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Field experimental data of five experiments covering a wide range of growing conditions are assembled for wheat growth and cropping systems modeling. The data include an experiment on interactive effects of elevated CO2 by water and elevated CO2 by nitrogen fertilizer application from a Free-Air Carbon Dioxide Enrichment experiment (FACE) at Arizona in USA; a nitrogen rate fertilizer experiment from three locations and two years in The Netherlands; water deficit experiments at Lincoln in New Zealand and at Cunderdin in Australia; and a temperature sensitivity experiment at Obregon in Mexico. The data consist of 65 experimental treatments with more than 1000 detailed observations, with time series of development and growth, soil water and soil nitrogen dynamics, yield and yield components, daily weather, soil characteristics, and cultivar descriptions. These data have been used in various previous agronomic and crop modeling studies. Assembled data are quality checked and supplied in Agricultural Model Inter-comparison and Improvement (AgMIP) format. Data access via DOI 10.7910/DVN/V4P6PU.

Potential impacts of climate change on grain sorghum (Sorghum bicolor) productivity were investigated using the CERES-sorghum model in the Decision Support System for Agrotechnology Transfer v4.5. The model was first calibrated for a sorghum cultivar grown in a free air CO2 enrichment experiment at the University of Arizona, Maricopa, Arizona, USA in 1998. The model was then validated with an independent dataset collected in 1999. The simulated grain yield, growth, and soil water of sorghum for the both years were in statistical agreement with the corresponding measurements, respectively. Neither simulated nor measured yields responded to elevated CO2, but both were sensitive to water supply. The validated model was then applied to simulate possible effects of climate change on sorghum grain yield and water use efficiency in western North America for the years 2080-2100. The projected CO2 fertilizer effect on grain yield was dominated by the adverse effect of projected temperature increases. Therefore, temperature appears to be a dominant driver of the global climate change influencing future sorghum productivity. These results suggest that an increase in water demand for sorghum production should be anticipated in a future high-CO2 world.

Sorghum [Sorghum bicolor (L.) Moench] is the world’s fifth most important crop, grown for forage, grain, and as a biofuel. Fast growing and drought tolerant, it is increasingly being planted as a climate change-ready alternative to maize. All parts of the sorghum plant except the grain contain the cyanogenic glucoside dhurrin, which breaks down to release hydrogen cyanide (prussic acid) when plant tissue is disrupted. Fresh forage, hay and silage may be toxic to stock when derived from plants that are young, droughted or heavily fertilized. Sorghum also stores nitrate, which can cause nitrite toxicity. The impact of elevated CO2 on dhurrin and nitrate concentration is unknown. It is important to understand how global environmental change will affect composition in order to be able to predict the safety of the crop in coming decades. Sorghum was grown experimentally at elevated CO2 in two free-air CO2 enrichment (FACE) experiments at ambient and elevated CO2 (ca. 550 ppm) and either irrigated regularly or only once after sowing in consecutive years and sampled at different stages of development. Since FACE-grown sorghum has been shown to have improved water status we hypothesized that they would contain less dhurrin. We found the most important factors governing cyanide concentration were (in decreasing order): plant age, irrigation treatment and tissue type. For nitrate, tissue type was by far the most important factor, followed by plant age, and then irrigation treatment. The concentration of CO2 in the atmosphere had no significant effect on the total nitrogen concentration, or the concentrations of cyanide and nitrate. As sorghum is becomes more widely used for forage, it will be important to have simple methods to assess the cyanide levels in the field or to develop new, low cyanogenic varieties to ensure that it is safe for grazing.

International audience; Despite widespread application in studying climate change impacts, most crop models ignore complex interactions among air temperature, crop and soil water status, CO2 concentration and atmospheric conditions that influence crop canopy temperature. The current study extended previous studies by evaluating Tc simulations from nine crop models at six locations across environmental and production conditions. Each crop model implemented one of an empirical (EMP), an energy balance assuming neutral stability (EBN) or an energy balance correcting for atmospheric stability conditions (EBSC) approach to simulate Tc. Model performance in predicting Tc was evaluated for two experiments in continental North America with various water, nitrogen and CO2 treatments. An empirical model fit to one dataset had the best performance, followed by the EBSC models. Stability conditions explained much of the differences between modeling approaches. More accurate simulation of heat stress will likely require use of energy balance approaches that consider atmospheric stability conditions.

Reductions in the protein and nitrogen content of plants grown under enhanced atmospheric CO2 concentrations could adversely affect the quality of food grown in the future, but the mechanisms of change remain unclear. Now research investigating plant responses to enhanced levels of atmospheric CO2 under field conditions finds that wheat nitrate assimilation was slower for elevated CO2 than for ambient CO2.

Agricultural system simulation models are key tools for assessment of possible impacts of climate change on crop production and environmental quality. In this study, the CERES-Wheat 4.0 module in the RZWQM2 model was calibrated and validated for simulating spring wheat grown under elevated CO2 conditions in the FACE (Free Air CO2 Enrichment) experiments conducted at Maricopa, Arizona, USA from 1992 to 1997. The validated model was then used to simulate the possible impacts of climate change on the crop for a 16-year period centered on 2050 with a projected atmospheric CO2 concentration of 550 ppm. Sixteen General Circulation Model (GCM) projections of climate in response to this CO2 concentration were used for this purpose. In the FACE experiment, the crops were grown under ambient (365–370 ppm) and elevated (∼550 ppm) CO2 concentrations with two irrigation treatments (wet and dry) in 1992–1993 and 1993–1994, and two nitrogen (N) treatments (high and low N) in 1995–1996 and 1996–1997 crop seasons. The model simulated crop growth and grain yield, and soil water responses to CO2 reasonably well, reproducing variations due to the treatments. Under ambient CO2 in 1992–1993 and 1995–1996, biomass was simulated better in the dry and low N treatments with root mean square difference (RMSD) of 181 and 161 kg ha−1, respectively, compared to the wet and high N treatments with RMSD of 259 and 268 kg ha −1 , respectively. The effects of water and N treatments were higher than those of CO2, and the model reproduced these effects well. Elevated CO2 effects on crop growth were counterbalanced by temperature effects, and projected precipitation had little effect on the simulated crop. The model results provide reasonable confidence for simulations of possible impacts of projected climate change on wheat crop growth in the region, within normal field data uncertainties.

Estimation of evapotranspiration (ET) is important for monitoring crop water stress and for developing decision support systems for irrigation scheduling. Techniques to estimate ET have been available for many years, while more recently remote sensing data have extended ET into a spatially distributed context. However, remote sensing data cannot be easily used in decision systems if they are not available frequently. For many crops ET estimates are needed at intervals of a week or less, but unfortunately due to cost, weather, and sensor availability constraints, high resolution (

Development and implementation of improved methodologies for crop water management will conserve valuable water resources in agricultural regions that depend on irrigation. To address this problem for conditions in central Arizona, we have evaluated the CSM-CROPSIM-CERES-Wheat model using measured wheat growth and soil water data from plot-level irrigation scheduling experiments conducted during the winters of 2003-2004 and 2004-2005. During each season, wheat plots were managed using two FAO-56-based irrigation scheduling approaches at three planting densities (~75, ~150, and ~300 plant m-2) and at two nitrogen application rates (~80 and ~215 kg ha-1 year-1). For these treatments, the calibrated model simulated wheat yield with relative root mean squared errors (RRMSE) of 7.4% and 1.7% for the 2003-2004 and 2004-2005 seasons, respectively. Time series plots of measured and simulated Zadoks number, leaf number, leaf mass, stem mass, spike mass, and green leaf area index demonstrated favorable wheat development and growth responses to experimental treatments and seasonal weather and management variability. The model was able to quantify average soil water contents in eight soil layers to a depth of 210 cm with RRMSEs ranging from 3.3% to 18.9% for the 2003-2004 season and from 2.7% to 11.3% for the 2004-2005 season. Evapotranspiration was simulated with RRMSEs of 2.4% and 3.2% for the 2003-2004 and 2004-2005 seasons, respectively. Deficiencies were demonstrated in the ability of the model's automatic irrigation routines to reproduce the FAO-56 irrigation schedules devised during field experimentation. With further development, the CSM-CROPSIM-CERES-Wheat model could become a valuable central component for decision tools designed to evaluate alternative water management scenarios and improve water management for irrigated agricultural systems.

