Cytokinins and fruit development in the kiwifruit (Actinidia deliciosa). II. Effects of reduced pollination and CPPU application

The significance of changes in cytokinin content during early fruit growth was examined in the kiwifruit ( Actinidia deliciosa var. deliciosa cv. Hayward). Fruit growth was modified by the reduction of seed number or by the application of the synthetic phenylurea cytokinin N -(2-chloro-4-pyridyl)- N -phenylurea (CPPU). The influence of these treatments on cell division was monitored by flow cytometry and changes in the endogenous cytokinins were measured at days 10 and 20 after anthesis, using high-performance liquid chromatography and radioimmunoassay. Total cytokinin levels appeared not to be limiting growth since the highest total cytokinin concentration was detected in unpollinated fruit, which abscised by day 25 after anthesis. However, compared with control fruit which had the highest concentration of zeatin (Z) 10 days post anthesis, Z levels were low in unpollinated fruit. It is hypothesised that an increase in Z is the critical change in cytokinin metabolism required for the initiation of cell division and fruit growth. The synthetic cytokinin CPPU promoted fruit development, but there was a decrease in the endogenous cytokinin concentration. Zeatin was not detected in CPPU-treated fruit. Cell division was reduced in unpollinated fruitlets but there was no significant difference ( P > 0.05) between the other treatments. Differences in final fruit size appeared to be due to cell expansion.     

Effect of temperature on the growth of individual cucumber fruits

In order to study the effect of temperature on the growth of individual fruits in cucumber (cucumis sativus L. cv. Corona), fruits were grown at 17. 5. 20,25 and 30°C continuously or the fruit temperature was changed from 17. 5 to 27.5°C or vice versa. Fruit development appeared to be closely related to the temperature sum. When the growth of a fruit was not constrained by assimilate supply, a decrease in growing period with increasing temperature was more than compensated for by a strong increase in growth rate, resulting in an increase in final fruit weight. However, when the growth of a fruit was constrained by assimilate supply, the increase in growth rate with increasing temperature was small and did not compensate for the decrease in growing period, resulting in a decrease in final fruit weight. Determinations of cell number and size showed that the effect of temperature on fruit growth was due to effects on cell expansion rather than on cell division. When growth was not constrained by assimilate supply. However, when assimilate supply did constrain fruit growth the number of cells per fruit decreased with increasing temperature, while the effect on cell size was negligible. In all stages of fruit development, the growth rate of a cucumber fruit responded within one day to a change in temperature. It was not irreversibly impaired by a low temperature (17. 5°C) during the early development of a fruit. A high temperature treatment (27. 5°C), however, had a great effect on the growth rate of a fruit after the temperature treatment had terminated. At all stages of fruit development (even before anthesis) a period of four days at 27. 5°C resulted in a pronounced stimulation of the growth rate afterwards at 17. 5°C.     

Effect of assimilate supply on the growth of individual cucumber fruits

The effects of assimilate supply on the growth of individual fruits during different stages of fruit development were analysed in cucumber ( Cucumis sativus L. cv. Corona). The assimilate supply was varied by maintaining different numbers of fruits per plant or maintaining different irradiances. The growth rate of a cucumber fruit strongly increased with increasing assimilate supply, but its growing period was not noticeably affected. At a low assimilate supply both cell number and cell size were reduced. Increasing the assimilate supply at different stages of fruit development showed that the early development of a cucumber fruit was not crucial for setting its growth potential. A small number of cells, due to a low assimilate supply, during early fruit development, was to a great extent compensated by an increased expansion rate of individual cells. It is concluded that cell number is not an important determinant of fruit size in cucumber, although fruit size is often positively correlated with cell number. In the early stages of fruit development the effect of irradiance on the fruit growth rate depends on the presence of an earlier developed fruit because of dom-inance between fruits. In later stages of fruit development, a decrease in irradiance reduced the growth rate of all fruits relatively to the same extent independent of age or presence of other fruits.     

