Kinetics of plant growth and metabolism

Direct measurements of plant growth rates in terms of volume, length, net photosynthate, etc. provide little information concerning the mechanism of adaptation of metabolism to an environment. To derive the mechanism, metabolic properties must be measured as functions of environmental variables. Growth rates may be limited by the availability of nutrients including fixed carbon, by climate, by other environmental factors including toxins, or by the genetically determined properties of the plant. But in all cases, growth rate is equal to a function of respiration rate and efficiency. For a plant to thrive, its respiratory metabolism as well as its photosynthetic metabolism must be closely adapted to the seasonal and daily variations in the environment. Thus, measurement of respiratory properties is necessary for understanding plant adaptation. In terms of readily measurable respiratory variables, the rate law for growth driven by aerobic respiration is RSG=RCO2ϵC1−ϵC=rRO2ϵC1−ϵC=−RCO2ΔHCO2ηHΔHB=−ΔHCO2RCO2−RqΔHBwhere RSG is the specific growth rate, RCO2 the specific rate of CO2 evolution, ϵC the fraction of substrate carbon converted into structural biomass or the substrate carbon conversion efficiency, r the respiratory quotient, RO2 the specific rate of O2 uptake, ΔHCO2 the enthalpy change for combustion of substrate per mole of CO2, ηH the fraction of enthalpy produced by oxidation of substrate that is conserved in the biomass synthesized through anabolism (i.e. the enthalpic efficiency), and ΔHB is the enthalpy change for conversion of substrate into structural biomass per C-mole. ΔHCO2 can be obtained from Thornton’s rule, and ΔHB from either heat of combustion or composition data or from growth measurements. Calorespirometric measurements can then be used to obtain values for ϵC and ηH. Measurements of RCO2(or of r and RO2) and the metabolic heat rate, Rq, as functions of environmental variables thus, can be used to rapidly ascertain the growth and metabolic responses of plants to environmental variables. This model and calorespirometric measurements are used to predict the responses of plant growth to differing climates, to predict the global gradient of plant species ranges and diversity, and to predict global treeline temperature conditions. Growth-season temperature and temperature variability are found to be major determinants of growth rates and distributions of plants. These findings may be useful in predicting the response of plants to climate changes.