Tree Growth Regulator Effect on Phototropism-Its Implication for Utility Forestry

"Pruning procedures such as V-trimming or side-trimming used by electric utilities in tree maintenance programs result in asymmetrical light exposure within tree canopies, producing the potential for phototropic growth response. The tree growth regulators (TGRs) paclobutrazol and flurprimidol are used to reduce the rate of shoot regrowth following pruning. The mode of action involves complexing of the growth retardant with cytochrome P450-dependent enzymes in the metabolic pathway for gibberellins, plant hormones responsible for shoot elongation. Because cytochrome P450 also is a part of the blue light receptor system that controls phototropism, it was hypothesized that shoot growth, as well as phototropic curvature, would be reduced by paclobutrazol and flurprimidol. Effects of soil-applied (0, 0.5, 1, and 5 ppm) paclobutrazol and flurprimidol on shoot growth and phototropism of greenhouse-grown seedlings were found to vary among species. Neither shoot growth nor phototropic curvature in American sycamore (Platanus occidentalis L.) was reduced by treatment with the TGRs. In contrast, shoot growth of silver maple (Acer saccharinum L.) was reduced by all concentrations of both paclobutrazol and flurprimidol, whereas phototropic curvature was reduced only by flurprimidol. Phototropic curvature of etiolated zinnia (Zinnia elegans Scarlet Jacq.) was reduced by both growth regulators."