Enhanced inflorescence development in tomato by growth substance treatments in relation to 14C-assimilate distribution

Inflorescence development in tomato plants ( Lycopersicon esculentum Mill., cv. King Plus) grown under a low-light regime is promoted by exogenous applications of a mixture of N⁶-benzyladenine (BA) and gibberellins A₄₊₇ (GA) directly on the inflorescence. The photosynthetic rate of the young mature leaf, which feeds the developing inflorescence, and the proportion of ¹⁴C-assimilates exported from the source leaf are not affected by the growth substance treatment, but the pattern of ¹⁴C-assimilate distribution is altered. Assimilate supply to the treated inflorescence increases concomitantly with a decrease in the ¹⁴C import into the apical shoot, reflecting a competition between these two plant parts. The increased assimilate accumulation in the treated inflorescence is apparent 1 day after the first application of BA+GA, and precedes any morphological changes in the reproductive structure. These results are discussed in relation to nutritional hypotheses that regard assimilate supply as limiting for reproductive development.     

The role of auxin and sugar signaling in dominance inhibition of inflorescence growth by fruit load

Dominance inhibition of shoot growth by fruit load is a major factor that regulates shoot architecture and limits yield in agriculture and horticulture crops. In annual plants, the inhibition of inflorescence growth by fruit load occurs at a late stage of inflorescence development termed the end of flowering transition. Physiological studies show this transition is mediated by production and export of auxin from developing fruits in close proximity to the inflorescence apex. In the meristem, cessation of inflorescence growth is controlled in part by the age-dependent pathway, which regulates the timing of arrest. Here, we show the end of flowering transition is a two-step process in Arabidopsis (Arabidopsis thaliana). The first stage is characterized by a cessation of inflorescence growth, while immature fruit continues to develop. At this stage, dominance inhibition of inflorescence growth by fruit load is associated with a selective dampening of auxin transport in the apical region of the stem. Subsequently, an increase in auxin response in the vascular tissues of the apical stem where developing fruits are attached marks the second stage for the end of flowering transition. Similar to the vegetative and floral transition, the end of flowering transition is associated with a change in sugar signaling and metabolism in the inflorescence apex. Taken together, our results suggest that during the end of flowering transition, dominance inhibition of inflorescence shoot growth by fruit load is mediated by auxin and sugar signaling.

Dominance inhibition of inflorescence shoot growth by fruit load involves auxin and sugar signaling during the end of flowering transition.     

Partitioning of (13)C-photosynthate from spur leaves during fruit growth of three Japanese pear (Pyrus pyrifolia) cultivars differing in maturation date

BACKGROUND AND AIMS: In fruit crops, fruit size at harvest is an important aspect of quality. With Japanese pears (Pyrus pyrifolia), later maturing cultivars usually have larger fruits than earlier maturing cultivars. It is considered that the supply of photosynthate during fruit development is a critical determinant of size. To assess the interaction of assimilate supply and early/late maturity of cultivars and its effect on final fruit size, the pattern of carbon assimilate partitioning from spur leaves (source) to fruit and other organs (sinks) during fruit growth was investigated using three genotypes differing in maturation date. METHODS: Partitioning of photosynthate from spur leaves during fruit growth was investigated by exposure of spurs to (13)CO(2) and measurement of the change in (13)C abundance in dry matter with time. Leaf number and leaf area per spur, fresh fruit weight, cell number and cell size of the mesocarp were measured and used to model the development of the spur leaf and fruit. KEY RESULTS: Compared with the earlier-maturing cultivars 'Shinsui' and 'Kousui', the larger-fruited, later-maturing cultivar 'Shinsetsu' had a greater total leaf area per spur, greater source strength (source weight x source specific activity), with more (13)C assimilated per spur and allocated to fruit, smaller loss of (13)C in respiration and export over the season, and longer duration of cell division and enlargement. Histology shows that cultivar differences in final fruit size were mainly attributable to the number of cells in the mesocarp. CONCLUSIONS: Assimilate availability during the period of cell division was crucial for early fruit growth and closely correlated with final fruit size. Early fruit growth of the earlier-maturing cultivars, but not the later-maturing ones, was severely restrained by assimilate supply rather than by sink limitation.     

The role of sink demand in carbon partitioning and photosynthetic reinvigoration following shoot decapitation

Photosynthesis, growth, and carbon partitioning of vigorous coppice shoots were compared with the slower growing intact shoots of Populus maximowiczii × nigra L. MN9 to determine the relationship between carbon partitioning and photosynthetic rate. Relative height growth rate of coppice shoots was 2.2 times that of intact shoots with net photosynthetic rate 1.9 times that of intact shoots. Coppice leaves exported a larger proportion of newly-fixed assimilate (11% compared with 6%) after a 4-h chase. The greater export from coppice leaves was correlated with a greater proportion of [¹⁴C]-labelled photosynthate deposited as starch in stems 4 cm below the point of label application. Coppice leaf assimilate levels were reduced to 15% that of leaves on intact plants, but coppice leaves had twice the concentration of labelled sucrose. Carbohydrates constituted 55% of the water-soluble [¹⁴C]-labelled photosynthate in leaves of coppice shoots compared with 40% in intact shoots. The results suggest that carbon allocation and partitioning in coppice shoots were altered towards production and export of new assimilate, and support the hypothesis that photosynthetic rate is responsive to sink demand for assimilates.     

