What determines rates of photosynthesis per unit nitrogen in Eucalyptus seedlings?

Species originating from xeric sites are characterised by slower rates of photosynthesis per unit nitrogen (PNUE) than species from mesic sites, but we lack mechanistic explanations for these interspecific differences. We examined N allocation to Rubisco and chlorophyll, and photosynthetic characteristics in seedlings of nine Eucalyptus species grown in a fully sunlit glasshouse with an optimal supply of nutrients. Species were selected from mesic (1800 mm year^-1 rainfall) through to semi-arid habitats (300 mm year^-1). All species were characterised by allocation of a large proportion of N to Rubisco (32-48%) with high in vivo specific activity. Intercellular CO₂ concentration (C_i) varied between 260 and 300 μmol mol^-1, and thus, stomatal limitations were low in all species. This combination of traits resulted in a PNUE (172-335 μmol mol^-1 s^-1) that was higher than is commonly observed in tree species and which may be related to the rapid growth, water-spender strategy of Eucalyptus seedlings. There were significant differences in photosynthetic parameters and N allocation among species, but these were only weakly related to rainfall at the site of seed origin. There were correlations of C_i with PNUE but a sensitivity analysis suggested that interspecific variation in C_i explained at most 7% of variation in PNUE. Photosynthesis and PNUE were also rather insensitive to large interspecific differences in RuBP-limited rate of electron transport per unit N (J_max / N), because photosynthesis was primarily limited by the maximum rate of carboxylation (V_cmax). PNUE was most sensitive to changes in N allocation to Rubisco and V_cmax / Rubisco.