Small flux, global impact: Integrating the nuances of leaf mitochondrial respiration in estimates of ecosystem carbon exchange

The balance of photosynthesis and respiration, their responses to a changing environment, and predictive models of these responses continue to be an active body of research. While photosynthesis is robustly described by a longstanding, scalable biochemical model (Farquhar et al., 1980), a similar mechanistic model of respiration remains an ongoing challenge. Respiration encompasses multiple cellular processes in the mitochondria and cytosol that drive energy and carbon skeleton production for plant growth and maintenance. Through glycolysis (cytosol), the tricarboyxlic acid (TCA) cycle (mitochondrial matrix), the electron transport chain/oxidative phosphorylation (mitochondrial inner membrane), and other associated pathways, metabolic products of photosynthesis are transformed into energy in the form of ATP, oxygen is consumed, and carbon dioxide is produced. Unlike its metabolic foil, photosynthesis, mitochondrial respiration takes place in all plant tissues, in all cells, at all times. Its ubiquity as an energy source in plants, its role promoting and maintaining efficient photosynthesis, and its contribution to the terrestrial carbon cycle warrant accurate quantification for scaling leaflevel fluxes of carbon. New strategies for measuring and modeling plant respiration across systems and scales are necessary to robustly characterize how carbon flows through terrestrial environments. Developments in measurement techniques, comprehensive fieldbased data sets, and crossscale research collaborations are directly addressing environmental sensitivities and biochemical nuances and, in turn, advancing how respiration is considered at the leaf and ecosystem levels. This essay covers current areas of plant respiration research and their integration into the broader terrestrial carbon cycle.