Carbon and water economy of Australian NAD-ME and NADP-ME C-4 grasses

C-4 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 difference s by comparing growth, water use efficiency (WUE, dry matter gain per unit water transpired) and gas exchange characteristics of NAD-ME and NADP-ME C- 4 grasses belonging to three taxonomic groups (main Chloroid assemblage, Pa niceae and Andropogoneae). We grew 28 C-4 grasses from seeds for 6 weeks in a glasshouse under ample water and nutrients in winter and summer. The int er-specific variation in plant dry mass (30-fold) was much greater than tha t in WUE (2-fold). There was no significant difference in average WUE betwe en 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 pres sure (p(i)/p(a)) were measured under standard conditions at high light. Ave rage 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 irradia nce. Dry matter accumulation correlated positively with leaf area ratio (LA R; plant leaf area per unit plant dry mass) and specific leaf area (SLA; le af area per unit leaf dry mass) in NAD-ME, but not NADP-ME, grasses. Variat ion in A (expressed on a per area basis) did not correlate with biomass acc umulation or SLA. When expressed on a dry mass basis, A correlated with SLA in all C-4 grasses. This study shows that there is large inter-specific va riation in growth among the C-4 grasses, but average WUE and A/g are simila r for NAD-ME and NADP-ME species under well-watered conditions.