Endogenous hormonal levels and growth of dark-incubated shoots of Catasetum fimbriatum

Apical shoots and Lateral buds of the epiphytic orchid Catasetum fimbriatum give rise to rootless etiolated stolons, when cultured in the presence of light and then transferred to the dark. The stolons are characterized by fast and continuous apical longitudinal growth. Measurements of four endogenous cytokinin, indole-3-acetic acid (IAA) and abscisic acid (ABA) levels in etiolated shoots and light-grown plants were low. However, after transfer of green plants to the dark, cytokinin Levels increased 3- and 7-fold by 10 and 30 days of incubation, respectively. IAA levels also increased significantly, but the increase was not as great as for cytokinins. A similar trend was observed in the roots. A close relationship seems to exist between both cytokinin accumulation and the formation of etiolated stolons. Variations in ABA levels were practically inconspicuous. The presence of paclobutrazol in the medium, a potent inhibitor of gibberellin synthesis, strongly inhibited etiolated and non-etiolated longitudinal shoot growth, although no apparent effect was observed on apical meristem activity.     

Apical dominance

Apical dominance is the control exerted by the apical portions of the shoot over the outgrowth of the lateral buds. The classical explanations for correlative inhibition have focused on hormone/nutrient hypotheses. The remarkable progress that has been made in the technology of endogenous hormone quantification in plant tissue has not been accompanied by comparable progress in the elucidation of mechanisms of hormone action in apical dominance. Evidence from hormonal studies suggests that apically produced auxin indirectly suppresses axillary bud outgrowth that is promoted by cytokinin originating from roots/shoots. Significant involvement with other hormones, although less likely, has not been ruled out. Possible changes in tissue sensitivity to hormones should not be overlooked. Auxin-induced oligosaccharide signals originating from the cell walls of shoot tips or polyamines may function as secondary inhibitors to bud growth. Alternatively, apically produced auxin may suppress lateral bud growth by inhibiting auxin export from these buds. Support for a critical role for nutrients in apical dominance keeps resurfacing, especially for auxin-directed nutrient transport and for water as a possible inducing signal for bud outgrowth. Histological and biochemical analyses of lateral buds recently released from apical dominance are urgently needed. The feasibility of manipulating endogenous auxin/cytokinin content in plant tissue by gene insertion and modulation opens the door to exciting approaches as does the use of hormone insensitive/resistant mutants. There is also need to recognize the existence of variability of apical dominance mechanisms among different plant types. The aesthetic and economic implications of understanding apical dominance for the modification of plant structure and form are extremely significant.     

Light cytokinin interactions in shoot formation in epicotyl cuttings of Troyer citrange cultured in vitro

Ilumination did not affect the pathway of shoot regeneration at the cut edges of epicotyl explants of Troyer citrange (Moreira-Dias et al. 2000, 2001), but signigficantly affected the number of developed shoots and the response to exogenous cytokinins. Shoot regeneration at the apical end occurred through a direct organogenic pathway without callus formation. For explants incubated in the light, this regeneration did not require cytokinin addendum, but the number of shoots formed was significantly increased by benzyl adenine, but not by zeatin or kinetin. Incubation in the dark almost suppressed shoot formation at the apical end. The addition of benzyl adenine or kinetin, but not of zeatin, restored shoot formation in the dark to the value obtained in the light. At the basal end of the explants shoot regeneration occurred through an indirect organogenic pathway after the formation of a primary callus. In explants incubated in the light, callus formation and shoot growth was supported by a low (0.5–1 mg l⁻¹) benzyl adenine concentration and by zeatin. Kinetin did not support callus growth. Shoot formation was higher in the presence of benzyl adenine (0.5–1 mg l⁻¹) than of zeatin, but was inhibited by a high (5 mg l⁻¹) benzyl adenine concentration. Incubation in the dark increased callus growth and shoot formation at the basal cut as compared to explants incubated in the light. The three cytokinins tested supported callus growth and shoot formation in the dark, zeatin being the most effective and kinetin the least. In terms of number of shoots developed, the optimum cytokinin addendum depended on the pathway of organogenesis and the conditions of incubation. The maximum number of shoots developed at the apical end was obtained when the incubation was performed in the light in the presence of benzyl adenine. At the basal end, the optimal conditions were incubation in the dark in the presence of zeatin. It was not always possible to define an optimal cytokinin concentration as the curve concentration/response varied from experiment to experiment, which seemed unrelated to the endogenous cytokinin concentration in the explants.     

