Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by Subject "Alternate fruiting cycle"

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    Critical factors concomitant to the physiological development of alternate bearing in citrus (Citrus spp.)
    (Stellenbosch : Stellenbosch University, 2018-12) Stander, Ockert Petrus Jacobus; Cronje, P. J. R.; Barry, G. H.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The significance of carbohydrates, mineral nutrients and phyto-hormones was investigated in relation to their possible roles in selected phenological events in alternate bearing ‘Nadorcott’ mandarin (C. reticultata Blanco) trees. Crop load in ‘Nadorcott’ mandarin trees was influenced by flowering intensity. The most important determinants of flowering intensity were the amount of new vegetative shoot growth and resulting number of new potential floral buds that developed during summer, and the influence of fruit on floral bud development during winter. The lack of development of summer vegetative shoots in “on” trees was not related to leaf carbohydrate concentration. In “off” trees, root sugar concentration peaked during full bloom and high root growth activity was observed prior to the vegetative shoot flush in summer. In “on” trees, fruit were the major carbohydrate sinks and probably disturbed the balance between vegetative shoot development and root growth. Sugar concentration in roots in “on” trees was 3-fold lower, root growth was absent, and shoot growth was halved. The concentration of mineral nutrients in leaves was a response to fruit load and not related to parameters of flowering or vegetative shoot growth. Measurements of phyto-hormones in leaves and roots confirmed that the inhibition of summer vegetative shoots was related to a high concentration of 1 H-indole-3-acetic acid (IAA) in leaves. High concentrations of dihydrophaseic acid and the abscisic acid (ABA) glucose ester suggested that IAA might have acted synergistically with ABA to create a growth inhibition in fruiting shoots. As a result, cytokinins did not contribute to the development of new summer vegetative shoots. High gibberellin concentration in leaves in May and June contributed to limited flowering in “on” trees. Consistent with this interpretation, treatment of “off” trees with 40 mg·L-1 gibberellic acid inhibited flowering, whereas soil and foliar treatments of “on” trees with 1000 mg·L-1 paclobutrazol or uniconazole, gibberellin biosynthesis inhibitors, increased flowering and resulted in fruit development from buds of “on” shoots.