Plant Water Use Efficiency of 17 Australian NAD–ME and NADP–ME C Grasses at Ambient and Elevated CO Partial Pressure

Aust. J. Plant Physiol., 2001, 28, 1207–1217 Plant water use efficiency of 17 Australian NAD–ME and NADP–ME C4 grasses at ambient and elevated CO2 partial pressure Oula GhannoumAC, Susanne von CaemmererB and Jann P. ConroyA ACentre for Horticulture and Plant Sciences, University of Western Sydney, Locked Bag 1797, South Penrith DC, NSW 1797, Australia. BMolecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, PO Box 475, Canberra, ACT 2601, Australia. CCurrent address: Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, PO Box 475, Canberra, ACT 2601, Australia. Corresponding author; email: [email protected] Abstract. 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) 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. Keywords: C4 photosynthesis, CO2 enrichment, NAD–ME, NADP–ME, water-use efficiency. Introduction 1998) account for a great deal of this variability. However, this does not explain the large variability in the growth It is becoming increasingly evident that both well-watered response of well-watered and -fertilised C plants. To date, and water-stressed C plants can accumulate more biomass at 4 4 three possible sources of variations have been investigated. elevated CO partial pressure (pCO : Poorter 1993; Poorter 2 2 First, C weeds were found to show a larger growth et al. 1996; Wand et al. 1999). However, not all well-watered 4 response to CO enrichment than C crops (Ziska and Bunce C plants show a growth response, and it is not clear whether 2 4 4 1997). This may reflect the greater physiological plasticity of all well-watered C4 plants use less soil water at high pCO2. weeds (relative to highly-bred crops) to most environmental There are numerous examples of responsive (e.g. Wong factors. Secondly, based on the major C acid decarboxyla- 1979; Sionit and Patterson 1984; Read and Morgan 1996; 4 tion enzyme operating in the bundle sheath, C4 plants are Ghannoum et al. 1997; Watling and Press 1997; Ziska and grouped into three biochemical subtypes [NAD–malic Bunce 1997; LeCain and Morgan 1998; Wand et al. 2001) enzyme (NAD–ME; EC 1.1.1.39), NADP–malic enzyme and non-responsive (e.g. Patterson and Flint 1980; Carter and (NADP–ME; EC 1.1.1.40) and phosphoenolpyruvate Peterson 1983; Morison and Gifford 1984b; Wilsey et al. carboxykinase (PCK; EC 4.1.1.49)], possessing character- 1994; Ward et al. 1999; Wand et al. 2001) C4 plants to istic leaf biochemistry, anatomy and physiology (Hatch elevated pCO2. The interaction with environmental factors, 1987; Hattersley 1992; Ghannoum et al. 2001). It has been particularly water (e.g. Owensby et al. 1993; Samarakoon hypothesised that NAD–ME grasses have bundle sheath and Gifford 1996; Seneweera et al. 1998) and nitrogen walls which are ‘leakier’ to CO2 diffusion (Hattersley 1982), supply (e.g. Owensby et al. 1994; Ghannoum and Conroy and hence ought to be more responsive to elevated pCO2 Abbreviations used: A, CO2 assimilation rate; g, stomatal conductance; LAR, leaf area ratio; ME, malic enzyme; pCO2, CO2 partial pressure; pi, intercellular pCO2; ppm, parts per million; SLA, specific leaf area ratio; VPDl, vapour pressure deficit at leaf surface; WUE, water-use efficiency. © CSIRO 2001 10.1071/PP01056 0310-7841/01/121207 1208 O. Ghannoum et al. than their NADP–ME counterparts (LeCain and Morgan nomic groups (main Chloroid assemblage, Paniceae and 1998; Ziska et al. 1999; Wand et al. 2001). However, using a Andropogoneae). We focussed on NAD–ME and small number of C4 grasses, it was shown that the growth NADP–ME subtypes because they are the floristically most response tends to be larger in NADP–ME than NAD–ME dominant in Australia and biochemically and anatomically, grasses (LeCain and Morgan 1998; Wand et al. 2001), and the most contrasting subtypes (Hattersley 1992). This also independent of leak rate (Ziska et al. 1999). Thirdly, the allowed us to use more species in each group. taxonomic origin of C4 grasses has also been proposed as a source of variation for the growth response to elevated pCO2 Materials and methods (Kellogg et al. 1999). With a few exceptions, the NAD–ME Plant culture and growth conditions and NADP–ME pathways originated in distinct taxa (Hattersley 1992; Kellogg 1999; Sage et al. 1999). Using a Soil was collected from bushland at