The effects of SO 2 dosage kinetics and exposure frequency on photosynthesis and transpiration of kidney beans ( Phaseolus vulgaris L.)

Abstract

The effects of variations in SO 2 dosage kinetics on bean plants ( Phaseolus vulgaris L. “red kidney”) were studied at SO 2 levels equal to or below the current secondary ambient air quality standard. Response of photosynthesis, transpiration and growth were measured to evaluate the extent and duration of effects following repeated treatment at weekly (Experiment I) and daily (Experiment II) intervals. Depression of photosynthesis was the most consistent response observed. Transpiration was either stimulated or depressed by SO 2 concentrations which reduced photosynthesis. This effect indicates a probable biochemical rather than stomatal basis for observed photosynthetic depressions noted in these experiments. Increasing the peak: mean concentration ratio from 1.0 to 1.7 during exposure had no obvious effect on short-term photosynthetic response but caused apparent stimulation of photosynthesis one day later. Increasing the peak: mean ratio from 1.0 to 6.0 caused a threefold average greater depression of short-term photosynthetic response even though the 3 hr av. concentration was reduced by 25% (0.50–0.37). Residual effects from this treatment were generally small on the day after exposure. No strong evidence was obtained to indicate that plants were sensitized to SO 2-induced suppression of photosynthesis by previous exposure to SO 2. Predisposition was not apparent when exposures were repeated at weekly or daily intervals. Visible foliar injury did not occur. Analysis of harvest data at the end of these experiments indicated that plants could undergo a repeated short-term (1 day) cycle of photosynthetic inhibition and still retain an essentially unimpaired capacity for growth during a subsequent recovery period. When exposures were spaced a week apart, some indication of a residual effect on growth, particularly with the highest peak: mean ratio (6.0), was apparent.