Tadahiro Ushijima

The relationship between absorption of sulfur dioxide (SO2) and inhibition of photosynthesis in several plants

Photosynthetic rate, transpiration rate and SO2 absorption rate were simultaneously measured under exposure to SO2 (0.1–1.0 μl l−1) for 5 or 8 hr in six species belonging to C4 or C3 plants (Zea mays, Sorghum vulgare, Amaranthus tricolor, Oryza sativa, Avena sativa andHelianthus annuus). Distinct interspecific differences were found as to the extent of inhibition of photosynthetic rate. Calculation of diffusive resistance to H2O(r) and SO2(r′) showed that the ratio of r′/r was 1.9 irrespective of species and coincided well with the theoretical value based on molecular diffusion. Thus it was made clear that the absorption of SO2 was dependent upon the gas exchange capacity of leaf blade. Using the ratio of r′/r the rate of SO2 absorption could be calculated from transpiration rate and was compared with the inhibition rate of photosynthesis. In three C4 species, the inhibition of photosynthesis increased linearly with the amount of SO2 absorbed during a 5-hour period. The pattern of inhibition of photosynthesis inA. sativa andH. annuus among C3 species was similar to that of C4 species until the amount of SO2 absorbed reached 60 mg-SO2 m−2 above which the inhibition abruptly increased. The inhibition of photosynthesis inO. sativa was exceptionally severe even with only a small amount of SO2 absorbed.