Effects of low concentrations of o(3) on net photosynthesis, dark respiration, and chlorophyll contents in aging hybrid poplar leaves.

Chronic exposure of hybrid poplar (Populus deltoides x 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 O(3) 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 CO(2) 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 O(3), and lightsaturated P(n) was linearly related to chlorophyll a + b contents. Differences in light-saturated P(n) between control and O(3)-treated leaves of the same age were mostly due to photosaturation in O(3)-treated leaves and to a much lesser extent to lowered apparent quantum yields. Also, since P(n) and dark respiration were most affected by O(3) at different leaf ages, distinct modes of action are suggested. The effects of leaf aging on CO(2) 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 O(3) exposure upon CO(2) exchange.