Application of an enthalpy balance model of the relation between growth and respiration to temperature acclimation of Eucalyptus globulus seedlings

The enthalpy balance model of growth uses measurements of the rates of heat and CO2 production to quantify rates of decarboxylation, oxidative phosphorylation and net anabolism. Enthalpy conversion efficiency (ηH) and the net rate of conservation of enthalpy in reduced biosynthetic products (RSGΔHB) can be calculated from metabolic heat rate (q) and CO2 rate (RCO2). ηH is closely related to carbon conversion efficiency and the efficiency of conservation of available electrons in biosynthetic products. RSGΔHB and η can be used, together with biomass composition, to describe the rate and efficiency of growth of plant tissues. q is directly related to the rate of O2 consumption and the ratio q:RCO2 is inversely related to the respiratory quotient. We grew seedlings of Eucalyptus globulus at 16 and 28°C for four to six weeks, then measured q and RCO2 using isothermal calorimetry. Respiratory rate at a given temperature was increased by a lower growth temperature but ηH was unaffected. Enthalpy conversion efficiency—and, therefore, carbon conversion efficiency—decreased with increasing temperature from 15 to 35°C. The ratio of oxidative phosphorylation to oxygen consumption (P/O ratio) was inferred in vivo from ηH and by assuming a constant ratio of growth to maintenance respiration with changing temperature. The P/O ratio decreased from 2.1 at 10-15°C to less than 0.3 at 35°C, suggesting that decreased efficiency was not only due to activity of the alternative oxidase pathway. In agreement with predictions from non-equilibrium thermodynamics, growth rate was maximal near 25°C, where the calculated P/O ratio was about half maximum. We propose that less efficient pathways, such as the alternative oxidase pathway, are necessary to satisfy the condition of conductance matching whilst maintaining a near constant phosphorylation potential. These conditions minimize entropy production and maximize the efficiency of mitochondrial energy conversions as growing conditions change, while maintaining adequate finite rates of energy processing.