The global impact of an increased concentration of CO2 in the atmosphere on plants has been studied extensively, but little information has been published on the effect of enrichment of atmospheric CO2 on the nutritive value of grain and straw used as ruminant feeds. This paper reports the chemical composition and nutritive value of grain and straw harvested from the drought tolerant hard red spring wheat (Triticum aestivum L.) variety Yecora Rojo managed with two carbon dioxide regimes (ambient, 350 μl/l and elevated, 550 μl/l), two rates of nitrogen application (low N: 53 kg N/ha and high N: 393 kg N/ha) grown under a water-fed (i.e., no deficit) regime. Accumulation of carbon in straw did not differ among crops grown under elevated CO2 and low N supplementation and crops grown under ambient CO2 with low levels of N supplementation. Increased N application increased sequestration of C (P

Various remote sensing indices have been used to infer crop nitrogen (N) status for field-scale nutrient management. However, such indices may indicate erroneous N status if there is a decrease in crop canopy density influenced by other factors, such as water stress. The Canopy Chlorophyll Content Index (CCCI) is a two-dimensional remote sensing index that has been proposed for inferring cotton N status. The CCCI uses reflectances in the near-infrared (NIR) and red spectral regions to account for seasonal changes in canopy density, while reflectances in the NIR and far-red regions are used to detect relative changes in canopy chlorophyll, a surrogate for N content. The primary objective of this study was to evaluate the CCCI and several other remote sensing indices for detecting the N status for cotton during the growing season. A secondary objective was to evaluate the ability of the indices to appropriately detect N in the presence of variable water status. Remote sensing data were collected during the 1998 (day of year [DOY] 114 to 310) and 1999 (DOY 106 to 316) cotton seasons in Arizona, in which treatments of optimal and low levels of N and water were imposed. In the 1998 season, water treatments were not imposed until late in the season (DOY 261), well after full cover. Following an early season N application in 1998 for the optimal (DOY 154) but not the low N treatment, the CCCI detected significant differences in crop N status between the N treatments starting on DOY 173, when canopy cover was about 30%. A common vegetation index, the ratio of NIR to red (RVI), also detected significant separation between N treatments, but RVI detection occurred 16 days after the CCCI response. After an equal amount of N was applied to both optimal and low N treatments on DOY 190 in 1998, the CCCI indicated comparable N status for the N treatments on DOY 198, a trend not detected by RVI. In the 1999 season, both N and water treatments were imposed early and frequently during the season. The N status was poorly described by both the CCCI and RVI under partial canopy conditions when water status differed among treatments. However, once full canopy was obtained in 1999, the CCCI provided reliable N status information regardless of water status. At full cotton cover, the CCCI was significantly correlated with measured parameters of N status, including petiole NO 3 -N (r = 0.74), SPAD chlorophyll (r = 0.65), and total leaf N contents (r = 0.86). For well-watered cotton, the CCCI shows promise as a useful indicator of cotton N status after the canopy reaches about 30% cover. However, further study is needed to develop the CCCI as a robust N detection tool independent of water stress.

Experimentation with dynamics of soil carbon pools as affected by elevated CO2 can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis (δ13C) were used to investigate labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998–1999 Arizona Maricopa free air CO2 enrichment (FACE) experiment, in which elevated CO2 (FACE: 560 μmol mol−1) and ambient CO2 (Control: 360 μmol mol−1) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO2 transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0–30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Similarly, old C in the recalcitrant pool under elevated CO2 was higher than that under ambient CO2, indicating that elevated CO2 reduces the decay of the old C in recalcitrant pool. Mean residence time (MRT) of bulk soil carbon at the depth of 0–30 cm was significantly longer in FACE plot than Control plot by the averages of 12 and 13 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO2 enhanced the shift of the carbon from labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO2 and mitigating harmful CO2 under future atmospheric conditions.

A method widely used for irrigation management determines crop evapotranspiration (ETc) from reference evapotranspiration (ETo) calculations and estimated crop coefficients. However, standard time-based crop coefficients may fail to represent the actual crop water use, for example, when deviations in weather or agronomic constraints appreciably change crop development patterns from typical conditions. In this study, the FAO-56 dual crop coefficient procedures were applied during experiments with wheat to calculate the estimated ETc for irrigation scheduling. The objective of this research was to determine whether basal crop coefficients (Kcb) determined from a normalized difference vegetation index (NDVI treatment) improve the prediction of ETc over a standard application with a locally developed time-based Kcb curve (FAO treatment). The experiments conducted for two seasons in central Arizona included subtreatments, equally replicated within the NDVI and FAO treatments, of three plant densities (typical, dense, and sparse) and two nitrogen levels (high and low) to provide a range of crop development and water use conditions. The effects of plant density and N level resulted in significant differences in measured seasonal ETc. Large variations that occurred in the observed Kcb and ETc trends between subtreatments were better correlated with the NDVI than the FAO treatment. The mean absolute percent difference for predicted ETc was significantly smaller for NDVI than FAO during both seasons. The treatment difference was 5% for the first season, but 10% for the second season when an unexpected early decline in ETc and Kcb was effectively predicted by the NDVI treatment but not by the FAO treatment. NDVI appears to be a robust approach for Kcb estimation of wheat, able to reliably predict actual ETc for both typical and abnormal water use conditions.

Current irrigation scheduling is based on well-established crop coefficient and reference evapotranspiration procedures to estimate daily crop evapotranspiration (ETc). Effective irrigation scheduling and efficient irrigation water use can occur when ETc is calculated with crop coefficients representative of actual crop water use conditions. The objective of this research was to evaluate irrigation scheduling using two approaches to estimate the basal crop coefficient (Kcb) during wheat experiments conducted in 2003-2004 and 2004-2005 at Maricopa, Arizona. Each Kcb approach (main treatment) included six subtreatment combinations (three plant densities and two N managements) imposed to create spatial and temporal variations in water use among experimental plots. The first approach (NDVI treatment) estimated Kcb separately for each plot based on normalized difference vegetation index (NDVI) data obtained by frequent canopy reflectance measurements. The second approach (FAO treatment) estimated Kcb uniformly for all plots based on a Kcb curve developed for standard wheat conditions. The Kcb estimates were incorporated within the FAO-56 dual crop coefficient procedures to calculate daily ETc and root zone soil water depletion (Dr). Plot irrigations were provided when the predicted Dr reached 45% of the available soil water. During both wheat experiments, considerable variations in measured soil water depletion were observed for subtreatments due to differences in crop water use rates. For the FAO treatment, mean absolute percent difference (MAPD) for predicted Dr was 27% and 40% for 2003-2004 and 2004-2005, respectively. Prediction of Dr was improved significantly for NDVI for both experiments where treatment MAPD was 17% (2003-2004) and 18% (2004-2005). Although mean irrigation application efficiency for NDVI (89%) and FAO (88%) was similar for 2003-2004, it was significantly higher for NDVI (86%) than FAO (77%) for 2004-2005. Differences for irrigation scheduling resulted in significantly lower seasonal irrigation water use for the NDVI than FAO treatment, 8% (2003-2004) and 13% (2004-2005), but did not result in appreciable treatment differences for seasonal ETc, final grain yield, and crop water use efficiency (yield per unit ETc). Consequently, a primary outcome for both experiments was significantly higher irrigation water use efficiency (yield per unit irrigation water) for NDVI than FAO. Incorporating Kcb estimates based on NDVI within existing crop coefficient algorithms provides an opportunity to improve wheat irrigation scheduling strategies for conserving irrigation water while maintaining grain yield potentials.

