Girdling for Increased "hass" Fruit Size and Its Effect on Carbohydrate Production and Storage

In Hass avocado trees, the girdling and scoring of about 50% of the fruit bearing branches per tree, at the end of November 1994, resulted in an initial accumulation of carbohydrate in the leaves and branches of the girdled limbs. The girdling of only 50% of the branches and also the width of the girdle are crucial to the process since healing of the girdle wound, and continued nutrient supply to the root system, is essential for maintaining tree health and vigor. During fruit development, the reserve carbohydrate levels are rapidly diminished in both the girdled and ungirdled fruit-bearing branches. Although no apparent restriction in upward sapflow could be detected, girdling nevertheless resulted in diminished nitrogen, phosphate and calcium levels, in the leaves of the girdled branches when compared with ungirdled branches. This is probably due mainly to the accumulation of carbohydrate in the leaves, which also caused a 50% reduction in chlorophyll content. Water uptake by the tree has been shown by Kurzmann (1966) to be little affected by girdling. Nitrogen uptake is apparently restricted by girdling but this is attributed to the lower carbohydrate levels in the roots and the consequent impairment of metabolism. The scoring wound should be completely healed four-to-five weeks after treatment. Photosynthesis rates were initially reduced when starch accumulated in the leaves on the girdled branches, as indicated by the yellowing of the leaves. However, as the carbohydrate levels in the girdled branches were reduced by the increasing demand of the developing fruits, photosynthesis rates recovered. Preliminary results showed that girdling when the fruit demand for carbohydrate was at its peak, gave an average increase in individual fruit mass of 36%, after eight weeks, when compared with fruit on the ungirdled branches of the same trees.