Summary• Atmospheric CO 2 concentration is expected to increase by 50% near the middle of this century. The effects the free air CO 2 enrichment (FACE) is presented here on growth and development of field-grown grain sorghum ( Sorghum bicolor ) at ample (wet) and limiting (dry) levels of irrigation water at Maricopa, AZ, USA.• Daytime CO 2 mole fractions were 561 and 368 µ mol mol -1 for the FACE and control treatments, respectively. Irrigation plus precipitation averaged 1132 mm for the wet plots and 396 mm in the dry plots.• During the growing season, FACE increased biomass accumulation in the dry plots but the effects in the wet plots were inconsistent. At final harvest, FACE increased total yield from 999 to 1151 g m -2 in the dry plots and had no effect in the wet plots.• If atmospheric CO 2 continues to increase, total sorghum yield is likely to be higher in the future in areas where water is limited.

Summary• The likely consequences of future high levels of atmospheric CO 2 concentration on wheat ( Triticum aestivum L.) grain nutritional and baking quality were determined.• Two free-air CO 2 enrichment (FACE; 550 mmol mol − 1 ) experiments were conducted at ample (Wet) and limiting (Dry) levels of irrigation, and a further two experiments at ample (High-N) and limiting (Low-N) nitrogen concentrations. Harvested grain samples were subjected to a battery of nutritional and bread-making quality tests.• The Dry treatment improved grain quality slightly (protein +2%; bread loaf volume +3%). By contrast, Low-N decreased quality drastically (protein − 36%; loaf volume − 26%). At ample water and N, FACE decreased quality slightly (protein − 5%; loaf volume − 2%) in the irrigation experiments and there was no change in the nitrogen experiments. At Low-N, FACE tended to make the deleterious effects of Low-N worse (protein − 33% and − 39%, at ambient CO 2 and FACE, respectively; loaf volume − 22% and − 29% at ambient CO 2 and FACE, respectively).• The data suggest that future elevated CO 2 concentrations will exacerbate the deleterious effects of low soil nitrogen on grain quality, but with ample nitrogen fertilizer, the effects will be minor.

Summary• The interactive effects of atmospheric CO 2 concentration and soil-water content on grain sorghum ( Sorghum bicolor ) are reported here.• Sorghum plants were exposed to ambient (control) and free-air CO 2 enrichment (FACE; ambient + 200 µmol mol -1 ), under ample (wet, 100% replacement of evapotranspiration) and reduced (dry, postplanting and mid-season irrigations) water supply over two growing seasons.• FACE reduced seasonal average stomatal conductance ( g s ) by 0.17 mol (H 2 O) m -2 s -1 (32% and 37% for dry and wet, respectively) compared with control; this was similar to the difference between dry and wet treatments. FACE increased net assimilation rate ( A ) by 4.77 µmol (CO 2 ) m -2 s -1 (23% and 9% for dry and wet, respectively), whereas dry decreased A by 10.50 µmol (CO 2 ) m -2 s -1 (26%) compared with wet. Total plant water potential ( ψ w ) was 0.16 MPa (9%) and 0.04 MPa (3%) less negative in FACE than in the control treatment for dry and wet, respectively. Under dry, FACE stimulated final shoot biomass by 15%.• By ameliorating the adverse effects of drought, elevated atmospheric CO 2 improved plant water status, which indirectly caused an increase in carbon gain.Key words: carbon dioxide, global change, stomatal conductance, net assimilation rate, water relations, free-air CO 2 enrichment (FACE). AbbreviationsA, instantaneous leaf net assimilation rate (µmol (CO 2 ) m -2 s -1 ); B, final shoot biomass (g m -2 ground area); B a , average accumulated shoot biomass (g m -2 ground area); CD, control-dry; CW, control-wet; C, carbon dioxide effect in ANOVA; C a , atmospheric CO 2 concentration (µmol (CO 2 ) mol -1 ); C i , intercellular CO 2 concentration (µmol (CO 2 ) mol -1 ); C i : C a , ratio of C i to C a ; C m , internal CO 2 concentration in mesophyll (µmol (CO 2 ) mol -1 ); C m : C a , ratio of C m to C a ; D, soil dehydration cycle effect in ANOVA; DAP, day after planting; D r , maximum depth of root penetration (m); ET, evapotranspiration rate (mm d -1 ); ET c , cumulative seasonal evapotranspiration (mm); e a , atmospheric water vapor pressure at T a (Pa); e s , atmospheric saturation water vapor pressure at T a (Pa); e*, atmospheric water vapor pressure deficit ( i.e. , e* = e s -e a ) at T a (Pa); FD, FACE-dry; FW, FACE-wet; G, growth stage effect in ANOVA; g s , stomatal conductance (mol (H 2 O) m -2 s -1 ); HI: harvest index (grain yield divided by total shoot biomass); I, irrigation effect in ANOVA; IWUE: A/g s , intrinsic water use efficiency (µmol (CO 2 ) mol (H 2 O)

