The inhibitory effect of gibberellic acid on flowering in Citrus

The application of gibberellic acid (GA₃) at any time from early November until bud sprouting, resulted in a significant inhibition of flowering in the sweet orange [ C. sinensis (L.) Osbeck] and the Satsuma ( C. unshiu Marc.) and Clementine ( C. reticulata Blanco) mandarins. Two response peaks were evident: the first occurred when the application was timed to the translocation of an unknown flowering signal from the leaves to the buds. The second occurred during bud sprouting, at the time the flower primordia were differentiating. From the pattern of flowering, it appears that the mechanism of inhibition was similar irrespective of the timing of GA₃ application. There was an initial reduction in bud sprouting affecting selectively those buds originating leafless inflorescences. An additional inhibition resulted in a reduction in the number of leafy inflorescences with an increase in the number of vegetative shoots, suggesting the reversion of a floral to a vegetative apex. The inhibited buds sprouted readily in vitro but invariably vegetative shoots were formed. A continuous influence of the sustaining branch is necessary to keep the flowering commitment of the buds; irreversible commitment occurs when the petal primordia are well differentiated.     

Dual effect of light on flowering and sprouting of rose shoots

Shade, caused by a dense leaf canopy in the light conditions of a normal greenhouse, reduced sprouting of the third axillary bud (from the top) on decapitated rose branches ( Rosa hybrida cv. Marimba) in comparison to less shaded buds on branches protruding above the canopy and sparsely spaced. Flowering of the third young shoot on shaded branches bearing 3 lateral shoots was totally inhibited. Mixed fluorescent and incandescent light in a growth chamber reduced sprouting of the third bud on decapitated rose branches in comparison to decapitated branches on rose plants held in fluorescent light of similar photon flux density. This was attributed to the higher R:FR ratio in fluorescent vs mixed light that reached the third bud, and in exposed vs shaded branches. Flowering of the third shoot was promoted by several factors: high photon flux density, 0.5 m M gibberellic acid (GA) or 0.2 m M benzyladenine (BA). BA was the most effective treatment. Treatments promoting flowering of the third shoot did not reduce growth or flowering of the upper shoots. However, spraying the uppermost shoot with BA suppressed the growth of the shoots below. It is concluded that light affects flowering in two ways. The effect on bud sprouting is related mainly to R:FR ratios, while the effect on flower development is related mainly to photon flux density. Cytokinins may substitute for the light effect on flower development.     

Fruit Regulates Bud Sprouting and Vegetative Growth in Field-Grown Loquat Trees (Eriobotrya japonica Lindl.): Nutritional and Hormonal Changes

The effects of fruit on bud sprouting and vegetative growth were compared on fruiting and defruited loquat trees from fruit set onward. Carbohydrate and nitrogen content in leaves and bark tissues and hormone concentrations were studied during the fruit development and vegetative growth periods. On defruited trees, a significant proportion of buds sprouted in winter, whereas buds from fruiting trees sprouted only in the spring when fruit reached its final size. Furthermore, when panicles were completely removed in autumn, the buds also sprouted. In addition, fruit directly affected vegetative growth by reducing shoot length. An effect of sink removal (flower or fruit) promoting bud sprouting, regardless of the season, was then demonstrated. Neither soluble sugar concentration nor nitrogen fraction concentration in leaves or bark tissues was related to bud sprouting, but a certain nutritional imbalance was observed during the most active period of fruit development. Moreover, fruit sink activity significantly modified hormone content by increasing indole-3-acetic acid (IAA) and reducing zeatin concentrations, resulting in a higher IAA/zeatin ratio parallel to the lower bud sprouting intensity. Therefore, these changes caused by fruit removal are all related to vegetative growth, but there is no evidence that they are responsible for bud burst.     

