SEASONAL PATTERNS OF NONSTRUCTURAL CARBOHYDRATES OF APPLE (MALUS-PUMILA MILL) FRUITS - RELATIONSHIP WITH RELATIVE GROWTH-RATES AND CONTRIBUTION TO SOLUTE POTENTIAL

The seasonal dry-weight accumulation of midseason (cv. Cox's Orange Pi ppin) and late maturing (cv. Golden Delicious) apple (Malus pumila Mil l.) fruits, followed the characteristic sigmoid growth pattern. Simila r to patterns reported for peach fruits, apple fruit relative growth r ates of dry-matter accumulation, calculated on a degree-day basis, dec lined rapidly in an exponential fashion early in the season, then decr eased slowly taking an asymptotic course later in the season. Unlike p each, the relative growth rate curve of apple fruits did not exhibit a distinct phase shift between two physiological phases of growth. Howe ver, seasonal changes of the relative nonstructural carbohydrate compo sition of apple fleshy tissue, in particular, the rapid increase of su crose later in the season, indicated that there may be two phases of f ruit sink activity in both cultivars. Seasonal patterns of nonstructur al carbohydrates of the two apple cultivars were different if expresse d on a dry weight basis. In 'Cox's Orange Pippin', sucrose was the mai n soluble carbohydrate later in the season, whereas 'Golden Delicious' accumulated high fructose concentrations. Estimated contribution of s oluble carbohydrates to fruit solute potential declined over the growi ng season in both apple cultivars. Fructose contributed the largest am ount to the solute potential, followed by sucrose.