Promoting fruit-set and yield in sweet orange using plant growth substances

Abstract

Experiments were done over four years to see if certain growth substances would increase the yields of sweet orange by increasing fruit-set. The main object was to find a method to increase yields during an off-year, and so control biennial bearing. Gibberellic acid (GA) was the most promising substance and under glasshouse and controlled environment conditions increased fruit-set when several applications were made, but had only a temporary effect when a single spray was applied. In the first instance a greater volume of fruit was produced although the mean size of the fruit was less. In the second instance some changes in fruit size and shape were found. In the field, a single spray (to part of a tree) of 400 p.p.m. GA at petal fall in one season on both Late Valencia and Washington Navel increased fruit-set, and in the following season 100 p.p.m. GA applied at petal fall increased fruit-set in Washington Navels. When a 50 p.p.m. GA spray was applied to whole Washington Navel trees in successive seasons there was a 20 per cent increase in the number of fruit harvested, but this was barely significant. However, there was a greater percentage of small fruit from the treated trees. This treatment had no effect the following season. Single applications of GA from 25 to 200 p.p.m. applied to Late Valencia trees from petal fall onwards in an 'off year' had relatively little effect. Some treatments retarded fruit-drop, but had insignificant or only minor effects on yield. Applying gibberellic acid to improve fruit-set is not a practical means for controlling yields of sweet orange trees.