Gibberellic Acid Reduces Flowering Intensity in Sweet Orange [Citrus sinensis (L.) Osbeck] by Repressing CiFT Gene Expression

In Citrus, gibberellic acid (GA₃) applied at the floral bud inductive period significantly reduces flowering intensity. This effect is being used to improve the fruit set of parthenocarpic cultivars that tend to flower profusely. However, the molecular mechanisms involved in the process remain unclear. To contribute to the knowledge of this phenomenon, adult trees of ‘Salustiana’ sweet orange were sprayed at the floral bud inductive period with 40?mg?L^?1 of GA₃ and the expression pattern of flowering genes was examined up to the onset of bud sprouting. Trees sprayed with paclobutrazol (PBZ, 2,000?mg L^?1), a gibberellin biosynthesis inhibitor, were used to confirm the effects, and untreated trees served as control. Bud sprouting, flowering intensity, and developed shoots were evaluated in the spring. GA₃ significantly reduced the number of flowers per 100 nodes by 72% compared to the control, whereas PBZ increased the number by 123%. Data of the expression pattern of flowering genes in leaves of GA₃-treated trees revealed that this plant growth regulator inhibited flowering by repressing relative expression of the homolog of FLOWERING LOCUS T, CiFT, whereas PBZ increased flowering by boosting its expression. The activity of the homologs TERMINAL FLOWER 1, FLOWERING LOCUS C, SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1, and APETALA1 was not affected by the treatments. The number of flowers per inflorescence, in both leafy and leafless inflorescences, was not altered by GA₃ but increased with PBZ; the latter paralleled LEAFY relative expression. These results suggest that GA₃ inhibits flowering in Citrus by repressing CiFT expression in leaves.     

The Influence of Ringing on Bud Development and Flowering in Satsuma Mandarin

Ringing of Satsuma mandarin (Citrus unshiu Marc.) trees showedincreased flowering in the following spring when performed duringSeptember and October, but not during November. Most of theeffect on flowering was due to an enhancement of both bud sproutingand the number of flowering shoots formed per node. In addition,a direct effect of ringing on flower initiation was demonstrated,since the number of vegetative shoots was reduced. The response of the buds to ringing was rapid as demonstratedby changes in bud weight, protein content, and electrophoreticpattern and behaviour when cultured in vitro. Buds from ringedtrees readily flowered in vitro when forced during the winterrest period and flower formation was enhanced by the additionof cytokinin. Buds from control trees formed a smaller numberof flowers in vitro, and flowering was much less enhanced bythe addition of cytokinin. It is concluded that ringing acceleratesflower initiation in the buds and this effect takes place beforethe winter rest period. Key words: Bud sprouting, Citrus unshiu Marc., flower initiation, flowering, in vitro flowering, Satsuma mandarin, ringing     

Low temperature influence on flowering in Citrus. The separation of inductive and bud dormancy releasing effects

The flowering response of Owari Satsuma mandarin ( Citrus unshiu Marc) to low temperature treatments has been determined using potted trees and in vitro bud cultures. In potted trees the chilling treatments released bud dormancy and enhanced both sprouting and flowering, but these two responses could not be separated. However, bud cultures showed no dormancy, and a specific effect of low temperature on flower induction was demonstrated. Low temperature appears to have a dual effect, releasing bud dormancy and inducing flowering. Potential flower buds have a deeper dormancy than vegetative buds, and the first stages of flower initiation seem to occur before the winter rest period.     

Dormancy and spring development of lateral buds in mulberry (Morus alba)

