The Effects of Low Concentrations of Sulphur Dioxide on Stomatal Conductance and Epidermal Cell Survival in Field Bean (Vicia fabaL.)

Abstract

Exposure of 3 week old field bean plants to concentrations of SO₂ from 50–500 μg m^-3 induced comparable 20–25% increases in mean leaf diffusive conductance regardless of whether the diffusive conductances were obtained by porometric measurement or calculation from gas exchange data. The stomatal conductances of the adaxial and abaxial leaf surfaces were both increased by exposure to SO₂. Microscopic examination of epidermal strips from control and polluted plants revealed that the stomatal opening observed in treated plants was associated with a sharp reduction in the proportion of living epidermal cells adjacent to the stomata. The proportion of surviving adjacent epidermal cells was invariably smaller on the lower epidermis and appeared to decrease as the SO₂ concentration was raised from 50 to 500 μg m⁻³. Although the guard cells appeared to be undamaged at concentrations below 200 μg m⁻³, structural disorganization or death of one or both guard cells was observed frequently at or above 500 μg m⁻³. The results are discussed in relation to the controversy concerning the effects of SO₂ on stomatal aperture.