Use of abscisic acid on table grapes: Effects on anthocyanins and color

Table grapes often fail to develop adequate color. Abscisic acid (ABA) is a plant growth regulator involved in grape berry color development, however its cost of production has been high. A new lower-cost ABA formulation promoted further research for its commercial use. Different ABA concentrations and application times were evaluated for maturity, firmness, anthocyanins and color (L*, C*, h°) on 'Flame Seedless', 'Redglobe' and 'Crimson Seedless'. Application of ABA had a small or none effect on maturity but decreased firmness. For 'Flame Seedless' and 'Redglobe' concentrations about 300 mg/l ABA at veraison gave the best results in anthocyanins and color while for 'Crimson Seedless' 150 to 300 mg/l ABA at or after veraison increased anthocyanins and improve color. Later applications had smaller effects on color but decreased the negative impact on firmness. Measurements of color showed inverse curvilinear relationships between anthocyanin content and L* or h°. Plant growth regulators like forchlorfenuron (CPPU) could have a big impact on the table grape industry because its ability to improve berry size, but it also suppresses color, restricting its use. 'Flame Seedless' is particularly sensitive to CPPU color problems and was used to evaluate the combined effect of CPPU and ABA. Clusters treated with CPPU (8 g/acre at fruit set) and ABA (300 or 600 mg/l at veraison) had higher soluble solids, lower acidity, higher anthocyanins and better color than clusters with only CPPU, and were similar to untreated fruit. Further work is necessary to establish optimal concentrations of CPPU and ABA to increase berry size and obtain good berry color. Finally, mRNA levels of the key anthocyanin gene UDP-glucose: flavonoid 3-O-glucosyltransferase (UFGT) were measured on 'Crimson Seedless' skins and compared between ABA concentrations (0, 150 or 300 mg/l at veraison). Level of mRNA UFGT increased markedly one day after ABA application but effect was short lived and differences diminished after three weeks. The rapid changes in mRNA were concurrent with color changes in ABA treated fruit, particularly L* and h°, which decreased after ABA application, but were relatively stable thereafter; while on control fruit color changed gradually throughout the season.