Leaf respiration in darkness and in the light under pre-industrial, current and elevated atmospheric CO₂ concentrations

Our study sought to understand how past, low atmospheric CO₂ concentrations ([CO₂]) impact respiration (R) of soybean (Glycine max), when compared to plants grown under current and future [CO₂]s. Experiments were conducted using plants grown under 290, 400 and 700ppm [CO₂]. Leaf R was measured in both darkness (R_D) and in the light (R_L; using the Kok method), with short-term changes in measurement [CO₂] and [O₂] being used to explore the relationship between light inhibition of leaf R and photorespiration. Root R, photosynthesis (A), leaf [N] and biomass allocation traits were also quantified. In contrast to the inhibitory effect of low growth [CO₂] on A, growth [CO₂] had no significant effect on leaf R_D or root R. Irrespective of growth [CO₂], R_L was always lower than R_D, with light inhibiting leaf R by 17-47%. Importantly, the degree of light inhibition of leaf R was lowest in plants grown under low [CO₂], with variations in R_L being positively correlated with R_D and photorespiration. Irrespective of whether leaf R was measured in the light or dark, a greater proportion of the carbon fixed by leaf photosynthesis was released by leaf R in plants grown under low [CO₂] than under current/future [CO₂]'s. Collectively, our results highlight the differential responses of A and R to growth of plants under low to elevated atmospheric [CO₂].