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

Abstract

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 (gm) of several Australian tropical and temperate wet-forest trees, grown in a common environment. Maximum Rubisco carboxylation capacity, Vcmax, was obtained from CO2 response curves. gm and the drawdown of CO2 across the mesophyll were both relatively constant. Vcmax estimated on the basis of intercellular CO2 partial pressure, Ci, was equivalent to that estimated using chloroplastic CO2 partial pressure, Cc, using ‘apparent’ and ‘true’ Rubisco Michaelis–Menten constants, respectively Having ruled out gm 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. Show more