Temperature dependency of acid-rain effect on photosynthesis of Pinus ponderosa

We measured the effects of acid rain and elevated ozone on net photosynthes is (A), stomatal conductance (g(1)), and internal CO2 concentration (C-i) i n 1-year-old and current-year foliage of field-grown trees of a single clon e of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) under varying, contro lled temperature and light combinations. Branches of trees were exposed to four combinations of two rain acidities (pH 5.1 and pH 3.0) by two ozone co ncentrations (ambient and twice-ambient). The simulated rain treatment was applied weekly to foliage only from January to April 1992. The stone treatm ent was applied from September 1991 to November 1992. Gas-exchange response s to nine combinations of three temperature (18, 25, 32 degrees C) by three light (250, 500, 1000 mu mol photons m(-2) s(-1)) regimes were measured da ily (0800 to 1900 hours) in August 1992. Increased ozone had no significant effect on physiological responses studied regardless of temperature/light regime or foliage age. Rain with pH 3.0 decreased A of 1-year-old foliage, from 11 to 25%, depending on the temperature regime. By contrast, pH 3.0 ra in had no significant effect on stomatal conductance or internal CO2 concen tration of I-year-old foliage suggesting that the decreased A was not linke d to altered stomatal behavior. It appears that a seasonal foliar applicati on of pH 3.0 rain had a persistent effect on net photosynthesis and that th e effect could only be detected when temperatures were either below or abov e an optimal temperature (about 25 degrees C). This suggests that assessmen ts of acid-rain effects on gas-exchange, made under optimal temperature con dition, may underestimate acid-rain effect under natural temperature regime s. (C) 1999 Elsevier Science B.V. All rights reserved.