Photosynthetic limitations: use in guiding effort in crop improvement

Methods for identifying the important processes limiting crop production and especially photosynthesis are discussed with special reference to the use of transgenic plants with altered expression of the small subunit of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). Recent advances in the use of metabolic control analysis are reviewed and some restrictions in their application to crop improvement are discussed. Metabolic control and other sensitivity approaches are compared with the opportunities offered by ‘elimination’ analyses where the contribution of one component is estimated from the change in assimilation rate when any limitation by that component is removed. It is concluded that although control or sensitivity analyses provide effective tools for describing the importance of component processes, especially when using transgenic approaches to generation of isogenic material, our present limited knowledge of the complex interactions between various metabolic pathways restricts the value of these approaches in guiding breeding effort.