Estimating and monitoring the spatial distribution of evapotranspiration (ET) over irrigated crops is becoming increasingly important for managing crop water requirements under water scarce conditions. The usual point-based approaches for estimating ET, however, do not provide enough data for precision farming applications, whereby irrigation schedules could be customized by crop conditions at sub-field scales. Needed in addition are spatially distributed ET modeling approaches, obtainable only through remote sensing, which can observe ET-related surface properties such as vegetation density and surface temperature. Although research using remote sensing to estimate ET has been pursued for many years, there are still few ground-validated, full-season ET studies at fine spatial scales. In this study, we assessed the ability of a remote sensing model to retrieve daily ET throughout an entire growing season for wheat. Image data with 0.5 m resolution were collected in 2005 over an irrigation scheduling research site in central Arizona (Maricopa). The 1.3 ha study area (FISE05) contained 32 leveled, flood-irrigated plots with treatments for irrigation scheduling, planting density, and fertilization. Daily ET was modeled using a two-source energy balance (TSEB) approach and airborne image observations in visible, near-infrared, and thermal infrared wavelengths for six dates throughout the growing season. Using independent soil water depletion observations, modeled daily ET values were accurate to within 0.4 mm d-1 for most of the FISE05 season and sensitive to changes in wheat canopy changes. Late-season ET estimates were less satisfactory, with accuracies to within 1.3 mm d-1. The significance of these results was supported by verifying agreement between ground and airborne estimates of vegetation indices, wheat canopy cover, and surface temperature. Results from FISE05 also showed that the value of this particular implementation of ET remote sensing was limited by inadequate temporal sampling, seasonal dependence of vegetation density estimators, and uncertain parameterization for the late-season. In each case, modeled and projected daily ET estimate errors could exceed 1 mm d-1. These ET/energy balance results can be improved, however, by revising the vegetation density estimators and by combining episodic high-spatial-resolution image data with continuous high-temporal-resolution ground data.

Multispectral data can meet many of the information requirements of site-specific farming. Examples from the literature are presented where multispectral data has been applied to agricultural management problems. Some of the examples are illustrated using remotely sensed estimates of green leaf area index for a cotton field during the 1994 growing season.

In order to determine the future consequences of the increasing atmospheric C02 concentration on vegetation, experimental plants have been exposed to controlled C02 concentrations in open-top chambers (OTCs) and in open fields using the recently developed free-air C02 enrichment (FACE) approach. The environment inside open-top chambkrs approaches that of the field outside, but generally it is warmer, more humid, shaded, and has altered air movement. These plus other environmental differences have caused plants to grow differently than outside, as shown by a review of experiments that reported such chamber effects. The 95% confidence interval for ratios of biomass accumulation inside to outside ranged from 0.70 to 1.73. Moreover, there was a significant average bias for about 10% better growth inside. Thus, absolute growth can not be determined with a high degree of confidence using open-top chambers. Recent cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) experiments in which both OTCs and FACE were used showed that the relative growth responses to elevated C02 were not significantly different between OTCs and FACE, but the absolute growth of the cotton was 30% greater inside OTCs, while that of the wheat was about the same for both OTCs and FACE. Thus, for many studies the FACE approach is prefened because both absolute and relative responses to elevated C02 can be obtained reliably. Furthermore, the large plot size suppo& multidisciplinary teams with much destructive sampling, and there is an economy of scale. Yet, OTCs remain a workable alternative in some experiments that appear technically difficult or too expensive with FACE. Copyright 8 1997. ASA, CSSA, and SSSA, 677 S. Segoe Rd., Madison, WI 53711, USA. Advances in Carbon Dioxide EfJeca Research. ASA Special Publication no. 61. 114 KIMBALL lh AL. ' The increasing C02 concentration of the atmosphere has led to major research efforts to determine what effects it will have on the vegetation of the Earth in the future (e.g., Dahlman et al., 1985). This research generally has involved exposing plants to elevated (or subambient) levels of C02 for various lengths of time and observing the responses of the plants. The techniques used for exposing plants to elevated C02 were reviewed by Drake et al. (1985), Lawlor and Mitchell (1991), and Allen et al. (1993); and recently the proceedings of a symposium on "Design and Execution of Experiments on C02 Enrichment" was edited by Schulze and Mooney (1993). For some studies the objective is to determine what effect varying C02 will have at specified levels of other variables, such as temperature. For such studies, controlled-environment chambers appear most suitable. Often however, the objective is to determine the effects of C02 on plant growth under conditions representative of future fields. These latter studies will be the subject of this chapter. Because the natural wind rapidly disperses any C02 released in an open field, researchers have frequently used a transparent wind barrier in the form of an open-top chamber (OTC) around their experimental plants in order to confine the C02 while transmitting most of the solar radiation. Such chambers prevent natural wind flow and also alter the environment from being the same as the surrounding field in several other ways. Until recently, no one had controlled the C02 concentration in an open field with any degree of precision. In 1993, however, Hendrey (1993) and colleagues reported that they had successfully developed free-air C02 enrichment (FACE) technology for controlling C02 concentrations under field conditions. They presented results with an initial cotton crop. Since that first study, we have conducted two more FACE experiments on cotton (Dugas & Pinter, 1994) and one on wheat (Pinter et al., 1993; Kimball et al., 1993b). Two questions arise when considering results from OTCs and FACE systems: (i) do plants grow the same in OTCs as they do in an open field, and (ii) even if they do not grow the same in an absolute sense, do they at least respond relatively the same to elevated C02 compared with control plants. Answering these two questions, using data from the literature as well as from the recent FACE cotton and wheat experiments, is the primary goal of this chapter. COMPARISON OF OPEN-TOP CHAMBER AND OPEN FIELD ENVIRONMENTS The environment inside an OTC can differ from that of the field outside in several ways (Tables 5-1 and 5-2), as reviewed previously by, for example, Heagle et al. (1988). As already mentioned, the transparent OTC walls are constructed in order to create a wind barrier and thereby make it easier to control the concentration of C02 (or other gas of interest) inside. Of course, an assumption is being made that the effects of air movement on plant growth are less important than those of C02 (or other gas) or at least that meaningful relative comparisons can be made between plants grown in treatment chambers and those from suitable control chambers; however, air movement itself can affect plant growth VEGETATION RESPONSE TO ELEVATED CARBON DIOXIDE 115 Table 5-1. Observations of open-top chamber conditions. Observers Range of Observations Air movement Weinstock et al., 1982 Drake et al., 1989 Ham & Owensbyt Solar radiation Heagle et al., 1979 Olszyk et al., 1980 Heagle & Letchworth, 1982 Weinstock et al., 1982 Kats et al., 1985 Olszyk et al., 1986b Unsworth, 1986 Drake et al., 1989 Sanders et al., 1991 Olszyk et a]., 1992 Fuhrer, 1993 Long-wave radiation Unsworth, 1986 Air temperature Heagle et al., 1979 Olszyk et al., 1980 Weinstock et al., 1982 Olszyk et al., 1986b Drake et al., 1989 Sanders et al., 1991 Nie et al., 1992 Olszyk et al., 1992 Fuhrer, 1993 Foliage temperature Weinstock et al., 1982 Kimball et al., 1983 Kats et al., 1985 Olszyk et al., 1986b Drake et al., 1989 Olszy k et al., 1992 Ham, 1993$ Air vapor pressure Weinstock et al., 1982 Olszyk et al., 1992 Transpiration Olszyk et al., 1980 Kimball et al., 1985 Dunin & Greenwood, 1986 Bremer & Ham, 19935 Reduced to 0.38 to 0.21 of outside Constant inside but variable outside Constant at 0.75 m s-', while outside winds twice as high in daytime and half as high at night Turbulent intensity 0.05 inside and 0.20 outside, both day and night 0.7S1.01 of outside 0.77-1.00 of outside 0.80-0.99 of outside 0.82-1.03 of outside 0.90 of outside 0.71 of outside 0.93-0.97 of outside 0.90 of outside 0.81 of outside 0.81 of outside 0.77-0.79 of outside 150 W m-2 greater than outside, which is typically 350 W m-2 0.6-0.9OC warmer than outside 0.0-0.8"C warmer than outside 0.4-3.7OC warmer than outside 0.0-2.0°C warmer than outside 1.9-2.7"C warmer than outside OA°C warmer than outside 1.4-2.7OC warmer than outside 0.9OC warmer than outside 1.3-3.6"C warmer than outside -1.1 to +3.1°C warmer than outside O.l°C warmer than outside 2B°C warmer than outside 0.0 to l.O°C warmer than outside lS°C warmer than outside 2.1°C warmer than outside cooler than outside when have ample ventilation, low vapor pressure, and high outside foliage temperatures 0-14% higher absolute vapor pressure than outside 0% higher absolute vapor pressure than outside 0.86 of outside using black atmometers 0.89 of outside in well-watered plots using small pans 0.96 of outside in dry plots using small pans no difference between lysimeter inside and Bowen ratio outside 0.80 of outside for ~orghastrurn nutans using sap flow gauges 0.82 of outside similarly for Andromgon gerardi

