Stomatal response to humidity and total gas exchange of untreated leaves of larrea tridentata (DC) Cov. were compared with those of leaves treated to remove most of their external resin by washing attached branches in an ethanol-water mixture. The ethanol-water mixture did not infiltrate the stomatal pores and produced no visible symptoms of tissue damage. Partial resin removal enhanced stomatal responsiveness to humidity in illuminated leaves and the responsiveness of leaf conductance to humidity in the dark. Diffusive conductances and transpiration of both illuminated and darkened leaves increased following resin removal. The net effect of resin removal on total gas exchange was to reduce water use efficiency by altering the coupling between assimilation and conductance. These results indicated that the resin behaved as an ideal antitranspirant because it diminished transpiration more than assimilation rates. Possible mechanisms for the responses to resin removal are discussed.
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