EFFECT OF SPRING DAY/NIGHT TEMPERATURES ON FLOWER DEVELOPMENT, FRUIT-SET AND FRUIT-QUALITY ON STRANGULATED POMELO TREES

The effect of different day/night temperatures during spring season on flower development, fruit set and fruit quality of 2-year-old potted 'Tosa Buntan' pummelo (Citrus grandis (L.) Osbeck) trees grafted on trifoliate orange (Poncirus trifoliata Raf.) rootstock was investigated. The experiment was conducted in growth chambers with setting day/night temperatures of 15 degrees/10 degrees C, 20 degrees/15 degrees C sad 30 degrees/25 degrees C; one plot was set in a plastic house (Max. 30 degrees/Min. 15 degrees C) condition. Temperature treatments were started on 1 April and terminated on 30 July 1992. After the temperature treatments in the growth chambers, all trees were grown in the same way in the plastic house until harvest, 25 January 1993. All the trunks of the potted trees were strangulated with a steel wire (1.6 mm diam.) at a tension of 60 Kgf.cm degree at the start of the temperature treatment; the wire was removed 4 months later. The number of inflorescences and flower buds increased as the day/night temperatures were lowered. Higher day/night temperature treatments led to a reduction in the number of days to anthesis, the end of flowering and ''June drop''. The greater the leaf area per flower bud, the higher was the fruit set. Sugar and carbohydrate contents in leaves increased while N content decreased under low day/night temperature treatments, except for 30 degrees/25 degrees C regimen. Higher day/night temperature treatments increased the number of vegetative shoots, promoted shoot growth, enhanced chlorophyll synthesis in the first flush, hastened the rate of trunk enlargement which deepened the girdles in the trunk, and decreased the weight of flower, petal and ovary as well as citric acid content in juice. Lower day/night temperature treatments produced smaller fruit and a lower TSS-TA ratio. The TSS content in fruit juice did not differ among the temperature treatments. Under the 30 degrees/25 degrees C regimen, fruits at harvest exhibited broken axial parenchyma tissue.