Ozone (O 3 )-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 O 3 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 O 3 -induced accelerated leaf senescence, whereas sugar maple and northern red oak did not. When the O 3 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 O 3 -induced accelerated senescence after the same exposure period as did black cherry, this species took up much greater quantities of O 3 to achieve the same response. The O 3 -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 O 3 uptake and lower rate of uptake. Sugar maple had the highest antioxidant potential of all four species, which may have contributed to O 3 tolerance of this species. Ascorbate peroxidase activity, when expressed on a fresh weight basis, could not account for differential sensitivity among the four species.

SUMMARYExperiments were conducted to determine the impact of nitrogen and ozone (O3) stress on the growth of domestic radish Raphanus sativus L. cv. Cherry Belle. Plants were grown in field chambers with sub-, optimal and supraoptimal levels of nitrogenous fertilizer. Chamber air was either charcoal-filtered, or supplemented with one of two levels of O3. The highest O3 treatment resulted in significant reduction in weight of hypocotyls and roots while elevated nitrogen treatments resulted in increased weight of all plant parts. Ozone did not affect the weight of plant foliage at any nitrogen level. Plants grown with lower levels of nitrogen had less leaf biomass but the tissue accounted for a greater percentage total weight than did the foliage of higher nitrogen treatments. Relative growth rate of whole plants was not aflFected by O3 or nitrogen treatments refiecting compensation in response to both stresses. Ozone-induced depression in biomass was observed in 03-treated plants grown with higher nitrogen supply but not in those grown with limiting nitrogen. This observation could reflect compensation at the lower levels of nitrogen supply or inability to detect changes in biomass due to reduced weights of plants grown at the lowest nitrogen supply. The dry weight ratio of sink organs (hypocotyl plus root)/shoot was significantly correlated with the total non-structural carbohydrate (TNC) content of these organs, regardless of treatment. Initially, O3 induced a significant decrease and nitrogen an increase in percent TNC of sink organs. At later sampling times, plants adjusted to stress as effects on percent TNC were no longer evident.

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Measurement of the electrical field along soybean hypocotyls shows the development of a positive electrical potential in the lower side approximately 1 min after horizontal placement. The time is as fast or faster than the geotropic presentation time of soybean seedlings. The maximum positive electrical field potential is produced in a zone 1-2 cm below the hook, which is the region showing the geotropic curvature.

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