Atmospheric CO 2 concentration (C a ) continues to rise. An imperative exists, therefore, to elucidate the interactive effects of elevated C a and drought on plant water relations of wheat (Triticum aestivum L.). A spring wheat (cv. Yecora Rojo) crop was exposed to ambient (Control: 370 μmol mol -1 ) and free-air CO 2 enrichment (FACE: ambient + 180 μmol mol -1 ) under ample (Wet), and reduced (Dry), water supplies (100 and 50% replacement of evapotranspiration, respectively) over a 2-yr study. Our objective was to characterize and quantify the responses of 26 edaphic, gas exchange, water relations, carbohydrate pool dynamics, growth, and development parameters to rising C a and drought. Increasing C a minimized the deleterious effects of soil-water depletion by increasing drought avoidance (i.e., lower stomatal conductance and transpiration rate, and growth and development of a more robust root system) and drought tolerance (i.e., enhanced osmoregulation and adaptation of tissue) mechanisms, resulting in a 30% reduction in water stress-induced midaftemoon depressions in net assimilation rate. An elevated C a -based increase in daily and seasonal carbon gain resulted in a positive feedback between source capacity (shoots) and sink demand (roots). Devoid of a concomitant rise in global temperature resulting from the rise in C a , improved water relations for a herbaceous, cool-season, annual, C 3 cereal monocot grass (i.e., wheat) are anticipated in a future high-CO 2 world. These findings are applicable to other graminaceous species of a similar function-type as wheat common to temperate zone grassland prairies and savannas, especially under dryland conditions.

Shadows are being used more frequently to estimate plant canopy biophysical characteristics. Typically, a zero value is assumed or a threshold value is derived from histogram analysis of imagery to determine the shadow endmember (EM). Here, two distinct shadow EMs were measured in situ for use in spectral mixture analysis of a cotton canopy on five dates in 2003. The four EMs used in the analysis were: sunlit green leaf, sunlit dry soil, self-shadowed leaf, shadowed dry soil. This 4-EM model was compared to a 3-EM model where a zero-value shade EM was used for unmixing with the two sunlit EMs. Multiple endmember spectral mixture analysis (MESMA) was used to allow EM composition to vary across each scene. The analysis and EMs were applied to fine-scale hyperspectral image data collected in the wavelength range, 440 to 810 nm. Ground data collected included percent cover, height, SPAD (a measure of leaf greenness), and chlorophyll a concentration. The normalized difference vegetation index (NDVI) was also compared to the unmixing results. Regression analysis showed that NDVI was equal to the 4-EM model for estimation of percent cover ( r 2 = 0.95, RMSE = 6.6) although the NDVI y -intercept was closer to zero. The 4-EM model was best for estimating height ( r 2 = 0.79, RMSE = 0.07 m) and chlorophyll a concentration ( r 2 = 0.46, RMSE = 7.0 μg/cm 2 ). The 3-EM model and NDVI performed poorly when estimating chlorophyll a concentration. Inclusion of two distinct shadow EMs in the model improved relationships to crop biophysical parameters and was better than assuming one, zero-value shade EM. Since MESMA operates at the pixel level and allows variable EM assignment to each pixel, mapping the spatial variability of shadows and other variables of interest is possible, providing a powerful input to canopy and ecosystem models as well as precision farming.

Crop coefficient methodologies are widely used to estimate actual crop evapotranspiration (ETc) for determining irrigation scheduling. Generalized crop coefficient curves presented in the literature are limited to providing estimates of ETc for “optimum” crop condition within a field, which often need to be modified for local conditions and cultural practices, as well as adjusted for the variations from normal crop and weather conditions that might occur during a given growing season. Consequently, the uncertainties associated with generalized crop coefficients can result in ETc estimates that are significantly different from actual ETc, which could ultimately contribute to poor irrigation water management. Some important crop properties such as percent cover and leaf area index have been modeled with various vegetation indices (VIs), providing a means to quantify real-time crop variations from remotely-sensed VI observations. Limited research has also shown that VIs can be used to estimate the basal crop coefficient (K cb) for several crops, including corn and cotton. The objective of this research was to develop a model for estimating K cb values from observations of the normalized difference vegetation index (NDVI) for spring wheat. The K cb data were derived from back-calculations of the FAO-56 dual crop coefficient procedures using field data obtained during two wheat experiments conducted during 1993–1994 and 1995–1996 in Maricopa, Arizona. The performance of the K cb model for estimating ETc was evaluated using data from a third wheat experiment in 1996–1997, also in Maricopa, Arizona. The K cb was modeled as a function of a normalized quantity for NDVI, using a third-order polynomial regression relationship (r 2=0.90, n=232). The estimated seasonal ETc for the 1996–1997 season agreed to within −33 mm (−5%) to 18 mm (3%) of measured ETc. However, the mean absolute percent difference between the estimated and measured daily ETc varied from 9% to 10%, which was similar to the 10% variation for K cb that was unexplained by NDVI. The preliminary evaluation suggests that remotely-sensed NDVI observations could provide real-time K cb estimates for determining the actual wheat ETc during the growing season.

Multispectral vegetation indices calculated from canopy reflectance measurements have been used to simulate real-time basal crop coefficients (Kcb), which have been validated to improve evapotranspiration (ETc) estimation for several crops. In this article, an application of the approach was evaluated for cotton using remote sensing observations of the normalized difference vegetation index (NDVI) to estimate Kcb as a function of NDVI. The dual crop coefficient procedures of FAO Paper 56 (FAO-56) were used to calculate ETc and determine irrigation scheduling using Kcb estimates from remote sensing (NDVI treatment) as well as from time-based Kcb curves (FAO treatment), which were developed locally for standard crop conditions using FAO-56 procedures. Two cotton experiments, conducted in 2002 and 2003 in central Arizona, included sub-treatments of three levels of plant density and two levels of nitrogen management to impose a wide range of crop development and water use. The NDVI-Kcb relationships used for 2002, developed from previous data for a different cotton cultivar, row orientation, and soil type, substantially underestimated ETc, resulting in significantly less irrigation water applied and lower lint yields for NDVI compared to the FAO treatment. The 2002 data were used to recalibrate the NDVI-Kcb relationships, which were then used for the NDVI treatments in 2003. The FAO Kcb curve used in 2002 described ETc and irrigation scheduling reasonably well for sparse plots, but consistently underestimated water use and soil water depletion for the higher plant densities during the first half of the season. Consequently, an adjusted FAO Kcb curve, based on 2002 results, was employed for the FAO treatment in 2003. For the 2003 experiment, estimated cotton ETc for the NDVI treatment resulted in a mean absolute error of 9% compared to 10% for the FAO treatment, where the difference was not significant between treatments. However, the NDVI-Kcb relations used in 2003 greatly improved estimates for ETc compared to the previous year, where the mean absolute error for the NDVI treatment in 2002 was 22%. Predicted ETc using the FAO Kcb curve of 2003 for typical planting density and high nitrogen conditions resulted in a mean absolute error of 10% compared to 15% in 2002. Final lint yields for 2003 were not significantly different between the two Kcb methods. Although additional research is needed to validate remote sensing Kcb estimation for other conditions than those in these experiments, this study did not show significant advantages for the NDVI approach over a carefully derived single FAO Kcb application. However, the NDVI approach has the potential to further extend our present crop coefficient estimation capabilities when weather, plant density, or other factors cause cotton canopy development and water use patterns to depart from typical conditions.

