Branching in Pea (Action of Genes Rms3 and Rms4)

The nonallelic ramosus mutations rms3-2 and rms4 of pea (Pisum sativum L.) cause extensive release of vegetative axillary buds and lateral growth in comparison with wild-type (cv Torsdag) plants, in which axillary buds are not normally released under the conditions utilized. Grafting studies showed that the expression of the rms4 mutation in the shoot is independent of the genotype of the root-stock. In contrast, the length of the branches at certain nodes of rms3-2 plants was reduced by grafting to wild-type stocks, indicating that the wild-type Rms3 gene may control the level of a mobile substance produced in the root. This substance also appears to be produced in the shoot because Rms3 shoots did not branch when grafted to mutant rms3-2 rootstocks. However, the end product of the Rms3 gene appears to differ from that of the Rms2 gene (C.A. Beveridge, J.J. Ross, and I.C. Murfet [1994] Plant Physiol 104: 953-959) because reciprocal grafts between rms3-2 and rms2 seedlings produced mature shoots with apical dominance similar to that of rms3-2 and rms2 shoots grafted to wild-type stocks. Indole-3-acetic acid levels were not reduced in apical or nodal portions of rms4 plants and were actually elevated (up to 2-fold) in rms3-2 plants. It is suggested that further studies with these branching mutants may enable significant progress in understanding the normal control of apical dominance and the related communication between the root and shoot.     

bushy, a dominant pea mutant characterised by short, thin stems, tiny leaves and a major reduction in apical dominance

The spontaneous, single-gene dominant, pea ( Pisum sativum L.) mutant bushy is characterised by short, thin stems, tiny leaves and a proliferation of basal lateral branches. We symbolised the dominant mutant allele bsh and the recessive wild-type allele BSH . Some effects were very large, e.g. the reduction in internode length was around 10-fold in pure mutant plants. The effect on branching was qualitative under our conditions as the wild-type did not branch and the mutant branched extensively. Analysis of epidermal cells indicated the reduction in internode length arose principally from a reduction in cell length. The bushy mutation also altered root morphology with a reduction in the number and length of lateral roots. Time to first open flower was increased but node of flower initiation was not affected. In a few cases, bushy plants died before producing an open flower even though tiny abortive flower buds were produced in the upper leaf axils. In pure mutant plants, individual seed weight was reduced by 30%, number of seeds per pod was reduced 3-fold, and seed number per plant was reduced 4-fold. However, pod size was essentially normal for a given seed content, and the flowers were fertile and of normal structure. Grafting studies showed the primary action of the bushy mutation occurred in the shoot. In summary, the reduced cell and shoot elongation, loss of apical dominance and a primary action in the shoot, all point toward auxin deficiency (or perceived deficiency) as a possible cause of the bushy phenotype. The overall characteristics of bushy make it a useful mutant for research on plant development.     

Long-distance signaling and the control of branching in the rms1 mutant of pea

The ramosus (rms) mutation (rms1) of pea (Pisum sativum) causes increased branching through modification of graft-transmissible signal(s) produced in rootstock and shoot. Additional grafting techniques have led us to propose that the novel signal regulated by Rms1 moves acropetally in shoots and acts as a branching inhibitor. Epicotyl interstock grafts showed that wild-type (WT) epicotyls grafted between rms1 scions and rootstocks can revert mutant scions to a WT non-branching phenotype. Mutant scions grafted together with mutant and WT rootstocks did not branch despite a contiguous mutant root-shoot system. The primary action of Rms1 is, therefore, unlikely to be to block transport of a branching stimulus from root to shoot. Rather, Rms1 may influence a long-distance signal that functions, directly or indirectly, as a branching inhibitor. It can be deduced that this signal moves acropetally in shoots because WT rootstocks inhibit branching in rms1 shoots, and although WT scions do not branch when grafted to mutant rootstocks, they do not inhibit branching in rms1 cotyledonary shoots growing from the same rootstocks. The acropetal direction of transport of the Rms1 signal supports previous evidence that the rms1 lesion is not in an auxin biosynthesis or transport pathway. The different branching phenotypes of WT and rms1 shoots growing from the same rms1 rootstock provides further evidence that the shoot has a major role in the regulation of branching and, moreover, that root-exported cytokinin is not the only graft-transmissible signal regulating branching in intact pea plants.     

