Growth responses of N and S deficient white clover and burr medic to SO 2, NO and NO 2

Abstract

White clover and burr medic plants were exposed to SO 2, NO and NO 2 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 SO 2 could be ameliorated by NO and NO 2. Burr medic was very sensitive to SO 2, as 80 ppb SO 2 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 NO 2 for 4 h day −1 for 149 days suggests a very high sensitivity to NO and NO 2. Although the unexposed plants were deficient or marginal in sulphur nutrition, exposure to SO 2 did not result in a corresponding improvement in plant growth. This is despite exposure to SO 2 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 SO 2 concentration suggests that the toxic effects of SO 2 overwhelmed any potential beneficial effects. The toxic effects of SO 2 on vegetative growth were found to be additively increased by the additional presence of NO and NO 2. Although they were probably nitrogen-deficient, when the white clover plants were fumigated with NO and NO 2, 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 NO 2 may partially relieve the nitrogen deficiency increasing stomatal conductance, causing enhanced uptake of SO 2 and growth inhibition.