Plant water use efficiency of 17 Australian NAD-ME and NADP-ME C₄ grasses at ambient and elevated CO₂ partial pressure

This study investigates the response to elevated CO₂ partial pressure (pCO₂) of C₄ grasses belonging to different biochemical subtypes (NAD-ME and NADP-ME), and taxonomic groups (main Chloroid assemblage, Paniceae and Andropogoneae). Seventeen C₄ grasses were grown under well-watered conditions in two glasshouses maintained at an average daily pCO₂ of 42 (ambient) or 68 (elevated) Pa. Elevated pCO₂ significantly increased plant water-use efficiency (WUE; dry matter gain per unit water transpired) in 12 out of the 17 C₄ grasses, by an average of 33%. In contrast, only five species showed a significant growth stimulation. When all species are considered, the average plant dry mass enhancement at elevated pCO₂ was 26%. There were no significant subtype (or taxa) X pCO₂ interactions on either WUE or biomass accumulation. When leaf gas exchange was compared at growth pCO₂ but similar light and temperature, high pCO₂-grown plants had similar CO₂ assimilation rates (A) but a 40% lower stomatal conductance than their low pCO₂-grown counterparts. There were no signs of either photosynthetic or stomatal acclimation in any of the measured species. We conclude that elevated pCO₂ improved WUE primarily by reducing stomatal conductance.