The Role of Gravity in Apical Dominance : Effects of Clinostating on Shoot Inversion-Induced Ethylene Production, Shoot Elongation, and Lateral Bud Growth

Shoot inversion-induced release of apical dominance in Pharbitis nil is inhibited by rotating the plant at 0.42 revolutions per minute in a vertical plane perpendicular to the axis of rotation of a horizontal clinostat. Clinostating prevented lateral bud outgrowth, apparently by negating the restriction of the shoot elongation via reduction of ethylene production in the inverted shoot. Radial stem expansion was also decreased. Data from experiments with intact tissue and isolated segments indicated that shoot-inversion stimulates ethylene production by increasing the activity of 1-aminocyclopropane-1-carboxylic acid synthase. The results support the hypothesis that shoot inversion-induced release of apical dominance in Pharbitis nil is due to gravity stress and is mediated by ethylene-induced retardation of the elongation of the inverted shoot.     

Shoot inversion-induced ethylene in pharbitis nil induces the release of apical dominance by restricting shoot elongation

Shoot inversion induces outgrowth of the highest lateral bud (HLB) adjacent to the bend in the stem in Pharbitis nil. In order to determine whether or not ethylene produced by shoot inversion plays a direct role in promoting or inhibiting bud outgrowth, comparisons were made of endogenous levels of ethylene in the HLB and HLB node of plants with and without inverted shoots. That no changes were found suggests that the control of apical dominance does not involve the direct action of ethylene. This conclusion is further supported by evidence that the direct application of ethylene inhibitors or ethrel to inactive or induced lateral buds has no significant effect on bud outgrowth. The hypothesis that ethylene evolved during shoot inversion indirectly promotes the outgrowth of the highest lateral bud (HLB) by restricting terminal bud (TB) growth is found to be supported by the following observations: (1) the restriction of TB growth appears to occur before the beginning of HLB outgrowth; (2) the treatment of the inverted portion of the shoot with AgNO₃, an inhibitor of ethylene action, dramatically eliminates both the restriction of TB growth and the promotion of HLB outgrowth which usually accompany shoot inversion; and (3) the treatment of the upper shoot of an upright plant with ethrel mimics shoot inversion by retarding upper shoot growth and inducing outgrowth of the lateral bud basipetal to the treated region.     

Shoot Inversion Inhibition of Stem Elongation in Pharbitis nil: A Possible Role for Ethylene-Induced Glycoprotein and Lignin

Inversion of the upper shoot of Pharbitis nil results in the inhibition of elongation in the inverted stem. The objective of the present study was to determine how shoot inversion-induced gravity stress inhibited elongation and to elucidate the possible role of ethylene-induced glycoprotein and lignin in this process. Determinations of hydroxyproline, peroxidase, phenylalanine ammonia-lyase (PAL), phenol, and lignin content/activity were carried out by appropriate spectrophotometric methods. It was found that inversion and Ethrel treatments of upright shoots caused significant increases in hydroxyproline content, peroxidase, and PAL activity in 12 hours and in phenol and lignin contents in 24 hours. All of these increases except for that of cytoplasmic peroxidase activity were partially reversed by AgNO₃, the ethylene action inhibitor. It is concluded that possible cross-linking associated with the accumulation of ethylene-induced hydroxyproline-rich glycoprotein and lignin may be responsible for the later stages of cessation of elongation in the inverted Pharbitis shoot.     

Gravistimulus direction, ethylene production and shoot elongation in the release of apical dominance in Pharbitis nil

Release of apical dominance can be induced in Pharbitis nil by the inversion of the upper shoot. This promotion of outgrowth of the highest lateral bud adjacent to the bend of the stem appears to be mediated by ethylene inhibition of growth of the inverted main shoot. In the present investigation the existence of a direct correlation between ethylene evolution and the direction of gravistimulus is demonstrated as well as an inverse correlation between ethylene production by the inverted upper shoot and its elongation. An inverse correlation also exists between elongation of the inverted upper shoot and the outgrowth of the highest lateral bud if the lower portion of the shoot (below the bend) is oriented in an upright position. The patent period for shoot-inversion induction of ethylene production is about 2 h. These results support the hypothesis of indirect ethylene control of apical dominance release by retardation of elongation of the inverted shoot.     

