Equisetum praealtum and E. hyemale have abundant Rubisco with a high catalytic turnover rate and low CO2 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 C₄ plants maize (Zea mays) and sorghum (Sorghum bicolor), whose k_cat^c values are high. Rubisco CO₂ affinity, estimated from the percentage of Rubisco carboxylase activity under CO₂ 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 CO₂ affinities of Rubisco in other ferns, including E. arvense and E. variegatum were comparable with those in C₃ plants. The N allocation to Rubisco in the ferns examined was comparable to that in the C₃ plants. These results indicate that E. praealtum and E. hyemale have abundant Rubisco with high k_cat^c and low CO₂ affinity, whereas the carboxylase activity and abundance of Rubisco in other ferns were similar to those in C₃ plants. Herein, the Rubisco properties of E. praealtum and E. hyemale were discussed regarding their evolution and physiological implications.