Integrating two physiological approaches helps relate respiration to growth of Pinus radiata

Summary•  Correlation methods originating in the growth and maintenance paradigm (GMP) are traditionally used to calculate a ‘growth coefficient’ ( g ) or the ‘growth potential’ (1/ g ) of entire plants. The enthalpy balance approach is usually applied to plant organs and relies on determination of both CO 2 release and O 2 reduction to provide a measure of instantaneous rates of enthalpic growth ( R SG Δ H B ). •  Aspects of both the approaches to explore physiological mechanisms that govern enthalpic growth (variation in rates of CO 2 release versus rates of O 2 reduction) were combined. •  Respiration and growth rates of apical buds of Pinus radiata were affected strongly by canopy position, and moderately by branching order. A linear relation between enthalpic growth and CO 2 respiration explained 69% of the observed variation. Despite faster rates of growth, enthalpic growth potential (1/ g H ) was comparatively low in the upper canopy. Low enthalpic growth potential entailed comparatively low enthalpy conversion efficiency (η H , ratio of R SG Δ H B to ; proportional to CO 2 :O 2 and to carbon conversion efficiency ɛ) at large R SG ΔH B . Maximizing enthalpic growth requires a large capacity for O 2 reduction. •  Relations between R SG Δ H B and η H could be described by hyperbolae using two parameters. One parameter, P 1 , is equivalent to enthalpic growth potential (1/ g H ).