Importance of the leaf respiratory quotient

Rates of leaf respiratory CO₂-release (R_CO2) are important for terrestrial biosphere models that estimate carbon exchange between plants and the atmosphere. Hitherto, models of R_CO2 have primarily been based on considerations of respiratory energy demand (particularly ATP) for maintenance and growth purposes. Respiratory ATP synthesis is closely tied to the rate of respiratory O₂-uptake (R_O2), with relative engagement of the alternative oxidase influencing the ATP:O ratio. However, the extent to which respiratory ATP synthesis is coupled to leaf R_CO2 depends on the respiratory quotient (RQ, mol CO₂ efflux per unit mol O₂ uptake), with models predicting leaf R_CO2 assuming that the RQ is at unity. Here, we show systematic inter-specific, temporal and temperature-dependent variation in leaf RQ, with values of RQ ranging from 0.51 to 2.2, challenging model assumptions on the RQ. We discuss possible mechanisms underlying the variation in leaf RQ, potential ways forward in terms of new measurement protocols, and perspectives for modelled R_CO2. Our analyses highlight a range of outstanding research questions that need to be answered before we can mechanistically model leaf R_CO2 at various scales.