Patterns of spring development of lateral buds of mulberry (Morus alba L. cv. Shin-ichinose) coppice shoots on 11-year-old low-pruned stumps varied in response to girdling, pruning and arching. The erect controls showed a weak acrotonic (apex-favoring) growth habit, in which the majority of the buds, including the basal ones, sprouted and elongated in mid- and late April, and hence there was a prolonged imposition of dominance on the upper laterals in mid- and late May. In contrast, early spring girdling or pruning enhanced the activity of the upper buds of the proximal (lower) halves of the girdled stems or of the pruned stems, resulting in considerable dominance of the laterals from such buds in late April. Arching markedly inhibited buds on the under side of the arched stems, leading to poor shoots. By late April, the buds on the adaxial (upper) side readily grew into new vertical shoots, which dominated over the lateral ones. When studied by a multiple-node-cutting test, increased length of segments of post-dormant mulberry stems was accompanied by decreased bud activity of the segments and by decreased breaking ability of the lower buds within the segments, suggesting the importance of roots in the weak acrotonic habit of the erect stem in spring. By contrast, the acropetal influences of the attached stems can in part affect dominance relationships, perhaps mediated through competition for factors translocated from the roots. Continuous basal applications of abscisic acid inhibited bud break and shoot growth of the postdormant stem segments, but these inhibitory effects could be reversed by applied gibberellic acid A₃ (GA₃). Two phases of lateral bud dormancy in erect mulberry coppice shoots were identified. The first was characterized by a smaller breaking capacity in the upper buds than in the lower ones and hence by a basitonic (base-favoring) gradient in bud growth potential. The second phase corresponded to a restoration of these capabilities in the upper buds and to a change towards a linear gradient in bud growth potential, with disappearance of the dormant condition, in February and March. This gradient change during dormancy release may represent the physiological basis for the weak acrotonic habit of erect mulberry stems in spring.     

Flower Bud Atrophy in Baccara Roses. V. The Effect of Different Growth Substances on Flowering

The effect of various growth regulators on the atrophy of terminal flower buds was tested on Baccara roses. Treatments with gibberellic acid (GA) and 2-chloroethyl trimethyl ammonium chloride (CCC) reduced the atrophy of the flowers. The application of 2-chloroethyl phosphonic acid (CEPA) to the buds enhanced abortion, and the effect was more marked on the lower than on the upper shoot. The stage most sensitive to CEPA was when the shoots were 8–35 cm long. Treatment with abscisic acid (ABA) had no effect of the degree of “blindness”, nor did kinetin applied to the apex affect flowering. Spraying with benzyl adenine increased both the rate of sprouting of the lateral buds and the extent of “blindness” of the sprouting shoots, but did not reduce the number of flowers per branch.     

Sprout initiation and growth for three years after cutting in an abandoned secondary forest in Kyoto,Japan

Secondary forests in Japan have been abandoned and the ecosystem has degraded since the high economic growth period. We carried out cutting in January in three small areas of a long-abandoned secondary forest and investigated the sprout initiation and growth of woody plants for three years in order to reveal the early stage of sprout regeneration and to understand the sprouting ability and characteristics of each species for effective management. The percentage of sprouted stumps and the number of sprouting shoots was substantially maximized in autumn in the first year. These results suggest that autumn monitoring in the first year after cutting shows the maximum percentage of sprouted stumps and the maximum sprout number when cutting was conducted in the dormant season. With regards to species characteristics, Eurya japonica showed a low percentage of sprouted stumps in the lower plot, where the mean diameter at breast height for this species was smaller than in the other plots. The sprouting ability of E. japonica was deemed to be influenced by parent tree size. Ilex pedunculosa and Lyonia ovalifolia var. elliptica had high percentages of sprouted stumps and many sprouts. These species are useful for obtaining sprouting shoots (e.g., for firewood), but it is difficult to control their sprouting.     

Lateral bud development and shoot growth in low-pruned Morus alba as affected by stem orientation

