Measurement of mesophyll conductance in tobacco, arabidopsis and wheat leaves with tunable diode laser absorption spectroscopy

In C₃ leaves, diffusion of CO₂ into leaves is restricted by stomata and subsequently by the intercellular airspaces and liquid phase into chloroplasts. This diffusion restriction within leaves has been termed mesophyll conductance, g_m, which has garnered attention because g_m limits photosynthesis at ambient CO₂. Recently, some reports have shown that g_m varies with CO₂ concentration and it may be caused by gating of aquaporins (cooporins) which are located in the plasma membrane and inner envelope of chloroplasts. However, there is still controversy about whether g_m responds to CO₂ concentration. In this study, the effects of CO₂ concentration on g_m were examined in tobacco, Arabidopsis and wheat leaves by combining gas exchange with carbon isotope discrimination measurements using tunable diode laser absorption spectroscopy. CO₂ was initially increased from 200 to 1,000 ppm and then decreased stepwise to 200 ppm and increased stepwise back to 1000 ppm. In 2% O₂, a step increase from 200 to 1,000 ppm significantly decreased g_m by 26%–40% in all three species. The CO₂ response of g_m was less in 21% O₂.