Response of soybean leaf water relations to tropospheric ozone

Tropospheric O3 and water stress cause significant reductions in crop growth and yield. The effects of chronic O3 exposures on leaf water relations have been less thoroughly studied. Soybeans were grown in two years in open-top field chambers equipped to control O3. The seasonal mean O3 concentrations for the charcoal-filtered controls and supplemental O3 treatments were 24 and 83 nL∙L−1 for the first year (1990) and 20 and 99 nL∙L−1 for the second year (1992). In 1990 leaves were sampled during four intervals of the 106-d growing season and subjected to potential–volume analysis. In 1992, leaves were sampled over a 3-week period, centered on 49 days after planting for potential–volume analysis as well as for midday xylem pressure potentials and leaf conductance. Ontogenetic changes in most of the parameters were large compared with treatment effects. O3 treatment consistently caused decreased symplastic volume, specific leaf mass, and tissue elasticity. In 1992, these effects were accompanied by decreased leaf conductances with no discernable change in xylem pressure potential, although midday turgor increased by 32% and stomatal competency was maintained. Tissue elasticity decreases may be related to O3-induced changes in cell wall structure during leaf expansion and may result in decreased symplastic volume. Key words: Glycine max, ozone, leaf water relations, pressure–volume analysis, elasticity, elastic modulus.