Osmotic adjustment induced by elevated ozone: interactive effe

and elevated ozone on tissue water relations of mature clones of a fast-growing genotype of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and their half-sib seedlings. Whole seedlings and branches of mature trees were exposed to acid rain (pH 5.1 and 3.0) and ozone (ambient and twice-ambient) treatments in open-bottomed chambers. The acid rain treatment was applied to foliage weekly from January to April 1992. The ozone treatment was applied daily from September 1991 to November 1992. The treatments had little effect on the water relations of branches of mature trees. In contrast, water relations of seedlings were affected by the treatments, particularly by the twiceambient ozone treatment, and in many instances, there was a significant acid rain × ozone interaction. A combination of twice-ambient ozone and pH 3.0 rain lowered seedling solute potential, turgor loss point and cell-wall elasticity but increased pressure potential and symplastic water content, whereas total water content was unchanged. We conclude that twice-ambient ozone caused osmotic adjustment in seedlings, and the response was magnified by pH 3.0 rain. The long-term consequences of this response are unclear because although osmotic adjustment and increased pressure potential might be advantageous to seedlings in resisting drought and maintaining growth, the concurrent decrease in cell-wall elasticity might induce adverse effects. We note that the effects of elevated ozone on water relations of ponderosa pine seedlings were similar to the effects of drought on water relations of many species.