Effect of Different Day Temperatures on Flowering and Fruiting in Tosa Buntan Pummelo (Citrus grandis (L.) Osbeck).

Abstract

The effect of different day temperatures during winter and spring seasons on the flowering and fruiting of 3-year-old potted Tosa Buntan pummelo (Citrus grandis (L.) Osbeck) trees grafted on trifoliate orange (Poncirus trifoliata Raf.) rootstock was investigat-ed. The experiment was conducted in growth chambers with setting day temperatures of 15°C, 20°C, 25°C, 30°C and 35°C, while night temperature in each chamber was set at a constant 10°C. The temperature treatments were started on December 18, 1989 and terminated on May 1, 1990, except for the 15°C day temperature treatment which lasted until June 11, 1990. After the temperature treatments in the growth chambers, all trees were grown in the same way in the field condition until harvesting time of November 15, 1990. The lowest day temperature treatment of 15°C needed considerably more days to flower bud initiation and greatly inhibited flower bud development, thus all flower buds in this regime dropped before and after their opening. Higher day temperature treatments led to a reduction in the number of days to flower bud initiation, flower opening and the end of flower opening and physiological fruit drop. A large number of leafy inflorescences were observed at higher day temperatures than at lower day temperature treatments. Higher temperatures also increased the rate of flower bud drop, decreased opening flower number, flower weight and petal length, and reduced fruit set on both leafless and leafy infloresc-nces. The fruit set of leafy inflorescences was higher than that of leafless inflorescences in all treatments in which setting of fruit was observed. At the 20°C day temperature regime, flower open number, flower weight, petal length and fruit set were highest among the treatments. The 20°C day temperature treatment produced fruit lower in weight and in rind coloring, compared to those at other higher day temperature treatments. Day temperature treatments over 20°C produced larger fruit with pyriform shape and thicker peel, increased TSS content except for 35°C, and decreased citric acid content in juice in mid-November 1990. 30°C and 35°C treatments gave gaps at axial parenchyma in all fruit, and 35°C treatment also gave granulation in juice sacs in all fruit at harvest.