Electron transport to assimilation ratio (ETR/<i>A</i>) of C3 plants is affected by atmospheric CO2

Abstract The linear electron transport to assimilation ratio (ETR/A) has been used to evaluate the stress status of plants and the ETR measurement accuracy. However, how ETR/A responds to short- and long-term CO2 variations remains uncertain. Here, we assessed the short-term, instantaneous CO2 response (seconds to minutes) of ETR/A in sunflower (Helianthus annuus) and cowpea (Vigna unguiculata) in a controlled experiment. Additionally, we compiled published data to investigate the effect of elevated growth CO2 (long-term) on ETR/A across plant functional groups. Our results showed that ETR, photosystem II photochemical efficiency, and photochemical quenching increase while ETR/A decreases with increasing CO2 for both sunflower and cowpea. The CO2 sensitivity of ETR and ETR/A is greater at low [CO2] (Rubisco-limiting condition) than at moderate to high [CO2] (electron transport-limiting stage). The meta-analysis showed that plant functional groups and long-term CO2 significantly affect ETR/A, which challenges the universal applicability of current empirical ETR/A thresholds across species and environments. Long-term CO2 elevation increased A without altering ETR, resulting in a 21% reduction in ETR/A compared to ambient CO2. Our results demonstrate an imbalance between electron transport and carboxylation under elevated CO2, potentially constraining photosynthetic performance under future climate scenarios.