Light and the maintenance of photosynthetic competence in leaves of Populus balsamifera L. during short-term exposures to high concentrations of sulfur dioxide

Abstract

Leaves of Populus balsamifera grown under full natural sunlight were treated with 0, 1, or 2 μl SO₂·1^-1 air under one of four different photon flux densities (PFD). When the SO₂ exposures took place in darkness or at 300 μmol photons·m^-2·s^-1, sulfate accumulated to the levels predicted by measurements of stomatal conductance during SO₂ exposure. Under conditions of higher PFD (750 and 1550 μmol·m^-2·s^-1), however, the predicted levels of accumulated sulfate were substantially higher than those obtained from anion chromatography of the leaf extracts. Light-and CO₂-saturated capacity as well as the photon yield of photosynthetic O₂ evolution were reduced with increasing concentration of SO₂. At 2 μl SO₂·1^-1 air, the greatest reductions in both photosynthetic, capacity and photon yield occurred when the leaves were exposed to SO₂ in the dark, and increasingly smaller reductions in each occurred with increasing PFD during SO₂ exposure. This indicates that the inhibition of photosynthesis resulting from SO₂ exposure was reduced when the exposure occurred under conditions of higher light. The ratio F _v/F _M (variable/maximum fluorescence emission) for photosyntem II (PSII), a measure of the photochemical efficiency of PSII, remained unaffected by exposure of leaves to SO₂ in the dark and exhibited only moderate reductions with increasing PFD during the exposure, indicating that PSII was not a primary site of damage by SO₂. Pretreatment of leaves with SO₂ in the dark, however, increased the susceptibility of PSII to photoinhibition, as such pretreated leaves exhibited much greater reductions inF _V/F _M when transferred to moderate or high light in air than comparable control leaves.