The capacity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to consume RuBP is a major limitation on the rate of net CO(2) 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 CO(2) 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 CO(2) concentrations below the current atmospheric value of 37 Pa, particularly near the temperature optimum. Below 20 degrees C, 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 degrees C, but exceeds the photosynthetic capacity above 25 degrees C, 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 CO(2) 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 CO(2) saturation, so that increases in k(cat) would enhance A. In warm-habitat C(4) species, Rubisco often operates at CO(2) concentrations below the K(m) for CO(2). 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 CO(2) in C(3) species from warm climates.