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

Abstract

C₄ grasses are grouped into three biochemical subtypes,NAD malic enzyme (NAD–ME), NADP malic enzyme (NADP–ME)and phosphoenolpyruvate carboxykinase (PCK), possessingcharacteristic leaf anatomy, biochemistry and physiology. This studyinvestigates 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–MEand NADP–ME C₄ grasses belonging to threetaxonomic groups (main Chloroid assemblage, Paniceae and Andropogoneae). Wegrew 28 C₄ grasses from seeds for 6 weeks in aglasshouse under ample water and nutrients in winter and summer. Theinter-specific variation in plant dry mass (30-fold) was much greater thanthat in WUE (2-fold). There was no significantdifference in average WUE between NAD–ME andNADP–ME grasses. Average plant dry mass andWUE were highest in the Paniceae (mostlyNADP–ME), lowest in the Andropogoneae (NADP–ME) and intermediate in the Chloroid (NAD–ME). CO₂assimilation 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. AverageA and g were slightly higher inNADP–ME than NAD–ME grasses, but p_i/p _awas similar for the two subtypes. A did not differbetween winter and summer experiments in spite of a 3-fold difference inmaximal daily irradiance. Dry matter accumulation correlated positively withleaf area ratio (LAR; plant leaf area per unit plant drymass) and specific leaf area (SLA; leaf area per unitleaf dry mass) in NAD–ME, but not NADP–ME, grasses.Variation in A (expressed on a per area basis) did notcorrelate with biomass accumulation or SLA. Whenexpressed on a dry mass basis, A correlated withSLA in all C₄ grasses. This studyshows that there is large inter-specific variation in growth among the C₄ grasses, but average WUE andA/g are similar forNAD–ME and NADP–ME species under well-wateredconditions.