Effect of the intensity and phenophase of defoliation and water stress on the rate of photosynthesis and the recovery of carbohydrate reserves in Acacia karroo Hayne.

Abstract

A field study was conducted with Acacia karroo plant to determine changes in relative photosynthetic rates, the extent of carbohydrate reserve depletion and the rate reserves take to recover, following defoliation by goats at different intensities and phenophases, at a "wet" and a "dry" site. The rate of photosynthesis of fully expanded leaves increased markedly following defoliation. Light defoliation increased photosynthetic rate the most. Total non-structural carbohydrate levels dropped significantly after defoliation. The magnitude of decrease was directly related to the intensity of defoliation. Following the heavy defoliations, recovery of carbohydrate levels was much faster than after the light defoliations. Rates of recovery were also faster following defoliation in the second half of the growing season, than in the first half. However, the plants that had been heavily defoliated in the second half of the growing season had not fully recovered carbohydrate levels before leaf fall in late autumn. Moisture stress had very little effect on carbohydrate levels in comparison with the defoliation treatments.

Keywords: acacia karroo; carbohydrate; defoliation; dohne research station; eastern cape; field study; goats; leaves; moisture stress; phenophases; photosynthesis; photosynthetic rate; recovery; south africa; stutterheim; water; water stress