Description:
<jats:p>Abscisic acid (ABA) and a synthetic analog, the 2‐<jats:italic>cis</jats:italic> acetylenic alcohol, were compared to evaluate their effectiveness in conditioning seedlings of Douglas‐fir [<jats:italic>Pseudotsuga menziesii</jats:italic> (Mirb.) Franco], Engelmann spruce (<jats:italic>Picea engelmannii</jats:italic> Parry) and lodgepole pine (<jats:italic>Pinus contorta</jats:italic> Dougl.). Following preconditioning with ABA and the analog, seedlings were water stressed with the osmoticum polyethylene glycol (PEG) 3350. The effects of the growth regulators on transpiration, net photosynthesis, their ratio, called water use efficiency, and cell water relations parameters were then compared in stressed and unstressed plants. The antitranspirant action of these compounds varied depending on the species, the growth regulator, and the level of stress. ABA promoted transpiration in unstressed seedlings for all 3 species seven days after application. The analog was superior to ABA as an antitranspirant in osmotically‐stressed lodgepole pine and Engelmann spruce, but neither compound was effective in Douglas‐fir. For Douglas‐fir and Engelmann spruce, net photosynthesis remained consistently higher in ABA‐treated plants during the two levels of osmotic stress, relative to control and analog treatments. Neither compound had any effect on root development or cell water relations. ABA, and to a lesser extent its analog, hastened terminal bud formation in seedlings exposed to short days and low temperatures.</jats:p>