Increasing atmospheric CO 2 concentrations are expected to increase plant production and demand for N and other nutrients. The objectives of this investigation were to characterize and quantify the temporal trends in soil mineral N and aboveground biomass N during the growing season of wheat (Triticum aestivum L.) with adequate N, ambient and elevated CO 2 , and two levels of water stress. The free-air CO 2 enrichment (FACE) technique was used to enrich the air from 370 to 550 μmol mol -1 CO 2 . Spring wheat was planted in late December of 1992 and 1993 and harvested at the end of May. Each main plot (CO 2 level) was split into two irrigation treatments to replace 100 and 50% of the potential evapotranspiration. Soil and plant samples were taken for N analysis six times each year. Elevated CO 2 lowered soil mineral N concentrations in the top 0.3 m of soil as much as 40% and increased aboveground biomass N by as much as 16% compared with the ambient treatment. Before anthesis, irrigation level had little effect on either soil mineral N or aboveground biomass N, but at harvest in 1992-1993 and at dough stage in 1993-1994 deficit-irrigated plots had higher soil mineral N (p < 0.05) and lower aboveground biomass N than plots that received adequate irrigation. There was little variation in the concentrations of N in the aboveground biomass among treatments within a sampling date. The data suggest elevated CO 2 may lead to rapid N uptake, which could result in increased early vegetative growth.

Elevated concentrations of atmospheric CO 2 (C a ) are believed to raise sorghum [Sorghum bicolor (L.) Moench] productivity by improving water relations. In ecosys, water relations are simulated by solving for the canopy water potential (ψ c ) at which water uptake from a model of soil-root-canopy water transfer equilibrates with transpiration from the canopy energy balance. Simulated water relations were tested with ψ c , water uptake, and energy exchange measured under ambient (363 μmol mol -1 ) and elevated (566 μmol mol -1 ) C a and high vs. low irrigation in a free air CO 2 enrichment experiment during 1998 and 1999. Model results, corroborated by field measurements, showed that elevated C, raised ψ c and lowered latent heat fluxes under high irrigation and delayed water stress under low irrigation. Changes in ψ c modeled under ambient vs. elevated C a varied diurnally, with lower ψ c causing earlier midafternoon stomatal closure under ambient C a . Modeled changes in sorghum water status caused elevated C a to raise seasonal water efficiency under high and low irrigation by 20 and 26% (vs. 20 and 13% measured) in 1998 and by 9 and 27% (vs. 6 and 26% measured) in 1999. Ecosys was used to generate an irrigation response function for sorghum yield, which indicated that yields would rise by 13% for a range of irrigation rates if air temperatures were to rise by 3°C and C a by 50%. Current high sorghum yields could be achieved with 120 mm or 20% less irrigation water if these rises in temperature and C a were to occur.

Increasing atmospheric carbon dioxide (CO 2 ) 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.) Moench, a C 4 grain crop) using a residual energy balance approach. During the summer and autumn of 1998 and 1999, sorghum was grown under free-air CO 2 enrichment (FACE) conditions near Maricopa, Arizona. Latent heat flux ( λ ET) was determined by subtracting soil heat flux ( G 0 ) and sensible heat flux ( H ) from net radiation ( R n ) values in both Control CO 2 plots (about 370 μmol mol −1 ) and FACE plots (Control + 200 μmol mol −1 ). R n was observed using net radiometers. G 0 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 λ ET by 13.8±1.8% in 1998, and 11.8±1.9% in 1999. Drought-stress resulted in a reduction in λ ET of 8.5±3.7% for the FACE treatments in 1998, but an increase in λ ET 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 CO 2 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 CO 2 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.

The cropping systems simulation model APSIM-Nwheat was tested against detailed field measurements representing possible growing conditions under future climate change scenarios. Increasing average temperatures by 1.7 °C observed over several seasons at Obregon, Mexico reduced the time to flowering by 11 days and resulted in a decline of total biomass and grain yield. These effects were reproduced by the model, except when the observed total biomass inexplicably rose again in the fourth and fifth year, despite higher temperature and a much shorter growing time. In a water stress experiment, the effects of different timing and duration of water deficit on crop growth and yield were reproduced with the model for a rain-shelter experiment at Lincoln, New Zealand where observed grain yields were reduced from 10 to 4 t ha −1 due to increased water deficit. In experiments from Western Australia, reduced growth and yields due to extreme terminal water deficit were also reproduced with the model where measured yields fall below 0.5 t ha −1 . In the Maricopa Free Air Carbon-Dioxide Enrichment (FACE) experiment in Arizona, USA, the largest yield increase occurred with elevated CO 2 in the dry and high N treatments, whereas little or no response was observed in the wet and low N supply treatments, as simulated with the model. Combining elevated CO 2 with increased temperature in a sensitivity analysis, two levels of water supply and a range of N applications indicated a positive effect of elevated CO 2 on yield as long as N was not limiting growth. Increased temperature and reduced water supply reduced yields and the yield response to N supply under ambient and elevated CO 2 . Grain protein concentrations were reduced under elevated CO 2 , but the difference was minor with ample N fertiliser. Evapotranspiration was reduced under elevated CO 2 . Higher temperatures increased evapotranspiration with low N input, but reduced it with ample N fertiliser, resulting in a reduction and an increase, respectively, in drainage below the root zone. In the Mediterranean environment of Western Australia the impact of elevated CO 2 and increased temperature on grain yield was in average positive, but varied with seasonal rainfall distribution. Based on the range of model testing experiments and the sensitivity analysis, APSIM-Nwheat was found suitable for studies on directional impacts of future climate change on wheat production. Due to some large discrepancies between simulated and observed data, field experiments representing only a limited range of possible climate change scenarios and the large possible range of factorial interactions not tested, simulated quantitative effects with the model should be interpreted cautiously.

Crop coefficients are a widely used and universally accepted method for estimating the crop evapotranspiration (ETc) component in irrigation scheduling programs. However, uncertainties of generalized basal crop coefficient (Kcb) curves can contribute to ETc estimates that are substantially different from actual ETc. Limited research with corn has shown improvements to irrigation scheduling due to better water-use estimation and more appropriate timing of irrigations when Kcb estimates derived from remotely sensed multispectral vegetation indices (VIs) were incorporated into irrigation-scheduling algorithms. The purpose of this article was to develop and evaluate a Kcb estimation model based on observations of the normalized difference vegetation index (NDVI) for a full-season cotton grown in the desert southwestern USA. The Kcb data used in developing the relationship with NDVI were derived from back-calculations of the FAO-56 dual crop coefficient procedures using field data obtained during two cotton experiments conducted during 1990 and 1991 at a site in central Arizona. The estimation model consisted of two regression relations: a linear function of Kcb versus NDVI (r2=0.97, n=68) used to estimate Kcb from early vegetative growth to effective full cover, and a multiple regression of Kcb as a function of NDVI and cumulative growing-degree-days (GDD) (r2=0.82, n=64) used to estimate Kcb after effective full cover was attained. The NDVI for cotton at effective full cover was ~0.80; this value was used to mark the point at which the model transferred from the linear to the multiple regression function. An initial evaluation of the performance of the model was made by incorporating Kcb estimates, based on NDVI measurements and the developed regression functions, within the FAO-56 dual procedures and comparing the estimated ETc with field observations from two cotton plots collected during an experiment in central Arizona in 1998. Preliminary results indicate that the ETc based on the NDVI-Kcb model provided close estimates of actual ETc.

The developmental pattern of C4 expression has been well characterized in maize and other C4 plants. However, few reports have explored the possibility that the development of this pathway may be sensitive to changes in atmospheric CO2 concentrations. Therefore, both the structural and biochemical development of leaf tissue in the fifth leaf of Sorghum bicolor plants grown at elevated CO2 have been characterized. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities accumulate rapidly as the leaf tissue differentiates and emerges from the surrounding whorl. Rubisco was not expressed in a cell-specific manner in the youngest tissue at the base of the leaf, but did accumulate before PEPC was detected. This suggests that the youngest leaf tissue utilizes a C3-like pathway for carbon fixation. However, this tissue was in a region of the leaf receiving very low light and so significant rates of photosynthesis were not likely. Older leaf tissue that had emerged from the surrounding whorl into full sunlight showed the normal C4 syndrome. Elevated CO2 had no effect on the cell-specific localization of Rubisco or PEPC at any stage of leaf development, and the relative ratios of Rubisco to PEPC remained constant during leaf development. However, in the oldest tissue at the tip of the leaf, the total activities of Rubisco and PEPC were decreased under elevated CO2 implying that C4 photosynthetic tissue may acclimate to growth under elevated CO2.

