© 1995 Heron Publishing—Victoria, Canada
Osmotic adjustment induced by elevated ozone: interactive effects of acid rain and ozone on water relations of field-grown
seedlings and mature trees of Pinus ponderosa
B. Momen (1, 2) and J. A. Helms (1)
1. Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA / 2. MRC, Biology Department, Rensselaer Polytechnic Institute, Troy, New York 12180, USA / Received January 3, 1995
Summary
We investigated the effects of simulated acid rain 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 twice-ambient 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.
Keywords:
branches, cell wall elasticity, drought, pH, ponderosa pine, pressure potential, solute potential, symplastic water content,
turgor loss point.
