On the significance of C 3 —C 4 intermediate photosynthesis to the evolution of C 4 photosynthesis

Evidence is drawn from previous studies to argue that C 3 —C 4 intermediate plants are evolutionary intermediates, evolving from fully‐expressed C 3 plants towards fully‐expressed C 4 plants. On the basis of this conclusion, C 3 —C 4 intermediates are examined to elucidate possible patterns that have been followed during the evolution of C 4 photosynthesis. An hypothesis is proposed that the initial step in C 4 ‐evolution was the development of bundle‐sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO 2 using RuBP carboxylase. The CO 2 ‐recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle‐sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO 2 . As the activity of PEP carboxylase increased to higher values, other enzymes in the C 4 ‐pathway are proposed to have increased in activity to facilitate the processing of the products of C 4 ‐assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C 4 ‐cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully‐expressed C 4 plants. Such metabolism would have limited benefit in terms of concentrating CO 2 at RuBP carboxylase and, therefore, also be of little benefit for improving water‐ and nitrogen‐use efficiencies. However, the development of such a limited C 4 ‐cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully‐expressed C 4 plants. Thus, during the initial stages of C 4 ‐evolution it is proposed that improvements in photorespiratory CO 2 ‐loss and their influence on increasing the rate of net CO 2 assimilation per unit leaf area represented the evolutionary ‘driving‐force’. Improved resourceuse efficiency resulting from an efficient CO 2 ‐concentrating mechanism is proposed as the driving force during the later stages.