Low level O₃ and/or SO₂ exposure causes a linear decline in soybean yield

A field fumigation system was modified from one described earlier (Reich et al., 1982) and used to expose unenclosed field-grown soybean plants Glycine max cv Hark to low levels of O₃ and/or SO₂. A 3 × 3 factorial design was employed, allowing for analysis of individual effects of and potential interactions between the two pollutants. During the experiment ambient O₃ was significant, but low (mean concentration of 0·04 μl litre^-1), and ambient SO₂ were not detected. Plants were exposed to ambient air with or without additional pollutants for about 5 h per day on 16 days between 20 August and 12 September, 1980. Exposure to greater than ambient levels of O₃ (mean concentration of 0·06 or 0·08 μl litre^-1) caused significant linear reductions of 2-5% in mass per seed and number of seeds per pod, 17-25% in number of seeds and pods per plant and 10-25% in seed yield (dry mass) per plant and per hectare. Exposure to greater than ambient levels of SO₂ (mean concentration of 0·06 or 0·11 μl litre^-1) caused significant linear reductions of 4-7% in mass per seed. High SO₂ treatment (mean concentration of 0·11 μl litre^-1) also resulted in non-significant declines of 7-12% in yield per plant and per hectare. no significant interactions between the pollutants were observed and O₃ had a severalfold greater impact on soybean than SO₂ on either a concentration or a dose basis.