Author index volume 38 (1985)

Mechanical perturbation (MP, rubbing) of internodes of Pharbitis nil shoots initiates release of lateral buds (LB) from apical dominance within 48 h. Evidence is presented which suggests that MP promotion of LB outgrowth is mediated by ethylene-induced restriction of main shoot growth. Ethylene production in the internodes is stimulated by MP within 2 h. Effects of MP are mimicked by treatments with 1-aminocyclopropane-1-carboxylic acid (ACC) and are negated by the inhibitors of ethylene production or action, aminoethoxy vinylglycine (AVG) and AgNO₃. The fact that effects of MP, ACC and ethylene inhibitors are observed to occur on main shoot growth at least 24 h before they are observed to occur on LB growth suggests a possible cause and effect relationship. MP also causes an increase in internode diameter. MP stimulation of ethylene production appears to be mediated by ACC synthase. The results of this study and our previous studies suggest that apical dominance may be released by any mechanism which induces ethylene restriction of main shoot growth.     

Shoot Inversion-induced Ethylene Production in the Diageotropica Tomato Mutant

Shoot inversion was found to inhibit shoot elongation over 24h in the diageotropica (dgt) mutant tomato and its isogenicparent, VNF8, by 55% and 51%, respectively. Previous studieswith normal tomato and other species would suggest that gravitystress-induced ethylene production in the inverted shoot retardselongation. Since exposure of the diageotropic shoot of thedgt mutant tomato to ethylene restores normal upward growth,it might appear that dgt is defective in its capacity to produceethylene. That shoot inversion did stimulate some ethylene productionand that the ethylene action inhibitor, AgNO₃, largely negatedthe inhibiting effect of shoot inversion on elongation in dgtstrongly suggest that the ethylene produced in the invertedshoot was responsible for its retardation of growth. Althoughethylene production in the slower growing dgt was much lessthan that in the faster-growing VNF8, the dgt shoot was foundto be much more sensitive to ethylene. Shoot inversion, ethylene, shoot elongation, diageotropica (dgt), auxin     

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     

A Developmental Pattern of Flowering in Colored <Emphasis Type="Italic">Zantedeschia</Emphasis> spp.: Effects of Bud Position and Gibberellin

The restricted flowering of colored cultivars ofZantedeschia is a consequence of developmental constraints imposed by apical dominance of the primary bud on secondary buds in the tuber, and by the sympodial growth of individual shoots. GA₃ enhances flowering inZantedeschia by increasing the number of flowering shoots per tuber and inflorescences per shoot. The effects of gibberellin on the pattern of flowering and on the developmental fate of differentiated inflorescences along the tuber axis and individual shoot axes were studied in GA₃ and Uniconazole-treated tubers. Inflorescence primordia and fully developed (emerged) floral stems produced during tuber storage and the plant growth period were recorded. Days to flowering, percent of flowering shoots and floral stem length decreased basipetally along the shoot and tuber axes. GA₃ prolonged the flowering period and increased both the number of flowering shoots per tuber and the differentiated inflorescences per shoot. Activated buds were GA₃ responsive regardless of meristem size or age. Uniconazole did not inhibit inflorescence differentiation but inhibited floral stem elongation. The results suggest that GA₃ has a dual action in the flowering process: induction of inflorescence differentiation and promotion of floral stem elongation. The flowering pattern could be a result of a gradient in the distribution of endogenous factors involved in inflorescence differentialtion (possibly GAs) and in floral stem growth. This gradient along the tuber and shoot axes is probably controlled by apical dominance of the primary bud. Online publication: 7 April 2005     

Ethylene-Mediated Regulation of Gibberellin Content and Growth in Helianthus annuus L

Elongation of hypocotyls of sunflower can be promoted by gibberellins (GAs) and inhibited by ethylene. The role of these hormones in regulating elongation was investigated by measuring changes in both endogenous GAs and in the metabolism of exogenous [³H]- and [²H₂]GA₂₀ in the hypocotyis of sunflower (Helianthus annuus L. cv Delgren 131) seedlings exposed to ethylene. The major biologically active GAs identified by gas chromatography-mass spectrometry were GA₁, GA₁₉, GA₂₀, and GA₄₄. In hypocotyls of seedlings exposed to ethylene, the concentration of GA₁, known to be directly active in regulating shoot elongation in a number of species, was reduced. Ethylene treatment reduced the metabolism of [³H]GA₂₀ and less [²H₂]GA₁ was found in the hypocotyls of those seedlings exposed to the higher ethylene concentrations. However, it is not known if the effect of ethylene on GA₂₀ metabolism was direct or indirect. In seedlings treated with exogenous GA₁ or GA₃, the hypocotyls elongated faster than those of controls, but the GA treatment only partially overcame the inhibitory effect of ethylene on elongation. We conclude that GA content is a factor which may limit elongation in hypocotyls of sunflower, and that while exposure to ethylene results in reduced concentration of GA₁ this is not sufficient per se to account for the inhibition of elongation caused by ethylene.     