Stimulatory effect of cytokinins and interaction with IAA on the release of lateral buds of pea plants from apical dominance

Lateral buds of pea plants can be released from apical dominance and even be transformed into dominant shoots when repeatedly treated with synthetic exogenous cytokinins (CKs). The mechanism of the effect of CKs, however, is not clear. The results in this work showed that the stimulatory effects of CKs on the growth of lateral buds and the increase in their fresh weights in pea plants depended on the structure and concentration of the CKs used. The effect of N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) was stronger than that of 6-benzylaminopurine (6-BA). Indoleacetic acid (IAA) concentration in shoot, IAA export out of the treated apex and basipetal transport in stems were markedly increased after the application of CPPU or 6-BA to the apex or the second node of pea plant. This increase was positively correlated with the increased concentration of the applied CKs. These results suggest that the increased IAA synthesis and export induced by CKs application might be responsible for the growth of lateral shoots in intact pea plants.     

The shoot controls zeatin riboside export from pea roots. Evidence from the branching mutant rms4

The rms4 mutant of pea ( Pisum sativum L.) was used in grafting studies and cytokinin analyses of the root xylem sap to provide evidence that, at least for pea, the shoot can modify the import of cytokinins from the root. The rms4 mutation, which confers a phenotype with increased branching in the shoot, causes a very substantial decrease (down to 40-fold less) in the concentration of zeatin riboside (ZR) in the xylem sap of the roots. Results from grafts between wild-type (WT) and rms4 plants indicate that the concentration of cytokinins in the xylem sap of the roots is determined almost entirely by the genotype of the shoot. WT scions normalize the cytokinin concentration in the sap of rms4 mutant roots, whereas mutant scions cause WT roots to behave like those of self-grafted mutant plants. The mechanism whereby rms4 shoots of pea cause a down-regulation in the export of cytokinins from the roots is unknown at this time. However, our data provide evidence that the shoot transmits a signal to the roots and thereby controls processes involved in the regulation of cytokinin biosynthesis in the root.     

Effect of apex excision and replacement by 1-naphthylacetic acid on cytokinin concentration and apical dominance in pea plants

As known from literature lateral buds from pea ( Pisum sativum ) plants are released from apical dominance when repeatedly treated with exogenous cytokinins. Little is known, however, about the endogenous role of cytokinins in this process and whether they interact with basipolar transported IAA, generally regarded as the main signal controlling apical dominance. This paper presents evidence that such an interaction exists.
The excision of the apex of pea plants resulted in the release of inhibited lateral buds from apical dominance (AD). This could be entirely prevented by applying 1-naphthylacetic acid (NAA) to the cut end of the shoot. Removal of the apex also resulted in a rapid and rather large increase in the endogenous concentrations of zeatin riboside (ZR), isopentenyladenosine (iAdo) and an as yet unidentified polar zeatin derivative in the node and internode below the point of decapitation. This accumulation of ZR and iAdo, was strongly reduced by the application of NAA. The observed increase in cytokinin concentration preceded the elongation of the lateral buds, suggesting that endogenous cytokinins play a significant role in the release of lateral buds from AD. However, the effect of NAA on the concentration of cytokinins clearly demonstrated the dominant role of the polar basipetally transported auxin in AD. The results suggest a mutual interaction between the basipolar IAA transport system and cytokinins obviously produced in the roots and transported via the xylem into the stem of the pea plants.     

