Changes in sugar content and sorbitol- and sucrose-related enzyme activities during development of loquat (Eriobotrya japonica Lindl. cv. Mogi) fruit

摘要

Seasonal changes in the sugar content and activities of enzymes associated with sucrose and sorbitol metabolism were followed throughout the development of loquat cv. Mogi fruits. Sucrose and fructose are the predominant sugars in mature fruits whereas fructose and sorbitol are the major ones in young fruits. Sorbitol dehydrogenase (SDH) [L-iditol 2-dehydrogenase] activity remained at a low level until 19 May when it and sorbitol-6-phosphate dehydrogenase (S6PDH) activity increased dramatically with sugar accumulation; the levels reached a plateau on 26 May. Cell wall bound acid invertase (bAIV) activity was noticeably higher than was soluble acid invertase (sAIV) throughout fruit development. Although bAIV was high in young fruits, it increased further with sugar accumulation from 19 May. Sucrose synthase (SS) activity rose on 19 May and again on 30 May, accompanied by peaks on 23 May and 2 June, respectively. Sucrose phosphate synthase (SPS) activity remained significantly lower than that of SSthroughout development but its activity increased in parallel with that of sucrose accumulation. The above rapid sugar uptake into the fruit was attributed to the increased unloading of sorbitol and sucrose from 19 to 26 May induced by the prominent rise in SDH activity from 23 May, the temporal rise in SS activity from 19 to 26 May, and the gradual increase of bAIV from 19 May. The rapid influx of sucrose was paralleled by a rise in SPS activity from 19 May and a second rise in SS activity from 26 May.