Equisetum praealtum and E. hyemale have abundant Rubisco with a high catalytic turnover rate and low CO 2 affinity.

The kinetic properties of Rubisco, a key enzyme for photosynthesis, have been examined in numerous plant species. However, this information on some plant groups, such as ferns, is scarce. This study examined Rubisco carboxylase activity and leaf Rubisco levels in seven ferns, including four Equisetum plants (E. arvense, E. hyemale, E. praealtum, and E. variegatum), considered living fossils. The turnover rates of Rubisco carboxylation (k cat c ) in E. praealtum and E. hyemale were comparable to those in the C 4 plants maize (Zea mays) and sorghum (Sorghum bicolor), whose k cat c values are high. Rubisco CO 2 affinity, estimated from the percentage of Rubisco carboxylase activity under CO 2 unsaturated conditions in k cat c in these Equisetum plants, was low and also comparable to that in maize and sorghum. In contrast, k cat c and CO 2 affinities of Rubisco in other ferns, including E. arvense and E. variegatum were comparable with those in C 3 plants. The N allocation to Rubisco in the ferns examined was comparable to that in the C 3 plants. These results indicate that E. praealtum and E. hyemale have abundant Rubisco with high k cat c and low CO 2 affinity, whereas the carboxylase activity and abundance of Rubisco in other ferns were similar to those in C 3 plants. Herein, the Rubisco properties of E. praealtum and E. hyemale were discussed regarding their evolution and physiological implications.