Growth Inhibition of Thompson Seedless and Napa Gamay Berries by Heat Stress and its Partial Reversibility by Applications of Growth Regulators

Berries on Thompson Seedless grapevines exposed to 40°C for four days during Stage I of berry development were delayed in ripening, smaller in size, and contained lower total soluble solids and titratable acidity at harvest as compared to those on vines grown in the greenhouse (non-stressed control). Berries on heat-stressed vines treated with either gibberellic acid-3 (GA3 or GA) alone or Promalin, a mixture of gibberellin-4 and gibberellin-7 (GA4 and GA7) and benzyladenine (BA), were consistently smaller in size and lower in soluble solids and acidity than comparably-treated berries on non-stressed vines. The endogenous level of GA-like substances in stressed berries was lower than that of non-stressed fruit, but it followed the same seasonal pattern indicating that heat stress reduces the capacity of berries to synthesize or import these hormones. Exogenous applications of hormones partially overcame the effect of high temperatures. Napa Gamay berries exposed to 43°C for four days or at 40°C for eight days responded similarly to those of Thompson Seedless.