RADIATION UTILIZATION EFFICIENCY AND THE GROWTH OF SOYBEANS EXPOSED TO OZONE - A COMPARATIVE-ANALYSIS

Effects of ozone (O3) on field-planted soybeans (Glycine max (L.) Merr. 'Davis') were examined using a simple crop-growth model that resolves the rate of increase in crop dry weight into the total amount of radiation intercepted by a crop canopy and the efficiency of conversion of this energy into dry matter (ε{lunate}, termed utilization efficiency). The objectives were : (1) to examine the effects of O3 on light interception and ε{lunate}, and compare these results with a similar study by Unsworth et al. to determine whether the effects of O3 are consistent from year to year, and (2) to determine whether ε{lunate} is more sensitive to O3 during reproductive than vegetative growth and whether allocation of aboveground biomass to seed is affected by O3. Five O3 concentrations (seasonal means of 30, 52, 57, 76 and 99 ppb) were maintained in open-top chambers over the growing season. All treatments were irrigated. Leaf, stem and pod weights and leaf area were measured weekly throughout the season. Total incident solar radiation and total radiation transmission through the canopy were measured with tube solarimeters and electronic integrating devices. Increased O3 concentrations were accompanied by depressed leaf area expansion and earlier leaf senescence, but did not affect total light interception over the entire season. Utilization efficiency (ε{lunate}) was reduced in high O3 treatments and was the most important factor in the reduction of yield; ε{lunate} for the whole season declined from 0.89 g MJ-1 in the 30-ppb treatment to a low of 0.68 g MJ-1 in the 99-ppb treatment. Efficiencies were higher during reproductive than vegetative growth, but were not more sensitive to O3. Seed yield decreased from 450 g m-2 in the 30-ppb treatment to 320 g m-2 in the 99-ppb treatment. Fractional allocation of aboveground dry weight to seed was not affected by O3. Reduced ε{lunate} values with increasing O3 concentrations were consistent with an earlier study by Unsworth et al. however, ε{lunate} values for the vegetative phase of growth in this study were lower at all O3 concentrations. Lower ε{lunate} values were attributed to canopy light saturation because of higher incident radiation and water stress caused by a shallow rooting zone during a hot, dry growing season. © 1990.