Foliar metabolic heat rate of seedlings and mature trees of Pinus ponderosa exposed to acid rain and ozone

We investigated the effects of acid rain and ozone on respiration rates of 1-year-old and current-year foliage of half-sib seedlings and mature clones of a ponderosa pine genotype by measurement of foliar metabolic heat rates, Two rain regimes (pH 5 . 1 and 3 . 0) were applied weekly to foliage only, from January to April 1992, Two ozone regimes (ambient and twice-ambient) were applied from September 1991 to November 1992, Metabolic heat rate was measured in April on 1-year-old foliage, in June on both 1-year-old and current-year foliage, and in November on current-year foliage in 1992, Except for current-year foliage in June, the metabolic heat rate was calculated per unit of both foliar dry mass and N mass, In seedlings, both measures of metabolic heat rate increased in late June for 1-year-old foliage exposed to twice-ambient ozone, and in November for current-year foliage exposed to the combination of twice-ambient ozone and pH 3 . 0 rain, In mature trees, metabolic heat rate was not affected significantly by ozone, rain acidity, or their interaction, In June, when both 1-year-old and current-year tissues were examined, the metabolic heat rate of expanding, current-year foliage was higher than that of fully expanded, 1-year-old foliage regardless of plant age or treatment combination.