Changes of sugar composition and related enzyme activities during fruit development of asian pear cultivars 'Niitaka' and 'Whangkeumbae'

Abstract

We report on the seasonal changes of soluble sugar accumulation and several enzyme activities related to sugar metabolism, in two Asian pear cultivars (Pyrus pyrifolia) cultivars 'Niitaka' and 'Whangkeumbae'. Soluble sugar content increased as the fruit developed and its total contents in matured fruits were 104.6 and 95.1 mg.g-1 fresh weight for 'Niitaka' and 'Whangkeumbae', respectively. However, the fructose-glucose-sucrose-sorbitol ratio was not significantly changed. Sorbitol-6-phosphate dehydrogenase activity was maintained at high level in leaves and low in fruits throughout the fruit development, although the enzyme activity highly fluctuated in young and mature fruits. NAD-dependent sorbitol dehydrogenase activity increased sigmoidally in both leaves and fruits during fruit development, and correlated with accumulation of total sugar in fruits. Sucrose synthase and sucrose phosphate synthase (SPS) activities remained low in leaves, but higher with two peak fluctuations in fruits. The change of soluble sugar contents positively correlated with SPS activity, based on the seasonal fluctuation of SPS activity and soluble sugar accumulation.