Long‐term effects of low levels of SO2 on bean plants (Phaseolus vulgaris). I. Immission‐response pattern of net photosynthesis and transpiration during life‐long, continuous measurements

The immission‐response effect of five low levels of sulfur dioxide on net photosynthesis and transpiration was studied during continuous measurements in near‐complete life cycles of whole bean plants (Phaseolus vulgaris L. cv. Processer) grown in a controlled environment. Sixteen plants were grown in individual water cultures in each of five 100 1 glass assimilation chambers with a new type of exposure system with separate root aeration. SO2 immission ranged from 10 μg m−3 to 950 μg m−3 during 12‐h day‐time exposure periods, five days a week, while a low, natural background of NOx was accepted. The SO2‐induced photosynthetic reductions were in the short term, but in particular on the long‐term level very closely related with stomatal conductance (significance level better than 0.0005). However, a causal coherence was not inferred. Physiological inhibitions were composed of: (1) A reversible component (night and week‐end recovery) and (2) an irreversible component (related to reduced green leaf area). The pattern of leaf growth was studied, with the conclusion that SO2 reduced leaf area by promoting senescence, rather than by interfering with leaf emergence and development.