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     

Timing of Growth Inhibition following Shoot Inversion in Pharbitis nil

Shoot inversion in Pharbitis nil results in the enhancement of ethylene production and in the inhibition of elongation in the growth zone of the inverted shoot. The initial increase in ethylene production previously was detected within 2 to 2.75 hours after inversion. In the present study, the initial inhibition of shoot elongation was detected within 1.5 to 4 hours with a weighted mean of 2.4 hours. Ethylene treatment of upright shoots inhibited elongation in 1.5 hours. A cause and effect relationship between shoot inversion-enhanced ethylene production and inhibition of elongation cannot be excluded.     

Stimulation of shoot regeneration from cotyledons of Helianthus annuus by the ethylene inhibitors,silver and cobalt

The effects of CoCl₂, AgNO₃ and ethylene released by exogenous 2-chloroethylphosphonic acid (Ethephon), were studied on shoot regeneration from cotyledons of Helianthus annuus cv. E8206R, a poorly regenerative cultivar. Inhibition of ethylene biosynthesis by CoCl₂, at concentrations of 20 K, provoked a substantial enhancement of shoot regeneration (30 %): the control was poorly regenerative. However, CoCl₂ had no effect when Ethephon was supplied. Inhibition of ethylene action by AgNO₃, at concentrations of 10–25 M, caused a significant increase in plant regeneration: 25 % instead of 1.2 % in the control. Furthermore, addition of Ethephon to AgNO₃-treated tissues failed to reduce the stimulation of shoot regeneration caused by AgNO₃. On the basis of these findings, it is suggested that ethylene inhibits the regeneration process from cotyledons of sunflower.Abbreviations NAA 1-naphthalene acetic acid - BAP 6-benzylamino-purine - GA₃ gibberellic acid - Ethephon 2-chloroethylphosphonic acid - MS Murashige and Skoog medium - AVG aminoethoxyvinylglycine     

Does Ethylene Play a Role in the Release of Lateral Buds (Tillers) from Apical Dominance in Oats?

The growth of lateral buds (tillers), which are undergoing release from apical dominance, was measured in upright and gravistimulated intact Avena sativa L. cv. `Victory' (oat) shoots as well as in isolated Avena stem segments treated with kinetin and sucrose. During release, the tiller bud initially shows a slow rate of elongation accompanied by swelling. It is followed by a more rapid rate of elongation. Ethylene (C₂H₄) production in shoot segments containing a tiller bud was found to occur at the onset of tiller swelling during gravistimulation as well as during inflorescence emergence. Exogenous application of indoleacetic acid or C₂H₄ inhibits kinetin-induced tiller bud swelling and elongation. However, stem segments pulsed for 24 hours in C₂H₄ or the C₂H₄ biosynthesis precursor, 1-amino-cyclopropane-1-carboxylic acid (ACC) and then transferred to kinetin and sucrose, showed a significant increase in swelling elongation as compared with segments maintained under the same conditions but without C₂H₄ or ACC in the pulse. Segments pulsed for 24 hours with kinetin and sucrose plus the ACC biosynthesis inhibitor, aminoethoxyvinylglycine, or the C₂H₄ action inhibitor, CO₂, then transferred to kinetin and sucrose medium, showed inhibition of tiller swelling during the pulse and of subsequent elongation. These results indicate that C₂H₄ plays a role in promoting tiller swelling during the onset of tiller release from apical dominance and may act as a modulator hormone in promoting tiller elongation in the presence of cytokinin.     

Shoot inversion-induced ethylene production: a general phenomenon?

