Changes in Dormancy and Sensitivity to Vernalization in Axillary Buds of Satsuma Mandarin Examined in vitro During the Annual Cycle

Effect of season and the presence of fruit on bud-endodormancyand the flowering response to low temperature treatments weredetermined using bud cultures of Owari satsuma mandarin (Citrusunshiu Marc). Bud dormancy was deeper in fruiting as comparedto defruited trees. In fruiting trees, the intensity of buddormancy was highest in spring, decreased to a low value byearly Jul. and then increased until early winter. This increasein dormancy during summer and early autumn did not occur innon-fruiting trees. No flowers formed in buds cultured betweenMay and Sep. Both in fruiting and defruited trees, buds becamecompetent to show a vernalization response to chilling by theend of Oct., at the time they also became capable of sproutingin vitro at low temperature (15/10 °C). There was a directeffect of fruit on the buds which persisted long after fruitremoval and resulted in a reduction of the flowering responseto chilling.Copyright 1995, 1999 Academic Press Citrus flowering, Citrus unshiu Marc., dormancy, flower induction, flowering, in vitro flowering, satsuma mandarin, vernalization     

Sugar and starch changes in pea cotyledons during germination

Changes in starch and sugar contents in the cotyledons during germination have been compared in a smooth (cv. Alaska) and a wrinkled (cv. Progress) cultivar of the garden pea ( Pisum sativum L.). In both cultivars there was an initial accumulation of sucrose due to the hydrolysis of sucrosyl oligosaccharides, but galactose did not accumulate in the cotyledons. Starch mobilization in the Progress pea was linear with time and started before the rise in α-amylase (EC 3.2.1.1) activity in the cotyledons; sucrose was synthesized in the cotyledons, and their excision from the axis resulted in an additional accumulation of this sugar. In the Alaska pea, the onset of starch hydrolysis coincided with the rise in α-amylase activity; no accumulation of sucrose was found in excised cotyledons, whilst the sucrose content decreased continuously in attached cotyledons.
The same sugars were found in the cotyledons of both cultivars, suggesting a common pathway for starch breakdown. Maltose, maltotriose and linear malto-dextrins were not present and only trace amounts of glucose were detected, suggesting a degradation of starch by phosphorylase after an initial attack by α-amylase. α-Amylase activity in the cotyledons was higher in the presence of the axis, but was influenced by the water content of the cotyledons. Transient changes in α-amylase activity correlated well with changes in the rate of starch hydrolysis, but after 2–3 days starch mobilization was reduced in excised cotyledons probably due to the resynthesis of starch.     

Genotype Affects the Morphogenic Response in vitro of Epicotyl Segments of Citrus Rootstocks

The influence of light, hormones and explant orientation onin vitro regeneration in epicotyl cuttings was compared in fourCitrus species (C. aurantium, C. macrophylla, C. reshni andC.sinensis ) and the hybrid Troyer citrange (C. sinensis x Poncirustrifoliata). In all cases, explants planted vertically regeneratedshoots at the apical end by a process of direct organogenesiswithout callus formation. When the Troyer citrange explantswere incubated horizontally, regeneration at the apical endoccurred by an indirect organogenic pathway after callus formation.This change in the pathway of regeneration did not occur inany of the Citrus species, and incubation horizontally resultedin a reduction in the number of buds and shoots formed throughthe direct organogenic pathway. Shoot formation through thedirect organogenic pathway was inhibited by darkness, and thisinhibitory effect was counteracted by the cytokinin benzyladeninein Troyer citrange and, partly, in C. sinensis, but not in C.macrophylla. A non-organogenic callus formed at the basal endof most of the cuttings of C. reshni. InC. sinensis and C. aurantium,a non-organogenic callus formed only in a low proportion ofexplants. Troyer citrange formed an organogenic callus in whichbuds or roots differentiated depending on the auxin/cytokininbalance. C. macrophylla formed callus in the dark but not inthe light. Root formation occurred both in the presence of theauxin naphthaleneacetic acid or low concentrations (2.2 to 4.4µM) of the cytokinin benzyladenine, but no buds were formed.These qualitative and quantitative differences in the organogenicresponse indicate that the conditions for regeneration mustbe optimized for each genotype. Copyright 2000 Annals of BotanyCompany Benzyladenine, citrus, Citrus aurantium, Citrus macrophylla, Citrus sinensis, Citrus sinensis x Poncirus trifoliata, naphthaleneacetic acid, organogenesis, rooting, shoot regeneration, Troyer citrange     

