Leaf temperature of C4 grasses is increased in elevated CO2 in a glasshouse

C4 plants accumulate more biomass at elevated than at ambient CO2 partial pressure although CO2 assimilation rates are already CO2 saturated at ambient CO2 partial pressures. Ghannoum et al. (2000 PCE 23: 931-942) proposed that reduced stomatal conductance in elevated CO2, might increase leaf temperature which in turn would increase photosynthesis and leaf growth. We tested this hypothesis in a glasshouse experiment with two C4 grasses Bothriochloa bladhii and Astrebla lappacea. Plants were grown in 5 L pots with well watered and nutrient supplemented soil in temperature controlled glasshouse compartments (22/24/26°C in night/morning/day) at three different CO2 partial pressures (120/70/35 Pa, average during the light period). The difference between leaf and air temperature was monitored with 7-12 thermocouples per species and glasshouse over a period of three weeks. Leaf temperature was light dependent reaching highest values at noon (up to 3.8°C in B. bladhii and 2.9°C in A. lappacea when averaged over 10 to 12 plants and 1 hour in 120 Pa CO2). The average leaf-air temperature difference at noon (average of one hour) was for B. bladhii 1.93/1.47/0.97 °C and for A. lappacea 1.53/1.23/0.80 °C at 120/70/35 Pa CO2, respectively. The average leaf-air temperature difference during the full light period was for B. bladhii 1.17/0.89/0.54 °C and for A. lappacea 1.00/0.77/0.52 °C at 120/70/35 Pa CO2 respectively. We conclude, that the leaf temperature is indeed increased with increased CO2 in C4 grasses as we proposed.