Thesis (MscAgric (Viticulture and Oenology))--University of Stellenbosch, 2006. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2784 |
| Date | 03 1900 |
| Creators | Strydom, Janene |
| Contributors | Raath, P. J., Avenant, J. H., University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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