Department of Horticulture

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Browsing Department of Horticulture by browse.metadata.advisor "Bester, Rachelle"

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    The manipulation of flowering time to produce lemon fruit out of season
    (Stellenbosch : Stellenbosch University, 2020-04) Krogsheepers, Cornelius; Stander, Ockert P. J.; Cronje, Paul J. R.; Maree, H. J.; Bester, Rachelle; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: South African lemon (Citrus limon) plantings have increased considerably during the past few years, leading to a possible oversupply of fruit in winter and subsequent declines in current export prices. To address this potential problem, an attempt was made to shift the major lemon fruit harvest to summer, a period during which prices are traditionally higher. This was done by physiologically manipulating the natural flowering habit of a lemon tree, firstly, through an inhibition of spring flowering with the application of foliar gibberellic acid (GA3) sprays during the floral induction period in autumn; and, secondly, to stimulate a late-summer or autumn flowering response through induced water-deficit stress in mid-summer in a manner similar to the Sicilian forzatura technique. An additional objective of the study was to evaluate the effect of these horticultural manipulations on the expression of key citrus flowering genes to shed light on how the reproductive processes are expressed at a molecular level. The lowest concentration of GA3 foliar sprays, viz. 10 mg·L⁻¹, consistently reduced spring flowering compared with untreated trees and to the same extent as the 20 and 40 mg·L⁻¹ treatments. Vegetative growth was stimulated in most cases compared with the control. The expression of the FLOWERING LOCUS T (FT) gene, which is known to integrate floral pathways to initiate the floral cascade, was decreased in buds of trees that received foliar GA3 treatments. Similarly, mRNA levels of APETALA 1 (AP1) were decreased in GA3 treated trees. As expected, AGAMOUS (AG) was not sufficiently expressed to quantify. Adaptation of the forzatura technique was however not consistently successful under South African conditions in this study. Varying climatic conditions across seasons and areas resulted in a lack of sufficient water-deficit stress, which translated in non-significant changes in the expression of citrus flowering genes and varying and limited floral induction. In cases where stem-water potential reached levels lower than -2.5 MPa, a significant flowering reaction was observed about three weeks after re-irrigation, similar to Sicilian observations. However, fruit set was very low due to rapid and extensive floral abscission, possibly as a result of low carbohydrate levels and/or wind and insect damage. This study confirmed the efficacy of GA3 in restricting lemon flowering, while proving that this inhibitive effect is due to downregulation of FT. Additionally, the forzatura technique was proven to be reproducible in Western Cape production regions, albeit with limited success. Future research focused on increasing fruit set after water-deficit stress conditions may support the practicality and commercial viability of the forzatura technique.