Carbon and water economy of Australian NAD-ME and NADP-ME C₄ grasses

C₄ grasses are grouped into three biochemical subtypes, NAD malic enzyme (NAD-ME), NADP malic enzyme (NADP-ME) and phosphoenolpyruvate carboxykinase (PCK), possessing characteristic leaf anatomy, biochemistry and physiology. This study investigates the physiological implications of these differences by comparing growth, water use efficiency (WUE, dry matter gain per unit water transpired) and gas exchange characteristics of NAD-ME and NADP-ME C₄ grasses belonging to three taxonomic groups (main Chloroid assemblage, Paniceae and Andropogoneae). We grew 28 C₄ grasses from seeds for 6 weeks in a glasshouse under ample water and nutrients in winter and summer. The inter-specific variation in plant dry mass (30-fold) was much greater than that in WUE (2-fold). There was no significant difference in average WUE between NAD-ME and NADP-ME grasses. Average plant dry mass and WUE were highest in the Paniceae (mostly NADP-ME), lowest in the Andropogoneae (NADP-ME) and intermediate in the Chloroid (NAD-ME). CO₂ assimilation rate (A), stomatal conductance (g) and the ratio of intercellular to ambient CO₂ partial pressure (p_i/p_a) were measured under standard conditions at high light. Average A and g were slightly higher in NADP-ME than NAD-ME grasses, but p_i/p_a was similar for the two subtypes. A did not differ between winter and summer experiments in spite of a 3-fold difference in maximal daily irradiance. Dry matter accumulation correlated positively with leaf area ratio (LAR; plant leaf area per unit plant dry mass) and specific leaf area (SLA; leaf area per unit leaf dry mass) in NAD-ME, but not NADP-ME, grasses. Variation in A (expressed on a per area basis) did not correlate with biomass accumulation or SLA. When expressed on a dry mass basis, A correlated with SLA in all C₄ grasses. This study shows that there is large inter-specific variation in growth among the C₄ grasses, but average WUE and A/g are similar for NAD-ME and NADP-ME species under well-watered conditions.