Changes in the activity and gene expression of sorbitol- and sucrose-related enzymes with leaf development of 'La France' pear

The activity and gene expression of sorbitol-6-phosphate dehydrogenase (S6PDH), NAD⁺-dependent sorbitol dehydrogenase (NAD-SDH), sucrose phosphate synthase (SPS), sucrose synthase (SS) and soluble acid invertase (S-AIV), in developing 'La France' pear leaves in relation to the metabolism of sorbitol and sucrose as translocated sugars, were investigated. The levels of activity and mRNA of S6PDH were significantly higher in mature leaves than in folded and young ones. S6PDH activity was more than ten times higher than SPS activity. SPS activity remained at an almost constant level throughout the developmental stages; it was undetectable except in the very young leaves although its mRNA was present. NAD-SDH and SS activities were higher in young leaves than in mature ones; the levels of mRNAs, however, exhibited an inverse patterns to that of activities. The levels of both activity and mRNA of S-AIV were significantly higher in folded and young leaves than mature ones. The distinctly higher expression of S6PDH than that of SPS in mature leaves suggests that the former is more important than the latter in relation to CO₂ assimilation (source activity). Thus, the most important change in the transition from sink to source of the pear leaf is the increasing expression of S6PDH gene with leaf maturation.