DOES ATMOSPHERIC HYDROGEN-PEROXIDE CONTRIBUTE TO DAMAGE TO FOREST TREES

H2O2 has two faces in plants: it is toxic at low concentrations in the chloroplasts, and it is a necessary cosubstrate for the production of biopolymers in the apoplastic compartment. Plants have evolved an antioxidant system that enables them to cope with high intrinsic production rates of H2O2. Measurements of H2O2 in air at a rural, forested site were used to calculate the influx of atmospheric H2O2 into spruce needles. The estimated uptake rates were compared with the capacity of protective systems present in the aqueous matrix of the cell wall and inside the cells in the symplastic space. Evidence is presented that the rate of H2O2 detoxification exceeds its uptake up to 10(6)-fold. Therefore, it is unlikely that atmospheric H2O2 in the absence of synergistic effects of other air pollutants can overwhelm the intrinsic protection of mesophyll cells, thereby contributing to damage to the spruce trees.