Growth stage and conditions affect sorbitol:sucrose ratio in peach source and sink organs

Abstract

Along with sucrose, sorbitol represents the major photosynthetic product and the main form of translocated carbon in peach. In leaves, the ability to synthesize and accumulate sorbitol and sucrose increases with leaf age or distance from the apex until full maturity. Also, in the internodes of a growing shoot sorbitol:sucrose ratio (SOR:SUC) increases with distance from the apex. In mature leaves, SOR:SUC increases in response to fruit sink removal and water deficit, due mainly to sorbitol rather than sucrose accumulation. Data from ‘Encore’ peach show that SOR:SUC remains fairly constant around 2:1 from the leaf blade all the way to the fruit peduncle and drops sharply in the fruit flesh and the seed. In root apices, SOR:SUC is not associated with tissue developmental stage and varies between 2.2:1 and 3:1. In the fruit flesh, SOR:SUC starts at values of about 1:1 during cell division stage, decreases sharply to 0.2:1 at pit hardening, and remains constant at those levels until maturation stage. This decline is primarily due to a greater accumulation of sucrose compared to that of sorbitol. In the seed, SOR:SUC is generally low and exhibits a decreasing trend from cell division (0.3:1) to pit hardening (0.03:1) and cell expansion (0.01:1). In this case, the observed trend seems to be mainly attributed to very active sorbitol degradation. Low crop loads imposed by fruit thinning seem to lower SOR:SUC in the flesh but not in the seed, due mainly to changes in sucrose content. Flesh SOR:SUC is also inversely related to changes in leaf:fruit ratio imposed by defoliating single shoots. At cell division, girdling reduces significantly SOR:SUC of the stem and fruit located downstream. In both those two cases, the observed trends are associated with reductions in sorbitol content, but not in sucrose content. Overall, our data show significant changes in the SOR:SUC which may provide in peach some adaptive response to varying growth and stress conditions generally associated to photoassimilate availability.