Carbohydrate Metabolism of Vegetative and Reproductive Sinks in the Late-maturing Cultivar Encore

Abstract

Activities of major sorbitol and sucrose metabolizing enzymes and carbohydrate contents were followed during the growth season of `Encore' peach fruits and developing shoot tips. In fruit flesh, sucrose synthase (SS) was present during stage I of growth, when cells are actively dividing, and NAD+-dependent sorbitol dehydrogenase (SDH) during stage III, when cells enlarge actively. Acid invertase (AI) revealed the best correlation with relative growth rate (RGR) of fruits during the entire season. Activities of all carbohydrate metabolizing enzymes were not detectable in fruit flesh during seed filling and pit hardening, when RGR of fruits was the slowest. The highest content of sucrose was in concert with low levels of sucrose metabolizing enzyme activities toward the end of fruit development. In shoot tips, SDH had low activity at the beginning and end of the season when vegetative growth was slowest and a 2:1 sorbitol: sucrose ratio was present. Mid-growing season was, on the other hand, characterized by an increase in SDH activity and a 1:1 sorbitol: sucrose ratio. In `Nemaguard' seedlings, only SDH activity showed a positive correlation with shoot growth. Also, in root tips of `Nemaguard' seedlings SDH exhibited its highest activity, even higher than AI activity in the same roots. We concluded that in peach sucrose may represent the major carbon form used for fruit growth, while sorbitol seems to play a predominant role in vegetative growth.