Effects of forchlorfenuron and abscisic acid on the quality of 'Flame Seedless' grapes

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'.