Abstract. In order to determine the likely effects of the increasing atmospheric GO 2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550/xmol/mol CO2 (or 200/xmol/mol above current ambient levels of about 360/xmol/mol) using a free-air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, R n; soil heat flux; air and soil temperatures; canopy temperature, Ts; and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime T s about 0.6 ø and 1.1øC at high and low N, respectively. Daily total R n was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively. IntroductionThe CO 2 concentration of the atmosphere is increasing, and climate modelers have predicted a consequent global warming as well as changes in precipitation patterns. The report of the IPCC [Intergovernmental Panel on Climate Change, 1996] projects CO2 increasing from present day concentrations of about 360/xmol/mol to about 500/xmol/mol by the end of the next century if emissions are maintained at 1994 levels. They further project that the increase in CO2 plus that of other radiatively active "greenhouse" gases (methane, nitrous oxide, chlorofluorocarbons (CFCs), ozone) will cause an increase in global mean temperature of 0.9 ø to 3.5øC depending on future emission rates. Some regions might receive increases in precipitation, while others might receive less. However, these projected changes in climate are very uncertain.Increasing CO2 concentration has been shown to cause partial closure of plant leaf stomata, which reduces the conductance of water vapor from inside the leaf stomatal cavities to the outside air [Morison, 1987] [1997] exposed grassland to elevated CO2 using open-top chambers and attempted to measure ET with smaller gas-exchange chambers. They found reductions of ET of 12-39% due to elevated CO2 on sandstone-derived soil, but on serpentinederived soil, ET actually increased from -1% to + 14%. They believe that the latter increase is because the serpentine canopy was sparse, so there was more E (evaporation from soil) than T, and E would not be affected by CO2. Thus the prior experimental work using chambers has been somewhat variable but explainable based on leaf area growth, canopy, and stomatal effects. Except for the results of Dugas et al. [1997], however, generally the effects of CO2 on ET have been small. 1179

Summary• Sorghum ( Sorghum bicolor ) was grown for two consecutive seasons at Maricopa, AZ, USA, using the free-air CO 2 enrichment (FACE) approach to investigate evapotranspiration of this C4 plant at ample and limited water supplies.• Crop evapotranspiration (ET) was measured using two CO 2 concentrations (control, c. 370 µmol mol -1 ; FACE, ambient + 200 µmol mol -1 ) and two irrigation treatments (well watered and water-limited). Volumetric soil water content was measured before and after each irrigation using neutron scattering techniques.• Averaged over both years, elevated CO 2 reduced cumulative ET by 10% when plants were given ample water and by 4% under severe drought stress. Water-use efficiency based on grain yield (WUE-G) increased, due to CO 2 enrichment, by 9% and 19% in wet and dry plots, respectively; based on total biomass, water-use efficiency (WUE-B) increased by 16% and 17% in wet and dry plots, respectively.• These data suggest that in the future high-CO 2 environment, water requirements for irrigated sorghum will be lower than at present, while dry-land productivity will increase, provided global warming is minimal.