Abscisic acid application timing and concentration affect firmness, pigmentation, and color of 'flame seedless' grapes

Poor coloration of red grapes grown in warm regions is a frequent problem that decreases production efficiency. Most table grape growers use ethephon to improve color, but its influence on color development is erratic, and it may reduce berry firmness. Application of S-abscisic acid (ABA) to grapes can increase the anthocyanins in their skins, but no protocols have been established regarding its potential commercial use. Therefore, we evaluated the effects of ABA and ethephon treatments on fruit quality characteristics, including those related to firmness and color, on 'Flame Seedless' grapes (Vitis vinifera L.) in several experiments over three consecutive seasons. Abscisic acid had few effects on berry weight or juice composition, but it increased berry softening and skin anthocyanin concentrations. The effect of ABA on berry firmness was similar to ethephon. With respect to skin anthocyanin concentration and fruit color characteristics, 300 mg.L-1 ABA applied at veraison was superior to the other ABA concentrations and to ethephon applied at any of the times tested. Moreover, any concentration of ABA between 75 and 300 mg.L-1 applied after veraison improved color better than ethephon applied at the same time. There was a highly significant inverse curvilinear relationship between skin anthocyanin concentration and the lightness and hue of the berries. Anthocyanin concentrations between 0.01 and 0.04 mg.cm(-2) had little effect on berry lightness and hue, so researchers should consider measuring color, not just anthocyanins, when evaluating the quality of red table grapes.