Influence of Defoliation, Leaf Darkening, and Cluster Shading on the Growth and Composition of Sultana Grapes

Abstract

One half of the total leaf area of 'Sultana' vines was eliminated by either physical leaf removal (defoliation) or by covering the leaves with aluminum bags (darkening). Treatments were applied about a month after anthesis either to all leaves on every other shoot per vine or to leaves on the basal or apical half on all shoots per vine. Half of the fruit clusters on each vine were covered to protect the fruit from direct solar radiation. A further series of vines receiving the defoliation treatments above, including no defoliation, were completely defoliated at véraison.

All leaf elimination treatments reduced berry weight and soluble solids concentration significantly, the reductions being already pronounced in the early stages of ripening and not increasing as the fruit matured. Elimination of apical leaves reduced both variables more than did elimination of basal leaves. Defoliation reduced berry weight more, and soluble solids concentration less, than did darkening. Covered clusters had a slightly higher soluble solids concentration in all treatments and a slightly higher berry weight in the control and defoliation treatments. They had a considerably higher berry weight in the darkening treatments, and this accounted for the difference in this respect between darkening and defoliation, the berry weight for uncovered clusters being the same for both.

All leaf elimination treatments led to a higher total acidity in the fruit in the early stages of ripening, but only when apical leaves were eliminated did it remain higher until maturity. At maturity, fruit from leaf darkened vines was slightly more acidic than fruit from defoliated vines, and fruit from covered clusters was slightly more acidic than fruit from uncovered clusters.

Fruit from vines completely defoliated at véraison had a drastically lower soluble solids concentration but less severely reduced berry weight by the end of the ripening period, than did corresponding vines not further defoliated at véraison. Acidity was scarcely affected. Berry weight and °Brix of fruit were significantly greater for covered clusters on vines entirely defoliated at véraison than for uncovered clusters.

Removal of apical leaves increased the total number of nodes per vine and per shoot over all other treatments. The percentage of mature nodes was greater on defoliated vines than on leaf-darkened vines. Shoots with no leaves always had fewer mature nodes than shoots with leaves, and vines with basal leaves removed had considerably higher percentages of mature nodes than vines with apical leaves removed.

Approximately 10 square centimeters of leaf tissue per gram of fruit was required to mature the crop fully without decreasing the total soluble solids in the berries.