Models relating subcellular effects of temperature to whole plant responses.

The analysis of the temperature dependence of the rate of photosynthetic carbon assimilation by leaves of C3 plants has involved subdivision into a diffusional (mainly stomatal) component and a biochemical one. The latter has been further analysed in terms of Rubisco activity and of the capacity for regeneration of RubP. Rubisco activity has been modelled in terms of the number of active (carbamylated) enzyme sites and the kinetics of these sites. RubP regeneration capacity has been modelled in terms of photosynthetic electron transport and photophosphorylation capacity and the rate of supply of inorganic phosphate. The short-term (minutes to hours) responses of assimilation rate to changes in temperature of Eucalyptus pauciflora leaves, in the range 15 to 35 degrees C, are examined using this framework. It is argued that the above approach has some useful features, even if these are mainly pedagogical. Problems with the simplified framework are also discussed, including heterogeneity of the light intensity and CO2 concentration within the leaf. The latter is discussed in terms of the resistances to diffusion within the leaf and observations of non-uniformity ('patches') of stomatal opening. The effects of temperature, couched in the above terms, appear to be mainly those on the supply of carbon as a substrate for growth. Yet the model (or description of carbon assimilation) can accommodate certain effects of demand for carbon (sink limitation). These occur, notably, via the supply of inorganic phosphate, but potentially also via Rubisco activation and other mechanisms. A striking example of effects of reduced temperature on assimilation and growth being caused by changed demand for carbon, is given for the tropical crop, Arachis hypogaea. This example prompts an appraisal of the effects of temperature on components of growth other than photosynthesis, for example, of leaf production rate.