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

Mesophyll resistance to CO diffusion (r) and the maximum carboxylation rate of Rubisco (V) affect photosynthetic rates, and can potentially also influence the percentage of respiratory and photorespiratory CO being refixated (P) 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 in leaves of mature forest trees. We further explored how r and V in turn affected P, and if these traits varied among species and leaves along a light gradient. Photosynthetic CO 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, r and P. 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 and LMA were significantly higher in the conifers, and r had a negative correlation with V. We found that P 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 and V in broadleaves, and we hypothesise that this effect might derive from a higher CO drawdown near Rubisco in leaves with high V.