1999 Volume 68 Issue 5 Pages 942-948
Seasonal changes in the sugar content and activities of enzymes, associated with sucrose and sorbitol metabolism, were followed throughout the development of loquat (Eriobotrya japonica Lindl. cv Mogi) fruit. Sucrose and fructose are the predominant sugars in mature fruit whereas fructose and sorbitol are the major ones in young fruit. Sorbitol dehydrogenase (SDH) activity remained at a low level until May 19 when it and sorbitol-6-phosphate dehydrogenase (S6PDH) activity increased dramatically with sugar accumulation ; the levels reached a plateau on May 26. 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 fruit, it further increased with sugar accumulation from May 19. Sucrose synthase (SS) activity rose on May 19 and again on May 30, accompanied by peaks on May 23 and June 2, respectively. Sucrose phosphate synthase (SPS) activity remained significantly lower than that of SS throughout the developmental stages but its activity increased in parallel with that of sucrose accumulation. The above rapid sugar uptake into the fruit is attributed to the increased unloading of sorbitol and sucrose from May 19 to 26 induced by the prominent rise in SDH activity from May 23, the temporal rise in SS activity from May 19 to 26, and the gradual increase of bAIV from May 19. The rapid influx of sucrose was paralleled by a rise in SPS activity from May 19 and the second rise of SS activity from May 26.
