Starting in 1970, an “on” year for fruit set, young Valencia orange fruit were thinned in July, September, and November. Degrees of thinning were 0, 33, 66, and 100 per cent. Leaf samples were taken in 1971 for carbohydrate analysis.

The fruiting cycle was almost completely reversed when all of the young fruit were removed in July. Less fruit removal or removal at a later date was progressively less effective. Thinning had no effect on the size of the fruit remaining on the tree. In the year after thinning, fruit size was inversely related to number of fruit on the tree. The amount of fall growth in 1970 was increased by fruit removal. Likewise, the spring growth in 1971 was earlier and more extensive on the trees that were 100 per cent thinned in July, 1970.

Carbohydrates in the leaves were mainly starch, sucrose and one unidentified compound. Also present were small amounts of fructose, glucose, malic acid, myo-inositol, and one other unidentified soluble carbohydrate. There was a positive correlation between the fruit set in the spring of 1971 and the concentration of starch in the leaves before the beginning of 1971 spring growth. During the June drop period, there was an increase in leaf starch, but not in sugar, associated with an increase in degree of thinning. At the end of the June drop period, there were no significant differences in starch or sugar content. Data presented in this study indicate that carbohydrates, in the amounts present, were not limiting for fruit set.