Elevated CO2 increases the leaf temperature of two glasshouse-grown C4 grasses

This study investigates the effect of elevated CO 2 partial pressure (pCO 2)-induced stomatal closure on leaf temperature and gas exchange of C 4 grasses. Two native Australian C 4 grasses, Astrebla lappacea (Lindl.) Domin and Bothriochloa bladhii Kuntze, were grown at three different pCO 2 (35, 70 and 120 Pa) in three matched, temperature-controlled glasshouse compartments. The difference between leaf and air temperature (AT) was monitored diurnally with thermocouples. AT increased with both step-increases of ambient pCO 2. Average noon leaf temperature increased by 0.4 and 0.3°C for A. lappacea with the 35-70 and 70-120 Pa steps of pCO 2 elevation, respectively. For B. bladhii, the increases were 0.5°C for both pCO 2 steps. ΔT was strongly dependent on irradiance, pCO 2 and air humidity. Leaf gas exchange was measured at constant temperature and high irradiance at the three growth pCO 2. Under these conditions, CO 2 assimilation saturated at 70 Pa, while stomatal conductance decreased by the same extent (0.58-fold) with both step-increases in pCO 2, suggesting that whole-plant water use efficiency of C 4 grasses would increase beyond a doubling of ambient pCO 2. The ratio of intercellular to ambient pCO 2 was not affected by short- or long-term doubling or near-tripling of pCO 2, in either C 4 species when measured under standard conditions.