Carbohydrate Accumulation in Leaves and Stems of 'Valencia' Orange at Progressively Colder Temperatures

Potted citrus trees, 1-yr-old `Valencia' orange (Citrus sinensis L. Osbeck) on 1.5-yr-old rough lemon (C. limon L. Burm. f.) rootstocks, were maintained for consecutive 7-day periods in controlled environments at progressively colder temperatures from 30 to 0 C. Temperatures were decreased abruptly by 5-C intervals, and leaves and stems from selected sample trees were removed for analyses immediately before each temperature decrease. Carbohydrates accumulated most rapidly between 15 and 5 C. At 10 C, carbohydrates accumulated in the leaves at an average rate of 8 mg·g dry wt-1·day-1. Slower rates of carbohydrate accumulation of 3 mg·g dry wt-1·day-1 were found in the stems. Reducing sugars increased with colder temperatures, but there was little starch hydrolysis above 0 C. At 0 C, total nonstructural carbohydrates stopped accumulating. Oxygen uptake, H2O content, and chlorophyll decreased with colder temperatures. Freeze tests of -6.7 C for 4 h at the end of the 0-C regime indicated some cold hardening of the trees. Results suggested that relatively low starch-sugar conversion restricts maximum cold hardening in citrus at low but nonfreezing temperatures.