The maximum carboxylation rate of Rubisco affects CO₂ refixation in temperate broadleaved forest trees

Mesophyll resistance to CO₂ diffusion (r_m) and the maximum carboxylation rate of Rubisco (V_cmax) affect photosynthetic rates, and can potentially also influence the percentage of respiratory and photorespiratory CO₂ being refixated (P_r) by mesophyll cells. Here we investigated how various leaf anatomical traits (e.g. leaf mass per area [LMA] and leaf dry matter content [LDMC]) influenced r_m in leaves of mature forest trees. We further explored how r_m and V_cmax in turn affected P_r, and if these traits varied among species and leaves along a light gradient. Photosynthetic CO₂ response of leaves grown in high-, medium-, and low-light environments was measured, from Pinus sylvestris [Scots pine], Picea abies [Norway spruce], Quercus robur [English oak], and Betula pendula [Silver birch] in southern Sweden. A modified version of the Farquhar-von Caemmerer-Berry model was fitted to the leaf gas exchange data to estimate V_cmax, r_m and P_r. We found that of all leaf traits measured, only LMA for Q. robur was significantly higher in leaves from high-light environments. When comparing species, both r_m and LMA were significantly higher in the conifers, and r_m had a negative correlation with V_cmax. We found that P_r was similar between different species and functional groups, with an average of 73.2% (and SD of ±10.4) across all species. There was a strong, positive correlation between P_r and V_cmax in broadleaves, and we hypothesise that this effect might derive from a higher CO₂ drawdown near Rubisco in leaves with high V_cmax.