Short- and Long-term Effects of Antitranspirants on Water Relations and Photosynthesis of Woody Plants1,2

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Abstract
Each of 8 antitranspirants reduced transpiration of 2 species of woody plants. Dow Silicone and CS 6432 were the most effective compounds on Fraxinus americana and Dow Silicone was effective on Pinus resinosa. Keykote, Folicote and Improved Wilt Pruf showed an increased effect on plant water loss and net photosynthesis (measured by net CO2 uptake) of P. resinosa up to 8 days after compound application. Thereafter there was no significant change in the effect of any compound on transpiration or photosynthesis. Effects on F. americana of all compounds except Improved Wilt Pruf decreased with time after application. Scanning electron micrographs of treated leaves suggested that antitranspirant films on F. americana leaves cracked over the guard cell pore, accounting for the decrease in compound effect with time. Antitranspirants apparently reduced water loss of P. resinosa by combining with wax in the stomatal pore and forming an impermeable plug. The compounds tested were toxic to F. americana seedlings and photosynthesis of treated plants decreased with time, even when direct physical effect of a compound had worn off. Pinus resinosa seedlings showed no decrease in photosynthesis with time. F. americana plants treated with Keykote exhibited low rates of water loss and transpiration/photosynthesis ratios that were not significantly different from those of control plants. Folicote was toxic, and Clear Spray increased water loss of F. americana seedlings. Dow Silicone reduced water loss of Pinus seedlings by about 80%, and plants treated with Dow Silicone, Improved Wilt Pruf, Keykote, or Folicote had favorable transpiration/photosynthesis ratios. The effects of antitranspirants on transpiration and photosynthesis were greatly influenced by environmental regimes and by species.

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