Sugar content and compartmentation in melon fruit and the restriction of sugar efflux from flesh tissue by ABA

Abstract

Using the compartmental analysis method, the distribution of sucrose, glucose, and fructose and their efflux from the free space, cytoplasm, and vacuole were determined in melon (Cucumis melo L. cv. Prince) fruits picked 14 days after pollination (DAP), 28 DAP and 42 DAP.
1. At all stages, the greatest percentage of total sugar was accumulated in the vacuole. The main sugars accumulated in melon early and late in the season were fructose and sucrose, respectively. Concentration of sugar in the free space increased from 44 mM at 14 DAP to 103 mM at 42 DAP, while that within the cell (cytoplasm+vacuole) increased slightly from 234 mM to 279 mM.
2. Permeability of the plasma membrane to sucrose, glucoseand fructose was higher than that of the tonoplast and increased with fruit maturation. 3. CCCP restricted the release of sugar by reducing the rate of efflux across the plasma membrane and tonoplast while PCMBS restricted the release of sugar by reducing efflux across the plasma membrane.
4. S-ABA of 10^-5 M as compared to 10^-4 M restricted the release of sugar from fruit discs; restriction was due to a reduction in the velocity constant of efflux of sugar across the plasma membrane. Thus, S-ABA may stimulate the accumulation of sugar in fruit flesh.