This review addresses the challenges and progress in sensor development and radiometric correction for agricultural applications with particular emphasis on activities within the U.S. Department of Agriculture (USDA) Agricultural Research Service (ARS). Examples of sensor development include on-site development of sensors and platforms, participation in cooperative research and development agreements (CRADA) with commercial companies, and membership on NASA science teams. Examples of progress made in sensor radiometric correction suitable for agriculture are presented for both laboratory and field environments. The direction of future sensor development includes integrated sensors and systems, sensor standardization, and new sensor technologies measuring fluorescence and soil electrical conductivity, and utilizing Light Detection and Ranging (lidar), hyperspectral, and multiband thermal wavelengths. The upcoming challenges include definition of the core spectral regions for agriculture and the sensor specifications for a dedicated, orbiting agricultural sensor, determination of an operational approach for reflectance and temperature retrieval, and enhanced communication between image providers, research scientists, and users. This review concludes with a number of avenues through which USDA could promote sensor development and radiometric correction for agricultural applications. These include developing a network of large permanent calibration targets at USDA ARS locations; investing in new technologies ; pooling resources to support large-scale field experiments: determining ARS-wide standards for sensor development, calibration, and deployment; and funding interagency agreements to achieve common goals.

Interagency programs have made it possible for scientists from the USDA, Agricultural Research Service (ARS) to conduct remote sensing research with researchers from various federal, state, and private organizations since 1965. Cumulative results from these cooperative studies have established the scientific basis for using remotely acquired imagery to better understand and manage the Earth’s agricultural and natural resources. Examples of these partnerships include the 1965 Statement of Agreement between USDA and NASA that formally began ARS’s remote sensing research, the Large Area Crop Inventory Experiment (LACIE, 1974), the ARS Wheat Yield Project (1976), Agriculture and Resources Inventory Surveys Through Aerospace Remote Sensing (AgRISTARS, 1980), the China Wheat Project (1983), and AG 20/20 (1999). This paper provides a brief overview of these collaborations.

with the Agricultural Research Service (ARS) and various government agencies and private institutions have provided a great deal of fundamental information relating spectral reflectance and thermal emittance properties of soils and crops to their agronomic and biophysical characteristics. This knowledge has facilitated the development and use of various remote sensing methods for non-destructive monitor- ing of plant growth and development and for the detection of many environmental stresses which limit plant productivity. Coupled with rapid advances in computing and position- locating technologies, remote sensing from ground-, air-, and space-based platforms is now capable of providing detailed spatial and temporal information on plant response to their local environment that is needed for site specific agricultural management approaches. This manuscript, which empha- sizes contributions by ARS researchers, reviews the biophysi- cal basis of remote sensing; examines approaches that have been developed, refined, and tested for management of water, nutrients, and pests in agricultural crops; and as- sesses the role of remote sensing in yield prediction. It con- cludes with a discussion of challenges facing remote sens- ing in the future.

We evaluated the influences of CO2[Control, ∼ 370 µmol mol−1; 200 µmol mol−1 above ambient applied by free-air CO2 enrichment (FACE)] and soil water (Wet, Dry) on above- and below-ground responses of C3 (cotton, Gossypium hirsutum) and C4 (sorghum, Sorghum bicolor) plants in monocultures and two density mixtures. In monocultures, CO2 enrichment increased height, leaf area, above-ground biomass and reproductive output of cotton, but not sorghum, and was independent of soil water treatment. In mixtures, cotton, but not sorghum, above-ground biomass and height were generally reduced compared to monocultures, across both CO2 and soil water treatments. Density did not affect individual plant responses of either cotton or sorghum across the other treatments. Total (cotton + sorghum) leaf area and above-ground biomass in low-density mixtures were similar between CO2 treatments, but increased by 17–21% with FACE in high-density mixtures, due to a 121% enhancement of cotton leaf area and a 276% increase in biomass under the FACE treatment. Total root biomass in the upper 1.2 m of the soil was not influenced by CO2 or by soil water in monoculture or mixtures; however, under dry conditions we observed significantly more roots at lower soil depths (> 45 cm). Sorghum roots comprised 81–85% of the total roots in the low-density mixture and 58–73% in the high-density mixture. CO2-enrichment partly offset negative effects of interspecific competition on cotton in both low- and high-density mixtures by increasing above-ground biomass, with a greater relative increase in the high-density mixture. As a consequence, CO2-enrichment increased total above-ground yield of the mixture at high density. Individual plant responses to CO2 enrichment in global change models that evaluate mixed plant communities should be adjusted to incorporate feedbacks for interspecific competition. Future field studies in natural ecosystems should address the role that a CO2-mediated increase in C3 growth may have on subsequent vegetation change.

The present study was carried out to test the hypothesis that elevated atmospheric CO 2 (Ca) will alleviate over-excitation of the C 4 photosynthetic apparatus and decrease non-photochemical quenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescence was monitored in Sorghum bicolor plants grown under a freeair carbon-dioxide enrichment (FACE) by water-stress (Dry) experiment. Under Dry conditions elevated Ca increased the quantum yield of photosystem II ( f PSII) throughout the day through increases in both photochemical quenching coefficient ( q p ) and the efficiency with which absorbed quanta are transferred to open PSII reaction centres ( F v ¢¢¢ ¢ / F m ¢¢ ). However, in the well-watered plants (Wets) FACE enhanced f PSII only at midday and was entirely attributed to changes in F v ¢¢¢ ¢ / F m ¢¢ . Under field conditions, decreases in f f f PSII under Dry treatments and ambient Ca corresponded to increases in NPQ but the de-epoxidation state of the xanthophyll pool (DPS) showed no effects. Water-stress did not lead to long-term damage to the photosynthetic apparatus as indicated by f PSII and carbon assimilation measured after removal of stress conditions. We conclude that elevated Ca enhances photochemical light energy usage in C 4 photosynthesis during drought and/ or midday conditions. Additionally, NPQ protects against photo-inhibition and photodamage. However, NPQ and the xanthophyll cycle were affected differently by elevated Ca and water-stress.

The rationale for this study is found in the probable higher temperatures and changes in rainfall patterns that are expected in the future as a result of increasing levels of CO2 in the atmosphere. In particular, higher air temperatures may cause an increase in evapotranspiration demand while a reduction in rainfall could increase the severity and duration of drought in arid and semi-arid regions. Representation of the water transfer scheme includes water uptake by roots and the interaction between evapotranspiration and CO2 enrichment. The predicted response of a spring wheat (Triticum aestivum L. cv. Yecora rojo) canopy in terms of energy exchange processes to elevated atmospheric CO2 level was tested against measurements collected at the FACE (Free Air Enrichment Experiment) site in 1994. Simulated and measured canopy conductances were reduced by about 30% under elevated [CO2] under optimum conditions of water supply. Reductions in latent heat fluxes under elevated instead of ambient [CO2] caused reductions in both simulated and measured seasonal water use of 6% under optimum and 2% under suboptimum irrigation. The soil-plant-atmosphere water transfer scheme proposed here offers several advances in the simulation of land surface interactions. First, the stomatal resistance model minimizes assumptions in existing land surface schemes about the effects of interactions among environmental conditions (radiation, temperature, CO2) upon stomatal behavior. These interactions are resolved in the calculation of CO2 in which processes are already well understood.

○ 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.

Summary • Sorghum (Sorghum bicolor) was grown for two consecutive seasons at Maricopa, AZ, USA, using the free-air CO2 enrichment (FACE) approach to investigate evapotranspiration of this C4 plant at ample and limited water supplies. • Crop evapotranspiration (ET) was measured using two CO2 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 CO2 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 CO2 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-CO2 environment, water requirements for irrigated sorghum will be lower than at present, while dry-land productivity will increase, provided global warming is minimal.

