Starch degradation characteristics in relation to physiological and biochemical properties during growth and maturation of apple fruit

Abstract: Fruit maturity indices, i.e. respiration rate and ethylene production, amylose (AM) and amylopectin (AP) content, total hydrolytic activity, and sugar content were investigated during the growth and maturation of 'Tsugaru' (early-maturing) and 'Fuji' (late-maturing) apples (Malus domestica Borkh.). Different starch degradation characteristics during the growth and maturation processes were observed between 'Tsugaru' and 'Fuji'. By iodine staining, the loss of starch in 'Tsugaru' was observed earlier than in 'Fuji'. The different degradation patterns of starch were also demonstrated through the observations on AM and AP content. In 'Tsugaru', AM and AP degraded rapidly between 95 to 110 days after full bloom (DAFB) and almost all starch were lost rapidly at 125 DAFB with simultaneous increases in rate of respiration and production of ethylene. However, in 'Fuji', starch degraded gradually throughout growth and maturation process and was clearly degraded at 170 DAFB with a low level of ethylene production and decreased respiration. In both the cultivars, content of AM and AP were highest in the outer cortex and lowest in the inner cortex. Starch degradation was observed simultaneously in 3 different tissue zones and there was little difference in the total hydrolytic activity among tissue zones in both cultivars. These results suggest that starch hydrolysis in the apple flesh began simultaneously rather than preferentially in any one tissue zone. For sugar content, although differences among tissue zones were not clear, it increased distinctly with loss of starch content. Moreover, sugars from the degradation of accumulated starch and sugar translocation seem to influence mainly the sweetness quality as the fruit ripens.