Browsing by Author "Verreynne, Johannes S."
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- ItemManipulation of internal fruit quality in citrus(Stellenbosch : Stellenbosch University, 1999-12) Verreynne, Johannes S.; Rabe, E.; Theron, K. I.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticultural Science.ENGLISH SUMMARY: Internal fruit quality in citrus IS becoming an important Issue in the deregulated marketing environment. Fruit with poor and variable taste would lead to low prices on the markets and loss of market share. Internal fruit quality is determined by a few parameters such as the sugar content (determined as total soluble solids (TSS)), titratable acid (TA) content, the TSS:TA ratio and juice percentage. The objective of this study was to manipulate these parameters by means of cultural practices in order to improve the internal quality of citrus fruit. The separate and combined effects of summer trunk girdling and deficit irrigation on internal fruit quality of 'Delta' Valencia and 'Marisol' Clementine trees were investigated over two consecutive seasons. On 'Marisol' Clementines, four trials were conducted over two seasons, at two different sites, Somerset West and Citrusdal. Girdling increased the TSS, although not always significantly, but there were no trends observed for T A and TSS:TA ratios. Deficit irrigation increased the TSS and the T A levels in both seasons and at both sites. On 'Delta' Valencias, deficit irrigation increased the TSS of fruit in 1998, but decreased the TSS in 1999. Deficit irrigation decreased the TA levels and increased the TSS:TA ratio in both years. Summer trunk girdling, performed at different times after the physiological fruit drop period, had no significant effect on the TSS and TA levels and consequently the TSS:TA ratios in both years. The timing of summer trunk girdling, perforn1ed at different times after the physiological fruit drop period on the internal fruit quality of 'Temple' tangor trees, was evaluated over two consecutive years. Girdling significantly decreased the TSS in the first year, but had no significant effect on the TA level and the TSS:TA ratio of the fruit. In the second year there was, however, a tendency towards an increase in TSS and TA levels with the later girdling treatments, although this was not significant. The effect of exposing internally-borne fruit to direct sunlight, by means of summer pruning, on the internal fruit quality of these normally-shaded fruit was evaluated on 'Mihowase' Satsuma and 'Nules' Clementine trees over two consecutive years. Summer pruning had no significant effect on fruit quality of 'Mihowase' Satsumas in the first year. In the second year, pruning significantly decreased T A levels and increased the TSS:TA ratio, but had no significant effect on fruit colour and TSS. Pruning on 'Nules' Clementines significantly increased the TSS:TA ratio of fruit in the first year, but had no significant effect on the colour, TSS and TA levels. In the second year, pruning had no significant effect on the colour, but significantly decreased the T A levels and increased the TSS levels and the TSS:TA ratio. Fruit quality differences between fruit from different sectors of 'Mihowase' Satsuma, 'Nules' Clementine, 'Fairchild' and 'Temple' tangor trees were investigated. In Satsumas, Clementines and 'Temple' tangors the top fruit were the largest and inside bottom fruit the smallest, but the opposite was true in 'Fairchild'. The TSS in all four cultivars was the highest in top fruit and the lowest in inside bottom fruit. Outside bottom fruit (Satsumas) or top fruit (Clementines, 'Fairchild' and 'Temple') had the highest TSS:TA ratios and inside bottom fruit had the lowest ratios. The north sector in all the cultivars had higher TSS:TA ratios than the south sector. The accumulation of reducing and non-reducing sugars in 'Mihowase' Satsuma and 'Nules' Clementine fruit from physiological fruit drop to maturation were quantified. In Satsumas, reducing and non-reducing sugars increased linearly over time, but plateaued out towards the last sampling date. In Clementines, reducing sugars increased cubically over time with a rapid decrease towards maturity and non-reducing sugars also increased over time until maturity. The timing of girdling and the type of girdling tool used on the healing ability of the girdle on 'Nules' Clementine trees were investigated in two trials. Slow healing and essentially at the same rate, took place when trees were girdled with a Stanley carpet knife in September, October and November, faster healing when girdled in December and January (the hottest period), followed by slower healing when girdled in February. The Outspan girdling tool resulted in girdles which took more days to heal than the Stanley carpet knife. The effect of ridging on the internal quality of fruit from 'Bahianinha' Navel trees was evaluated over two consecutive years. Ridging increased the TSS and the TSS:TA ratio of fruit in both years.