Carbohydrate Availability in Relation to Fruitlet Abscission in Citrus

Abscission of flowers and fruitlets in the Washington navelorange (Citrus sinensis [L.] Osbeck) has been characterizedin relation to carbohydrate availability. A main wave of flowerabscission occurs shortly after anthesis while the carbohydratereserves in the tree are high. Fruitlet abscission starts approx.30 d after the commencement of flowering, while carbohydrates(mainly starch) are being accumulated in the leaves. Flowerand early fruitlet abscission are not caused by carbohydrateshortage. During late fruitlet abscission sucrose concentrationin the leaves falls to a low value demonstrating a limitationin supply and competition among the developing fruitlets forcarbohydrates. Concentrations of sucrose and reducing sugarsin the peel of the fruitlets also fall to low values, and arelationship could be demonstrated between these free sugarlevels and abscission. Ringing increases carbohydrate supplyto fruit and reduces late fruitlet abscission, but only hasa marginal effect on the growth of the fruitlets, which seemsless sensitive than abscission to carbohydrate shortage. Thelimitation of carbohydrate supply to the fruitlets occurs whilestarch levels in the leaves remain high. Slow mobilization ofstarch reserves may be one factor limiting set in Citrus. Copyright2001 Annals of Botany Company Carbohydrate supply, citrus, fruit growth and abscission, ringing, navel orange, starch, sugar metabolism     

Inhibition of flowering in vivo by existing fruits and applied growth regulators in Citrus unshiu

In the Satsuma mandarin ( Citrus unshiu Marc.) the presence of the fruit results in a gradual inhibition of flowering and of bud sprouting. This inhibitory effect starts several months before the onset of the winter rest period and lasts until the end of the accumulation of carotenoids in the fruit peel, more than one month after the completion of fruit growth. During all this time and until natural bud sprouting, flowering and bud sprouting are inhibited by exogenous gibberellic acid. Peak responses to this growth regulator coincide with periods of maximal rates of flowering inhibition by the fruit. Kinetin and abscisic acid, applied at the time of peak response to gibberellic acid, inhibited flowering and reduced the number of shoots developed through the reduction of the number of shoots formed per sprouted node, but failed to reduce the number of nodes which sprouted. The same pattern of sprouting was obtained in trees treated with gibberellic acid during the winter rest period or several months earlier. It is concluded that some step leading to flowering and which determines the differences in sensitivity of the buds to this growth regulator has taken place already at this early date.     

Dry matter accumulation in citrus fruit is not limited by transport capacity of the pedicel

The vascularization of the pedicel in Marisol clementine (Citrus clementina Hort. ex Tanaka) has been characterized in relation to fruit growth. Phloem and xylem formation occurred during the first half of the period of fruit growth. Phloem cross-sectional area reached its maximum value by the end of fruitlet abscission, 78 d after anthesis (DAA), shortly after the rate of accumulation of dry matter in fruitlets reached its maximum value. Secondary xylem formation occurred until day 93, well after the end of fruitlet abscission. At fruit maturity, xylem accounted for 42-46 % of the cross-section of the pedicel. Vessels differentiated in this late-formed xylem. Formation of phloem and early xylem was directly related to fruitlet size (and growth rate). Differences in the rate of formation of conductive tissues in the pedicel of the developing fruitlets followed rather than preceded the differences in growth rate. Specific mass transfer (SMT) in the phloem was highest in the fastest growing fruitlets, and peaked during the late stages of fruitlet abscission (72-78 DAA) and during the main period of fruit growth (107-121 DAA). Application of a synthetic auxin to developing fruits, either at the end of flowering (2,4-D) or by day 64 after flowering (2,4-DP), increased the growth rate of the fruit and fruit size at maturity (8-13 % increase in fruit diameter at maturity). These auxin applications also enhanced the formation of conductive tissues in the pedicel, with a specific effect on phloem formation. Applying auxin at flowering resulted in a reduction in the phloem SMT by days 72-78, whereas auxin application on day 64 increased this parameter. Despite this difference in behaviour, which resulted from the different time-course of the growth response of the fruit to auxin applications, these applications increased fruit size to a similar extent. Severing 37 % of the phloem of the pedicel during the main period of fruit growth resulted in an increase in the specific mass transfer in the phloem but had no influence on fruit growth. These observations demonstrate that the transport capacity in the phloem of the pedicel does not limit fruit growth and, within the limits of our experiments, an increase in demand by the fruit appeared to be matched by an increase in SMT. The dependence of late xylem formation (after the period of fruitlet abscission) on fruitlet growth was demonstrated in Salustiana orange [Citrus sinensis (L.) Osbeck] by means of controlling fruit growth through the manipulation of leaf area. Fruit growth at this time was more closely related to leaf area than to carbohydrate levels, suggesting that it may be limited by current photosynthesis.     

Effects of gibberellin A3 and cytokinins on natural and post-harvest, ethylene-induced pigmentation of Satsuma mandarin peel

Natural and post-harvest ethylene-induced pigment changes in the rind of Satsuma mandarin ( Citrus unshiu Marc.) fruits respond differently to the exogenous application of growth regulators. Both gibberellin A₃ and the synthetic cytokinins N⁶-benzyladenine and kinetin opposed the ethylene-induced chlorophyll destruction, while the loss of chlorophyll during natural maturation was retarded by the gibberellin but not by the cytokinins. This different behaviour suggests that ethylene may not be playing a central role in the endogenous control of ripening. Carotenoid accumulation during natural maturation is apparently controlled through a different mechanism than chlorophyll loss since it is reduced both by the cytokinins and gibberellin A₃.
Kinetin and gibberellin A₃ increased to a similar extent the accumulation of reducing sugars and free amino acids, and reduced that of non-reducing sugars in the peel during natural maturation. Their differential effect on chlorophyll loss may not be explained through their effects on sugar accumulation.