The effects of SO2, and water stress, singly and in combination on potato plants (Solanum tuberosum L. cv. Russet Burbank) were studied by exposing them to one of three SO2 concentrations (2, 110 and 300 nl 1−1) and one of two levels of soil moisture (well-watered and water-stressed). Visible leaf injury attributable to SO2 toxicity was observed after 6 weeks of fumigation with 300 nl 1−1 SO2 and 9 weeks of fumigation with 110 nl 1−1 SO2. Leaf, stem and tuber growth were significantly reduced by 300 nl 1−1 SO2 and water stress. Exposure to 300 nl 1−1 SO2 under well-watered conditions induced an increase of the shoot: root (including tuber) ratios but not for the ratios (excluding tuber) early in the season, indicating a strong effect of the treatment on tuber growth. With progress of the season, water stress usually reduced both ratios.
SO2−induced growth reductions in well-watered plants were mostly significant, but usually not in water-stressed plants, indicating a protective function of soil water stress in the response of plants to SO2. This could be caused by reduced SO2 uptake in water-stressed plants, as the well-watered plants showed much higher leaf sulphur concentrations than did the water-stressed plants at the same SO2 concentrations.