Hormone physiology of pea mutants prevented from flowering by mutations gi or veg1

The veg1 ( vegetative ) mutant in pea ( Pisum sativum L.) does not flower under any circumstances and gi ( gigas ) mutants remain vegetative under certain conditions. gi plants are deficient in production of floral stimulus, whereas veg1 plants lack a response to floral stimulus. During long days in particular, these non-flowering mutant plants eventually enter a stable compact phase characterised by a large reduction in internode length, small leaves and growth of lateral shoots from the upper-stem (aerial) nodes. The first-order laterals in turn produce second-order laterals and so on in a reiterative pattern. The apical bud is reduced in size but continues active growth. Endogenous hormone measurements and gibberellin application studies with gi-1 , gi-2 and veg1 plants indicate that a reduction in gibberellin and perhaps indole-3-acetic acid level may account, at least partially, for the compact aerial shoot phenotype. In the gi-1 mutant, the compact phenotype is rescued by transfer from a 24- to an 8-h photoperiod. We propose that in plants where flowering is prevented by a lack of floral stimulus or an inability to respond, the large reduction in photoperiod gene activity during long days may lead to a reduction in apical sink strength that is manifest in an altered hormone profile and weak apical dominance.     

The Effect of Chilling of the Physiological and Simulated Apex of 2- and 3-leaf Plants of Pisum sativum L. cv. Alaska on Lateral Shoot Growth, C-14 IAA Movement and P-32 Accumulation

The apex of a 3-leaf pea plant was chilled in cold chambers maintained at 5–7°C. The lateral shoots 1 through 5 grew, and shoot 5 eventually dominated other lateral shoots. The apex when returned to the ambient temperature did not reimpose apical dominance. The growing lateral shoots competed with the stem apex. The apices of 2- and 3-leaf plants were chilled and P-32 distribution in these plants was studied in the entire plant, at various intervals of time. Phosphorus-32 accumulation followed the growth pattern of the plant. The lateral shoots accumulated P-32 activity and very little activity was accumulated by the apex. The dominating shoots 2 and 5 accumulated the maximum amount of activity in 2- and 3-leaf plants, respectively. Labeled-IAA moved basipetally through the stem when applied to the cut stump simulating the apex. By cold treatment the translocation of IAA was influenced more than its absorption. The plant seems to metabolize this compound in the later periods of application. The plant now becomes “insensitive” to auxin and the lateral shoots grow.     

Effects of the auxin-inhibiting substances raphanusanin and benzoxazolinone on apical dominance of pea seedlings

The effects of the auxin-inhibiting substances raphanusanin ((3R*,6S*)-3-[methoxy (methylthio) methyl]-2-pyrrolidinethione, raphanusanin B)and benzoxazolinone (6-methoxy-2-bezoxazolinone, MBOA) on apical dominance of pea(Pisum sativum L. cv. Alaska) seedlings were studied.Application of raphanusanin B or MBOA to the apical bud, internode, or lateralbud of pea seedlings released apical dominance in either intact orindole-3-acetic acid (IAA )-treated, decapitated plants. These results suggestthat the auxin-inhibiting substances raphanusanin B and MBOA have activity inreleasing apical dominance. Conversely, the auxin transport inhibitors2,3,4-triiodobenzoic acid (TIBA) and 1-naphthylphthalamic acid (NPA) did notstimulate lateral bud growth when they were applied directly to the lateralbud,although application to the apical bud or internode released apical dominance.Therefore, the mode of action of raphanusanin B and MBOA in apical dominance isclearly different from that of auxin transport inhibitors. Raphanusanin B andMBOA may suppress the synthesis of growth-inhibiting factor(s) of the lateralbud induced by endogenous auxin transported from the apical bud or exogenouslyapplied auxin, and/or the action of the factor(s).     

