Interaction between sublethal pollution by sulphur dioxide and drought stress. The effect on photosynthetic capacity

Five-year-old Picea abies L. plants were grown in growth cabinets in the presence (3.1 μmol m^?3) or absence of SO₂. After 5 weeks, the photosynthetic capacity of mature needles produced in the year was the same in both conditions. Trees were then submitted in situ to drought stress by withholding water. The decline of leaf photosynthetic capacity was greatest in the presence of SO₂. Chlorophyll decreased only when trees were submitted to dehydration in the presence of SO₂; however, this al-one could not account for the large decline in photosynthetic capacity observed under that condition. Needle water content was the lowest during dehydration in the presence of SO₂. It is concluded that the critical factor in the interaction between pollution by SO₂ and drought stress is the greater dehydration of the tissue found in stressed plants grown in the presence of SO₂. The large decline in photosynthetic capacity under such conditions might be due to this greater dehydration.