneous auxin and gibberellin t r e a t m e n t of decapitated plants. Some workers

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observed, as might be expected from the effects of individual application of auxin or gibbereliin, t h a t when GA 3 was supplied along with I A A then the inhibiting influence of I A A on bud growth was diminished (KATo, 1958; WICKSO~ and TtIIMANN, 1958; NAKAMUI~A, 1965). More recently, others have found t h a t GAa can enhance inhibition of axillary buds by I A A (JAcoBs and CASW, 1965; SCOTT, Case and JACOBS, 1967). I n the course of experiments on apical dominance in dwarf bean plants (PHILLIPS, 1968), it Was noticed t h a t t r e a t m e n t of decapitated plants with a mixture of I A A and GA 3 led to greater growth of axillary buds t h a n t h a t seen in similar plants treated with I A A alone. I n view of the results obtained b y JACoBs and co-workers (loc. cit.), this was unexpected. However, these workers experimented with a tall variety of pea (Pisum sativum, c.v. Alaska). Since certain genetically dwarfed plants exhibit iarge growth responses to exogenous gibberellins, it seemed possible t h a t m y results differed from those obtained with Alaska pea because a dwarf bean variety was being studied. Thus, the effects of IAA, GAs, and mixtures of the two, were examined with respect to axillary bud growth in decapitated dwarf and tall ("runner") bean plants. Results obtained were similar with both types of bean, and it was therefore desirable to re-examine the responses to I A A and GA 3 of decapitated Alaska pea seedlings under experimental conditions identical with those used for the bean seedlings. I n addition, a dwarf pea variety (P. sativum e.v. Meteor) was similarly investigated. Materials and Methods Plants used were, dwarf bean (Phaseolus vulgaris, c.v. Canadian Wonder), runner bean (P. multi/lotus, c.v. Scarlet Emperor), dwarf pea (Pisum sativum, e.v. Meteor), and tall pea (P. ~ativum, c.v. Alaska). Cultivation was in John Innes No. 2 compost in clay pots (beans in 4-inch, and pea in 3-inch pots), kept in a heated glasshouse with supplementary illumination by mercury vapour lights. Certain experimental details are given in Table 1. Hormones were incorporated into hydrous lanolin at concentrations of 1.0 per cent w/w. Lanolin preparations containing both IAA and GAs contained both substances at 1.0 per cent. Equal quantities of the lanolin-hormone preparations were injected into gelatine halfcapsules, and these were used to cap the cut surface of the stem in decapitated plants. Decapitated plants not treated with either hormone were capped with half-capsules filled with plain hydrous lanolin. Lengths of axillary buds were determined at intervals of time (see graphs) to the nearest millimetre, using a pair of dividers. Results The experiments with bean and pea plants, both tall and dwarf varieties, yielded essentially similar results (Figs. 1--4). I n all cases, decapitation b r o u g h t about release of the ~xillary buds from correlative inhibition, and an apical application of I A A prevented or delayed bud

Auxin-Gibberellin Interaction in Apical Dominance

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80

dwarf bean

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Fig. 1. Axillary bud elongation growth in dwarf bean (Phaseolus vulgaris, c.v. Canadian Wonder) after decapitation and application of lanolin-hormone preparations to the upper end of the stem Fig. 2. Axillary bud elongation growth in runner bean (Phaseolus multi]lorus, c.v. Scarlet Emperor) Table 1. Certain experimental details by species and variety Plant material

Age at start of experiment

No. of plants per treatment

Point of decapitation

Measured buds

Hormone applications

Dwarf bean

14 days

20

3 cm above primary ]eavers

both buds of primary leaves

Tall (runner, or climbing) bean

15 days

21

3 cm above primary leaves

both buds of primary leaves

Dwarf and tall pea

14 days

25

immediately below 4th node

at nodes 1, 2 and 3

1st day and every 2nd day thereafter 1st day and every 2nd day thereafter 1st day and every 2nd day thereafter

growth. I n dwarf bean, I A A completely p r e v e n t e d o u t g r o w t h of the axillary buds over the n i n e days of the e x p e r i m e n t (Fig. 1), b u t i n tall b e a n (Fig. 2) a n d b o t h pea varieties (Figs. 3 a n d 4) I A A h a d r a t h e r less effect. 22