Shoot inversion induction of ethylene production was found in inverted shoots of corn, peas, soybean, sunflower, tomato, andPharbitis nil. The increases in ethylene production were found to range from two- to threefold in soybean to eightfold in corn and sunflower. The occurrence of peaks of ethylene production ranged from 16 h following shoot inversion in corn to 72 h inPharbitis. That the enhanced ethylene production was due to activation of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase is supported by the finding of increased ACC content in inverted shoots of all species tested. Shoot inversion inhibition of elongation was found in inverted shoots of pea, soybean, sunflower, tomato, andPharbitis nil. This inhibition is thought to be mediated via increased ethylene production in the inverted shoots. That shoot inversion induction of ethylene is not a persistent effect is supported by the finding that ethylene synthesis could be terminated by reorientation of shoots to the upright position and could be reinitiated by the subsequent inversion of the shoots. The effects of shoot inversion on the enhancement of ethylene production and on the inhibition of elongation of the inverted shoot appear to be general phenomena.     

Role of Ethylene on de Novo Shoot Regeneration from Cotyledonary Explants of Brassica campestris ssp. pekinensis (Lour) Olsson in Vitro

The promotive effect of AgNO₃ and aminoethoxyvinylglycine (AVG) on in vitro shoot regeneration from cotyledons of Brassica campestris ssp. pekinensis in relation to endogenous 1-amino-cyclopropane-1-carboxylic acid (ACC) synthase, ACC, and ethylene production was investigated. AgNO₃ enhanced ACC synthase activity and ACC accumulation, which reached a maximum after 3 to 7 days of culture. ACC accumulation was concomitant with increased emanation of ethylene which peaked after 14 days. In contrast, AVG was inhibitory to endogenous ACC synthase activity and reduced ACC and ethylene production. The promotive effect of AVG on shoot regeneration was reversed by 2-chloroethylphosphonic acid at 50 micromolar or higher concentrations, whereas explants grown on AgNO₃ medium were less affected by 2-chloroethylphosphonic acid. The distinctive effect of AgNO₃ and AVG on endogenous ACC synthase, ACC, and ethylene production and its possible mechanisms are discussed.     

Inhibition of the growth of peas by tris-(2-diethylaminoethyl)-phosphate trihydrochloride

The effects of Tris-(2-diethylaminoethyl)-phosphate trihydrochloride (SK&F 7997-A₃) on the development of 4 varieties of Pisum sativum were investigated. The compound inhibited shoot elongation of all 4 varieties by as much as 50% or more when seeds were soaked in solutions of the inhibitor for 12 hours before planting. Seed treatment also affected flowering by causing an increase in the number of nodes to the first flower in the early varieties Little Marvel and Alaska. The number of nodes preceding the first flower in the late varieties Dwarf and Tall Telephone was not affected by high concentrations of SK&F 7997-A₃, but low concentrations appeared to cause a slight reduction in the number of nodes to flower.

The inhibitor had little effect on growth when applied to established seedlings; some slight inhibition was noted when high doses were applied to the shoot tip.

SK&F 7997-A₃ suppressed the growth response of dwarf and tall peas to exogenous GA₃. The compound did not inhibit biosynthesis of gibberellin by Fusarium moniliformc when present in shaken liquid cultures at concentrations as high as 10 mg/ml. The inhibitor suppressed the action of applied GA₃ on shoot elongation when the 2 chemicals were applied in 3 ways: 1) inhibitor on lowermost compound leaf and GA₃ on shoot tip; 2) GA₃ on lowermost leaf and inhibitor on shoot tip; and 3) soaking of seeds in the 2 compounds combined for 12 hours prior to planting. The third method of dual treatment yielded evidence that SK&F 7997-A₃ interacts noncompetitively with GA₃ in the regulation of shoot elongation.

     

Histomorphological changes in shoot apices of <Emphasis Type="Italic">Lactuca sativa</Emphasis> treated with gibberellic acid

Lettuce plants were treated with gibberellic acid (GA₃) and uniconazole (UZ; a gibberellin synthesis inhibitor) to investigate the influence of GA₃ on cell division frequency in the shoot apical meristem (SAM) during stem elongation and flower initiation in lettuce (Lactuca sativa) grown in a greenhouse. GA₃ (0.1 mM) was sprayed on the surface of outer leaves and uniconazole solution (0.86 mM) was applied to the soil. GA₃ increased cell division frequency in the peripheral zone and the rib meristem of shoot apices, and this was associated with the stimulation of stem elongation. UZ treatment decreased cell division frequency in the peripheral zone, rib meristem and subapical pith, and this was associated with restricted stem elongation. Treatment with UZ and GA₃ together induced minor stem elongation. Flower induction occurred 3 d earlier in the GA₃ and UZ+GA₃ treatments than in the control, while the UZ treatment delayed flower initiation for more than 9 d relative to the control.     

