Carbon Dioxide Diffusion inside Leaves

Abstract

CO2 reaches Rubisco by diffusion down a concentration gradient. The gradient forms during photosynthesis as a result of restrictions to free movement imposed by stomata and leaf anatomy. Recent techniques have enabled the internal conductance, gi, of leaves, that is, from sub-stomatal cavities to the sites of carboxylation, to be measured (1). A strong correlation has been found between gi and the rate of CO2 assimilation under high irradiance and ambient CO2 partial pressure, Ca (2,3,4). Unfortunately, the title of the paper by Lloyd et al. (3), low conductances for CO, diffusion from stomata to the sites of carboxylation in leaves of woody species’ has been taken to mean that there is a correspondingly low partial pressure of CO2 in the chloroplast, which contributes to the low CO2 assimilation rate of sclerophyllous leaves. The claim was repeated by Epron et al. (5) ‘high internal resistances to CO2 transfer may account for the low net CO2 assimilation rates that often characterise tree leaves.