MODEL ANALYSIS OF INTERACTIVE EFFECTS OF OZONE AND WATER-STRESS ON THE YIELD OF SOYBEAN

The interactive effects of ozone and water stress on the yield of soybean (Glycine max (L.) Merr. 'Davis) were addressed with a growth model of soybean. Two simulations were conducted, using the data from the exposures of soybean to ozone in open-top chambers under two soil moisture regimes, and the results of the simulations were compared In the original simulation, soil moisture content was calculated based on a water budget using the actual precipitation and irrigation data. In the modified simulation, the soil water content was given as input data. In this case, soil moisture content was maintained al the same level across the ozone treatments regardless of different water use by the plants. Both simulations included the effect of reduced ozone flux to the leaves due to water stress, whereas only the original simulation included the effect of mitigated water stress due to reduced water use by the plants under higher ozone concentration. The water stress reduced ozone impact on soybean yield in the orignal simulation on the basis of the ozone dose-crop yield response relationship, but not in the modified simulation. The ozone uptake rate was reduced by water stress in the original simulation, but the relationship between seasonal mean ozone uptake rate and relative yield still showed reduced impact of ozone due to water stress. These results indicated that the alleviation of water stress by ozone due to reduced plant water use in ozone-treated plots can be a contributing factor in the reduction of ozone impact by water stress. The above conclusion was partly confirmed by the actual data for soil water content, which was significantly lower in the lowest ozone treatment than in the higher ozone treatments. Further experimental and modelling studies are needed to elucidate the mechanism of the ozone X water stress interaction.