Acclimation of photosynthesis and respiration to elevated atmospheric CO₂ in two scrub oaks

For two species of oak, we determined whether increasing atmospheric CO₂ concentration (C_a) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated C_a, or both. In particular, we tested whether acclimatory decreases in leaf-Rubisco content in elevated C_a would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of Quercus myrtifolia Willd. and Quercus geminata Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of Q. myrtifolia. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 μmol mol^-1) C_a, in open-top chambers (OTCs). For Q. myrtifolia, an increase in C_a from 360 to 710 μmol mol^-1 had no direct effect on R at any time during the year. In April 1999, R in young Q. myrtifolia leaves was significantly higher in elevated C_a - the only evidence for an indirect effect of growth in elevated C_a. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated C_a significantly reduced maximum RuBP-saturated carboxylation capacity (V_{c max}) for both the sun and shade leaves of only Q. geminata. However, we estimated that only 11-12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit left area or unit dry weight, and did not support the hypothesis that elevated C_a decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.