Browsing by Author "Mupambi, Giverson"

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    Studies to reduce the size of the navel-end opening of navel oranges
    (Stellenbosch : University of Stellenbosch, 2010-03) Mupambi, Giverson; Verreynne, J. S.; Cronje, P. J. R.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The size of the navel-end opening is an important parameter for external fruit quality in navel oranges [Citrus sinensis (L.) Osbeck]. The application of 2,4-dichlorophenoxyacetic acid (2,4-D) to increase the percentage of closed navel-ends and reduce the size of the navel-end opening was conducted on six different navel orange cultivars. Treatments were applied at full bloom (FB), 100% petal drop (PD), as well as 2 weeks (2 WAPD) and 4 weeks after 100% petal drop (4 WAPD), at 15 mg·L-1 to 45 mg·L-1, to determine the most effective timing and concentration. The application of 2,4-D at FB consistently decreased the average navel-end size (all fruit) and increased the percentage of closed navel-ends in all the cultivars, with later applications at PD, 2 WAPD and 4 WAPD being generally ineffective, regardless of the concentration applied. There were no major negative side effects on internal and external fruit quality, except for the reduction in juice content (%), especially with the later treatments. Therefore, 15 mg·L-1 2,4-D at FB can be applied to increase the percentage of closed navel-ends and possibly increase export packouts. Navel oranges have a small secondary fruit located inside the primary fruit at the stylar-end and an opening at the stylar-end called the navel-end opening or the stylar-end aperture. Fruit growth and development was studied in three navel orange cultivars by measuring the primary fruit diameter, the secondary fruit diameter and the navel-end opening fortnightly, using both destructive and non-destructive sampling methods. The relationships between the primary fruit size, the secondary fruit size and the navel-end opening size were studied using correlation analysis. In addition, the effect of 2,4-D on fruit morphology, when applied as a treatment to reduce the size of the navel-end opening, was also evaluated on the same cultivars. The primary fruit, the secondary fruit and the navel-end opening followed a similar developmental pattern, although the navel-end opening developed later, about six weeks after FB. The primary fruit size was not related to the size of the secondary fruit or the navel-end opening. Similarly, the size of the navel-end opening was not related to the size of the secondary fruit. No negative effects were noted on the primary fruit morphology when 2,4-D was applied. Fruit splitting is a major physiological disorder of ‘Marisol’ Clementine mandarin (Citrus reticulata) fruit. The effect of application of 2,4-D on fruit splitting and fruit quality was evaluated on ‘Marisol’ Clementine mandarin trees grafted on Troyer citrange rootstock. Treatments included an untreated control, 2,4-D applied at 15 mg·L-1 or 25 mg·L-1 at FB and 15 mg·L-1 or 25 mg·L-1 at PD. The application of 2,4-D reduced fruit splitting in ‘Marisol’ Clementine fruit. Internal fruit quality was not affected by the treatments, however, the fruit developed a coarse rind due to enlarged oil glands and the styles stayed attached on the fruit until harvest. Therefore, although 2,4-D reduced fruit splitting, it cannot be recommended at the timings and concentrations evaluated.
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    Water relations and sunburn in apple fruit
    (Stellenbosch : Stellenbosch University, 2017-12) Mupambi, Giverson; Steyn, Willem J.; Schmeisser, M.; Dzikiti, Sebinasi; Stellenbosch University. Faculty of AgriSciences. Dept. Horticulture
    ENGLISH SUMMARY: Sunburn in apple is a major problem in South Africa with losses of up to 50% of total yield. The primary objective of this study was to investigate the relationship between tree water relations and sunburn development under South African conditions. Cultural practices that may affect sunburn development through modification of plant water status and the effect of microclimate on fruit peel anatomy was also studied. Induced water stress increased sunburn necrosis and incidence in ‘Granny Smith’ and sunburn severity in ‘Cripps’ Pink’ apple. Although not very pronounced, chlorophyll fluorescence pointed to photochemical changes within the peel under water stress. Water stress influenced ascorbic acid levels in peel, indicating the occurrence of oxidative stress. Leaf net carbon assimilation was reduced due to lower stomatal conductance and possibly also photoinhibition. Pulsing irrigation reduced the incidence of sunburn in ‘Golden Delicious’ and ‘Granny Smith’ compared to an under irrigated control. The reduction in sunburn might be attributed to either the improved plant water status, which reduced tree stress compared to the under irrigated control, or the comparatively more vegetative growth resulting in shading of fruit. Canopy ambient temperature was lower under pulsing irrigation and plant water status was improved. Foliar S˗ABA (active form of abscisic acid) application had no effect on sunburn incidence and severity under South African conditions over three growing seasons. The main apple growing regions in South Africa (33-34º S latitude) experience high radiation, while temperatures often exceed 40 ºC during the growing season. S-ABA application resulted in lower stomatal conductance with a consequent decrease in gas exchange and reduction in fruit size. Tree water status in terms of stem water potential was improved due to the reduction in transpiration. Fruit transpiration and water loss was higher in young compared to mature fruit, which seemingly corresponds with increasing susceptibility of fruit to sunburn during fruit growth. Stomata are present in young fruit and are under regulatory control, which makes them more effective for transpiration. In mature fruit, the predominant lenticel type was partially open and not effective for transpiration. Lenticel density and morphology varied considerably within and between cultivars, between different canopy positions, and between production areas. Although fruit transpiration and water loss decreased over the growing season, it did not always follow the same pattern as fruit surface morphology. In addition, the contribution of transpiration and water loss to the fruit heat balance is negligible. Hence, differences in fruit surface morphology may not explain differences in cultivar and fruit sunburn sensitivity. In conclusion, this study provides insight into ecophysiological and biochemical changes that occur under water stress and that might have a bearing on sunburn development in apple fruit. The study provides further proof of the link between plant water status and sunburn development and shows that good management of plant water status through irrigation is instrumental to controlling sunburn in a warm production region.