Resistance analysis of sulphur dioxide fluxes to Vicia faba

Before plants respond directly to a gaseous air pollutant, the gas must diffuse into the leaves through stomatal pores or through the cuticle. Some gases are also absorbed or adsorbed on the surface of leaves1 and this flux elicits no direct plant response. In principle, responses to a pollutant may depend either on the rate at which the gas is absorbed, or on the total quantity absorbed over a period of time. In practice, workers have commonly related responses either to the gas concentration surrounding the plant or to the dose, defined as the product of the gas concentration and the duration of exposure2. Plants exposed in an environment where the pollutant flux to leaves is restricted because there is little air movement3 or because stomata are partly closed, are regarded as having the same dose as plants absorbing the pollutant rapidly. Consequently the response to a specified dose depends strongly on the conditions of exposure3. We report here laboratory measurements of fluxes of sulphur dioxide to plants of the field bean, Vicia faba L. and relate the observed inhibition of net photosynthesis to this flux. Measured fluxes are analysed to show the several pathways for SO2 into and on to leaves, and to demonstrate that the internal concentration of SO2 in the bean leaves was close to zero.