Competition for assimilates and fruit position affect fruit set in indeterminate greenhouse tomato

Localization and characterization of fruit set in winter tomato crops was investigated to determine the main internal and external controlling factors and to establish a quantitative relationship between fruit set and competition for assimilates. Individual fruit growth and development was assessed on a beef tomato cultivar during the reproductive period (first nine inflorescences). A non-destructive photograph technique was used to measure fruit growth from very early stages of their development and then calliper measurements were made on big fruits. From these measurements we determined the precise developmental stage at which fruit growth stopped. Fruit potential growth, which is defined as the growth achieved in non-limiting conditions for assimilate supply, was also assessed by this method on plants thinned to one flower per inflorescence. The latter was used to calculate the ratio between actual and potential growth, which was found to be a good index of the competition for assimilates. Time lags of fruit set were observed mainly on distal organs. When more than three flowers were left on each inflorescence, distal organs developed at the same time as proximal organs of the following inflorescence. Consequently they were submitted to a double competition within one inflorescence and among inflorescences. It was shown that, what is commonly named 'fruit set failure', is not an irreversible death of the organ and that a small fruit could resume growth after a delay of several weeks as soon as the first fruits ripened and thus ceased to compete for assimilates. In that case proximal fruits resumed growth before distal ones. The delayed fruits contained only few seeds but a germination test confirmed that fertilization took place before fruit set failed. Competition for assimilates was calculated during plant development by the ratio between actual and potential fruit growth. Potential growth of proximal fruits was strongly dependent on the position of the inflorescence on the stem, whereas potential growth of distal fruits was lower than or equal to that of proximal fruits of the same inflorescence and it was independent on the inflorescence position. We took into account both inflorescence and fruit positions to establish a quantitative relationship between fruit set of individual inflorescences and the ratio between actual and potential fruit 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     

Enhanced inflorescence development in bougainvillea "san diego red" by removal of young leaves and cytokinin treatments

Removal of young leaves and application of the cytokinin, N-benzyla-α-(tetrahydro-2H-pyran-2yl)-adenine promote inflorescence development in Bougainvillea “San Diego Red”. Defoliation greatly increased the amount of assimilate accumulated at the shoot tip 1 to 2 days after treatment. Cytokinin applications further increased the amount accumulated and this increase was apparent 4 days before morphological changes could be detected at the inflorescence axes. Short days promoted inflorescence development and also increased assimilate accumulation at the reproductive axes; thus, it is suggested that the role of short day induction in bougainvillea may be that of redirecting the flow of assimilates, perhaps by its influence on cytokinin synthesis and distribution.     

Apical control of branch growth and angle in woody plants

Apical control is the inhibition of a lateral branch growth by shoots above it (distal shoots). If the distal shoots are cut off to remove apical control, the lateral branch can grow larger and may bend upwards. Apical control starts when new lateral buds grow after passing through a period of dormancy. Buds initially break and produce leaves, then apical control is exerted and the lower (proximal) laterals stop growing. Apical control also inhibits growth of large, old branches. Gravimorphism and restricted water and nutrient transport can inhibit branch growth, but they are not primary mechanisms of apical control. Apical control may reduce branch photosynthesis. Under apical control allocation of branch-produced assimilate to the stem is relatively high, so low assimilates in the branch may limit branch growth even though hormone levels are adequate for growth. Hormones appear to be involved in apical control, but it is not known how. One role of hormones may be to maintain the strength of the stem sink for branch-produced assimilate. Upward bending of a woody branch after release from apical control requires both new wood production and production of wood cells that can generate an upward bending moment. Apical control inhibits radial growth of branches and, in some species, may regulate the production of wood with an upward bending moment.     

