Acclimation of photosynthesis and respiration to elevated atmospheric CO 2 in two Scrub Oaks

For two species of oak, we determined whether increasing atmospheric CO 2 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 leaf 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.