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 dwarf Citrus unshiu (cvs. Clausellina and Okitsu) were determined from bloom up to the end of fruit set. Full and partial (50%) defoliations were carried out at anthesis and at the beginning of the cell enlargement period. Carbohydrate reserves in woody storage tissues were also analyzed soon after defoliation. In control fruits, sucrose, glucose, fructose 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 carbohydrate status in either fruits or woody tissues during the cell division period, although the first fruit abscission wave, which takes place at this stage, was increased. Fruit growth was not altered either. However, at the onset of the second phase of growth, soluble sugars were reduced in fruits and sucrose and starch were lower in woody tissues from defoliated plants. In these plants, fruit abscission at this stage (June drop) was also increased, while fruit growth was arrested or delayed. Once the June drop was overcome, fruits remaining on either defoliated or control plants were similar in size and contained analogous high levels of sugars. Taken together, the results suggest that developing fruitlets are utilization sinks during the cell division period and act as storage sinks during the cell enlargement stage. At this critical transition, sucrose levels correlated positively with fruit growth and negatively with fruit abscission. These results are compatible with the proposal that sucrose supply is a major factor of the regulatory mechanism for citrus fruit abscission during the June drop.