Canopy manipulation practices for optimum colour of redglobe (V.Vinifera L.)
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Under certain South African conditions, Redglobe develops a colour that is too dark and thus unacceptable for the Far Eastern markets. These markets require a pink colour instead of a dark red colour. The cultivation of grapes with an acceptable colour involves amongst other, canopy management practices. This generally includes the removal of leaves and/or lateral shoots. Hereby, the leaf area and the microclimatic conditions in the canopy are altered. The aim of this study was to test the usefulness of leaf and lateral shoot removal at different defoliation times after anthesis in order to obtain a pink coloured Redglobe crop. Other quality aspects, namely total soluble solids (TSS), total titratable acidity (TTA), berry mass and total yield, were also evaluated. A canopy management trial was conducted on six year old Redglobe vines with moderate vigour. The treatment design was a 2 x 3 x 4 factorial and involved two leaf removal (L) levels (L0 = 0% leaf removal; L33 = 33% leaf removal) in combination with three lateral shoot removal (LS) levels (LS0 = 0 % lateral shoot removal; LS50 = 50% lateral shoot removal; LS100 = 100% lateral shoot removal). Four defoliation times (DT) were selected: 36 (pea berry size), 69 (véraison), 76 (one week after véraison) and 83 (two weeks after véraison) days after anthesis (DAA). A total of 24 treatment combinations, replicated in four blocks, were applied. Generally, treatment combinations involving 33% leaf removal lowered the main shoot leaf area. Likewise, the lateral shoot leaf area was decreased by increasing levels of lateral shoot removal at any defoliation time. As expected, 33% leaf removal applied in combination with any level of lateral shoot removal, always resulted in a lower total vine leaf area compared to where 0% leaf removal was part of the treatment combination. Compensation reactions occurred and in this regard the main shoot leaf size increased due to 33% leaf removal applied at 1 week after véraison and 2 weeks after véraison. Treatment combinations involving lateral shoot removal increased the ratio of main shoot leaf area to the total leaf area. On the other hand, the main shoot leaf area percentage was lowered by the application of 33% leaf removal at 2 weeks after véraison compared to no leaf removal at the same defoliation time. It can therefore be assumed that the contribution of lateral shoot leaves to grape composition might have increased in cases where the main shoot leaf area was lowered at a later stage (e.g. 2 weeks after véraison). The bunches were visually evaluated and divided into classes from dark (class one) to light (class nine). This visual bunch evaluation showed that the mean bunch colour was in class three (lighter than class two) due to the defoliation time. The lateral shoot removal x leaf removal interaction resulted in a mean bunch colour that was in classes 2 and 3. However, within these classes, there was a tendency that bunch colour decreased for defoliation times later than pea berry size. The lateral shoot removal x leaf removal interactions showed that bunch colour was darker when the treatment combinations involved 0% leaf removal. The percentage of bunches with the desired colour was increased by application of the treatments at véraison, compared to the other defoliation times, and also with 50% lateral shoot removal and 100% lateral shoot removal compared to 0% lateral shoot removal. Biochemical analyses confirmed that increased levels of lateral shoot removal generally lowered the anthocyanin concentration regardless of defoliation time. A similar effect on TSS was observed, i.e. from véraison onwards, the application of 50% lateral shoot removal and 100% lateral shoot removal tended to lower TSS. The effect of these levels of lateral shoot removal at véraison was significant. The role of the lateral shoots in colour development and sugar accumulation is therefore emphasized. Furthermore, the special role that lateral shoots also play in berry development is illustrated in that berry mass tended to decrease when 100% lateral shoot removal in combination with 33% leaf removal and 100% lateral shoot removal in combination with 0% leaf removal were applied at véraison. This, together with the positive relationship obtained between grape colour and the lateral shoot leaf area:fruit mass ratio, accentuates the role of active leaf area during the ripening period. The possible effect of the microclimatic light environment on colour must also be considered. However, although the light intensity increased with increased levels of LS, the colour that was obtained was probably not associated with the differences in light intensity. It was found that it is possible to manipulate the colour of Redglobe grapes with defoliation treatments. However, the treatments that have a decreasing effect on grape colour also affected other quality parameters like TSS and berry size negatively. Although, it is possible to reduce the colour of Redglobe through the application of leaf and lateral shoot removal at different defoliation times, the question arises whether the treatment combinations used in this study are worthwhile to pursue because the mean bunch colour that was obtained was still too dark. However, it was possible to increase the percentage of bunches with the desired colour. Therefore, if such treatments are applied, it must be approached cautiously, keeping in mind that assimilate supply has to be sustained throughout the ripening period.