Micrografting techniques for testing long-distance signalling in Arabidopsis

Grafting in species other than Arabidopsis has generated persuasive evidence for long-distance signals involved in many plant processes, including regulation of flowering time and shoot branching. Hitherto, such approaches in Arabidopsis have been hampered by the lack of suitable grafting techniques. Here, a range of micrografting methods for young Arabidopsis seedlings are described. The simplest configuration was a single-hypocotyl graft, constructed with or without a supporting collar, allowing tests of root-shoot communication. More complex two-shoot grafts were also constructed, enabling tests of shoot-shoot communication. Integrity of grafts and absence of adventitious roots on scions were assessed using plants constitutively expressing a GUS gene as one graft partner. Using the max1 (more axillary growth) and max3 increased branching mutants, it was shown that a wild-type (WT) rootstock was able to inhibit rosette branching of mutant shoots. In two-shoot grafts with max1 and WT shoots on a max1 rootstock, the mutant shoot branched profusely, but the WT one did not. In two-shoot grafts with max1 and WT shoots on a WT rootstock, neither shoot exhibited increased branching. The results mirror those previously demonstrated in equivalent grafting experiments with the ramosus mutants in pea, and are consistent with the concept that a branching signal is capable of moving from root to shoot, but not from shoot to shoot. These grafting procedures will be valuable for revealing genes associated with many other long-distance signalling pathways, including flowering, systemic resistance and abiotic stress responses.     

Control of Internode Length in Pisum sativum (Further Evidence for the Involvement of Indole-3-Acetic Acid)

The effects of altered endogenous indole-3-acetic (IAA) levels on elongation in garden pea (Pisum sativum L.) plants were investigated. The auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) and 9-hydroxyfluorene-9-carboxylic acid (HFCA) were applied to elongating internodes of wild-type and mutant lkb plants. The lkb mutant was included because elongating lkb internodes contained 2- to 3-fold less free IAA than those of the wild type. In the wild type, TIBA reduced both the IAA level and internode elongation below the site of application. Both TIBA and HFCA strongly promoted the elongation of lkb internodes and also raised IAA levels above the application site. The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) also markedly increased internode elongation in lkb plants and virtually restored petioles and tendrils to their wild-type length. In contrast, treatment of wild-type plants with TIBA, HFCA, or 2,4-D caused little or no increase in elongation above the application site. The ethylene synthesis inhibitor aminoethoxyvinylglycine also increased stem elongation in lkb plants, and combined application of HFCA and aminoethoxy-vinylglycine restored lkb internodes to the wild-type length. It is concluded that the level of IAA in wild-type internodes is necessary for normal elongation, and that the reduced stature of lkb plants is at least partially attributable to a reduction in free IAA level in this mutant.     

Involvement of auxin and CKs in boron deficiency induced changes in apical dominance of pea plants (Pisum sativum L.)

It has previously been shown that boron (B) deficiency inhibits growth of the plant apex, which consequently results in a relatively weak apical dominance, and a subsequent sprouting of lateral buds. Auxin and cytokinins (CKs) are the two most important phytohormones involved in the regulation of apical dominance. In this study, the possible involvement of these two hormones in B-deficiency-induced changes in apical dominance was investigated by applying B or the synthetic CK CPPU to the shoot apex of pea plants grown in nutrient solution without B supply. Export of IAA out of the shoot apex, as well as the level of IAA, Z/ZR and isopentenyl-adenine/isopentenyl-adenosine (i-Ade/i-Ado) in the shoot apex were assayed. In addition, polar IAA transport capacity was measured in two internodes of different ages using 3H-IAA. In B-deficient plants, both the level of auxin and CKs were reduced, and the export of auxin from the shoot apex was considerably decreased relative to plants well supplied with B. Application of B to the shoot apex restored the endogenous Z/ZR and IAA level to control levels and increased the export of IAA from the shoot apex, as well as the 3H-IAA transport capacity in the newly developed internodes. Further, B application to the shoot apex inhibited lateral bud growth and stimulated lateral root formation, presumably by stimulated polar IAA transport. Applying CPPU to the shoot apex, a treatment that stimulates IAA export under adequate B supply, considerably reduced the endogenous Z/ZR concentration in the shoot apex, but had no stimulatory effect on IAA concentration and transport in B-deficient plants. A similar situation appeared to exist in lateral buds of B-deficient plants as, in contrast to plants well supplied with B, application of CKs to these plants did not stimulate lateral bud growth. In contrast to the changes of Z/ZR levels in the shoot apex, which occurred after application of B or CPPU, the levels of i-Ade/i-Ado stayed more or less constant. These results suggest that there is a complex interaction between B supply and plant hormones, with a B-deficiency-induced inhibition of IAA export from the shoot apex as one of the earliest measurable events.     

