Ozone and Water Stress Interactive Effects on the Physiology and Productivity of Soybean (Glycine max, [L.] Merr.) Plants
The phytotoxicity of ozone to plants is conditioned by a number of environmental factors, including the availability of soil water. These experiments were conducted to determine the interactive effects of plant water stress and ozone on the productivity(biomass) and physiology (photosynthesis, stomatal conductance) of soybean (Glycine max[L.] merr. cv "Davis") plants under controlled laboratory conditions. Two similar experiments were designed to examine plant response to two possible ozone - water stress interactions. The first experiment examined the influence of water stress in preconditioning plants to ozone exposure. Plants were maintained in a greenhouse under water-stressed (via soil drying) or non-stressed conditions for two weeks, followed by 8 hour daily exposures to 0, 0.2, 0.3, and 0.5 ppm ozone in controlled gas exchange chambers. Results indicated no consistent patterns in plant physiological response to ozone over two weeks of ozone exposures. In terms of foliar injury, water-stressed plants were less sensitive to ozone damage compared to watered plants. The mechanism of protection appears to be mediated by drought-induced stomatal closure which inhibits entrance of ozone into leaf tissues. Plant biomass reductions were linear functions of ozone dose. Watered plants exhibited the greatest yield reductions, however, greater reductions in biomass on a percentage basis were found in plants subject to both water stress and ozone exposure. These larger % reductions in growth are the result of reduced photosynthetic capacity via long-term stomatal closure, and reflect the additive interaction of ozone and water stress. A second experiment was conducted to address the hypothesis that ozone exposure influences a plant's ability to withstand subsequent water stress. Soybean plants were exposed to ozone(O, 0.1, 0.2, 0.3, 0.4 ppm) for one or two weeks, and then split into watered and water-stressed treatments. There was a significant (p=.05) depression in photosynthesis due to ozone after two weeks of exposure, whereas one week of ozone had no effect on photsynthesis. In terms of productivity, watered plants exhibited greater yield reductions compared' to water-stressed plants, both in terms of net change in biomass and as a percent of controls. Plants preconditioned with ozone were much less responsive to a subsequent period of water stress. The decreased susceptibility of ozone pre-treated plants is the result of an induced stress acclimation, whereby ozone reduces the ability of the stomata to respond to environmental changes. In both experiments, water stress and ozone influenced plant physiology and yield through their effect on the functioning of the stomatal mechanism, and indirectly by predisposing plant tissues to subsequent environmental stresses. These results are discussed in terms of possible applications for the improvement of agriculture and plant ozone resistance.