Influence of irradiance and period of exposure on fruit set of French-American hybrid grapes

Abstract

Vines of container grown 'DeChaunac', 'Vidal blanc', 'Seyval blanc' and 'Chambourcin' grapes were subjected to 5 days of 80% shade at prebloom, bloom or 2 and 4 weeks after bloom. Fruit set, cluster weight, berries per cluster and juice components [soluble solids concentration (SSC), pH and titratable acidity] of 'DeChaunac' and 'Vidal blanc' were not affected by a short period of intensive shade. 'Chambourcin' was sensitive to a shade period near the time of bloom for most of the aforementioned factors, while 'Seyval blanc' was intermediate in sensitivity. Shot (green, hard, and undersized) berries of 'Chambourcin' and 'Seyval blanc' were increased by a 5-day period of shade 2 or 4 weeks after bloom. In a second study, container-grown 'Chambourcin' on 3309C (V. riparia × V. rupestris) with one or two clusters and 'Vidal blanc' with one cluster were subjected to the following light regimes beginning at bloom for 5 weeks: supplemental light, ambient greenhouse light and 30%, 50% or 80% shade. Yield, fruit set, specific leaf weight (leaf dry weight/leaf area), saturation index, and total leaf chlorophyll increased linearly with increasing irradiance. 'Chambourcin' juice pH, SSC, leaf chlorophyll a/b ratio, cluster color development and hue angle decreased as irradiance increased, likely related to crop reduction. Responses in 'Vidal blanc' followed similar trends, but differences were not as great. Results demonstrate that light is an important determining factor in fruit set of French-American hybrid grapes and fruit set of some cultivars are sensitive to short periods of intense shade.