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 NO_x 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.