Different models provide equivalent predictive power for cross-biome response of leaf respiration to temperature

Three physiological parameters capture variation in leaf respiration of Eucalyptus grandis, as elicited by short-term changes in ambient temperature, and differing nitrogen supply

We used instantaneous temperature responses of CO₂ -respiration to explore temperature acclimation dynamics for Eucalyptus grandis grown with differing nitrogen supply. A reduction in ambient temperature from 23 to 19 °C reduced light-saturated photosynthesis by 25% but increased respiratory capacity by 30%. Changes in respiratory capacity were not reversed after temperatures were subsequently increased to 27 °C. Temperature sensitivity of respiration measured at prevalent ambient temperature varied little between temperature treatments but was significantly reduced from ~105 kJ mol^-1 when supply of N was weak, to ~70 kJ mol^-1 when it was strong. Temperature sensitivity of respiration measured across a broader temperature range (20-40 °C) could be fully described by 2 exponent parameters of an Arrhenius-type model (i.e., activation energy of respiration at low reference temperature and a parameter describing the temperature dependence of activation energy). These 2 parameters were strongly correlated, statistically explaining 74% of observed variation. Residual variation was linked to treatment-induced changes in respiration at low reference temperature or respiratory capacity. Leaf contents of starch and soluble sugars suggest that respiratory capacity varies with source-sink imbalances in carbohydrate utilization, which in combination with shifts in carbon-flux mode, serve to maintain homeostasis of respiratory temperature sensitivity at prevalent growth temperature.

Macromolecular rate theory (MMRT) provides a thermodynamics rationale to underpin the convergent temperature response in plant leaf respiration

Temperature is a crucial factor in determining the rates of ecosystem processes, for example, leaf respiration (R) - the flux of plant respired CO₂ from leaves to the atmosphere. Generally, R increases exponentially with temperature and formulations such as the Arrhenius equation are widely used in earth system models. However, experimental observations have shown a consequential and consistent departure from an exponential increase in R. What are the principles that underlie these observed patterns? Here, we demonstrate that macromolecular rate theory (MMRT), based on transition state theory (TST) for enzyme-catalyzed kinetics, provides a thermodynamic explanation for the observed departure and the convergent temperature response of R using a global database. Three meaningful parameters emerge from MMRT analysis: the temperature at which the rate of respiration would theoretically reach a maximum (the optimum temperature, T_opt ), the temperature at which the respiration rate is most sensitive to changes in temperature (the inflection temperature, T_inf ) and the overall curvature of the log(rate) versus temperature plot (the change in heat capacity for the system, ΔCP‡). On average, the highest potential enzyme-catalyzed rates of respiratory enzymes for R are predicted to occur at 67.0 ± 1.2°C and the maximum temperature sensitivity at 41.4 ± 0.7°C from MMRT. The average curvature (average negative ΔCP‡) was -1.2 ± 0.1 kJ mol^-1  K^-1 . Interestingly, T_opt , T_inf and ΔCP‡ appear insignificantly different across biomes and plant functional types, suggesting that thermal response of respiratory enzymes in leaves could be conserved. The derived parameters from MMRT can serve as thermal traits for plant leaves that represent the collective temperature response of metabolic respiratory enzymes and could be useful to understand regulations of R under a warmer climate. MMRT extends the classic TST to enzyme-catalyzed reactions and provides an accurate and mechanistic model for the short-term temperature response of R around the globe.

Implications of improved representations of plant respiration in a changing climate

Land-atmosphere exchanges influence atmospheric CO₂. Emphasis has been on describing photosynthetic CO₂ uptake, but less on respiration losses. New global datasets describe upper canopy dark respiration (R _d) and temperature dependencies. This allows characterisation of baseline R _d, instantaneous temperature responses and longer-term thermal acclimation effects. Here we show the global implications of these parameterisations with a global gridded land model. This model aggregates R _d to whole-plant respiration R _p, driven with meteorological forcings spanning uncertainty across climate change models. For pre-industrial estimates, new baseline R _d increases R _p and especially in the tropics. Compared to new baseline, revised instantaneous response decreases R _p for mid-latitudes, while acclimation lowers this for the tropics with increases elsewhere. Under global warming, new R _d estimates amplify modelled respiration increases, although partially lowered by acclimation. Future measurements will refine how R _d aggregates to whole-plant respiration. Our analysis suggests R _p could be around 30% higher than existing estimates.