Growth of Lucerne (Medicago sativa L.) Exposed to Sulphur Dioxide

The effect of exposure to sulphur dioxide (0 or 96 µgm^–3 SO₂) on the growth and sulphur content of lucerne(Medicago sativa L.) was examined in a period of 135 d duringwhich the plants were harvested four times. The lucerne wasgrown in pots of soil with and without the addition of sulphateand of nitrate. Evidence of sulphur deficiency, including areduction in the weight, the number and the sulphur contentof 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 generallyincreasimg the concentration of sulphur in the shoots, exposureto SO₂ had no significant effect on plants grown with addedsulphate. The yield of shoots was greater, and was reduced toa lesser extent with sulphur deficiency, in plants grown withadded nitrate than in those dependent on rhizobia. Whilst thetranspiration coefficient increased greatly in sulphur-deficientplants, it was reduced where otherwise similarly treated plantswere exposed to SO₂; this treatment did not alter the coefficientin plants with an adequate supply of sulphur from the soil.