Effects of light reduction at bloom on fruit set in black magic table grape cultivar in early and late production cycles

Abstract

A shading experiment in Black Magic cultivar was conducted in Sicily aiming to reduce the number of berries/cluster, maximising quality and value of the production. A reduction of intercepted light was imposed at the 65 stage of the BBCH scale (50% cap fall) for 12 days. The soilless greenhouse conditions allowed two growing cycles occurring under different climate conditions, an early and a late production cycle, with flowering beginning in March and August, respectively. During the shade treatment, net photosynthetic rate was significantly reduced by 90,4% and 98.6% in the late and early cycles. The sum of flowers and berries dropped in the untreated vines was 633±105 in the late and 62±20 in the early cycle, increasing to 957±92 and 241±60 in the shaded vines. This supports the hypothesis that C-starvation during bloom induces berries abscission. As a consequence of the berry number reduction at harvest (46.2±4.6 and 93.4±8.9 berries/bunch obtained in the shade treatment versus 96.8±7.8 and 173.0±11.3 berries/bunch in the control, in late and early cycles), the yield dropped to 47.4% and 64.2% of control vines in the late and early cycle. Bunch compactness was also reduced, 5.1±0.4 and 10.5±0.6 berries/cm of rachis in shaded vines while 8.0±0.7 and 15.1±1.2 berries/cm of rachis in the control, in late and early production cycle. Total soluble solids in shaded vines were higher than control in both cycles. Under the viticulture viewpoint, control vines in the late production cycle had already an adequate fruit set, so thinning practice is not economically advantageous. In contrast, in the early cycle control vines produced an excessive number of berries/bunch, so, in this case, shading stands for a clear agronomic benefit. The fruit set in ‘Black Magic’ vines can be considered sensitive to incident light reduction at bloom, making shading an effective non-chemical thinning method.