Variation in the kcat of Rubisco in C3 and C4 plants and some implications for photosynthetic performance at high and low temperature

The capacity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to consume RuBP is a major limitation on the rate of net CO2 assimilation (A) in C-3 and C-4 plants. The pattern of Rubisco limitation differs between the two photosynthetic types, as shown by comparisons of temperature and CO2 responses of A and Rubisco activity from C-3 and C-4 species. In C-3 species, Rubisco capacity is the primary limitation on A at light saturation and CO2 concentrations below the current atmospheric value of 37 Pa, particularly near the temperature optimum. Below 20 degreesC, C-3 photosynthesis at 37 and 68 Pa is often limited by the capacity to regenerate phosphate for photophosphorylation. In C-4 plants, the Rubisco capacity is equivalent to A below 18 degreesC, but exceeds the photosynthetic capacity above 25 degreesC, indicating that Rubisco is an important limitation at cool but not warm temperatures. A comparison of the catalytic efficiency of Rubisco (k(cat) in mol CO2 mol(-1) Rubisco active sites s(-1)) from 17 C-3 and C-4 plants showed that Rubisco from C-4 species, and C-3 species originating in cool environments, had higher k(cat) than Rubisco from C-3 species originating in warm environments. This indicates that Rubisco evolved to improve performance in the environment that plants normally experience. In C-4 plants, and C-3 species from cool environments, Rubisco often operates near CO2 saturation, so that increases in k(cat) would enhance A. In warm-habitat C-4 species, Rubisco often operates at CO2 concentrations below the K-m for CO2. Because k(cat) and K-m vary proportionally, the low k(cat) indicates that Rubisco has been modified in a manner that reduces K-m and thus increases the affinity for CO2 in C-3 species from warm climates.