Gravistimulus Direction, Ethylene Production and Shoot Elongation in the Release of Apical Dominance in Pharbitis nil

Prasad, T. K. and Cline, M. G. 1985. Gravistimulus direction,ethylene production and shoot elongation in the release of apicaldominance in Pharbitis nil.—. exp. Bot. 36: 1969–1975.Release of apical dominance can be induced in Pharbitis nilby the inversion of the upper shoot. This promotion of outgrowthof the highest lateral bud adjacent to the bend of the stemappears to be mediated by ethylene inhibition of growth of theinverted main shoot. In the present investigation the existenceof a direct correlation between ethylene evolution and the directionof gravistimulus is demonstrated as well as an inverse correlationbetween ethylene production by the inverted upper shoot andits elongation. An inverse correlation also exists between elongationof the inverted upper shoot and the outgrowth of the highestlateral bud if the lower portion of the shoot (below the bend)is oriented in an upright position. The latent period for shoot–inversioninduction of ethylene production is about 2 h. These resultssupport the hypothesis of indirect ethylene control of apicaldominance release by retardation of elongation of the invertedshoot. Key words: Shoot inversion, gravistimulus, ethylene, latent period, bud outgrowth, pharbitis nil     

Control of Lateral Bud Growth inPhaseolus vulgarisL. by Ethylene in the Apical Shoot

Elongation growth of the first trifoliate leaf axillary budwas induced by physical restriction of the apical growth orby treating the apical shoot with ethylene or ethephon. Thereis evidence to suggest that the promotion of axillary bud developmentby ethylene action on the apical shoot was associated with theavailability of freely diffusible ethylene in the tissues ofthe treated shoot. Loss of apical dominance was not, apparently,directly dependent on either the internal ethylene concentration(i.e. concentration in the vacuum – extracted gases) oron the rate of ethylene emanation. The effects of aminoethoxyvinylglycine (AVG) and silver nitrate on ethylene production wereexamined. Although treatment of the shoot with tri – iodobenzoicacid (TIBA) induced various morphological responses in the plant,including axillary bud outgrowth, which appeared similar tothe responses to ethylene, the initial effect of TIBA is notthought to be ethylene – mediated.     

乙烯在豌豆植株顶端优势中的作用

用人工合成细胞分裂素BAP和CPPU处理豌豆植株叶腋可诱导处理部位侧芽的生长,同时伴有大量乙烯产生;用乙烯合成抑制剂AVG处理或植株去顶同样可导致创芽生长,但乙烯释放量却明显少于对照,表明侧芽的生长与乙烯释放量的多少无关。而3种物质处理后诱导产生的侧芽的数目、长度及其鲜重与处理部位内源IAA含量的增加则呈正相关。     

Correlative Inhibition of Lateral Bud Growth in Phaseolus vulgaris L. Ethylene and the Physical Restriction of Apical Growth

Restriction of apical growth in Phaseolus by enclosing the upperpart of the shoot in sealed or ventilated tubes induced developmentof axillary buds beneath the enclosure. Enclosed parts of shootsshowed a reduction of leaf growth and, in experiments wherethe tubes were sealed, of internode extension. Enclosure ofthe shoots in large vessels that did not restrict leaf expansion,but which contained 0?5 vols 10^–6 ethylene, similarlyinduced axillary bud growth. Analysis of the gaseous extractof physically restricted shoots showed a 2?5-fold increase inethylene concentration. The results suggest involvement of ethylenein the release of correlative inhibition brought about by physicalrestriction of apical growth.     

Gravimorphism in Trees: 1. Effects of Gravity on Growth and Apical Dominance in Fruit Trees

The effect of shoot-orientation on the vegetative growth andapical dominance of rootstocks of apply, cherry, plum, and blackcurrantwas investigated. In all the species studied the total annualextension growth of horizontally grown trees was markedly lessthan that of vertical trees, and the effect was accentuatedwhen the plants were rotated. In cherry and plum the reducedtotal growth was due both to a smaller number of internodesand also to reduced internode length; in apple and blackcurrantthe difference in extension growth was mainly due to differencesin internode length. In experiments with laternal shots trainedin various positions it was found that the apical dominancenormally exhibited by the uppermost shoot is only manifestedwhen it occupies a vertical position. The normal apical dominancerelationships can be reversed if the uppermost shoot is grownhorizontally and the second shoot vertically. Gravitationaleffects appear to play a role in the normal apical dominancerelationships of the tree. In all species there is a markedtendency for laterals to grow out on the upper side of horizontaland arched plants. The effects of training arched trees into various positionsare considered in relation to current concepts regarding therole of auxin in apical dominance and the redistribution ofauxin in horizontal organs; it is concluded that the outgrowthof laterals in arched shoots cannot be interpreted primarilyin terms of these current concepts. The observed results suggestthat the outgrowth of lateral buds occurs at the nearest pointto the roots at which the shoot is diverted from the verticalposition, and that nutrients are diverted to the highest upwardlydirected meristem. It is proposed that the term gravimorphismshould be applied to these diverse effects of gravity on plantgrowth.     

