Temperature Dependence of Whole-Leaf Photosynthesis in Eucalyptus pauciflora Sieb. Ex Spreng

Abstract

The temperature dependence of net photosynthetic assimilation of CO₂ by snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) was investigated. CO₂ assimilation was divided into its component processes, stomatal and biochemical. The biochemical limitation was investigated with gas-exchange techniques and found to conform well to a recent model of C₃ photosynthesis. In line with the model, net assimilation was further divided into ribulose 1,5-bisphosphate (RuP₂) regeneration or electron-transport/photophosphorylation limitation, limitation by RuP₂ carboxylase/oxygenase (Rubisco; EC 4.1.1.39) activity, together with loss of CO₂ in non-photorespiratory respiration. The estimated temperature dependence of electron transport agreed well with one published for uncoupled electron transport, while the estimated temperature dependence of the catalytic activity of Rubisco was slightly less than that reported from biochemical determinations. The estimated rate of non-photorespiratory respiration was about 0.6 times the rate of respiration at night and appeared to have the same temperature dependence. With this information the temperature dependence of the biochemical limitation was modelled. Stomatal conductance was assumed to follow the theory of constant marginal water cost of carbon assimilation (δE/δA) and net assimilation rate at ambient concentration of CO₂ was predicted. It was concluded that, for a given Rubisco activity and RuP₂-regeneration capacity, both temperature optimum and net assimilation rate at the optimum temperature increased with increasing stomatal conductance.