Fruit size: Towards an understanding of the metabolic control of fruit growth using avocado as a model system

Final fruit size is the consequence of complex metabolic events that occur between fruit set and maturation. Disruption of these biochemical and molecular processes at any stage during fruit growth will impact on final fruit size. Because fruit size is a function of cell number rather than cell size, factors affecting cell division cycle activity assume importance. In this paper, we focus attention on the metabolic control of fruit growth using avocado as a model system. Three areas of current interest are highlighted, viz. the contribution by isoprenoid metabolism in the control of cell proliferation, the role played by carbohydrate content and composition in signalling changes in metabolite status and gene expression and maintenance of plant hormone homeostasis. Central to the process of fruit growth and control of final fruit size by cell division is 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) and activity of the sucrose non-fermenting 1-related protein kinase (SnRK1) complex. It is argued that sugar content and composition of sink cells impact on SnRK1 (and hexokinase) to modulate expression of sugar-metabolizing enzymes, HMGR and molybdenum cofactor (MoCo)-containing enzymes. These changes, in turn, impact on hormone metabolism by affecting allocation of the purine-derived MoCo to aldehyde oxidase and thus the endogenous concentration of indole-3-acetic acid, abscisic acid and cytokinin (CK) to alter plant hormone homeostasis. These aspects are integrated into a model to explain the metabolic control of avocado fruit growth and final fruit size.     

Tobacco cells transformed with the fission yeast Spcdc25 mitotic inducer display growth and morphological characteristics as well as starch and sugar status evocable by cytokinin application.

In plants, the G2/M control of cell cycle remains an elusive issue as doubts persist about activatory dephosphorylation--in other eukaryotes provided by CDC25 phosphatase and serving as a final all-or-nothing mitosis regulator. We report on the effects of tobacco (Nicotiana tabacum L., cv. Samsun) transformation with fission yeast (Schizosaccharomyces pombe) cdc25 (Spcdc25) on cell characteristics. Transformed cell suspension cultures showed higher dry mass accumulation during the exponential phase and clustered more circular cell phenotypes compared to chains of elongated WT cells. Similar cell parameters, as in the transformants, can be induced in WT by cytokinins. Spcdc25 cells, after cytokinin treatment, showed giant cell clusters and growth inhibition. In addition, Spcdc25 expression led to altered carbohydrate status: increased starch and soluble sugars with higher sucrose:hexoses ratio, inducible in WT by cytokinin treatment. Taken together, the Spcdc25 transformation had a cytokinin-like effect on studied characteristics. However, endogenous cytokinin determination revealed markedly lower cytokinin levels in Spcdc25 transformants. This indicates that the cells sense Spcdc25 expression as an increased cytokinin availability, manifested by changed cell morphology, and in consequence decrease endogenous cytokinin levels. Clearly, the results on cell growth and morphology are consistent with the model of G2/M control including cytokinin-regulated activatory dephosphorylation. Nevertheless, no clear link is obvious between Spcdc25 transformation and carbohydrate status and thus the observed cytokinin-like effect on carbohydrate levels poses a problem. Hence, we propose that Spcdc25-induced higher CDK(s) activity at G2/M generates a signal-modifying carbohydrate metabolism to meet high energy and C demands of forthcoming cell division.     

Within-bunch Variability in Banana Fruit Weight: Importance of Developmental Lag Between Fruits

Within-bunch (inflorescence) variability in banana fruit weightis of great importance: distal fruits (at the bottom of thebunch) are 30 to 40% smaller than basal fruits at the top. Wehypothesize that this variability is related to a developmentallag between fruits. To validate this hypothesis, histologicalstudies (evolution in number of cells along the fruit radius,starch granule number and size) associated with physiologicalmeasurements (pulp dry weight, dry matter and starch concentration)were carried out. Fruit development stages were dated in cumulativedegree-days (dd) from flower emergence to 3 weeks after theharvest stage (1300 dd). For a fruit located at the top of thebunch, cell divisions ceased around 350 dd and cells began tofill with starch as soon as they appeared. A developmental lagbetween fruits at the top and bottom of the bunch was observed:cell divisions started and stopped approx. 70 dd later in bottom(distal) compared to top (basal) fruits. At the end of celldivisions, basal fruits had a higher number of cells along thefruit radius. This difference in cell number may be due to increasedcompetition for assimilates between fruits when cell divisionoccurs in distal fruits. Variability in cell number may be relatedto variability in pulp dry weight. We conclude that within-bunchvariability in banana fruit weight is related to a differencein cell number and age. Copyright 2001 Annals of Botany Company Musa acuminata, banana, fruit development, fruit growth, cell number, starch accumulation, fruit quality, fruit green-life, fruit-fruit competition     

