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The interactive effects of atmospheric CO[sub 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[sub 2] enrichment (FACE; ambient + 200 pmol mol[sub -1]), under ample (wet, 100% replacement of evapo-transpiration) and reduced (dry, postplanting and mid-season irrigations) water supply over two growing seasons. FACE reduced seasonal average stomatal conductance (g[sub s]) by 0.17 mol (H[sub 2]O) m[sup -2] s[sup -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[sub 2]) m[sup -2] s[sup -1] (23% and 9% for dry and wet, respectively), whereas dry decreased A by 10.50 μmol (CO[sub 2]) m[sup -2] s[sup -1] (26%) compared with wet. Total plant water potential (ψ[sub 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[sub 2] improved plant water status, which indirectly caused an increase in carbon gain. [ABSTRACT FROM AUTHOR]

Increasing atmospheric carbon dioxide (CO2) likely will affect future water requirements of most plants, including agricultural crops. This research quantifies such effects on the energy balance and evapotranspiration (ET) of sorghum (Sorghum bicolor (L.) Möench, a C4 grain crop) using a residual energy balance approach. During the summer and autumn of 1998 and 1999, sorghum was grown under free-air CO2 enrichment (FACE) conditions near Maricopa, Arizona.Latent heat flux (λET) was determined by subtracting soil heat flux (G0) and sensible heat flux (H) from net radiation (Rn) values in both Control CO2 plots (about 370 μmol mol-1) and FACE plots (Control + 200 μmol mol-1). Rn was observed using net radiometers. G0 was measured with soil heat flux plates at a depth of 10 mm, then corrected for heat storage above the plates. H was determined using measurements of air temperature from aspirated psychrometers, leaf temperature from infrared thermometers, and wind data from a three-cup anemometer. Both FACE and Control plots were divided into semicircular halves to allow a well-watered (Wet) treatment and a drought-stressed (Dry) treatment. This allowed comparisons of the FACE effect on ET in normal and water-stressed conditions. Under Wet conditions, FACE decreased λET by 13.8±1.8% in 1998, and 11.8±1.9% in 1999. Drought-stress resulted in a reduction in λET of 8.5±3.7% for the FACE treatments in 1998, but an increase in λET of 10.5±5.1% in 1999. When soil water was readily available, midday canopy temperatures in the FACE plots were increased by 1.47±0.09 °C in 1998, and 1.85±0.20 °C in 1999, indicative of increased stomatal resistance due to CO2 enrichment. These data suggest that soil water availability is a determining factor for the FACE effect.Water use efficiency (WUE) increased about 28% due to elevated CO2 under Wet conditions due to a savings of water for about the same growth, whereas under Dry conditions it increased about 16% due to much greater relative growth on only a slightly higher amount of water. [Copyright &y& Elsevier]

Effects of increasing carbon dioxide concentration [CO2] on wheat vary depending on water supply and climatic conditions, which are difficult to estimate. Crop simulation models are often used to predict the impact of global atmospheric changes on food production. However, models have rarely been tested for effects on crops of [CO2] and drought for different climatic conditions due to limited data available from field experiments.Simulations of the effects of elevated [CO2] and drought on spring wheat (Triticum aestivum L.) from three crop simulation models (LINTULCC2, AFRCWHEAT2, Sirius), which differ in structure and mechanistic detail, were compared with observations. These were from 2 years of free-air carbon dioxide enrichment (FACE) experiments in Maricopa, Arizona and 2 years of standardised (in crop management and soil conditions) open-top chamber (OTC) experiments in Braunschweig and Giessen, Germany. In a simulation exercise, models were used to assess the possible impact of increased [CO2] on wheat yields measured between 1987 and 1999 at one farm site in the drought prone region of Andalucia, south Spain.The models simulated well final biomass (BM), grain yield (GY), cumulative evapotranspiration (ET) and water use efficiency (WUE) of wheat grown in the FACE experiments but simulations were unsatisfactory for OTC experiments. Radiation use efficiency (RUE) and yield responses to [CO2] and drought were on average higher in OTC than in FACE experiments. However, there was large variation among OTC experiments. Plant growth in OTCs was probably modified by several factors related to plot size, the use (or not use) of border plants, airflow pattern, modification of radiation balance and/or restriction of rooting volume that were not included in the models. Variation in farm yields in south Spain was partly explained by the models, but sources of unexplained yield variation could not be identified and were most likely related to effects of pests and diseases that were not included in the models. Simulated GY in south Spain increased in the range between 30 and 65% due to doubling [CO2]. The simulated increase was larger when a [CO2]×drought interaction was assumed (LINTULCC2, AFRCWHEAT2) than when it was not (Sirius).It was concluded that crop simulation models are able to reproduce wheat growth and yield for different [CO2] and drought treatments in a field environment. However, there is still uncertainty about the combined effects of [CO2] and drought including the timing of drought stress and about relationships that determine yield variation at farm and larger scales that require further investigation including model testing. [Copyright &y& Elsevier]