Kinetics of plant growth and metabolism

Abstract

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 R SG =R CO 2 ϵ C 1−ϵ C =rR O 2 ϵ C 1−ϵ C =−R CO 2 ΔH CO 2 η H ΔH B = − ΔH CO 2 R CO 2 −R q ΔH B where R SG is the specific growth rate, R CO 2 the specific rate of CO 2 evolution, ϵ C the fraction of substrate carbon converted into structural biomass or the substrate carbon conversion efficiency, r the respiratory quotient, R O 2 the specific rate of O 2 uptake, Δ H CO 2 the enthalpy change for combustion of substrate per mole of CO 2, η H the fraction of enthalpy produced by oxidation of substrate that is conserved in the biomass synthesized through anabolism (i.e. the enthalpic efficiency), and Δ H B is the enthalpy change for conversion of substrate into structural biomass per C-mole. Δ H CO 2 can be obtained from Thornton’s rule, and Δ H B from either heat of combustion or composition data or from growth measurements. Calorespirometric measurements can then be used to obtain values for ϵ C and η H . Measurements of R CO 2 (or of r and R O 2 ) and the metabolic heat rate, R q, 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.