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A new hypothesis for temperature acclimation by the photosynthetic apparatus is presented. An optimization model is developed to examined effects of changes in the organization of photosynthetic components on leaf photosynthesis under various growth temperatures where the photosynthetic apparatus is not damaged. In this model, photosynthetic rate is limited either by the capacity of ribulose bisphosphate carboxylase (RuBPCase) to consume ribulose bisphosphate (RuBP), or by the capacity of RuBP regeneration. For temperature dependence of the RuBPCase activity, data from Spinacia oleracea L., which have a temperature optimum of 30 degrees C, are used. For temperature dependence of the capacity of RuBP regeneration, two contrasting curves that have temperature optima of 30 degrees C (Eucalyptus pauciflora Sieb. ex Spreng) and 40 degrees C (Larrea divaricata Cav.) are applied. The temperature dependence of each process is fixed for respective species, but the rate of each process varies with changes in the amounts of components. The cost of proteins, in terms of nitrogen, required to carry out each process is calculated when nitrogen is partitioned differently among photosynthetic components. The optimal nitrogen partitioning that maximizes daily photosynthesis at a given temperature is obtained. The predicted temperature optimum of the photosynthetic rate in Larrea divaricata exhibits large shifts with changes in target temperature, while shifts are negligible in Eucalyptus pauciflora. It is suggested that the shift in temperature optimum of photosynthetic rate is large when the temperature dependences of the capacities of RuBPCase and RuBP regeneration differ from each other. (C) 1997 Annals of Botany Company.