Does the direct effect of atmospheric CO₂ concentration on leaf respiration vary with temperature? Responses in two species of Plantago that differ in relative growth rate

The objective of this study was to investigate the direct effect of elevated atmospheric CO₂ concentrations on leaf respiration in darkness (R) over a broad range of measurement temperatures. Our aim was to further elucidate the underlying mechanism(s) of the often-reported inhibition of leaf R by a doubling of the atmospheric CO₂ concentration. Experiments were conducted using two species of Plantago that differed in maximum relative growth rate (fast-growing Plantago lanceolata L. and the slow-growing P. euryphylla Briggs, Carolin & Pulley). Rates of leaf respiration (R) were measured at atmospheric CO₂ concentrations ranging from 75 to 2000 μmol mol^-1 at temperatures from 12 to 42°C. R was measured as CO₂ release with a portable gas exchange system with infrared gas analysers. Our hypothesis was that the changes in temperature alter the flux coefficient (i.e. the extent to which changes in potential enzyme activity has an effect on the rate of a reaction) of enzymes potentially affected by CO₂. Initial analysis of our results suggested that R was inhibited by elevated CO₂ in both species, with the apparent degree of inhibition being greatest at low temperature. Moreover, the apparent degree of inhibition following a doubling of atmospheric CO₂ concentration from 350 to 700 μmol mol^-1 was similar to that reported by several previous studies (approximately 14% and 26% for P. lanceolata and P. euryphylla, respectively) at a temperature equal to the mean of the previous studies. However, subsequent correction for diffusion leaks of CO₂ across the gas exchange's cuvette gaskets revealed that no significant inhibition had occurred in either species, at any temperature. The inhibitory effect of elevated CO₂ on leaf respiratory CO₂ release reported by previous studies may therefore have been overestimated.