A Top-Down Hierarchy in Fruit Set on Inflorescences in Iris fulva (Iridaceae)

Abstract: In Iris fulva, the apical flower on an inflorescence opens first, and the flowering sequence then proceeds from the most basal flower upward to the apex (acropetally). Flowering order and flower position are thus partially decoupled, and this provides an opportunity to separately investigate the effects of these two factors upon fruit formation. We recorded natural patterns of fruit set and seed production, and found that fruit set patterns were determined by what appears to be a form of apical dominance. The first, apical flower had the highest fruit set and seed production. Fruit set decreased towards the base of the inflorescence, with later-opening flowers having a higher fruit set. This is contrary to the pattern usually observed in other acropetally flowering plants, which is a higher fruit set for basal, early flowers. By performing additional hand pollinations and counting pollen grains on naturally pollinated flowers, we found that pollen deposition was not a major factor limiting fruit set, and that it could not explain the large difference in fruit set between the apical and basal flowers. Removing the first flower after it had wilted increased fruit set in the remaining flowers, but mainly in the more apical flowers. Only by removing the two topmost flowers could we obtain an increase in fruit set for the basal flower. The fact that the basal flower rarely sets fruit, despite being closer to resources, suggests that the apical meristem is either a strong sink for resources or produces hormones that form a gradient along the inflorescence, which is comparable with apical dominance.     

Dormancy and Flowering Are Regulated by the Reciprocal Interaction Between Cytokinin and Gibberellin in <Emphasis Type="Italic">Zantedeschia</Emphasis>

Floral productivity of Zantedeschia is dependent on the conversion rate of buds to shoots, which is controlled by varying intensities of para- (apical dominance), endo- (dormancy), and ecodormancy. We present evidence of cross-talk between cytokinin and gibberellin in their complementary roles to alleviate bud dormancy and enhance flowering in a perennial geophyte. We assessed the impact of cytokinin and gibberellin, applied alone and in sequential combinations, on bud fate during three phases along the ontogeny of growth, which coincide with the progressive transition of buds from apical dominance to dormancy. Given that cytokinin can stimulate branching and gibberellin can induce flowering in Zantedeschia, we measured these phenotypic responses as parameters of bud commitment. The efficacy of cytokinin alone to stimulate branching declined with the transition to dormancy (phase 1 = 3.8 ± 0.2 shoots; phase 3 = 1.0 ± 0.3 shoots). To sustain branching during this transition, a sequential application of gibberellin was necessary. Gibberellin alone failed to stimulate branching. The efficacy of gibberellin alone to stimulate flowering diminished with the transition to dormancy. Any flowering during this transition occurred only after the sequential application of cytokinin. Cytokinin alone failed to stimulate flowering. Alleviating bud dormancy and enhancing flowering in Zantedeschia, achieved by the reciprocal cross-talk between cytokinin and gibberellin, contributes to the pool of evidence drawing common mechanisms between dormancy and flowering and may have commercial implications.     

The gravity-regulated growth of axillary buds is mediated by a mechanism different from decapitation-induced release

When the upper part of the main shoot of the Japanese morning glory (Pharbitis nil or Ipomoea nil) is bent down, the axillary bud situated on the uppermost node of the bending region is released from apical dominance and elongates. Here, we demonstrate that this release of axillary buds from apical dominance is gravity regulated. We utilized two agravitropic mutants of morning glory defective in gravisensing cell differentiation, weeping (we) and weeping2 (we2). Bending the main shoots of either we or we2 plants resulted in minimal elongation of their axillary buds. This aberration was genetically linked to the agravitropism phenotype of the mutants, which implied that shoot bending-induced release from apical dominance required gravisensing cells. Previous studies have shown that basipetal translocation of auxin from the apical bud inhibits axillary bud growth, whereas cytokinin promotes axillary bud outgrowth. We therefore compared the roles of auxin and cytokinin in bending- or decapitation-induced axillary bud growth. In the wild-type and we plants, decapitation increased cytokinin levels and reduced auxin response. In contrast, shoot bending did not cause significant changes in either cytokinin level or auxin response, suggesting that the mechanisms underlying gravity- and decapitation-regulated release from apical dominance are distinct and unique.