Differential response of four tree species to ozone-induced acceleration of foliar senescence

Ozone (O₃)-induced accelerated senescence of leaves was measured in four tree species: black cherry (Prunus serotina), hybrid poplar (Populus maximowizii x trichocarpa, clone 245), northern red oak (Quercus rubra) and sugar maple (Acer saccharum). Seedlings or ramets of the four species were subjected to chronic O₃ exposures and designated leaves harvested periodically from emergence to senescence. Gas exchange was analysed, and concentrations of total soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase were measured as indicators of leaf senescence. Total antioxidant potential and ascorbate peroxidase and glutathione reductase activities also were determined. Black cherry and hybrid poplar exhibited O₃-induced accelerated leaf senescence, whereas sugar maple and northern red oak did not. When the O₃ effects were related to cumulative uptake of the gas, black cherry was the most sensitive of the four species. Although hybrid poplar exhibited similar symptoms of O₃-induced accelerated senescence after the same exposure period as did black cherry, this species took up much greater quantities of O₃ to achieve the same response. The O₃-induced increase in glutathione reductase activity in hybrid poplar was consistent with the capacity of this species to take up high concentrations of the gas. Relative tolerance of northern red oak and sugar maple could be explained only in part by lower cumulative O₃ uptake and lower rate of uptake. Sugar maple had the highest antioxidant potential of all four species, which may have contributed to O₃ tolerance of this species. Ascorbate peroxidase activity, when expressed on a fresh weight basis, could not account for differential sensitivity among the four species.