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.