Variation in carbon content and size in developing fruit of Actinidia deliciosa genotypes

This study identifies the developmental processes contributing to variation in green-fleshed kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson var. deliciosa) fruit dry matter content (DM) and fresh weight (FW) by comparing genotypes with either high or low final DM. Results are compared with the model for fruit development, the tomato (Solanum lycopersicum L.). Differences in final composition were attributable to a higher rate of starch accumulation from 70 days after anthesis in high DM genotypes, with no other consistent differences in accumulation of soluble sugars or organic acids. High DM genotypes had 70% higher starch content and differed from low DM genotypes in the allocation of carbon between storage and other components. DM was negatively correlated with final fruit FW only in high DM genotypes, whereas starch was a constant proportion of dry weight (DW), suggesting a dilution effect rather than an interaction between fruit size and carbohydrate metabolism. Compared with tomato, the organic acids, particularly quinic acid, contributed more to estimated osmotic pressure during growth in FW than the soluble sugars, regardless of final composition or size. Seed mass per unit FW was highest in high DM genotypes, suggesting a previously unrecognised role for kiwifruit seeds in accumulation of carbohydrate by the pericarp. Anatomical comparisons also identified a role for differences in the packing of the two principal cell types, with an increased frequency of the larger cell type correlated with reduced DM. These genotypes demonstrate that kiwifruit differs from tomato in the role of starch as the principal stored carbohydrate, the reduced importance of dilution by growth in FW and the more minor role of the sugars compared with the organic acids during fruit development.