GROWTH-RESPONSES OF N-DEFICIENT AND S-DEFICIENT WHITE CLOVER AND BURRMEDIC TO SO2, NO AND NO2

White clover and burr medic plants were exposed to SO2, NO and NO2 und er 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. Bu rr medic was very sensitive to SO2, as 80 ppb SO2 decreased shoot weig ht by about 70%, and stem length by 30%. The halving of shoot weight o f white clover by exposure to 29 ppb NO and NO2 for 4 h day(-1) for 14 9 days suggests a very high sensitivity to NO and NO2. Although the un exposed plants were deficient or marginal in sulphur nutrition, exposu re to SO2 did not result in a corresponding improvement in plant growt h. This is despite exposure to SO2 causing a substantial increase in l eaf sulphur concentrations, and an improvement in nutrition to adequat e or high levels. A linear decrease in shoot weight with increase in S O2 concentration suggests that the toxic effects of SO2 overwhelmed an y potential beneficial effects. The toxic effects of SO2 on vegetative growth were found to be additively increased by the additional presen ce 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, ironi cally, the uptake of NO2 may partially relieve the nitrogen deficiency increasing stomatal conductance, causing enhanced uptake of SO2 and g rowth inhibition.