Effect of gibberellic acid on growth and photosynthesis in Abies balsamea

Abstract

The hypothesis that exogenous gibberellic acid (GA3) increases height growth in tree species partly because it stimulates the rate of net photosynthesis (PN) was investigated in the conifer balsam fir (Abies balsamea L.). In long-term experiments, the normal growth pattern of 4- and 5-year-old potted trees was changed by GA3 applied in both an aqueous soil drench (20 mg per tree, thrice weekly; or 50 mg per tree, twice weekly) and a foliar spray (1000 ppm, weekly) for about 3 months. Effects included increased longitudinal and radial growth in the leader and longest lateral in the uppermost whorl, enhancement of shoot apical dominance, increased needle length, and decreased root weight – top weight ratio. Nevertheless, the Pn and productive capacity of the current and 1-year-old needles in the GA3-treated and control trees were the same, measured during and at the end of the treatment period. In short-term experiments, the 1000-ppm GA3 spray did not affect PN of current-year needles, measured 3 h up to 7 days after one or two applications. PN of current-year cuttings also was not altered by feeding through the basal end (1) 0.25, 2.5, 25, or250 ppm GA3 for 4 h; or(2) 10 or 100 ppm GA3 for24 h. Feeding25 or250 ppm GA3 for 48 h through the distal portion of an attached, current shoot decreased PN in the proximal portion. Collectively, the results indicate that exogenous GA3 increases height growth in balsam fir by altering the normal distribution of photosynthate, not through increasing photosynthate production.