Modelling the role of Rubisco activase in limiting non-steady-state photosynthesis

The roles of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase in limiting the approach of photosynthesis to steady-state following a step increase from a low to a saturating value of photon flux density (PFD) are reviewed. This information, along with the effect of Rubisco on steady-state photosynthetic rate and the effect of Rubisco activase on maximum Rubisco activation state, is then used to construct a model to predict the optimum allocation of protein between Rubisco and Rubisco activase for plants exposed to different light environments. The model predicts that the distribution of protein that produces the maximum steady-state rate of photosynthesis does not produce the maximum activation rate for Rubisco or the maximum steady-state activation state. The latter conclusion may explain why Rubisco is rarely found to be fully activated in leaves, even at saturating PFD values. The former suggests that plants exposed to fluctuating PFD should allocate more protein to Rubisco activase than plants exposed to constant PFD. This aspect of the model is explored in more detail for lightflecks of differing duration.