AXR1 acts after lateral bud formation to inhibit lateral bud growth in Arabidopsis

The AXR1 gene of Arabidopsis is required for many auxin responses. The highly branched shoot phenotype of mature axr1 mutant plants has been taken as genetic evidence for a role of auxin in the control of shoot branching. We compared the development of lateral shoots in wild-type Columbia and axr1-12 plants. In the wild type, the pattern of lateral shoot development depends on the developmental stage of the plant. During prolonged vegetative growth, axillary shoots arise and develop in a basal-apical sequence. After floral transition, axillary shoots arise rapidly along the primary shoot axis and grow out to form lateral inflorescences in an apical-basal sequence. For both patterns, the axr1 mutation does not affect the timing of axillary meristem formation; however, subsequent lateral shoot development proceeds more rapidly in axr1 plants. The outgrowth of lateral inflorescences from excised cauline nodes of wild-type plants is inhibited by apical auxin. axr1-12 nodes are resistant to this inhibition. These results provide evidence for common control of axillary growth in both patterns, and suggest a role for auxin during the late stages of axillary shoot development following the formation of the axillary bud and several axillary leaf primordia.     

Pea rms6 mutants exhibit increased basal branching

Our studies on two branching mutants of pea ( Pisum sativum L.) have identified a further Ramosus locus , Rms6, with two recessive or partially recessive mutant alleles: rms6-1 (type line S2-271) and rms6-2 (type line K586). Mutants rms6-1 and rms6-2 were derived from dwarf and tall cultivars, Solara and Torsdag, respectively. The rms6 mutants are characterized by increased branching from basal nodes. In contrast, mutants rms1 through rms5 have increased branching from both basal and aerial (upper stem) nodes. Buds at the cotyledonary node of wild-type (WT) plants remain dormant but in rms6 plants these buds were usually released from dormancy. Their growth was either subsequently inhibited, sometimes even prior to emergence above ground, or they grew into secondary stems. The mutant phenotype was strongest for rms6-1 on the dwarf background. Although rms6-2 had a weak single-mutant phenotype, the rms3-1 rms6-2 double mutant showed clear transgression and an additive branching phenotype, with a total lateral length almost 2-fold greater than rms3-1 and nearly 5-fold greater than rms6-2 . Grafting studies between WT and rms6-1 plants demonstrated the primary action of Rms6 may be confined to the shoot. Young WT and rms6-1 shoots had similar auxin levels, and decapitated plants had a similar magnitude of response to applied auxin. Abscisic acid levels were elevated 2-fold at node 2 of young rms6-1 plants. The Rms6 locus mapped to the R to Gp segment of linkage group V (chromosome 3). The rms6 mutants will be useful for basic research and also have possible agronomical value.     

Growth patterns and endogenous indole-3-acetic acid concentrations in current-year coppice shoots and seedlings of two Betula species

Apical dominance, internode elongation, radial growth and xylem cell size in coppice and apical shoots of Betula pubescens B. Pendula were determined and related to endogenous indole-3-acetic acid (IAA) levels, measured by gas chromatography-selected ion monitoring in the apical bud and at three positions along the stem. The effects of defoliation and debudding on morphological and anatomical characters and endogenous IAA levels were also investigated. The coppice shoots displayed superior stem elongation and increased branching during the initial phase of growth, after which their growth pattern was similar to that of the seedlings; however, their radial growth was greater throughout the experiment. Both plant types produced smaller-sized xylem cells at the top of the shoot than at the bottom with coppice shoots tending to form larger tracheids and smaller vessels than the seedlings. There was no consistent difference in IAA concentration between the coppice shoots and the seedlings. Defoliation and debudding reduced the IAA level in the stem within 36 h and it was still low after 25 days. Although the extent of the IAA decrease was similar in both coppice shoots and seedlings, the treatments affected the morphological and anatomical characters differently in the two plant types. The results suggest that the observed differences between seedlings and coppice shoots were not mediated through a drastic change in IAA level.     

