SEASONAL-VARIATION OF GAS-EXCHANGE AND PIGMENTATION IN BRANCHES OF 3 GRAFTED CLONES OF MATURE PONDEROSA PINE EXPOSED TO OZONE AND ACID-RAIN

Gas exchange and pigmentation responses of mature ponderosa pine (Pinu s ponderosa Laws.) branches to ozone and acid rain exposure were inves tigated using three grafted clones growing in a managed seed orchard. Exposure of one-year-old foliage to twice ambient ozone (2 x AMB) resu lted in significant decreases in net photosynthesis (P-n), stomatal co nductance (g(sw)) and pigmentation relative to charcoal-filtered (CF) and ambient (AMB) ozone treatments. Ozone effects on gas exchange and pigmentation were most pronounced during late-season and differed sign ificantly among clones. Environmental parameters (e.g. light, vapor pr essure deficit, and temperature) accounted for more variation in P-n t han did cumulative ozone exposure. Minimal differences in g(sw) and P- n among ozone treatments occurred during seasonal periods of high temp erature and evaporative demand. Negative effects of 2 x AMB ozone on g (sw) and pigmentation were greatest for the clones having highest and lowest phenotypic vigor under ambient conditions; the clone of moderat e phenotypic vigor under ambient conditions was least sensitive to ozo ne. Application of simulated acid rain of pH 3.0, pH 5.1 or no rain (N R) had little impact on gas exchange or pigmentation. (C) 1997 Elsevie r Science Ltd.