Implication of Ethylene in the Release of Secondary Dormancy in Amaranthus caudatus L. Seeds by Gibberellins or Cytokinin

Ethephon (Eth), gibberellin A₃, A_{4 + 7} (GA₃, GA_{4 + 7}), and 6-benzyladenine (BA) removed secondary dormancy of Amaranthus caudatus seeds. The GAs and BA potentiated the effect of ethephon or 1-aminocyclopropane-1-carboxylic acid (ACC), an ethylene biosynthesis precursor, in terms of the rate or final percent of germination. Aminoethoxyvinylglycine (AVG), an ACC synthase activity inhibitor, was observed to simultaneously inhibit the release from dormancy effected by GA₃ or BA as well as the ethylene production stimulated by these regulators. Breaking of secondary dormancy by GA₃, GA_{4 + 7} or BA was prevented by 2,5-norbornadiene (NBD), an inhibitor of ethylene binding. Ethylene completely or markedly reversed the inhibitory effect of NBD. We thus conclude that the removal of secondary dormancy in Amaranthus caudatus seeds by gibberellin or benzyladenine involves ethylene biosynthesis and action.     

Effect of Morphactin, Gibberellic Acid and Auxin and on Growth and Development of Bryophyllum tubiflorum

six month old LD- and SD- grown plants of Bryophyllum tubiflorum were treated with morphactin (n-buty1-9-hydroxy-fluorene-(9)-carboxylate), singly and in combination with GA₃ and IAA. Morphactin and IAA decreased stem elongation and number of leaves, the effect increasing with concentration concentation. Morphactin also caused lateral buds to develop into branches and the fusion of upper leaves to form structures of different shapes. GA₃ enhanced stem elongation, increased leaf number and induced floral buds under SD conditions. It reversed the inhibitory effect of morphactin on stem elongation, leaf number and leaf fusion and also restored apical dominance when applied simultaneously with morphactin. The stimulaneous application of IAA also reversed the morphactin effects on leaf fusion and on apical dominance. The results have been discussed in the light of literature available on the subject.     

Ethylene and cytokinin interaction in the morphogenesis of Pelargonium × hortorum L.H. Bailey in vitro

Pelargonium × hortorum ‘Grand Prix’ which is susceptible to leaf yellowing and ‘Bergpalais’ which is not susceptible to leaf yellowing were chosen for the experiments. Ethylene production and action as well as the associated morphological response of Pelargonium shoots grown in the presence of a precursor of ethylene biosynthesis 1-aminocyclopropane-1-carboxylic acid (ACC), ethylene inhibitors: α-aminooxyacetic acid (AOA) and silver nitrate (AgNO₃) and different cytokinins: (meta-topolin) (mT) or 6-benzylaminopurine (BAP) were studied. It was found that ‘Grand Prix’ was more sensitive to ethylene than ‘Bergpalais’ and it showed the leaf yellowing in response to 0.1 mg l^?1 ACC. Moreover, it was noted that ACC added separately or together with cytokinin influenced Pelargonium morphogenesis. Depending on the concentration of ACC (0.1–2.0 mg l^?1), it either stimulated or inhibited shoot and root formation as well as the growth of shoots and leaf blades. ACC-induced leaf yellowing in ‘Grand Prix’ was effectively inhibited by mT. In contrast, BAP did not enhance shoot quality. Simultaneously, the presence of mT in the medium resulted in up to a twofold increase in the ethylene production by ‘Grand Prix’ shoots throughout the culture period compared with the shoots growing on the BAP-medium. The inhibitor of ethylene action (AgNO₃) added with cytokinin prevented the yellowing of Pelargonium shoots, but simultaneously influenced the formation of mature shoots with limited long-term multiplication potential. The shoots of P. × hortorum ‘Grand Prix’ treated with AgNO₃ and mT emitted two- and sevenfold more ethylene after 11th and 21st day of culture compared with those treated with AgNO₃ and BAP. It is suggested that mT inhibits the early senescence of Pelargonium in vitro by decreasing its sensitivity to ethylene.