Ozone and Water Stress Interactive Effects on the Physiology and Productivity of Soybean (Glycine max, [L.] Merr.) Plants
Abstract
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.