Summary • To determine the response of C4 plants to elevated CO2 it is necessary to establish whether young leaves have a fully developed C4 photosynthetic apparatus, and whether photosynthesis in these leaves is responsive to elevated CO2. • The effect of free-air CO2 enrichment (FACE) on the photosynthetic development of the C4 crop Sorghum bicolor was monitored. Simultaneous measurements of chlorophyll a fluorescence and carbon assimilation were made to determine energy utilization, quantum yields of carbon fixation (φCO2) and photosystem II (φPSII), as well as photorespiration. • Assimilation in the second leaf of FACE plants was 37% higher than in control plants and lower apparent rates of photorespiration at growth CO2 concentrations were exhibited. In these leaves, φPSII : φCO2 was high at low atmospheric CO2 concentration (Ca) due to overcycling of the C4 pump and increased leakiness. As Ca increased, φPSII : φCO2 decreased as a greater proportion of energy derived from linear electron transfer was used by the C3 cycle. • The stimulation of C4 photosynthesis at elevated Ca in young leaves was partially due to suppressed photorespiration. Additionally, elevated Ca enhanced energy-use efficiency in young leaves, possibly by decreasing CO2 leakage from bundle sheath cells, and by decreasing overcycling of the C4 pump.

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 • Atmospheric CO2 concentration is expected to increase by 50% near the middle of this century. The effects the free air CO2 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 CO2 mole fractions were 561 and 368 µmol 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 CO2 continues to increase, total sorghum yield is likely to be higher in the future in areas where water is limited.

Summary • Stable-carbon isotopic tracers were enlisted in 1996 and 1997 wheat (Triticum aestivum) free-air CO2 enrichment (FACE) experiments to detect entry of new C into soil organic carbon (SOC) pools. Any enhanced soil inputs might mitigate rising atmospheric CO2. • The CO2 used to enrich FACE plots (to ambient +190 µmol mol−1) resulted in 13C-depleted wheat relative to ambient plants and the native SOC. To trace new C in control plots C4-plant-derived exotic soils were placed into subplots in high-N FACE and control treatments, and a 13CO2 gas tracer was pulsed to subplots in high-N control replicates. • Under high-N, isotopic mass balance showed 6% (P = 0.003) and 5% (P = 0.04) new C in 0–15-cm and 15–30-cm FACE SOC, respectively, after 2 yr. Results from the C4-soil subplots were ambiguous, but the 13CO2 tracer induced a SOC δ13C increase (P = 0.08) at 15–30 cm in control-high N consistent with 6% new C. • We infer c. 3% year−1 (30–40 g C m−2 yr−1) SOC turnover in surface soils at high-N under both ambient and elevated CO2. The 13CO2-tracer result, however, is less reliable because of lower significance, fewer replicates and heterogeneous isotopic distribution within plants.

Changes in mass and energy exchange by crops under rising atmospheric CO 2 concentration (C a ) may be affected by N and weather; C, interacts with weather on mass and energy exchange through limitations on latent heat flux imposed by stomatal conductance, which is affected by C a , and aerodynamic conductance, which is affected by weather. We examined the bases for these interactions with the ecosystem model ecosys. Simulation results were tested with energy flux data from a Free Air CO 2 Enrichment (FACE) experiment in which wheat (Triticum aestivum L.) was grown under 548 vs. 363 μmol mol -1 C, and fertilized with 7 vs. 35 g N m -2 . Both model and experimental results indicated that raising C a from 363 to 548 μmol mol -1 reduced midday latent heat fluxes by ca. 50 W m -2 for wheat fertilized with 35 g N m -2 , and by ca. 100 W m -2 for wheat fertilized with only 7 g N m -2 when N deficits developed later in the growing season. These reductions were smaller under low wind speeds (

Summary • Sorghum bicolor was exposed to free-air CO2 enrichment (FACE) and drought at the Maricopa Agricultural Center, AZ, USA, in summer 1998. We predicted that bundle sheath leakiness (Φ) would be insensitive to FACE under well-irrigated (wet) conditions, but would be lower in FACE compared with control-CO2 treatments when irrigation was withheld (dry). • Leaf and air δ13C values and leaf pi/pa from gas exchange were measured to estimate carbon isotope discrimination (Δ) and F. Midday leaf water potential (Ψ) and photosynthetic rate were simultaneously measured to evaluate the influence of plant water status on Φ and the association between Φ and carbon gain. • Irrigation treatments affected Ψ, pi/pa, Δ and Φ in control CO2 and FACE rings. Differences in leaf Δ between wet- and dry-treatment plots resulted from changes in Φ and to stomatal influences on pi/pa. FACE had very little effect on Ψ, Δ and Φ in wet-treatment plots. However, Φ and Δ in dry plots were higher in control than in FACE rings. • FACE ameliorated the effects of drought on bundle sheath leakiness and Δ by reducing transpiration, prolonging soil water availability and enhancing plant water status. Direct effects of CO2 enrichment on C4 photosynthetic metabolism in Sorghum apparently are minimal and indirect effects depend on soil water supply.

While soil biota and processes are becoming increasingly appreciated as important parameters for consideration in global change studies, the fundamental characteristic of soil structure is a neglected area of research. In a sorghum [Sorghum bicolor (L.) Moench] field experiment in which CO2 [supplied using free-air CO2 enrichment (FACE) technology] was crossed factorially with an irrigation treatment, soil aggregate (1‐2 mm) water stability increased in response to elevated CO2. Aggregate water stability was increased by 40% and 20% in response to CO2, at ample and limited water supply treatments, respectively. Soil hyphal lengths of arbuscular mycorrhizal fungi (AMF) increased strongly (with a threefold increase in the dry treatment) in response to CO2, and the concentrations of one fraction (easily extractable glomalin, EEG) of the AMF-produced protein glomalin were also increased. Two fractions of glomalin, and AMF hyphal lengths were all positively correlated with soil aggregate water stability. The present results further support the hypothesis that AMF can become important in global change scenarios. Although in this field study a causal relationship between hyphal length, glomalin and aggregate stability cannot be demonstrated, the present data do suggest that AMF could mediate changes in soil structure under elevated CO2. This could be of great importance in agricultural systems threatened by erosional soil loss.

Summary Understanding the response of soil carbon (C) dynamics to higher atmospheric CO 2 concentrations is critical for evaluating the potential for soil C sequestration on time scales of decades to centuries. Here, we report on changes in soil respiration under Free-Air CO 2 Enrichment (FACE) where spring wheat was grown in an open field at two CO 2 concentrations (ambient and ambient+200 μmol mol −1 ), under natural meteorological conditions. FACE increased soil respiration rates by 40—70% during the peak of wheat growth. On the FACE plots, stable C isotopic composition of soil CO 2 was used to partition the soil CO 2 flux into C from rhizosphere respiration and decomposition of pre-existing C. Decomposition contributed 100% of the soil CO 2 flux before crop growth commenced, and only 35—45% of the flux at the peak of the growing season. Decomposition rates were not correlated with soil temperature, but rhizosphere respiration rates were strongly correlated with green leaf area index. Ein Verstandnis der Antwort der Kohlenstoff-Dynamik (C) im Boden auf hohere CO 2 -Konzentrationen in der Atmosphare ist bedeutsam fur die Bewertung des Potentials fur die C-Sequestration in Zeitraumen von Jahrzehnten bis Jahrhunderten. Hier berichten wir uber Veranderungen in der Bodenatmung unter Free-Air CO 2 Enrichment (FACE), bei dem Sommerweizen in einem offenen Feld unter zwei CO 2 -Konzentrationen (Umgebung und Umgebung + 200 (mol mol −1 ) und unter naturlichen meteorologischen Bedingungen angebaut wurde. FACE erhohte die Bodenatmungsraten um 40—70% wahrend des Maximums des Weizenwachstums. Auf den FACE Plots wurde die Zusammensetzung an stabilen C Isotopen des Boden-CO 2 genutzt, um den Boden CO 2 -Fluss zu C durch Rhizospharen-Atmung von der Zersetzung von zuvor existierendem C zu trennen. Die Zersetzung trug 100% des Boden-CO 2 -Flusses vor dem Beginn des Weizenwachstums bei, und nur 35—45% des Flusses wahrend des Maximums des Wachstums. Die Zersetzungsraten waren nicht mit der Bodentemperatur korreliert, aber die Rhizospharen-Atmungsraten waren eng korreliert mit dem grunen Blattflachen-Index.

