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

Diurnal changes in carbohydrates in leaves, phloem exudate and within fruit were determined in apple trees grown under high, low or no crop load. In leaves, diurnal patterns of carbohydrate concentrations were most distinct in high-cropping trees and least so in non-cropping trees. In phloem exudate, sorbitol comprised about 65-70% of sugars and sucrose 30-35%, with clear diurnal patterns in the amounts. Phloem exudates from stalks of sun and shade fruit were similar. Fruit carbohydrates showed little diurnal variation. Crude fruit extracts showed little difference in sucrose synthase (SS, EC 2.4.1.13) activity between crop loads, while sucrose phosphate synthase (SPS, EC 2.4.1.14) activity in high-cropping trees peaked at night and in low-cropping trees in the morning. SPS activity in shade fruit from high-cropping trees was only half of that in sun fruit (at midday). Results indicate that the greater weight of individual fruit from low crop loads is primarily due to greater supply from the source, rather than higher metabolic activity in the sink, compared to fruit from high crop loads. In contrast, fruit size differences in sun and shade fruit of high-cropping trees appear to be due to differences in sink metabolism rather than supply from the source.