Mesophyll conductance does not contribute to greater photosynthetic rate per unit nitrogen in temperate compared with tropical evergreen wet-forest tree leaves

Globally, trees originating from high-rainfall tropical regions typically exhibit lower rates of light-saturated net CO₂ assimilation (A) compared with those from high-rainfall temperate environments, when measured at a common temperature. One factor that has been suggested to contribute towards lower rates of A is lower mesophyll conductance. Using a combination of leaf gas exchange and carbon isotope discrimination measurements, we estimated mesophyll conductance (g_m) of several Australian tropical and temperate wet-forest trees, grown in a common environment. Maximum Rubisco carboxylation capacity, V_cmax, was obtained from CO₂ response curves. g_m and the drawdown of CO₂ across the mesophyll were both relatively constant. V_cmax estimated on the basis of intercellular CO₂ partial pressure, C_i, was equivalent to that estimated using chloroplastic CO₂ partial pressure, C_c, using ‘apparent’ and ‘true’ Rubisco Michaelis–Menten constants, respectively Having ruled out g_m as a possible factor in distorting variations in A between these tropical and temperate trees, attention now needs to be focused on obtaining more detailed information about Rubisco in these species.