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Summary
Seasonal fluctuations of carbohydrate composition and concentrations, and the activities of related enzymes of three domesticated tomato cultivars including two large (Lady First and Momotaro) and one cherry-fruited (Minicarol) cultivar were examined at 45 d interval with seven sowing times. Fruits picked in cool seasons (early winter to spring seasons) had higher sugar concentrations than those of warm seasons. Fructose and glucose in nearly equal amounts were the predominant sugar in all seasons. Sucrose was present in trace quantities, but cherry tomato cultivar ‘Minicarol’ accumulated more than the large fruited types. Acid invertase (AI) (EC 3.2.1.26) activity was highest during cool seasons at the red stage of ripening, while fruit matured during warm seasons (May to August) had lowest activity. The sucrose synthase (SuSy) (EC 2.4.1.13) showed significantly higher activity during cool seasons in rapidly growing fruits followed by very low activity in mature non-growing fruits. There were no significant differences in sucrose phosphate synthase (SPS) (EC 2.4.1.14) activity. A highly significant positive correlation was found with sucrose accumulation in the case of SuSy but there was a negative correlation with AI activity and none with SPS. Therefore, in all growing seasons, AI and SuSy, rather than SPS could play a central role in regulating sugar accumulation in both large and cherry tomatoes. The result suggested planting season had no adverse effect on the trends of carbohydrate accumulation and metabolism in developing tomato cultivars, but in cool-season crops there is an improvement in fruit carbohydrate accmulation with more enhanced enzyme activities than in warm-season crops. These observations may help in selecting molecular genetic targets, and provide information for biotechnologists who wish to improve desired quality criteria of domesticated tomato genotypes.