Two phases of bud activity were identified in the new growth of one-year-old erect coppice shoots on 11-year-old low-pruned stumps of mulberry (Morus alba L. cv. Shin-ichinose) in spring, the sprouting phase in which the majority of the buds, including the basal ones, sprout and elongate, and the dominance phase (starting 4–5 weeks after sprouting) during which the upper laterals begin to assert dominance and suppress the growth of lower laterals, becoming new leading shoots. In contrast, arching before sprouting markedly inhibited buds on the under side, leading to poor shoots. By late April, the sprouts on the upper side grew readily into new erect shoots, resulting in considerable dominance over those from the lateral sides. Of these erect shoots, those located closer to the stem base grew more in May and June. The effects of arching made during the sprouting phase (late April) on bud activity and shoot lengths were generally similar to those of earlier archings before spring bud bursting. Separation of the shoots from the upper and under sides by longitudinal, horizontal splitting of the arched stems in late April did not affect the inhibited elongation of the shoots from the under side. These results suggest that in the response to arching before and in late April, the effects are related to spring bud bursting and gravimorphism. In contrast, arching during and after the dominance phase (May) had no gravimorphic effects on growth of the shoots on the upper side, although there was a stimulation of outbreak of the buds on the upper side, which remained dormant during spring bud bursting. Continuous basal applications of abscisic acid in aqueous solution inhibited bud break and shoot growth of the postdormant erect stem segments, and defoliation of the new shoots markedly. In contrast, similar applications of an ethylene-releasing compound, Ethephon, depressed shoot elongation slightly, but enhanced defoliation greatly. Gibberellic acid (GA₃) stimulated shoot elongation, but depressed leaf enlargement.     

Dormancy in bulbils of several herbaceous plants: Effects of photoperiod,light, temperature,oxygen and gibberellic acid

Effects of some environmental conditions (photoperiod, white and colored lights, temperature, partial oxygen pressure) and growth regulators (gibberellic acid, 2-chloroethyltrimethylammonium chloride) on induction and release of dormancy of the bulbils ofDioscorea batalas, Laportea bulbifera, Elatostema involucratum andSedum bulbiferum were investigated. Bulbils were formed under short-day conditions inLaportea andElatostema, under long-day conditions inSedum, and irrespective of photoperiods inDioscorea. In all species exceptSedum, immature bulbils required light, particularly blue or far red, for sprouting (photo-sprouting stage), and mature bulbils required a cold treatment (thermo-sprouting stage). The duration of photo-sprouting and thermo-sprouting stages and the degree of dependency on light or low temperature of sprouting differed from species to species. Sprouting of chilled mature bulbils of these species was promoted by light, especially by red or green light. Both immature and mature bulbils ofSedum sprouted under short-day conditions. Continuous irradiation with blue, far-red and green light markedly inhibited their sprouting. Oxygen at high concentration inhibited the sprouting of immature bulbils inDioscorea; in the other species it promoted sprouting regardless of the maturation of the bulbils. Applications of gibberellic acid caused the sprouting of bulbils the absence of light, chilling or photoperiodic treatment in all species exceptDioscorea, in which gibberellic acid inhibited sprouting. Polyphenol oxidase activity was very high in the homogenates ofDioscorea bulbils, and increased further when the bulbils had been treated with gibberellic acid. In the other species, little or no such activity was observed.     

Bud dormancy breaking affects respiration and energy balance of bilberry shoots in the initial stage of growth

Respiration and heat production in the shoots of bilberry (Vaccinium myrtillus L.) were studied at the beginning of growth after breaking bud dormancy by means of transfer of the shoots to indoor conditions (November–April) and upon natural sprouting in spring (May). The buds released from dormancy at the beginning of winter sprouted slower and showed lower respiratory activity than the buds that started growing in May. In May, cytochrome respiratory pathway in sprouting buds was 1.3 times more active than energetically ineffective alternative pathway, whereas activity of cytochrome pathway in December was 1.4 times lower as compared with the alternative. In November–December, the rate of heat evolution by the buds was 3–5 times lower than in April–May. In case of early breaking of bud dormancy, the share of respiration energy dissipated as heat was 30% on average. In the buds whose growth was induced later, the value of this parameter was twice as much. The ratio between heat evolution and respiration depended on temperature. High temperature more intensely activated heat evolution than respiration, which caused a decrease in the level of metabolic energy available for growth. In the temperature range of 5–15°C characteristic of the beginning of vegetation, the share of respiration energy dissipated as heat was 2–3 times lower than at 20–30°C, which reflects a great adaptability of V. myrtillus to climatic conditions of the region. Our data suggest that progression through a full cycle of winter dormancy is physiologically important for shoot growth. Early dormancy release brought about changes in respiration and energy balance of the shoots in the initial stage of extra-bud growth.     

