Growth responses of N and S deficient white clover and burr medic to SO2, NO and NO2

White clover and burr medic plants were exposed to SO2, NO and NO2 under conditions of probable nitrogen and sulphur deficiency in open top chambers for 4 h day−1, 7 days week−1 for 149 days to determine if the growth changes in nitrogen and sulphur deficient plants resulting from long term exposure to SO2 could be ameliorated by NO and NO2. Burr medic was very sensitive to SO2, as 80 ppb SO2 decreased shoot weight by about 70%, and stem length by 30%. The halving of shoot weight of white clover by exposure to 29 ppb NO and NO2 for 4 h day−1 for 149 days suggests a very high sensitivity to NO and NO2. Although the unexposed plants were deficient or marginal in sulphur nutrition, exposure to SO2 did not result in a corresponding improvement in plant growth. This is despite exposure to SO2 causing a substantial increase in leaf sulphur concentrations, and an improvement in nutrition to adequate or high levels. A linear decrease in shoot weight with increase in SO2 concentration suggests that the toxic effects of SO2 overwhelmed any potential beneficial effects. The toxic effects of SO2 on vegetative growth were found to be additively increased by the additional presence of NO and NO2. Although they were probably nitrogen-deficient, when the white clover plants were fumigated with NO and NO2, shoot weight was halved but the rate of sulphur accumulation by leaves was doubled. Nitrogen deficiency may induce partial closure of stomata, but, ironically, the uptake of NO2 may partially relieve the nitrogen deficiency increasing stomatal conductance, causing enhanced uptake of SO2 and growth inhibition