Apical Dominance in Marsilea, with Particular Reference to the Effects of 3-Indolylacetic Acid,3-Indolylacetonitrile, and Coumarin on Lateral Bud Development

An account is given of the effects of 3-indolylacetic acid,3-indolylaceto-nitrile, and coumarin on the development of thelateral buds of intact plants, decapitated plants, and excisednodes of Marsilea Drummondii in aseptic culture. In all the plant materials used, lateral buds grew out at allphysiological concentrations of the two auxins, an observationwhich is considered to refute the hypothesis that correlativeinhibition is caused by direct inhibition by auxi. Other hypothesesand recent work on the subject are also discussed.     

The Effect of the Apical Bud on the Growth of Lateral Buds on Subterranean Shoots

The influence of the apical bud on the growth of the lateral buds on subterranean shoots was studied in Stachys sieboldiiMig. and Helianthus rigidus(Gass.) Desv. Removing and damaging the apical parts of subterranean shoots or their treatment with 2% chlorocholine chloride shoot enhanced shoot branching. The response to light of the apical bud was invariably negative: the stolons, which came out or were extracted from the soil, grew back into the ground (negative phototropism). The response to light of lateral buds was autonomous and depended on the conditions of their initiation. The lateral buds developed in darkness manifested negative phototropism when withdrawn from the soil and exposed to the light, whereas the buds developed in the light showed positive phototropism. The author concludes that the concept of apical dominance, thoroughly studied in aboveground shoots, is also valid for subterranean shoots. However, in contrast to the former, in the latter case, the apical bud does not control the growth orientation of the lateral buds.     

The Effect of Hadacidin on Bud Development and its Implications for Apical Dominance

Hadacidin, which is known to inhibit the enzyme adenylo-succinate synthetase, was applied locally to lateral buds on decapitated Pisum sativum L. plants. In controls the buds grew out normally, but the treated buds were almost completely inhibited. After a delay, the buds overcame the inhibition, and their subsequent outgrowth could be further hastened by the local application of a cytokinin. The inhibition, unlike that due to IAA, was not transported to a second bud, and it could be largely reversed by kinetin or dimethylallylaminopurine. Adenine itself produces little or no reversal, however. There was some reversal by aspartic acid, which is known to reverse, at least partially, the hadacidin inhibition of the isolated enzyme. It is deduced that lateral bud development is dependent on the synthesis of a cytokinin, which may take place by a biochemical route similar, but probably not identical, to that for adenine. It follows that this synthesis takes place locally in the bud itself.     

Apical Dominance, Water Deficit and Axillary Inflorescence Growth in Zea mays: The Role of Abscisic Acid

In the sweet corn cultivar, Iochief, an episode of water deficitduring early tassel development results in a subsequent promotionof the growth of the lower axillary inflorescences. This responseis also produced by the application of abscisic acid (ABA) atthis period of growth to well-watered plants, and the hypothesisthat the response to water deficit was due to an increase inendogenous ABA concentration was examined. The ABA contentsof the tassel, leaf and axillary inflorescences were found toincrease during water stress, the increase in the tassel andaxillary buds being most rapid in the first 2 days of waterdeficit. This increase in free ABA content was followed after4 days of water deficit by a progressive increase in the concentrationof ‘bound’ ABA in the tissues. There was littleincrease in free ABA concentration after 4 days water deficit;this paralleled the subsequent growth response of the axillaryinflonscences which also was unaffected by prolonging the epidoseof water deficit beyond 4 days. In order to establish whether the response of the axillary inflorescencesto ABA was dependent upon the presence of the tassel, ABA wasapplied to watered plants with or without the developing tassel.As had been previously found with water stress, removing thetassel inhibited the response of the plant to applied ABA. Zea mays, apical dominance, water stress, inflorescence growth, abscisic acid     

Apical Dominance Control in Ipomoea nil: The Influence of the Shoot Apex, Leaves and Stem

The outgrowth of lateral buds is known to be controlled by theupper shoot tissues, which include the apex, the young leavesand the upper stem. An analysis of the influence of these plantparts on axillary bud elongation in Ipomoea nil was carriedout by various treatments on these specific tissues. A restriction of elongation in the main shoot due to eitherdecapitation or shoot inversion resulted in the release of apicaldominance A non-linear type of compensating growth relationshipwas observed between the 13 cm apical growing region of thestem and the lateral buds. It was determined by decapitation,defoliation and AgNO₃ treatments that both the 13 cm stem-growthregion and the young leaves (1–5 cm in length) had a muchgreater inhibitory influence on the outgrowth of specified lateralbuds than did the stem apex (consisting of the terminal 0.5cm of the shoot). The specified lateral buds which were analyzedfor outgrowth were located a number of nodes below the shootapex. The intervening nodes were debudded. Although the importanceof young leaves in the control of apical dominance has beenpreviously recognized, the most significant result from thepresent study with Ipomoea was the strong influence of the 13cm apical growth region of the stem on the out growth of thelateral buds. Apical dominance, Ipomoea nil L., Pharbitis nil, growth region, lateral bud outgrowth, decapitation, defoliation, shoot inversion     