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318

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86 tall pea

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Fig. 3. Axillary bud elongation growth in dwarf pea (Pisum sativum, c.v. Meteor) Fig. 4. Axillary bud elongation growth in tall pea (Pisum sativum, c.v. Alaska) The addition of GA 3 to the lanolin-IAA preparation resulted in more rapid elongation of buds t h a n when only I A A was present (Figs. 1 ~[). The greater length of buds in I A A - k GAs-treated plants, compared with those in IAA-treated plants was, in tall bean and both pea varieties, due solely to a higher growth rate; the time elapsed from decapitation to the first signs of bud growth was similar in both IAA- and I A A + GA 3treated plants (Figs. 2 4). Thus, GA s did not shorten the period for which L&A inhibited bud growth. I n dwarf bean, however, the buds did not elongate at all over nine days when I A A alone was applied, so t h a t the buds of I A A 4- GAs-treated plants grew out earlier, as well as faster, t h a n buds in the IAA-treated plants (Fig. 1). I n the experiments with bean plants, the effect of adding kinetin along with I A A was also studied, for it has been reported t h a t apically applied kinetin can prolong the inhibitory effect of I A A on axillary bud growth in decapitated dwarf bean plants ( D A v i s , S~Tg and WAgv,mG,

9 10 days

Auxin-Gibberellin Interaction in Apical Dominance

319

1966). However, in neither tall bean (Fig. 2) nor dwarf bean (Fig. 1) did kinetin modify the effect of IAA over a period of seven and nine days respectively. Nevertheless, DAWES et al. (1966) found that it was not until about eight days after decapitation that kinetin could be seen to enhance the inhibitory effect of IAA, so that the results reported in this paper do not contradict those of DAWES et al. (1966). Axillary buds and shoots were harvested from the tall bean plants seven days after decapitation, and their dry weights determined in order to compare the effects of the treatments on bud elongation growth and dry matter accumulation (Table 2). I t can be seen that results for bud length and bud dry weight correlate, except that the dry weight of buds from the decapitated GAs + IAA-treated plants was only about twothirds of that of buds from plain lanolin-treated decapitated plants, whereas elongation growth was almost identical in the two treatments. This suggests that apically-applied GAa entered the axillary buds, for stimulatory effects of GA 3 on stem elongation at the expense of radial growth are commonly seen. Table 2. Dry weights and lengths o/ primary lea~ axiUary buds in runner bean ( Phaseolus multiflorus c.v. Scarlet Emperor) a/ter one week o/treatments

Treatment

Mean individual bud length (mm)

Mean individual bud dry wt. (mg)

Decapitated + plain lanolin Decapitated + IAA/GA a Decapitated -~ IAA Decapitated + IAA/kinetin Apex intact control

68.5 69.5 12.7 14.5 3.7

35.4 22.8 6.8 6.5 1.3

Discussion

The results reported above appear to contradict those obtained by JAcoBs and CASE (1965) and ScoTT et al. (1967), but are in general agreement with several earlier published findings (KATo, 1958; WICKSO~ and T H I ~ A ~ , 1958; NAKA~URA, 1965). I t is not only impossible to provide an explanation for these discrepancies on available evidence (see PHILLIPS, 1969), but also difficult to unequivocally identify inconsistencies in techniques used by different workers which could account for conflicting results on the effect of GA 3 upon IAA-induced inhibition of axillary buds. However, experiments reported in this paper were all conducted on plants growing in a soil-compost mix, whereas SCOTTetal. (1967) experimented with pea seedlings growing in unsterilized vermiculite, 22*

