FRUIT-SET, YOUNG FRUIT AND LEAF GROWTH OF CITRUS-UNSHIU IN RELATION TO ASSIMILATE SUPPLY

Fruit set, fruitlet and young leaf growth at an early stage of fruit ontogeny were promoted significantly by girdling at the base of autumn-shoots of the preceding year on Day 10 pre-full bloom. In contrast, defoliation or darkening of the old leaves on the autumn-shoots aggravated fruit drop and inhibited fruitlet and young leaf growth. During this period the net photosynthetic rate (Pn) of old leaves rose sharply. These observations clearly indicate that assimilate supply to fruitlets and young leaves of Citrus unshiu during this period is mainly affected by the current photosynthates from the old leaves instead of from the remobilization of stored reserves within the trunks. On Day 20 after full bloom the young leaves reached 48% of their final area and increased their Pn rapidly. Subsequently, the Pn of old leaves began to decrease dramatically. The impeding effect of darkening of old leaves on fruit growth from Day 20 after full bloom was absent. The results show that the young leaves gradually substitute for the old leaves to supply photosynthates to the developing fruits from about Day 20 after full bloom.