Using tunable diode laser spectroscopy to measure carbon isotope discrimination and mesophyll conductance to CO 2 diffusion dynamically at different CO 2 concentrations

In C3 leaves, the mesophyll conductance to CO2 diffusion, gm, determines the drawdown in CO2 concentration from intercellular airspace to the chloroplast stroma. Both gm and stomatal conductance limit photosynthetic rate and vary in response to the environment. We investigated the response of gm to changes in CO2 in two Arabidopsis genotypes (including a mutant with open stomata, ost1), tobacco and wheat. We combined measurements of gas exchange with carbon isotope discrimination using tunable diode laser absorption spectroscopy with a CO2 calibration system specially designed for a range of CO2 and O2 concentrations. CO2 was initially increased from 200 to 1000ppm and then decreased stepwise to 200ppm and increased stepwise back to 1000ppm, or the sequence was reversed. In 2% O2 a step increase from 200 to 1000ppm significantly decreased gm by 26-40% in all three species, whereas following a step decrease from 1000 to 200ppm, the 26-38% increase in gm was not statistically significant. The response of gm to CO2 was less in 21% O2. Comparing wild type against the ost1 revealed that mesophyll and stomatal conductance varied independently in response to CO2. We discuss the effects of isotope fractionation factors on estimating gm