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

Abstract

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 C3 and C4 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 C3 and C4 species. In C3 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°C, C3 photosynthesis at 37 and 68 Pa is often limited by the capacity to regenerate phosphate for photophosphorylation. In C4 plants, the Rubisco capacity is equivalent to A below 18°C, but exceeds the photosynthetic capacity above 25°C, indicating that Rubisco is an important limitation at cool but not warm temperatures. A comparison of the catalytic efficiency of Rubisco (kcat in mol CO2 mol-1 Rubisco active sites s-1) from 17 C3 and C4 plants showed that Rubisco from C4 species, and C3 species originating in cool environments, had higher kcat than Rubisco from C3 species originating in warm environments. This indicates that Rubisco evolved to improve performance in the environment that plants normally experience. In C4 plants, and C3 species from cool environments, Rubisco often operates near CO2 saturation, so that increases in kcat would enhance A. In warm-habitat C4 species, Rubisco often operates at CO2 concentrations below the Km for CO2. Because kcat and Km vary proportionally, the low kcat indicates that Rubisco has been modified in a manner that reduces Km and thus increases the affinity for CO2 in C3 species from warm climates.