Fruit growth, firmness and cell wall hydrolytic enzyme activity during development of sweet cherry fruit treated with gibberellic acid (GA₃)

Four genotypes of sweet cherry (Prunus avium L.) producing fruits characterized as early season/soft texture ('Merpet'), early season/firm texture ('Celeste'), late season/soft texture (13S-27-17) and late season/firm texture ('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 (i.e. cell division, lag phase, cell enlargement). Fruits on early maturing genotypes had a shortened period in the lag phase compared to the late maturing cultivars. Fruit firmness in all genotypes increased from the earliest green-coloured stage (about 5 mm in fruit diameter) until the middle of the lag phase, then decreased rapidly and continuously from that stage throughout maturation. Application of 20 mg 1^-1 GA₃ increased fruit firmness at harvest, decreased the rate of fruit softening and delayed fruit maturity 5-8 d for the two late-maturing genotypes, but had no significant effect on the early maturing fruits. GA₃-treated fruits, at final harvest, had a greater concentration of titratable acidity in all genotypes, but there was no effect on the total soluble solids. The activities of the enzymes polygalacturonase, endo β-1, 4-glucanase, β-galactosidase, and β-glucosidase were compared throughout fruit development.