Lateral Bud Growth in Phaseolus vulgaris L. and the Levels of Ethylene in the Bud and Adjacent Tissue

Ethephon and the ethylene inhibitors Ag⁺ and aminoethoxyvinylglycine (AVG) inhibited outgrowth of the axillary bud of thefirst trifoliate leaf in decapitated plants of Phaseolus vulgaris.Endogenous ethylene levels decreased in the stem upon decapitationalthough it is not conclusive that a causal relationship existsbetween this decrease and the release of axillary buds frominhibition. The proposition that auxin-induced ethylene is responsiblefor the suppression of axillary bud growth in the decapitatedplant when the apical shoot is replaced by auxin is not borneout in this study. Application of IAA directly to the axillarybud of intact plants gave rise to a transient increase in budgrowth. This growth increment was annulled when AVG was suppliedwith IAA to the bud despite the fact that the dosage of AVGused did not affect the normal slow growth rate of the bud ofthe intact plant or bud outgrowth resulting from shoot decapitation.     

The Presentation Time for Shoot Inversion Release of Apical Dominance in Pharbitis nil

The presentation time for shoot inversion release of apicaldominance in Pharbitis nil is between 1 and 1.5 d. Five to 6d of shoot inversion are required for persistent outgrowth ofthe highest lateral bud. Pharbitis nil, apical dominance, shoot inversion, lateral bud growth, presentation time     

Brassinolide Effect on Growth of Apical Meristems, Ethylene Production, and Abscisic Acid Content in Potato Tubers

Treatment of intact potato (Solanum tuberosum L., cv. Nevskii) tubers with 24-epibrassinolide (EB) resulted in prolonged deep dormancy, increased production of ethylene and higher contents of free and bound abscisic acid (ABA) in buds. EB at the most efficient concentration 0.021 mg dm^–3, applied immediately after tuber harvest, inhibited sprouting by 36 – 38 d, increased ethylene formation after 1 and 7 d of storage by almost 300 and 150%, respectively, and increased the content of both free and bound ABA during the whole period of storage (on average by about 80%). Electron microscopic and morphometric studies showed that EB brings about a decrease in cell volume in tunica and all types of meristems and an increase in the number of vacuoles, accompanied by a decrease in their volume.     

Effects of Brassinolide and IAA on Ethylene Production and Elongation in Maize Primary Roots

We examined the effects of brassinolide (BL) and/or an auxin (indole-3-acetic acid) on ethylene production and elongation in the primary roots of maize (Zea mays). When these two hormones were applied exogenously, both increased ethylene production. Before the tenth hour after treatment began, the influence of IAA was more evident than that of BL; the reverse was found beyond 10 h. When these hormones were treated simultaneously, the increase in level of ethylene was greater than the sum of effects by each hormone. Such a positive interaction was also recorded for changes in the activity of ACC synthase and the expression of its gene. For ACC oxidase, however, the two hormones had no apparent influence. When applied separately, neither affected root elongation nor proton extrusion. However, when given in combination, both phenomena occurred. Our results suggest that BL interacts with IAA to promote ethylene biosynthesis and elongation in roots. Therefore, it is possible that brassinolide acts by inducing auxin, which then stimulates both ethylene production (at the early stage) and root development.     

The Role of Ethylene in Growth and Endopolygalacturonase Production by Sclerotium rolfsii

Growth of and endopolygalacturonase production by Sclerotium rolfsii was better on a defined mineral medium than on a medium containing segments of tomato leaf petioles. The effect of treatment with ethylene (10μl/l) upon endopolygalacturonase activity with investigated at various stages of growth, in a mineral defined medium. Addition of ethylene to a 10 days-old culture of S. rolfsii resulted in a decrease in activity by day 14, whereas the addition of ethylene to a 4, 6 and 8 days old cultures resulted in an increase in endopolygalacturonase activity. Ethylene seems to have little or no stimulating effect upon growth of S. rolfsii when applied after 8 days. However, inhibited fungal growth, after the addition of ethylene at earlier stages of growth, was obtained due to the depletion of oxygen from sealed culture flasks. Endopolygalacturonase was extracted and purified from control cultures after 14 days of growth. Fractionation of this enzyme protein on Sephadex G-100 gel filtration columns resulted in two peaks of activity measured by the release of reducing sugars from polygalacturonic acid (PGA).     

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.     

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     

Abscisic Acid and Apical Dominance in Phaseolus coccineus L. The Role of Tissue Age

Abscisic acid (ABA) applied to the decapitated second internodeof runner bean plants enhanced outgrowth of lateral buds onlywhen internode stumps were no longer elongating. Applied toelongating internodes of slightly younger plants, ABA causesinhibition of bud outgrowth. Together with 10^–4 M indol-3-ylacetic acid (IAA), ABA stimulated internode elongation and interactedadditively in the inhibition of bud outgrowth. A mixture of10^–5 M ABA and 10^–6 M gibberellic acid (GA₃ ) causedsimilar effects on internode growth as IAA + ABA, but was mutuallyantagonistic in effect on growth of the lateral buds. Abscisic acid, apical dominance, gibberellic acid, indol-3yl acetic acid, Phaseolus coccineus, bean