Diurnal changes in non-structural carbohydrates in leaves, phloem exudate and fruit in ‘Braeburn’ apple

Abstract

Diurnal changes in carbohydrates in leaves, phloem exudate and within fruitwere determined in apple trees grown under high, low or no crop load. Inleaves, diurnal patterns of carbohydrate concentrations were most distinct inhigh-cropping trees and least so in non-cropping trees. In phloem exudate,sorbitol comprised about 65–70% of sugars and sucrose30–35%, with clear diurnal patterns in the amounts. Phloemexudates from stalks of sun and shade fruit were similar. Fruit carbohydratesshowed little diurnal variation. Crude fruit extracts showed little differencein sucrose synthase (SS, EC 2.4.1.13) activity between crop loads, whilesucrose phosphate synthase (SPS, EC 2.4.1.14) activity in high-cropping treespeaked at night and in low-cropping trees in the morning. SPS activity inshade fruit from high-cropping trees was only half of that in sun fruit (atmidday). Results indicate that the greater weight of individual fruit from lowcrop loads is primarily due to greater supply from the source, rather thanhigher metabolic activity in the sink, compared to fruit from high crop loads.In contrast, fruit size differences in sun and shade fruit of high-croppingtrees appear to be due to differences in sink metabolism rather than supplyfrom the source.