Oxygen response of leaf CO₂ compensation points used to determine Rubisco specificity factors of gymnosperm species

Rubisco specificity factor (S_c/o), a measure of the relative capacities of an enzyme to catalyze carboxylation and oxygenation of ribulose-1,5-bisphosphate, determines the extent of photosynthetic CO₂ assimilation and photorespiratory CO₂ release. The current model of C3 photosynthesis, the Farquhar-von Caemmerer-Berry (FvCB) model, requires a species-specific S_c/o. However, S_c/o values have never been reported in conifers, likely because in vitro kinetic analysis of conifer Rubisco presents difficulties. To estimate the S_c/o of conifers and compare it with angiosperm S_c/o, we measured changes in leaf CO₂ compensation points (Γ) in response to O₂ partial pressure for a variety of leaves, with different rates of day respiration (R_day) and maximum Rubisco carboxylation (V_cmax) in gymnosperms (Ginkgo biloba), conifers (Metasequoia glyptostroboides and Cryptomeria japonica), and angiosperms (Nicotiana tabacum and Phaseolus vulgaris). As predicted by the FvCB model, the slope of a linear function of Γ vs O₂ partial pressure, d, increased alongside increasing R_day/V_cmax. The S_c/o was obtainable from this relationship between d and R_day/V_cmax, because the d values at R_day/V_cmax = 0 corresponded to α/S_c/o, where α was the photorespiratory CO₂ release rate per Rubisco oxygenation rate (generally assumed to be 0.5). The calculated S_c/o values of N. tabacum and P. vulgaris exhibited good agreement with those reported by in vitro studies. The S_c/o values of both conifers were similar to those of the two angiosperm species. In contrast, the S_c/o value of G. biloba was significantly lower than those of the other four studied species. These results suggest that our new method for S_c/o estimation is applicable to C3 plants, including those for which in vitro kinetic analysis is difficult. Furthermore, results also suggest that conifer S_c/o does not differ significantly from that of C3 angiosperms, assuming α remains unchanged.