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 Pippin) and late maturing (cv. Golden Delicious) apple (Malus pumila Mill.) fruits, followed the characteristic sigmoid growth pattern. Similar to patterns reported for peach fruits, apple fruit relative growth rates of dry-matter accumulation, calculated on a degree-day basis, declined rapidly in an exponential fashion early in the season, then decreased slowly taking an asymptotic course later in the season. Unlike peach, the relative growth rate curve of apple fruits did not exhibit a distinct phase shift between two physiological phases of growth. However, seasonal changes of the relative nonstructural carbohydrate composition of apple fleshy tissue, in particular, the rapid increase of sucrose later in the season, indicated that there may be two phases of fruit sink activity in both cultivars. Seasonal patterns of nonstructural carbohydrates of the two apple cultivars were different if expressed on a dry weight basis. In 'Cox's Orange Pippin', sucrose was the main soluble carbohydrate later in the season, whereas 'Golden Delicious' accumulated high fructose concentrations. Estimated contribution of soluble carbohydrates to fruit solute potential declined over the growing season in both apple cultivars. Fructose contributed the largest amount to the solute potential, followed by sucrose.