Activation of CO2 assimilation during photosynthetic induction is slower in C4 than in C3 photosynthesis in three phylogenetically controlled experiments

Introduction Despite their importance for the global carbon cycle and crop production, species with C 4 photosynthesis are still somewhat understudied relative to C 3 species. Although the benefits of the C 4 carbon concentrating mechanism are readily observable under optimal steady state conditions, it is less clear how the presence of C 4 affects activation of CO 2 assimilation during photosynthetic induction. Methods In this study we aimed to characterise differences between C 4 and C 3 photosynthetic induction responses by analysing steady state photosynthesis and photosynthetic induction in three phylogenetically linked pairs of C 3 and C 4 species from Alloteropsis , Flaveria , and Cleome genera. Experiments were conducted both at 21% and 2% O 2 to evaluate the role of photorespiration during photosynthetic induction. Results Our results confirm C 4 species have slower activation of CO 2 assimilation during photosynthetic induction than C 3 species, but the apparent mechanism behind these differences varied between genera. Incomplete suppression of photorespiration was found to impact photosynthetic induction significantly in C 4 Flaveria bidentis , whereas in the Cleome and Alloteropsis C 4 species, delayed activation of the C 3 cycle appeared to limit induction and a potentially supporting role for photorespiration was also identified. Discussion The sheer variation in photosynthetic induction responses observed in our limited sample of species highlights the importance of controlling for evolutionary distance when comparing C 3 and C 4 photosynthetic pathways.