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

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