The effects of long-term open-air fumigation with SO2 on a field crop of broad bean (<em>Vicia faba</em> L.) III. Quantitative analysis of damage components.

Effects of SO2 on growth and production of broad bean, observed in three open-air fumigation experiments, were interpreted in terms of damage components with a mechanistic simulation model. The model consisted of an elementary model for crop growth, extended with submodels for the microclimate in the crop and with a submodel for uptake of SO2 by leaves and for effects on leaf photosynthesis. The major part of the observed reduction in total dry matter production could be explained by leaf injury observed in the oldest leaves of the fumigated plants at the end of the growing period. The effect consisted of dry matter loss through leaf abscission and a reduced growth rate at the end of the growth period due to reduction of the amount of absorbed radiation by the canopy. Direct effects of SO2 on leaf photosynthesis explained an extra 10% of the observed yield loss (losses ranged from 7 to 17% of control yield). This small effect was confirmed by field measurements which showed no detectable effects of SO2 on leaf and canopy photosynthesis. Increased leaf respiration, which was observed in the 1988 experiment, explained another 10% of the observed yield reduction. Total SO2-sulphur uptake by the fumigated crop, which is an important component of dry deposition of SO2, was accurately simulated by the model. [ABSTRACT FROM AUTHOR]