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

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.