Ladino clover (Trifolium repens L. cv. Regal) was exposed to mean SO2 concentrations of 215, 78 or 2.8 μg m−3* in open top fumigation hambers for up to 175 d. Exposure to 215 μg m−3 for 28 d significantly increased leaf area and number of leaves per plant, and exposure to 78 μg m−3 significantly reduced the shoot: root ratio, and increased the leaf area ratio, compared with control plants exposed to 2.8 μg m−3. Exposure to 215 or 78 μg m−3 had no significant effect on shoot or root weight, and all other effects had decreased beyond the level of statistically significant differences between treatment and control after 63 d of fumigation.
Exposure to SO2 for 175 d had no effect on leaf protein concentration but exposure to 215 or 78 μg m−3 significantly reduced the concentrations of L-ascorbic acid and starch in leaves, and increased the sulphur concentrations in the shoot. The specific energy of leaves was not significantly changed by 78 μg m−3 but was significantly increased by exposure to 215 μg m−3 of SO2 for 28 d and significantly decreased by the same concentration after 175 d. Exposure to SO2 for 175 d had no significant effect on the specific energy of shoots. The SO2-induced reduction in ascorbic acid concentration may suggest a mechanism for reduced freezing resistance in plants exposed to SO2.