Short-term (4 hours) effect of different concentrations of SO2 fumigation on in vivo photochemical activities of sugar maple (Acer saccharum Marsh.) leaves was investigated using photoacoustic spectroscopy. The relative quantum yield of 02 evolution (ratio of 02 signal to the photothermal signal) and photochemical energy storage are increased by 0.05 microliter per liter of SO2. This increase is more pronounced in 5 to 7 year old saplings than in 3 month old seedlings. Both oxygen-relative quantum yield and energy storage of seedlings are inhibited by increased concentrations of SO2 and the inhibition is concentration dependent. The inhibition is greater in seedlings than in saplings at 2 microliters per liter of SO2, indicating the more susceptible nature of seedlings. The present study indicates a concentration dependent differential effect of SO2 on photochemical activities of sugar maple leaves.Sulfur dioxide is a widespread gaseous pollutant affecting plant growth and yield (6). It reduces net photosynthetic CO2 assimilation by affecting stomatal and nonstomatal components of photosynthesis (6,12,22,23,27). Several investigators reported that SO2 inhibits photosynthetic electron transport and inactivates Calvin-cycle enzymes in chloroplasts isolated from SO2 fumigated or sulfite-treated leaves (2,22,23,27). However, there are also reports indicating increased rates ofelectron transport, NADP reduction, and CO2 fixation in sulfite-treated chloroplasts or chloroplasts isolated from SO,-fumigated leaves (13,15,30). Recently, it has been demonstrated that sulfite shows an artificial inhibitory effect on photosynthetic electron transport by interacting with oxygen and certain electron acceptors in the assay medium (3). So, the observed inhibition of photosynthetic electron transport in the in vitro studies may be overestimated.Limited information is available on the effect of different concentrations of SO, on in vivo photochemical activities of an intact leaf. Hence, in the present study, we examined the short term SO2 fumigation effect on the photochemical activities of sugar maple leaves using a relatively recent technique of photoacoustic spectroscopy. This technique has been successfully applied to study the energy storage and relative '