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

Abstract

Introduction: Despite their importance for the global carbon cycle and crop production, species with C₄ photosynthesis are still somewhat understudied relative to C₃ species. Although the benefits of the C₄ carbon concentrating mechanism are readily observable under optimal steady state conditions, it is less clear how the presence of C₄ affects activation of CO₂ assimilation during photosynthetic induction.

Methods: In this study we aimed to characterise differences between C₄ and C₃ photosynthetic induction responses by analysing steady state photosynthesis and photosynthetic induction in three phylogenetically linked pairs of C₃ and C₄ species from Alloteropsis, Flaveria, and Cleome genera. Experiments were conducted both at 21% and 2% O₂ to evaluate the role of photorespiration during photosynthetic induction.

Results: Our results confirm C₄ species have slower activation of CO₂ assimilation during photosynthetic induction than C₃ species, but the apparent mechanism behind these differences varied between genera. Incomplete suppression of photorespiration was found to impact photosynthetic induction significantly in C₄ Flaveria bidentis, whereas in the Cleome and Alloteropsis C₄ species, delayed activation of the C₃ 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 C₃ and C₄ photosynthetic pathways.

Keywords: C3 photosynthesis; C4 photosynthesis; CO2 assimilation; light response; non-steady state; photorespiration; photosynthetic induction.