Modelling short-term effects of sulphur dioxide. 1. A model for the flux of SO2 into leaves and effects on leaf photosynthesis

Abstract

A model for the nux of atmospheric S0 2 into leaves and the effects of S0 2 metabolites (S(IV) compounds) on leaf photosynthesis and stomatal resistance is presented. The S(IV) balance in the leaf is determined by the rate of S0 2 uptake and S(IV) removal by oxidation to sulphate. Toxic S(IV) compounds reduce the rate of photosynthesis and induce stomatal closure as a result of feed back control of stomatal resistance by photosynthesis. Other proposed mechanisms, like effects through a pH reduction, are not likely to play a role in short-term effects of realistic S0 2 concentrations. The model contains two key parameters which desC'ribe biochemical characteristics: a time coefficient for S(IV) oxidation and a parameter describing the sensitivity of photosynthesis for S(IV).Simulation results demonstrate the potential of plants to avoid extremely toxic concentrations of S(IV) in the leaf by three mechanisms: (1) rapid oxidation of S(IV) to less toxic sulphate, (it) relatively high resistance to S0 2 uptake and (iii) feed back control between photosynthesis and stomatal resistance. S(IV) concentrations in the leaf and S0 2 concentrations in the stomatal cavities in stable situations are less than I O!o of concentrations which build up without these mechanisms. Leaf thickness appears to be an important factor determining the susceptibility of plants to air pollutants. Thin leaves should be more sensitive than thicker leaves. It is concluded that effects of so2 on photosynthesis should be related to the uptake per unit of leaf volume instead of the commonly used nux per unit leaf area. The model accurately described the time course of photosynthetic reduction during a short fumigation period and subsequent recovery period.

Modelling short-term effects of sulphur dioxide. 2. Quantification of biochemical characteristics determining the effect of SO2 on photosynthesis of leaves

“…The sensitivity parameter was derived from experimental data on photosynthetic reduction in relation to the intracellular S(IV) concentration from Sakaki and Kondo (1985) who used isolated protoplasts and chloroplasts of Viciafaba L., and Alscher et al (1987) who studied the effects of S0 2 exposure on two cultivars of Piswn sativum L. Other data sets on both effects on photosynthesis and S(IV) concentrations were not available * S(IV) = [S0 2 ]aq + + [SO~-] 214 in literature. Surprisingly, the same relationship betvveen reduction in the rate of photosynthesis and internal sulphite concentration was observed for both data sets (Kropff, 1989). Especially the results of Alscher et al (1987) suggest that differences in sensitivity for so2 between plants is not based on differences in the sensitivity of photosynthesis to internal S(IV).…”

“…A summary version of a model for the S0 2 flux into leaves and effects of S0 2 on the rate of photosynthesis (Kropff, 1989) was used to analyse experimental data on the effects of S0 2 on the rate of photosynthesis with standard statistical techniques. Values for the two key parameters of the model, a sensitivity parameter relating intracellular S(IV) concentration (S0 2 , bisulphite and sulphite) to photosynthetic reduction, and a time coefficient for S( IV) oxidation, were estimated from data on photosynthesis during fumigation and subsequent recovery period, by combined non-linear regression of both equations.…”

“…A model was presented earlier, simulating S0 2 flux from the atmosphere into leaves and the effects of toxic S0 2 metabolites on the rate of photosynthesis and on stomatal resistance (Kropff, 1989). The model contains two key parameters, a sensitivity parameter relating photosynthetic reduction to intracellular S(IV)* concentration, and a time coefficient for S(IV) oxidation determining the rate of S(IV) removal to the less toxic sulphate.…”

“…For experimental determination of S(IV) in plants during the exposure of leaves to realistic so2 concentrations ( < 200 jlg so2 m -3 ), biochemical methods are needed to detect very low S(IV) concentrations in leaf tissue [ < 0.05 mmol S(IV) l-1 ] (Kropff, 1989;Huber et al, 1987). However, the detection limit is about 0.04 mmol S(IV) I-1 (Huber et al, 1987) and experimental determination in leaf tissue is extremely difficult because S(l V) quickly oxidizes to sulphate.…”

“…In order to gain more insight in the quantitative values of these two parameters, the model presented in a preceding paper (Kropff, 1989) was fitted to experimental data on the short-term effects of so2 on photosynthesis of various species.…”