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

This study investigates the effect of elevated CO₂ partial pressure (pCO₂)-induced stomatal closure on leaf temperature and gas exchange of C₄ grasses. Two native Australian C₄ grasses, Astrebla lappacea (Lindl.) Domin and Bothriochloa bladhii Kuntze, were grown at three different pCO₂ (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 pCO₂. 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 pCO₂ elevation, respectively. For B. bladhii, the increases were 0.5°C for both pCO₂ steps. ΔT was strongly dependent on irradiance, pCO₂ and air humidity. Leaf gas exchange was measured at constant temperature and high irradiance at the three growth pCO₂. 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 pCO₂, suggesting that whole-plant water use efficiency of C₄ grasses would increase beyond a doubling of ambient pCO₂. The ratio of intercellular to ambient pCO₂ was not affected by short- or long-term doubling or near-tripling of pCO₂, in either C₄ species when measured under standard conditions.