Plant water use efficiency of 17 Australian NAD-ME and NADP-ME C ₄ grasses at ambient and elevated CO ₂ partial pressure

This study investigates the response to elevated CO ₂ partial pressure (pCO ₂) of C ₄ grasses belonging to different biochemical subtypes (NAD-ME and NADP-ME), and taxonomic groups (main Chloroid assemblage, Paniceae and Andropogoneae). Seventeen C ₄ grasses were grown under well-watered conditions in two glasshouses maintained at an average daily pCO ₂ of 42 (ambient) or 68 (elevated) Pa. Elevated pCO ₂ significantly increased plant water-use efficiency (WUE; dry matter gain per unit water transpired) in 12 out of the 17 C ₄ 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 pCO ₂ was 26%. There were no significant subtype (or taxa) X pCO ₂ interactions on either WUE or biomass accumulation. When leaf gas exchange was compared at growth pCO ₂ but similar light and temperature, high pCO ₂-grown plants had similar CO ₂ assimilation rates (A) but a 40% lower stomatal conductance than their low pCO ₂-grown counterparts. There were no signs of either photosynthetic or stomatal acclimation in any of the measured species. We conclude that elevated pCO ₂ improved WUE primarily by reducing stomatal conductance.