Temperature responses of mesophyll conductance differ greatly between species

<jats:title>Abstract</jats:title><jats:p>The temperature responses of mesophyll conductance (<jats:italic>g</jats:italic><jats:sub>m</jats:sub>) were investigated for nine species using carbon isotope techniques combining tunable diode laser spectroscopy and gas exchange measurements. Species included the evergreen trees <jats:styled-content style="fixed-case"><jats:italic>E</jats:italic></jats:styled-content><jats:italic>ucalyptus pauciflora</jats:italic> and <jats:styled-content style="fixed-case"><jats:italic>Q</jats:italic></jats:styled-content><jats:italic>uercus engelmannii</jats:italic>; the tropical evergreen tree <jats:styled-content style="fixed-case"><jats:italic>L</jats:italic></jats:styled-content><jats:italic>ophostemon confertus</jats:italic>; as well as the herbaceous species <jats:styled-content style="fixed-case"><jats:italic>N</jats:italic></jats:styled-content><jats:italic>icotiana tabacum</jats:italic>, <jats:styled-content style="fixed-case"><jats:italic>O</jats:italic></jats:styled-content><jats:italic>ryza sativa</jats:italic>, <jats:styled-content style="fixed-case"><jats:italic>T</jats:italic></jats:styled-content><jats:italic>riticum aestivum</jats:italic>, <jats:styled-content style="fixed-case"><jats:italic>G</jats:italic></jats:styled-content><jats:italic>ossypium hirsutum</jats:italic>, <jats:styled-content style="fixed-case"><jats:italic>G</jats:italic></jats:styled-content><jats:italic>lycine max</jats:italic> and <jats:styled-content style="fixed-case"><jats:italic>A</jats:italic></jats:styled-content><jats:italic>rabidopsis thaliana</jats:italic>. Responses varied from a two‐ to threefold increase in mesophyll conductance between 15 and 40 °C observed for <jats:styled-content style="fixed-case"><jats:italic>N</jats:italic></jats:styled-content>. <jats:italic>tabacum</jats:italic>, <jats:styled-content style="fixed-case"><jats:italic>G.</jats:italic></jats:styled-content> <jats:italic>hirsutum</jats:italic>, <jats:styled-content style="fixed-case"><jats:italic>G.</jats:italic></jats:styled-content> <jats:italic>max</jats:italic> and <jats:styled-content style="fixed-case"><jats:italic>E.</jats:italic></jats:styled-content> <jats:italic>pauciflora</jats:italic> to almost no change in <jats:styled-content style="fixed-case"><jats:italic>L.</jats:italic></jats:styled-content> <jats:italic>confertus</jats:italic> and <jats:styled-content style="fixed-case"><jats:italic>T.</jats:italic></jats:styled-content> <jats:italic>aestivum</jats:italic>. To account for the different temperature responses between species, we suggest that there must be variation in both the activation energy for membrane permeability and the effective pathlength for liquid phase diffusion. Stomatal conductance was relatively independent of increases in leaf temperature and concomitant increases in leaf to air vapour pressure difference. Two exceptions were <jats:styled-content style="fixed-case"><jats:italic>E</jats:italic></jats:styled-content><jats:italic>ucalyptus</jats:italic> and <jats:styled-content style="fixed-case"><jats:italic>G</jats:italic></jats:styled-content><jats:italic>ossypium</jats:italic>, where stomatal conductance increased with temperature up to 35 °C despite increasing leaf to air vapour pressure. For a given species, temperature responses of stomatal and mesophyll conductance were independent of one another.</jats:p>