Radiation Inteception and the Growth of Soybeans Exposed to Ozone in Open- Top Field Chambers

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Abstract
Soybeans (Glycine max (L.) Merr. ''Davis'') were grown in the field in North Carolina [USA] in open-top chambers which were ventilated with either charcoal-filtered air, or non-filtered air, or non-filtered air to which about 0.02, 0.04 or 0.06 ppm ozone (O3) was added for 7 h day-1 (10.00-17.00 h). Interception of total solar radiation by the canopy in each treatment was measured with tube solarimeters, integrating the signals electronically on a daily basis. From regular harvests throughout the season, components of above-ground biomass and leaf area index (LAI) were determined. Non-destructive assessments of seed development were also made. During vegetative growth (the first 9 wk from emergence) the relation between dry matter and radiation intercepted from emergence was linear for all treatments. The slopes of the linear regressions consistently decreased as O3 concentration increased; in the highest O3 exposure the slope was about 30% less than that in filtered air. As O3 concentration increased, the maximum radiation interception attained during the season decreased, and eariler senescence gave a shorter persistence of leaf cover. The seasonal mean value of the fraction of radiation intercepted decreased from 0.68 in filtered air to 0.65 in the highest O3 treatment. Rates of seed growth were not significantly influenced by O3 but the duration of seed growth decreased with increased O3. Exposure to O3 reduced pod number per plant and mean seed weight, but the number of seeds per pod decreased only at the highest O3 concentration. Seed yields in the highest O3 treatment were about 50% lower than in filtered air. Analysis of environmental and biological factors limiting seed yield showed that over the whole season the smaller light interception and shorter duration of growth (emergence to cessation of seed growth) each accounted for less than 7% of the yield loss. More important factors restricting yield were the action of O3 in lowering efficiencies of dry matter production early in the season and in altering seed and pod development. The analysis showed that detailed studies of the interactions in the field between air quality and other environmental factors which determine plant growth can help to identify the mechanisms and processes which air pollutants modify.