Growth of lucerne (Medicago sativa L.) exposed to sulphur dioxide

The effect of exposure to sulphur dioxide (0 or 96 μg m−3 SO2) on the growth and sulphur content of lucerne (Medicago sativa L.) was examined in a period of 135 d during which the plants were harvested four times. The lucerne was grown in pots of soil with and without the addition of sulphate and of nitrate. Evidence of sulphur deficiency, including a reduction in the weight, the number and the sulphur content of shoots, was found in plants grown without added sulphate. Deficiency was alleviated through exposure of plants to SO2. Apart from reducing shoot weight at one harvest and generally increasimg the concentration of sulphur in the shoots, exposure to SO2 had no significant effect on plants grown with added sulphate. The yield of shoots was greater, and was reduced to a lesser extent with sulphur deficiency, in plants grown with added nitrate than in those dependent on rhizobia. Whilst the transpiration coefficient increased greatly in sulphur-deficient plants, it was reduced where otherwise similarly treated plants were exposed to SO2; this treatment did not alter the coefficient in plants with an adequate supply of sulphur from the soil.