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.

Key words: chlorophyll fluorescence, climate change, electron transport, elevated CO₂, photorespiration, photosynthesis