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except in one of their experiments where sterilized vermiculite-soilcompost was used. No details of the inorganic nutrient conditions adopted in the experiments of JACOBS and CASE (1965) are given in the paper, but it would appear that vermiculite was used rather than a soil mix. In the one experiment of SCOTTet al. (1967) which involved seedlings grown in vermiculite-soil-compost, a different pattern of response to IAA -5 GA 3 occurred than was seen in the other experiments using vermiculite alone. Whereas, with vermiculite, Alaska pea axillary bud growth was clearly reduced by IAA-5 GA 3 as compared with IAA alone (JAcoBS and CAs~, 1965; SCOTTet al., 1967), when soilcompost was added to the vermiculite the effect of GA a in enhancing the inhibitory activity of IAA was very much less obvious. Thus, SCOTT et al. (1967) record in their figure 4 that in the presence of soilcompost, GA a increased the inhibitory effect of IAA only very slightly, and that after six days from decapitation bud length was considerably greater in IAAq-GA3-treated plants than in the IAA-treated plants. This was in marked contrast to the prolonged inhibition by IAA-5 GA3 seen in their other experiments on plants grown in vermiculite, and was rather more similar to the present results and those of KATO (1958), WICKSON and T~MA~S (1958), and I%TAKAMUI%A(1965). Experimental conditions used in the work reported here (see Materials and Methods), undoubtedly differed from those of JACOBS and CASE (1965) and SCOTTet al. (1967) in respects other than the rooting medium (e.g. ScoTT et al. cultivated the pea seedlings under controlled conditions of 8 hour photoperiods at 24~ and a fight intensity of 2,000 ft-candles, and decapitated the seedlings midway along the sixth internode), but, in the light of the matters discussed above, it appears likely that differences in the rooting medium influence the effect of GA s on IAAinduced bud inhibition. Indeed, SCOTT et al. (1967) concluded that apical dominance in the Alaska pea seedling can be described in terms of compensatory growth between stem and buds, so that increased inhibition of buds when decapitated stems were treated withlAA -5 GA s would be due to nutrient-consuming enhanced growth in the hormone treated internode. Although the data of SCOTTet al. (1967) undoubtedly supports such a compensatory growth concept, and there is substantial evidence that nutrient availability can affect the degree of correlative inhibition of lateral buds (Pn-mLrrs, 1968; 1969), JAcoBs and CAs~ (1965) found in experiments essentially identical with those of SCOTT, JAOOBS and CAsE (1967), that there was no indication of a compensatory relationship between stem elongation and axillary bud growth. In fact, JACOBS and CAS~ (1965) concluded, " T h e r e is no reason to think that gibberellic acid is inhibiting side shoots indirectly by stimulating elongation of the main shoot".

Auxin-Gibberellin Interaction in Apical Dominance

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I t is obviously not possible with available information to clarify the relationship between auxin and gibberellin in apical dominance, but the results reported here for both tall and dwarf varieties of bean and pea are in themselves unambiguous in demonstrating antagonism between the two classes of plant growth hormones. Enhancement of auxin-induced inhibition of lateral bud growth in decapitated Alaska pea seedlings growing in sterilized vermiculite has received two interpretations. JACOBS and CAs~ (1965) found higher levels of IAA-14C present in the stem when GA 3 was also supplied t h a n when IAA-laC alone was applied to the stem stump, and concluded t h a t GA3 had an "auxin-sparing" effect which caused more IAA to be present and effective far from the site of its application. SCOTTet al. (1967), on the other hand, considered t h a t local growth promoting effects of auxin and gibberellin upon the decapitated stem were the cause of axillary bud inhibition (however, see previous paragraph). I n summary, it appears from the experiments reported in this paper t h a t apically-applied GA 3 reduces the inhibitory effect of auxin on axillary bud growth in decapitated plants growing under conditions of adequate mineral nutrition. Gibberellin treatment of intact plants, however, does normally result in increased apical dominance (LoNA and Boccn~, 1956; BnlAN, HV,MML~G and Low~, 1959; STODDAI~T, 1959; B~ADLEY and C~ANv,, 1960; B~uI~SMA and PAT~, 1963; NA~AMU~A, 1965), but the physiological basis of this is not known. However, effects of exogenous GA 8 on auxin synthesis and/or inactivation mechanisms have been reported frequently (HYAsY~ and MU~AKAML 1953 ; P:[LV,T and WVnGL~, 1958; PHILLIPS, Vr,ITOS and CUTLER, 1959; BRIA~, 1959; GAr,STO~ and McCu~v,, 1961 ; KVnAIS~I and Muir, 1962, 1963, 1964a, b; H~L~v~Y, 1963; S~srnY and Myra, 1965), and it is possible t h a t gibberellins influence apical dominance in intact plants indirectly through such effects on auxin metabolism.

References :BRADLEY,V., and J. C. C~A~-E: Gibberellin induced inhibition of bud development in some species of Prunus. Science 131, 825--826 (1960). B~IA~, P. W. : Effects of gibberellins on plant growth and development. Biol. Rev. ~4, 37--84 (1959). - - H. G. H~,~I~r and D. LOWE: The effect of gibberellic acid on shoot growth of cupid sweet peas. Physiol. Plant. 12, 15--29 (1959). - - - - and M. I~ADLEY: A physiological comparison of gibberellic acid with some auxins. Physiol. Plant. 8, 899--912 (1955). Bn~I~SMA, J., and S. S. PATIL: The effects of 3-indoleacetic acid, gibberellic acid and vitamin E on flower initiation in unvernalized Petkus winter rye plants. :Naturwissenschaften 50, 505 (1963).