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     

The bud and root sprouting capacity of Alternanthera philoxeroides after over-wintering on sediments of a drained canal

Alternanthera philoxeroides (Mart.) Griseb. is one of many aggressive invasive plants that can grow in diverse habitats. Aquatic A. philoxeroides forms dense floating mats over the water surface. However, when water levels decrease during winter, some mats become stranded on exposed sediments and are thus exposed to air. Do the stems of these mats possess the capacity to develop new shoots during the next growing season? In this study, we examined the sprouting of sediment-stranded over-wintering mats of A. philoxeroides. Stems of the over-wintering mats were divided into three types (dry, withered, and fresh stems) depending on moisture content and were immersed in water for 4 weeks to observe the sprouting of axillary buds and roots. The results showed that withered stems yielded much more biomass than dry or fresh stems. Stem moisture content significantly affected the sprouting rate and the length growth rate of buds and roots. Dry stems lacked reproductive capacity. The sprouting rate and length growth rate of the buds and roots were higher in fresh stems than in withered stems. Furthermore, the mean values of the bud sprouting rate and the bud length growth rate were highest during the first week, i.e., most of buds sprouted within 1 week or less. Our results suggest that more than 70% (on a dry weight basis) of the stems in stranded mats possessed rapid sprouting capacity even after over-wintering on the sediment for more than 2 months. This strategy may be an adaptation to the fluctuations inherent in many aquatic habitats, and it possibly explains why A. philoxeroides can flourish even after a dry winter. Handling editor: S. M. Thomaz     

Gibberellic acid-induced prolongation of the dormancy in tubers or rhizomes of several species of East Asian Dioscorea

Effects of the application of gibberellic acid on sprouting of tubers or rhizomes were tested in seven species of the genus Dioscorea that are native to the temperate regions of East Asia. The lowest concentrations for significant inhibition of sprouting in these species varied from 0.1–1 µM Application of gibberellic acid at 100 µM inhibited sprouting for more than 500 days at 20 °C. Some responses to the application of gibberellic acid differed between species and between sections of the genus. In D. japonica, the application of gibberellic acid inhibited sprouting of tubers and bulbils while it promoted seed germination.     

Apical dominance in mulberry (Morus alba): Effects of position of lateral and accessory buds and leaves

Suzuki, T. 1990. Apical dominance in mulberry ( Morus alba ): Effects of position of lateral and accessory buds and leaves. – Physiol. Plant. 78: 468-474.
Removing apical portions of current growth coppice shoots from field-grown, low-pruned stumps of mulberry ( Morus alba L. cv. Shin-ichinose) caused sprouting of one or more upper main buds, almost concurrently with that of accessory buds. However, removal of the new sprouts, including those from accessory buds, slightly enhanced the sprouting of buds immediately below them, and did not affect buds lower down. In contrast, mature leaves inhibited the buds in their axils. Budless, leafy nodes on the upper part of pruned shoots tended to swell after treatment, perhaps due to the accumulation of substances translocated from the roots and possibly from the remaining leaves. Lateral buds at different positions along the shoot differed in their sprouting ability with buds lower on the shoot being more inhibited. This inhibition gradient dissappeared when all coppice shoots on one stump were pruned to the same bud position, suggesting inhibition from neighboring, actively growing shoots. These results demonstrate that acropetal influences are important in bud dominance relationships.     

Plant growth regulator and graft control of axillary bud formation and development in the TO-2 mutant tomato

The torosa-2 tomato mutant is characterized by a strong inhibition of release of axillary shoots, that is not under the control of the main apex and IAA. Microscopic examination indicated that about 70% of leaf axils do not have axillary buds. Of the growth regulators tested, gibberellic acid and cytokinins were able to modify the to-2 phenotype: increasing bud number (GA₃ treated) and developing shoots (both substances). Sequential application of growth regulators demonstrated that bud production was only affected by treatments given between sowing time and 32 days after germination. Grafting experiments indicated that endogenous root factors have no essential role in the lateral branching of the genotypes investigated. The control of axillary bud differentiation and the branching pattern in the to-2 appears to be dependent of a complex mechanism involving gibberellins and cytokinins.     

