The Effects of Low Concentrations of Sulphur Dioxide on Stomatal Behaviour in Vicia Faba

SUMMARY The stomatal resistance of leaves of Vicia faba L., measured using a diffusion porometer, rapidly decreased when plants were exposed to air containing SO 2 . The stomatal resistance decreased by 20% for the SO 2 concentrations used (140, 290 and 1430 μg m ‐3 ) and the minimum concentration to which stomata responded was 72 μg m ‐3 . Although stomatal resistance increased with leaf age, when exposed to 290 μg ‐3 SO 2 , the opening response was larger and more rapid in the older leaves. Exposure to 290 μg m ‐3 SO 2 throughout a day suppressed the normal diurnal variation in stomatal behaviour and reduced the stomatal resistance by 25%. It was estimated that the measured stomatal responses to SO 2 would increase the transpiration rate by 32% for single leaves in a growth room and by 23% for a field crop.