Differences in fruit growth and ripening of early-, mid- and late-season maturing Japanese plum cultivars

Introduction. Plum cultivars exhibit differences in their fruit quality; however, very little is known about the changes in fruit growth and quality of plum cultivars maturing in different seasons. This study was conducted to contribute to a better understanding of the fruit growth and ripening behavior of Japanese plum (Prunus silicina Lindl.) varieties according to their maturing season. Materials and methods. Three Japanese plum cultivars were studied: early- ['Black Amber' (BA)], mid-['Amber Jewel' (AJ)] and late-season ['Angelino' (AG)] maturing. The increase in fruit weight and diameter was determined from days after full bloom (DAFB) to harvest, whilst changes in fruit pulp, skin and seed dry weights, fruit firmness, soluble solid contents (SSC), and titratable acidity (TA) were studied during days after stone hardening (DASH). Ethylene production, respiration rate, activities of 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) enzymes, fruit firmness, fruit color, SSC, TA, ascorbic acid, total antioxidant and carotenoid contents were determined at alternate days during fruit ripening. Results and discussion. All plum cultivars exhibited three distinct phases of fruit growth and development in the DAFB. The AG fruit exhibited the longest duration of phase-III, about 64 days and 12 days more, as compared with BA and AJ fruits, respectively. Fruit skin dry weight (%) was higher for AG fruit, whilst, AJ and BA fruits exhibited the highest increase in seed and pulp dry weight (%) during DASH. The BA and AJ fruits showed climacteric fruit ripening with a sharp rise in ACS and ACO enzymes; whereas AG fruit showed suppressed climacteric ripening. During the fruit ripening period AG fruit remained more firm with higher SSC, and SCC: TA ratio as compared with BA and AJ fruits. The level of ascorbic acid (24.8 mg 100 g(-1)) was higher in AG fruit. Whilst, total carotenoids (0.66 mg 100 g(-1)) and total antioxidants (215 mu mol Trolox 100 g(-1)) were higher in AJ and BA fruit respectively. Conclusion. The physico-chemical changes during fruit ripening vary with cultivar and are associated with the level of endogenous ethylene production. Japanese plum cultivars exhibit varying levels of SSC, SSC: TA ratio, levels of individual and total antioxidants of their ripe fruits.