Seasonal variation in the ultrastructure of the cambium in young stems of willow (Salix viminalis) in relation to phenology

The growth period of Salix viminalis L. (clone 683) plants near Stockholm, Sweden, (59.5°N, 18.3°E) started in April with flowering and ended in October with abscission of the shoot tips. Cell divisions in the vascular cambium started almost two months before sprouting and ceased at about the same time as the elongation growth of the shoots. Phloem cells were apparently produced before flowering, while new xylem production started at the time of flushing. Cytodifferentiation in immature xylem continued until November. Thick-walled cells with protoplasm were observed adjacent to xylem mother cells in the cambium during the winter. The number of radially arranged cells in the cambial zone increased from 3–4 during dormancy to about 18 during the mitotic maximum in July. Seasonal variation was apparent in vacuolization, wall thickness and presence of storage material in the cells. Lipid bodies and protein bodies occurred in both fusiform and ray initials, while starch was observed in ray initials, ray cells and in the phloem. In September the ultrastructure of the cambium showed anatomical features characteristic for both active and dormant cells. Dictyosomes with vesicles and rough ER were present in thick-walled cells that contained lipid bodies and starch granules. Nuclear divisions in the cambium ended in October.     

The Influence of Growth Regulators on Sprouting of Hydrilla Tubers and Turions

The effects of different growth regulators on the sproutingof hydrilla reproductive propagules (Hydrilla verticillata (L.F.)Rovle) were studied. Some growth regulators either had no effector inhibited sprouting. Ethephon, gibberellic acid, and thioureaincreased tuber sprouting significantly. Thiourea was less effectivein inducing sprouting in turions than in tubers. Maintainingtubers at 5 ± 2°C enhanced sprouting significantly.Tubers harvested in summer responded differently to growth regulatorsthan some of the tubers harvested in winter. Therefore, it wasconcluded that two types of dormancy exist in hydrilla tubers.     

Flowering Habit and Vegetative Behaviour in Tea (Camellia sinensis L) Seed Trees in North-East India

Apical growth of a tea shoot occurs by a succession of flushesseparated by short periods of rest. This paper describes theexternal morphology of flowering, fruiting, and abscission ofleaves of the tea plant in north-east India in relation to thephasic activity of shoot apices. All shoots on a tree make leafy growth when a new cycle of growthbegins in the spring, but terminal buds apparently become dormantas the season advances. Apparently dormant terminal buds shedbud scales, leaving on the stem a considerable number of scars,representing leafless cataphyllary flushes. These cataphyllaryflushes are produced at the same time as the leafy flushes onother shoots. A flower is formed only in the axil of a bud scale. Flowerswhich appear to develop in leaf axils are in fact inserted inthe axils of bud scales of the axillary buds. A distal leafy flush is without flowers. Flowers appear in itsleaf axils only when the terminal bud starts growth for thenext higher flush. A distal floriferous cataphyllary flush appearsas a terminal cluster of flowers. Thus, there is an acropetalsuccession of flowers, flush by flush on a caulome, determinedby the phasic activity of the apical bud. The main crop of flowers exposes anthers from the end of thethird flush (late September to early October) until the endof the winter period of growth (late January to early February).In some plants a second, minor crop of flowers appears in thespring between the end of the first and beginning of the secondflushes. In spite of considerable time lag between anthesis,the fruits produced by these two crops of flowers mature anddehisce at the same time during October to November. Abscission of leaves is also dependent upon the phasic activityof the apical buds. Only the top two flushes of a shoot possessleaves. Resumption of apical growth for a third flush, leafyor cataphyllary, causes the abscission of leaves on the lowermostof the three flushes. Two cataphyllary flushes therefore resultin the loss of all leaves on a shoot.