Robertson, NSW, left to dry in air, then mixed with the following nutrients (g kg–1 soil): N (0.6); P (0.27); few representative species of three lineages, Kellogg et al. Ca (2.5); Mg (0.36); K (1.67); Zn (0.04) and Cu (0.005) added as (1999) found that inter-specific variations in the growth (NH4)2SO4; CaHPO4; CaCO3, CaSO4; MgCO3; K2CO3; ZnSO4; and response of C4 grasses to CO2 enrichment outweigh those CuSO4. To prevent water leakage, plastic bags were placed inside between subfamilies. Nevertheless, significant inter- 3.5 or 10 L cylindrical pots. The amended soil (3.2 or 6.7 kg) was taxonomic differences in the growth response were also added to the pots, which were watered to 100% capacity, then observed (Kellogg et al. 1999). It is likely that variations in transferred to one of two naturally-lit, temperature-controlled glasshouses, maintained at an average daily pCO of either 42 the growth response are dependent on all of the afore 2 (ambient) or 68 (elevated) Pa (Fig. 1a). pCO2 was controlled by mentioned factors, but the small number of screened species injecting CO2 (Fogg grade; BOC Gases, North Ryde, Australia) from makes it difficult to distinguish general trends from pressurized cylinders through solenoid valves connected to an inter-specific variations. Differences in the growth response infra-red gas analyser (IRGA) (Fuji Electrics, Hokkaido, Japan). CO2 may also be related to other physiological or morphological was first passed through a Purafil column (Purafil Inc., Doraville, GA, USA) to eliminate possible ethylene contamination. pCO2 was traits, which may be causally or coincidentally associated monitored in both glasshouses by logging the voltage output, every with the biochemical subtype or evolutionary origin. 15 min, from the IRGAs with an ADAM-4018M analog data logger Another point of interest is the impact of elevated pCO2 (Priority Electronics, Sydney, Australia) interfaced with a PC. The on soil water availability and whole-plant water-use IRGAs were calibrated weekly with pure N2, and two CO2 calibration efficiency (WUE; dry matter gain per unit water transpired) (352 ± 7 and 735 ± 15 ppm) gases (BOC Gases). Air temperature and relative humidity were recorded every 15 min using Tinytag loggers of C4 plants. This is important for the prediction not only of (HDL, Port Macquarie, Australia), and averaged 30/23°C and 45/56% the growth response of C4 plants to a changing climate, but day/night, respectively (Figs 1b, c). Midday photosynthetically-active also their competitiveness and ultimately, geographic distri- radiation was measured daily with a LI–189 light meter (LI-COR Inc., –2 –1 bution (Polley et al. 1997; Howden 1999a; b). Due to the Lincoln, NE, USA), and averaged 860 µmol m s . Seventeen C4 grass species, belonging to different biochemical subtypes (NAD–ME near CO2-saturation of C4 photosynthesis in normal air, the effect of CO enrichment on C plants is thought to be and NADP–ME) and taxonomic groups (main Chloroid assemblage, 2 4 Paniceae and Andropogoneae) were used (Table 1). A more detailed mediated mainly through stomatal closure, and the subse- description of the species is given in Ghannoum et al. (2001). About quent reductions in leaf transpiration rate (Ghannoum et al. 10 seeds of each species were sown in each pot. Pot size and number of 2000). Therefore, it is assumed that C4 plants will spare soil plants per pot (2–4) were chosen in such a way as to maintain a water and become more water-use efficient at elevated relatively similar proportion between water (and hence other soil pCO . This assumption is mainly based on extrapolations resources) use and plant size. This was based on the rate of water use as 2 noted in a previous drought experiment (O. Ghannoum, unpublished from leaf-based instantaneous measures of WUE. Few data). There were four pots per species and pCO2 treatment. Two studies made direct measurements of whole-plant water use control pots, without plants but weighed with the same amount of and WUE in C4 plants at elevated pCO2. In general, these fertilised soil as planted pots, were included in each pCO2 treatment. Pots were randomised between the glasshouses every week to minimise studies indicated that CO2 enrichment leads to significant increases in WUE and soil water availability (Morison and possible glasshouse effects.

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