Enzymatic and quantitative properties of Rubisco in C

Plants adapted to cool habitats have been suggested to possess Rubisco with a high carboxylation capacity and low CO2 affinity, thereby facilitating efficient photosynthesis at low temperatures. Rubisco with a low CO2 affinity is believed to be less abundant owing to its concomitant high carboxylation capacity. This study aimed to provide information on these subjects by examining plants with early-spring persistent leaves and alpine plants. In these plants, the turnover rates of Rubisco carboxylation under CO2-saturated conditions (kcatc) were low to moderate, similar to those of the control C3 crops. The predicted Rubisco CO2 affinity, that is, the ratio of Rubisco carboxylase activity under CO2-unsaturated conditions (vcu), measured using reaction mixtures saturated with N2 gas containing CO2 and O2 at present atmospheric levels to kcatc, tended to be moderate, as observed in the control C3 crops. Some of these plants contained Rubisco with a low kcatc and moderate CO2 affinity, although Rubisco with a low kcatc was reported to exhibit a high CO2 affinity. N allocation to Rubisco in these plants was comparable to or slightly lower than that in control C3 crops. No significant correlations were observed between N allocation to Rubisco and kcatc or predicted Rubisco CO2 affinity. These results suggest that, for plants with early-spring persistent leaves and alpine plants, Rubisco kinetics is not necessarily advantageous for photosynthesis at low temperatures, and that Rubisco abundance is not necessarily dependent on Rubisco kinetics. The physiological implications of these phenomena are discussed.