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.

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Forchlorfenuron (CPPU), a synthetic cytokinin, applied after fruit set increases the size and firmness of table grapes (Vitis vinifera L.) beyond what is possible without CPPU treatment. However, treatment with CPPU may inhibit coloring of ‘Flame Seedless’ grapes, limiting its use in growing areas where color has been consistently poor. In contrast, application of abscisic acid (ABA) to ‘Flame Seedless’ grapes may cause fruit softening, which is undesirable, but its primary effect is to increase anthocyanin content and fruit color. Thus, we hypothesized that application of CPPU followed by ABA might increase the size and firmness of ‘Flame Seedless’ grapes without excessively inhibiting coloring. Grapes were treated with 0 or 20 g·ha−1 CPPU (applied at fruit set) and 0, 300, or 600 mg·L−1 ABA (applied at veraison) in 2005 and with 0, 5, 10, 15, or 20 g·ha−1 CPPU and 0, 200, 400, or 600 mg·L−1 ABA in 2006. Both plant growth regulators (PGRs) increased berry mass, but grapes treated with CPPU were as firm, or firmer, than nontreated grapes, whereas those treated with ABA were of similar or lesser firmness. Treatment with CPPU generally reduced soluble solids and red berry color, whereas treatment with ABA reduced titratable acidity and increased red color. The PGRs did not interact to affect any of the fruit quality variables measured, so beneficial effects of CPPU or ABA were apparent whether the grapes were treated with either or both PGRs. Thus, the combined use of CPPU and ABA may be a desirable cultural practice for ‘Flame Seedless’.

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SUMMARYThis study tested the relationship between stem water potential (ψ stem ) and vapor pressure deficit (VPD) in order to evaluate its use as reference for irrigation control of table grape (Vitis vinifera) cvs. Thompson Seedless and Redglobe. Two trials were carried out on consecutive seasons. In the first season the treatments consisted of four different irrigation regimes: T1, plants irrigated at 100% of ETc; T2, irrigated until fruit set the same as T1 but after fruit set every other time T1; T3, plants with no irrigation from fruit set to harvest; and T4, plants irrigated at 50% of ETc throughout the growing season. Vines from T1 always showed a higher ψ stem for a wide VPD range, therefore the ψ stem and VPD relationship was established using T1 vines, obtaining a logarithmic function with a high determination coefficient (R 2 =0.85). The following season this relationship was used to control irrigation frequency on two table grape cvs. Irrigation started when vines had 20 cm shoots and then the irrigation amount was set to replenish 10% of soil available water. Trial 1 was conducted on cv. Thompson Seedless and consisted of two treatments: T1 plants irrigated to satisfy 100% ETc and T2 plants irrigated according to the previously obtained ψ stem -VPD reference line. Trial 2 was established with Redglobe cv. plants using three treatments: T1 plants irrigated to satisfy 100% Etc; T2, plants irrigated the same as T1 but after fruit set every other time T1; and T3, plants irrigated according to the ψ stem -VPD reference line previously obtained. Irrigation frequency determined using the ψ stem -VPD relationship reduced total water volume compared to T1 on both trials, with no effects on yield or quality, showing the feasibility of using this relationship to control irrigation frequency. RESUMOEste trabalho incidiu na análise da relação entre o potencial hídrico xilemático (ψ stem ) e o déficit de pressão de vapor (VPD), para utilizá-la como referência na programação de irrigação em uva de mesa (Vitis vinifera) das variedades Thompson Seedless e Redglobe. Foram realizados dois ensaios em temporadas consecutivas. Na primeira temporada os tratamentos consistiram em quatro regimes de irrigação: T1, plantas irrigadas com 100% de ETc; T2, plantas irrigadas igual ao T1 até a frutificação e depois uma vez a cada duas irrigações deT1: T3, plantas sem irrigação entre a frutificação e a colheita; e T4, plantas irrigadas com 50% de ETc durante toda a temporada. As plantas de T1 apresentaram sempre valores mais altos de ψ stem para uma ampla gama de VPD, portanto, a relação ψ stem -VPD foi estabelecida utilizando plantas de T1, obtendo uma função logarítmica com um alto coeficiente de determinação ((R 2 =0.85). Na subsequente temporada esta relação foi utilizada para controlar a frequência de irrigação em duas variedades de uva de mesa. A quantidade de irrigação foi estabelecida para repor 10% de humidade aproveitável. O ensaio 1 foi realizado na variedade Thompson Seedless e baseou-se em dois tratamentos: T1, plantas...