• carbon reactions;
  • CO2;
  • heat;
  • photorespiration;
  • photosynthesis.


The partial pressure of CO2 at the sites of carboxylation within chloroplasts depends on the conductance to CO2 diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (gm). We investigated the temperature response of gm in tobacco (Nicotiana tabacum) by combining gas exchange in high light, ambient CO2 in either 2 or 21% O2 with carbon isotope measurements using tuneable diode laser spectroscopy. The gm increased linearly with temperature in 2 or 21% O2. In 21% O2, isotope discrimination associated with gm decreased from 5.0 ± 0.2 to 1.8 ± 0.2‰ as temperature increased from 15 to 40 °C, but the photorespiratory contribution to the isotopic signal is significant. While the fractionation factor for photorespiration (f = 16.2 ± 0.7‰) was independent of temperature between 20 and 35 °C, discrimination associated with photorespiration increased from 1.1 ± 0.01 to 2.7 ± 0.02‰ from 15 to 40 °C. Other mitochondrial respiration contributed around 0.2 ± 0.03‰. The drawdown in CO2 partial pressure from ambient air to intercellular airspaces was nearly independent of leaf temperature. By contrast, the increase in gm with increasing leaf temperature resulted in the drawdown in CO2 partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.