Effect of gibberellic acid during development of sweet cherry fruit: Physiological and molecular changes

Sweet cherry (Prunus avium L.) fruit respond to treatment with gibberellic acid (GA₃) by a delay in ripening date and increased firmness at maturity. In the Pacific Northwest of North America this effect is utilized commercially to enhance fruit quality for a significant quantity of production. The present study investigates the relationship between date of ripening and fruit softening and the differential effects of gibberellic acid treatment on early and late maturing cultivars. Four sweet cherry genotypes producing fruits characterized as early/soft (Merpet), early/firm (Celeste), late/soft (13S-27-17) and late/firm (Lapins) were evaluated for differences in fruit growth, firmness and hydrolytic enzyme activity with or without gibberellic acid treatment. The fruit developed in three distinct stages after fertilization corresponding to the standard Prunus profile (cell division, lag phase, cell enlargement). Fruits on early maturing genotypes had a shortened period in lag phase compared to the late maturing cultivars. Fruit firmness increased from the earliest green-colored stage (about 5 mm in fruit diameter) until the middle of lag phase, then decreased rapidly and continuously from that stage throughout maturation. Application of 20 mg l^-1 GA₃ decreased the rate of fruit softening and delayed fruit maturity 5-8 days only for the two late-maturing genotypes, but had no significant effect on early maturing fruits. The activities of the enzymes; endo β-1, 4-glucanase, β- galactosidase, and β-glucosidase were increased during the decrease of fruit firmness in the early fruit developmental stage. This result suggests that these enzyme actions can contribute to sweet cherry softening between the cell elongation stage and the onset of fruit ripening. However, fruit softening was in advance of increasing poly-galacturonase (PG) activity. PG activity may not contribute to early fruit softening but contributes to the disassembling of pectin in the late stage of fruit ripening.