DEFOLIATION INCREASES FRUIT ABSCISSION AND REDUCES CARBOHYDRATE-LEVELS IN DEVELOPING FRUITS AND WOODY TISSUES OF CITRUS-UNSHIU

Carbohydrate levels in fruitlets of control and defoliated trees of dw arf Citrus unshiu (cvs. Clausellina and Okitsu) were determined from b loom up to the end of fruit set. Full and partial (50%) defoliations w ere carried out at anthesis and at the beginning of the cell enlargeme nt period. Carbohydrate reserves in woody storage tissues were also an alyzed soon after defoliation. In control fruits, sucrose, glucose, fr uctose and starch showed a transient increase at anthesis and remained low during the cell division phase. Soluble sugars accumulated at the onset of the cell enlargement phase. Defoliation did not modify carbo hydrate status in either fruits or woody tissues during the cell divis ion period, although the first fruit abscission wave, which takes plac e at this stage, was increased. Fruit growth was not altered either. H owever, at the onset of the second phase of growth, soluble sugars wer e reduced in fruits and sucrose and starch were lower in woody tissues from defoliated plants. In these plants, fruit abscission at this sta ge (June drop) was also increased, while fruit growth was arrested or delayed. Once the June drop was overcome, fruits remaining on either d efoliated or control plants were similar in size and contained analogo us high levels of sugars. Taken together, the results suggest that dev eloping fruitlets are utilization sinks during the cell division perio d and act as storage sinks during the cell enlargement stage. At this critical transition, sucrose levels correlated positively with fruit g rowth and negatively with fruit abscission. These results are compatib le with the proposal that sucrose supply is a major factor of the regu latory mechanism for citrus fruit abscission during the June drop.