Impacts of ozone on the growth and yield of field-grown winter oilseed rape

Five cultivars of winter oilseed rape (Brassica napus ssp. oleifera var. biennis L.) were screened for their relative sensitivity to ozone (O-3) by exposing seedlings to charcoal-Purafil(R) filtered air (CFA) or CFA plus 75 nmol mol(-1) O-3 (6.5 h day(-1), 16 days) in controlled environment chambers, and effects on relative growth rate (R) determined. Based on this preliminary screening under standardised conditions, the most sensitive cultivar ('Eurol')-exhibiting a 10% reduction in R-was selected for held studies. Seed was sown in mid-August in eight field plots. During August and September, four of these plots were exposed to elevated O-3 concentrations on 17 days for 6 h day(-1) (mean daily O-3 concentration was 30 and 77 nmol mol(-1) in plots exposed to ambient air (AA) and AA + O-3, respectively). After overwintering, the same plots were exposed to elevated O-3 concentrations on 32 days for 7 h day(-1) between May and June (mean daily O-3 concentration was 31 and 80 nmol mol(-1) in plots exposed to AA and AA+O-3, respectively). Plants harvested in September revealed that exposure to AA + O-3 resulted in an 8% decrease in plant Dw, and reduced root growth relative to that of the shoot. Assessment of yield characteristics, determined following pod-ripening, revealed that plants exposed to AA + O-3 produced fewer flowering branches (-38%). However, the individual pods borne on these branches were heavier and contained more seed. As a consequence of these compensatory responses, exposure to AA + O-3 resulted in a smaller (-14%) decline in seed yield (tonne ha(-1)) than would be predicted from the O-3-induced reduction in the number of flowering branches. Seed quality, in terms of crude protein and oil content, was reduced by elevated O-3 Findings are discussed in relation to the potential impacts of ambient levels of O-3 on winter oilseed rape production in the UK. (C) 1999 Elsevier Science Ltd. All rights reserved.