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DAWES, C. R., A. K. SETh, and P. F. WAREING: Auxin and kinetin interaction in apical dominance. Science 151, 468--469 (1966). GALSTON, A.W., and D. C. MoCuxE: An analysis of gibbercllin-auxin interaction and its possible metabolic basis. I n : Plant growth regulation, p. 611--625 (R. M. KLEIn, ed.). Iowa State University Press 1961. H ~ E w r , A . H . : Interaction of growth-retarding compounds and gibberellin on indoleaectic acid oxidase and peroxidase of cucumber seedlings. Plant'Physiol. 38, 731--737 (1963). HYASHI, T., and Y. MVRAXA•I: The biochemistry of the bakanai fungus. The physiological action of gibberellin. The effect of gibbercllin on the straight growth of etiolated pea epicotyl sections. J. Agr. Chem. Soc. Japan 27, 675 (1953). JACOBS, W. P. : Longevity of plant organs: Internal factors controlling abscission. Ann. Rev. Plant Physiol. 13, 403--436 (1962). - - , and D. B. CAs~.: Auxin transport, gibberellin and apical dominance. Science 148, 1729--1731 (1965). Jo~Es, R.L., and I . D . J . Pm~LLIPS: Organs of gibberellin synthesis in lightgrown sunflower plants. Plant Physiol. 41, 1381--1386 (1966). K~TO, J. : Studies on the physiological effect of gibberellin: I. On the differential activity between gibberellin and auxin. Mem. Coll. Sci. Kyoto, Ser. B 20, 189--193 (1953). - - Studies on the physiological effect of gibberellin: II. On the interaction of gibberellin with auxins and growth inhibitors. Physiol. Plant. 11, 10--15 (1958) Kv~s~, S , and R. M. M~-I~: Increase in diffusible auxin after treatment with gibberellin. Science 137, 760--761 (1962). - - Paper chromatographic study of diffusible auxin. Plant Physiol. 89, 23--28 (1963). - - - - The relationship of gibberellin and auxin in plant growth. Plant and Cell Physiol. 5, 61--68 (1964a). - - - - The mechanism of gibberellin action in the dwarf pea. Plant and Cell Physiol. 5, 259--271 (1964b). Lo~A, F., e A. BoccHI: Caratteristiche d'acerescimento e sviluppo di aleune rasse invernale di cereali tratti con acido gibberellico. Riv. intern. Agr. 6, 44--47 (1956). NAKAMVRA, E. : Studies on the branching in Pisum sativum L. Special Report of the Laboratory of Horticulture, Shiga Agrie. Coll., Japan (1965). PHILLIPS, I. D. J.: Nitrogen, phosphorus and potassium distribution in relation to apical dominance in dwarf bean (Phaseolus vulgaris c.v. Canadian Wonder). J. exp. Bot. 19, 617--627 (1968). - - A p i c a l Dominance. I n : The physiology of plant growth and development, p. 163--202 (M. B. W~KI~S, ed.). London: McGraw-Hill 1969. - - A. J. VLITOS, and H. CUTLE~: The influence of gibberellic acid upon the endogenous growth substances of the Alaska pea. Contr. Boyce Thompson Inst. 20, 111--120 (1959). PILET, P . E . , and W. WVRGT.ER: Action des gibberellines sur la eroissance du Tri/olium ochroleucum ttUDSO~T. Bull. Soc. bot. Suisse 68, 54--63 (1958). SAST~u K. S. K., and R. M. MUIR: Effects of gibberellic acid on utilization of auxin precursors by apical segments of the Avena coleoptile. Plant Physiol. 40, 294--298 (1965). SCOTT, T . K . , D . B . CASE, and W. P. JACOBS: Auxin-gibberellin interaction in apical dominance. Plant Physiol. 42, 1329--1333 (1967). -

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STODDART,J. L.: The effects of gibberellic acid on growth habit and heading in late-flowering red clover (Tri/olium l)ratenseL.). J. Agr. Sci. 52, 161--167 (1959). Th~MA~N, K. V., and F. SKOOG: Studies on the growth hormone of plants. III. The inhibitory action of the growth substance oll bud development. Proc. nat. Aead. Sci. (Wash.) 19, 714--716 (1933). - - - - On the inhibition of bud development and other functions of growth substance in Vicia /aba. Proc. roy. Soc. B 114, 317--339 (1934). WIcxso~, M. E., and K. V. THIMA~N: The antagonism of auxin and kinetin in apical dominance. Physiol. Plant. 11, 62--74 (1958). I. D. J. PmLL~S Forestry Commission Research Branch Government Buildings, Bankhead Avenue Edinburgh 11, Scotland