Feedback regulation of xylem cytokinin content is conserved in pea and Arabidopsis

Increased-branching mutants of garden pea (Pisum sativum; ramosus [rms]) and Arabidopsis (Arabidopsis thaliana; more axillary branches) were used to investigate control of cytokinin export from roots in relation to shoot branching. In particular, we tested the hypothesis that regulation of xylem sap cytokinin is dependent on a long-distance feedback signal moving from shoot to root. With the exception of rms2, branching mutants from both species had greatly reduced amounts of the major cytokinins zeatin riboside, zeatin, and isopentenyl adenosine in xylem sap compared with wild-type plants. Reciprocally grafted mutant and wild-type Arabidopsis plants gave similar results to those observed previously in pea, with xylem sap cytokinin down-regulated in all graft combinations possessing branched shoots, regardless of root genotype. This long-distance feedback mechanism thus appears to be conserved between pea and Arabidopsis. Experiments with grafted pea plants bearing two shoots of the same or different genotype revealed that regulation of root cytokinin export is probably mediated by an inhibitory signal. Moreover, the signaling mechanism appears independent of the number of growing axillary shoots because a suppressed axillary meristem mutation that prevents axillary meristem development at most nodes did not abolish long-distance regulation of root cytokinin export in rms4 plants. Based on double mutant and grafting experiments, we conclude that RMS2 is essential for long-distance feedback regulation of cytokinin export from roots. Finally, the startling disconnection between cytokinin content of xylem sap and shoot tissues of various rms mutants indicates that shoots possess powerful homeostatic mechanisms for regulation of cytokinin levels.     

A reappraisal of the role of abscisic acid and its interaction with auxin in apical dominance

BACKGROUND AND AIMS: Evidence from pea rms1, Arabidopsis max4 and petunia dad1 mutant studies suggest an unidentified carotenoid-derived/plastid-produced branching inhibitor which moves acropetally from the roots to the shoots and interacts with auxin in the control of apical dominance. Since the plant hormone, abscisic acid (ABA), known to inhibit some growth processes, is also carotenoid derived/plastid produced, and because there has been indirect evidence for its involvement with branching, a re-examination of the role of ABA in apical dominance is timely. Even though it has been determined that ABA probably is not the second messenger for auxin in apical dominance and is not the above-mentioned unidentified branching inhibitor, the similarity of their derivation suggests possible relationships and/or interactions. METHODS: The classic Thimann-Skoog auxin replacement test for apical dominance with auxin [0.5 % naphthalene acetic acid (NAA)] applied both apically and basally was combined in similar treatments with 1 % ABA in Ipomoea nil (Japanese Morning Glory), Solanum lycopersicum (Better Boy tomato) and Helianthus annuus (Mammoth Grey-striped Sunflower). KEY RESULTS: Auxin, apically applied to the cut stem surface of decapitated shoots, strongly restored apical dominance in all three species, whereas the similar treatment with ABA did not. However, when ABA was applied basally, i.e. below the lateral bud of interest, there was a significant moderate repression of its outgrowth in Ipomoea and Solanum. There was also some additive repression when apical auxin and basal ABA treatments were combined in Ipomoea. CONCLUSION: The finding that basally applied ABA is able partially to restore apical dominance via acropetal transport up the shoot suggests possible interactions between ABA, auxin and the unidentified carotenoid-derived branching inhibitor that justify further investigation.     

