GROWTH ANALYSIS OF SOYBEAN EXPOSED TO SIMULATED ACID RAIN AND GASEOUS AIR POLLUTANTS

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Abstract
SUMMARY: Greenhouse‐grown soybean plants [Glycine max (L.) Merr. cv. Davis] were exposed to simulated acid rain (pH 2.6 to 5.6) and gaseous air pollutants (SO2+ O3) to determine how effects on short‐term physiological processes are integrated into whole‐plant response. Injury to expanding leaves, characterized by marginal necrosis and leaf deformation, was induced by twice‐weekly exposure to simulated rain of pH 2.6 and to a lesser extent pH 3.4. Growth inhibition resulted only from exposure to rain of pH 2.6, and there were no significant pH x air pollution interactions. Growth analysis from periodic harvests indicated that inhibitory effects of rain of pH 2.6 were mediated through a reduction in leaf area ratio (LAR). The alteration in LAR was not attributable to reduced allocation of dry matter to leaves but to lower specific leaf area caused by leaf deformation. There were no effects of rain pH on unit leaf rate. A second experiment confirmed the lack of growth inhibition from simulated rain with pH > 3.4. Cumulative water use, a measure that integrated aspects of root size, leaf area, and physiology, was remarkably similar in plants exposed to pH 3.4, 4.2, or 5.6 simulated rain. These experiments suggest that vegetative growth of soybeans may be adversely affected by acid rain if pH is low enough to cause physical injury to leaves and loss of photosynthetic area.