Elevated CO2 increases the leaf temperature of two glasshouse-grown C-4 grasses

This study investigates the effect of elevated CO2 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 (DeltaT) was monitored diurnally with thermocouples. DeltaT increased with both step-increases of ambient pCO(2). Average noon leaf temperature increased by 0.4 and 0.3degreesC for A. lappacea with the 35-70 and 70-120 Pa steps of pCO(2) elevation, respectively. For B. bladhii, the increases were 0.5degreesC for both pCO(2) steps. DeltaT 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, CO2 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.