Nauclea diderrichii: rooting of stem cuttings,clonal variation in shoot dominance,and branch plagiotropism

Summary Nauclea diderrichii (De Wild, and Th. Dur.) Merill (Rubiaceae), an indigenous hardwood of West Africa, is increasingly being grown commercially. This study investigates the potential for vegetative propagation and clonal selection, and raises some fundamental questions about the physiology of apical dominance and of plagiotropism. Rooting ability was high, with up to 100% rooting in 2–4 weeks, when different Indole-3-butyric acid (IBA) concentrations and leaf areas were tested. Auxin applications greatly increased the numbers of roots per cutting. The decapitation of unbranched plants revealed clonal variation in apical dominance and also in the establishment of outright dominance by the two shoots formed from the outgrowth of the axillary buds of the opposite leaves at the top node. Regression analysis of the Dominance Ratio (length of dominant: length of the sub-dominant shoot at the time of achieving dominance) against overall lateral bud activity (r = 0.82), showed that when the two top shoots co-dominate they provide a more powerful source of Correlative Inhibition than when one of the top shoots dominates the other. The imposition of plagiotropism in the axillary bud occurred over a period of a few days as the terminal and axillary buds emerged from the stipule. Growth of accessory buds on intact plants and debranched cuttings was orthotropic. These results are discussed with regard to the role of the leaf in root formation and the understanding of dominance relationships, branching and crown development in trees.     

Axillary Bud Formation in Two Isogenic Lines of Tomato Showing Different Degrees of Apical Dominance

Grafting experiments have been carried out in which rootstocksof the cultivar Craigella were paired with scions of an isogenicline Craigella Lateral Suppressor (ls ls) and vice versa, andthe levels of hormones in the roots and shoots of the graftedplants examined. The roots of Craigella plants differed from those of LateralSuppressor in that they contained a higher proportion of a cytokininthat co-chromatographed with N⁶ - (²—isopentenyl) adenosine.Reciprocal grafts did not lead to any qualitative or quantitativechanges in the cytokinins in the roots of either line. GraftingLateral Suppressor scions on Craigella rootstocks led to anincrease in the IAA content of the apical region and the ABAcontent of the stem tissue immediately below it, but when Craigellascions were grafted on Lateral Suppressor rootstocks there wereno changes in the level of either hormone. Cytokinins applied to the leaf axils of Lateral Suppressor plantsresulted in lateral bud initiation in the axils above the pointof treatment but not if the plants were also given a short periodof far-red light at the end of the photoperiod. Cytokinins wereineffective in initiating lateral buds in grafted Lateral Suppressorscions. It is suggested that root-produced cytokinins influence lateralbud outgrowth indirectly by way of their effect on the levelsof IAA and ABA in the shoot. Lycopersicon esculentum Mill., tomato, apical dominance, growth regulation, indol-3yl acetic acid, abscisic acid, cytokinins     

Differential Responses of Buds along the Shoot to Factors Involved in Apical Dominance

Intact and decapitated 6-node shoots of Hygrophila sp. weregrown aseptically immersed in liquid half-strength Knop's solutionwith microelements and 2% (w/v) sucrose (control medium), andin medium with 0.1 mg l^–1 benzyladenine (BA). In intactshoots grown in control medium apical dominance suppressed outgrowthof the lateral buds; in decapitated shoots buds grew out atseveral of the most apical nodes, increasing in size acropetally.There was a lag in outgrowth of the bud at the most apical node,attributable to its initially smaller size. Lateral shoots grewout first at basal nodes of intact shoots in BA medium, decreasingin size acropetally; in decapitated shoots in BA medium lateralshoots of approximately equal size grew out at all nodes. Differentialeffects of decapitation and cytokinin treatment on lateral shootoutgrowth along the shoot could be interpreted by postulatinga basipetally decreasing gradient of endogenous auxin concentrationin the intact shoot. Application of 20 mg l^–1 indoleaceticacid (IAA) in agar to decapitated shoots completely preventedbud outgrowth for at least 7 d in control medium, inhibitingit thereafter, and inhibited bud outgrowth in BA medium, thussupporting the hypothesis. Comparison of lateral shoot outgrowthin whole decapitated shoots and severed decapitated shoots (isolatednodes) lent no support to the alternative hypothesis that theremight be an acropetally decreasing concentration gradient ofa bud-promoting substance in the intact shoot, and demonstratedmuch greater lateral shoot growth in isolated nodes. The resultsemphasize important correlative relationships between the partsof a shoot with several nodes.