Effects of Low Concentrations of O3 on Net Photosynthesis, Dark Respiration, and Chlorophyll Contents in Aging Hybrid Poplar Leaves

Abstract

Chronic exposure of hybrid poplar (Populus deltoides × trichocarpa) plants to low concentrations of ozone had negative impact upon net photosynthetic capacity, dark respiration, and leaf chlorophyll contents. Exposure to as much as 0.20 microliters per liter O₃ had no immediate effect on net photosynthesis (P_n), but chronic exposure to 0.125 or 0.085 microliters per liter had a number of gradual effects on CO₂ exchange. These included increased dark respiration and consequently increased light compensation points in very young leaves (4-6 days old); and decreased P_n, leaf chlorophyll a and b contents, light saturation points, and apparent quantum yields in leaves 10 to 70 days old. Decreased P_n was partially due to accelerated aging in leaves exposed to O₃, and lightsaturated P_n was linearly related to chlorophyll a + b contents. Differences in light-saturated P_n between control and O₃-treated leaves of the same age were mostly due to photosaturation in O₃-treated leaves and to a much lesser extent to lowered apparent quantum yields. Also, since P_n and dark respiration were most affected by O₃ at different leaf ages, distinct modes of action are suggested. The effects of leaf aging on CO₂ exchange were considerable, but typical of other species. However, careful monitoring of the interacting effects of leaf age and pollutant exposure was needed in order to characterize the impact of chronic O₃ exposure upon CO₂ exchange.