Masters Degrees (Horticulture)

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Browsing Masters Degrees (Horticulture) by browse.metadata.advisor "Cronje, Paul J. R."

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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.
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    Studies on the seedless fruit set of Valencia oranges
    (Stellenbosch : Stellenbosch University, 2021-12) Van Zyl, Carel Petrus; Cronje, Paul J. R.; Theron, K.; Stander, O. P. J.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The profitability of a citrus orchard is determined by consistent fruit quality and yield, but seedless Valencia oranges have variation in this regard due to irregular fruit set. Environmental factors and cultural practices on Valencia fruit set under South African conditions are not well understood, and contradictory results between climatic regions exist. This study assessed factors that may influence the fruit set of seedless Valencia oranges. A review of weather conditions and production trends in two climatically diverse seedless Valencia production regions of South African (Kirkwood and Letsitele) revealed that tracking weather extremes help explain the variation in yield between orchards during growing seasons and shed some light on when the weather or the tree's physiology is the cause of a poor yield. Even so, the study highlighted the lack of targeted climatic studies in the various South African citrus-producing regions to identify possible weather factors contributing to inconsistent fruit set in these regions. Improved understanding should help manage their financial risk in high-value orchards by allowing them to take preventative measures. Secondly, the study included research into the cost of low set and yield of seedless Valencia cultivars in the Letsitele area Limpopo, using the orchard level budget model. The model highlighted that high fruit prices would decrease the possible financial loss an orchard incurs, but to reach these high fruit prices for high-quality fruit, the producers must use innovative horticultural practices in the orchard. This reduces the risk of the orchard not sustainably producing economically viable levels of high-quality seedless Valencia oranges. The effects of nitrogen (N), GA3 and uniconazole on ‘Midknight’ and ‘Delta’ Valencia to improve fruit set were tested and delivered inconsistent results but indicated that GA₃ application should be applied as standard practice to seedless Valencia oranges. Uniconazole applications can increase the yield, and if high N applications were made, it should be applied to counteract the possible excessive vegetative growth that it may cause. Higher levels N consistently led to the production of seedless Valencia oranges with a decreased internal quality. Finally, the study aimed to measure the effect of the soil potential and root development on fruit set of seedless Valencia oranges and provide an accurate classification system specifically for citrus, which should help the producer’s decision making, especially in making pre-establishment corrections, the optimisation of fertilisation and irrigation. The study also showed that higher potential soils improved the fruit size distribution produced, improving the orchards profitability. The improvement is possible because of improved nutrients, oxygen and water allocation to the root zone, and the tree and fruit itself during fruit development.