Crop simulation models are an essential tool for testing whether predicted global atmospheric changes are likely to have impact on food production. Any confidence in model predictions must be based on their ability successfully to predict performance in experiments. Accordingly, the predictions of three daily time step wheat simulation models (AFRCWHEAT2, FASSET and Sirius) were tested against data from wheat (Triticum aestivum L.) experiments in AZ in which the amount of applied N and the atmospheric CO2 concentration were both varied. Although there were differences between predicted and observed yields, all the three models predicted yield trends with treatments very similar to those observed. They all predicted, both in absolute terms and in the magnitude of responses, very similar effects of the variations on green area index (GAI), shoot and grain biomass accumulation, and shoot and grain biomass yield to observations and to each other. Comparison of simulated and observed results showed that CO2 effects were expressed through effects on light use efficiency (LUE), whereas N effects were expressed by causing variations in GAI. The exercise showed that the models used have potential for assessing climate change impacts on wheat production.

Wheat grain weight is a function of rate and duration of grain growth and is affected by photosynthate supply. Drought stress reduces photosynthate production because of stomatal closure. However, this might be partially overcome by an increase in air CO 2 concentration. This study was conducted to evaluate elevated CO 2 and drought stress effects on grain-filling rate and duration for spring wheat (Triticum aestivum L.). Spring wheat (cv. Yecora Roja) was grown at two CO 2 concentrations, 550 (elevated) or 370 (ambient) μmol mol -1 and two water treatments in a Free-Air CO 2 Enrichment (FACE) system at the University of Arizona Maricopa Agricultural Center. Plant samples were collected every 3 to 4 d from 6 d after anthesis until plant maturity. Main stem spikes were separated into upper, middle, and lower sections. Grain weight data for the intact main stem spike, each of its sections, and intact tiller spikes were fitted to a cumulative logistic model. Both elevated CO 2 and water treatments significantly influenced the grain-filling processes. Under drought stress conditions, elevated CO 2 increased grain weight in the upper and lower sections of the main stem spike by 10 and 24%, respectively. In well-watered plants, final grain weight in the midsection of the main stem spike was 8% higher than that measured under drought stress conditions. Grain weight increase under elevated CO 2 was due to a faster rate of grain filling. Effects of elevated CO 2 on the statistically derived duration of grain filling were inconclusive because of the confounding effect of blower-induced temperature changes on the process. An increase in grain weight of well-watered plants was due to a longer grain-filling period. Later-formed tiller spikes were more responsive to elevated CO 2 and drought stress than main stem spikes. Information from this study will help us understand the grain growth of wheat and provide information to establish grain growth mechanism.

The mid-day responses of wheat ear CO 2 and water vapour exchange to full-season CO 2 enrichment were investigated using a Free-Air CO 2 Enrichment (FACE) apparatus. Spring wheat [Triticum aestivum (L). cv. Yecora Rojo] was grown in two experiments under ambient and elevated atmospheric CO 2 (C a ) concentrations (approximately 370 μmol mol -1 and 550 μmol mol -1 , respectively) combined first with two irrigation (Irr) schemes (Wet: 100% and Dry: 50% replacement of evapotranspiration) and then with two levels of nitrogen (N) fertilization (High: 350, Low: 70 kg ha -1 N). Blowers were used for C a enrichment. Ambient C a plots were exposed to blower induced winds as well the C a x N but not in the C a x Irr experiment. The net photosynthesis for the ears was increased by 58% and stomatal conductance (g s ) was decreased by 26% due to elevated C a under ample water and N supply when blowers were applied to both C a treatments. The use of blowers in the C a -enriched plots only during the C a x Irr experiment (blower effect) and Low N supply restricted the enhancement of net photosynthesis of the ear due to higher C a . In the latter case, the increase of net photosynthesis of the ear amounted to 26%. The decrease in g s caused by higher C a was not affected by the blower effect and N treatment. The mid-day enhancement of net photosynthesis due to elevated C a was higher for ears than for flag leaves and this effect was most pronounced under ample water and N supply. The contribution of ears to grain filling is therefore likely to increase in higher C a environments in the future. In the comparison between Wet and Dry, the higher C a did not alter the response of net photosynthesis of the ear and g s to Irr. However, C a enrichment increased the drought tolerance of net photosynthesis of the glume and delayed the increase of the awn portion of net photosynthesis of the ear during drought. Therefore, the role of awns for maintaining high net photosynthesis of the ear under drought may decrease when C a increases.

Evapotranspiration (ET) and water use efficiency (WUE) were evaluated for two spring wheat crops, grown in a well-watered, subsurface drip-irrigated field under ambient (about 370 mmol mol 1 during daytime) and enriched (200mmol mol 1 above ambient) CO2 concentrations during 1995‐1996 and 1996‐1997 in Free-Air CO2 Enrichment (FACE) experiments in central Arizona. The enriched (FACE) and ambient (Control) CO2 treatments were replicated in four, circular plots, each 25 m in diameter. Two soil nitrogen (N) treatments, ample (High N) and limited (Low N), were imposed on one-half of each circular plot. Wheat ET, determined using soil water balance procedures, was significantly greater in High N than Low N treatments starting in late-March (anthesis) during both years. Differences in ET between CO2 treatments during the seasons were generally small and not statistically significant, however, there was a tendency for the ET to be lower for FACE than Control under the High N treatment. The reduction in the cumulative seasonal ET due to FACE averaged 3.7 and 4.0% under High N and 0.7 and 1.2% under Low N in the first and second years, respectively. However, WUE (grain yield per unit seasonal ET) was significantly increased for the FACE treatment under both soil N treatments. For the High N treatment, the WUE was 19 and 23% greater for FACE than Control and for the Low N treatment the WUE was 12 and 7% greater for FACE than Control in the 2 years, respectively. Published by Elsevier Science B.V.

Atmospheric CO2 concentration continues to rise. It is important, therefore, to determine what acclimatory changes will occur within the photosynthetic apparatus of wheat (Triticum aestivum L. cv. Yecora Rojo) grown in a future high-CO2 world at ample and limited soil N contents. Wheat was grown in an open field exposed to the CO2 concentration of ambient air [370 μmol (CO2) mol−1; Control] and air enriched to ∼200 μmol (CO2) mol−1 above ambient using a Free-Air CO2 Enrichment (FACE) apparatus (main plot). A High (35 g m−2) or Low (7 and 1.5 g m−2 for 1996 and 1997, respectfully) level of N was applied to each half of the main CO2 treatment plots (split-plot). Under High-N, FACE reduced stomatal conductance (gs) by 30% at mid-morning (2 h prior to solar noon), 36% at midday (solar noon) and 27% at mid-afternoon (2.5 h after solar noon), whereas under Low-N, gs was reduced by as much as 31% at mid-morning, 44% at midday and 28% at mid-afternoon compared with Control. But, no significant CO2 × N interaction effects occurred. Across seasons and growth stages, daily accumulation of carbon (A′) was 27% greater in FACE than Control. High-N increased A′ by 18% compared with Low-N. In contrast to results for gs, however, significant CO2 × N interaction effects occurred because FACE increased A′ by 30% at High-N, but by only 23% at Low-N. FACE enhanced the seasonal accumulation of carbon (A′′) by 29% during 1996 (moderate N-stress), but by only 21% during 1997 (severe N-stress). These results support the premise that in a future high-CO2 world an acclimatory (down-regulation) response in the photosynthetic apparatus of field-grown wheat is anticipated. They also demonstrate, however, that the stimulatory effect of a rise in atmospheric CO2 on carbon gain in wheat can be maintained if nutrients such as nitrogen are in ample supply.

We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO2 concentration, and ample and limiting N supplies, within a field experiment using free-air CO2 enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal CO2 concentration (Ci) was used to estimate the in vivo carboxylation capacity (Vcmax) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (Asat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO2 did not alter Vcmax in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vcmax and Asat was observed, and was found to correlate with reduced Rubisco activity and content. The dependency of acclimation on N supply was different at each developmental stage. With adequate N supply, acclimation to elevated CO2 was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in all leaves, although an increased LHC II/Rubisco ratio under elevated CO2 was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding the acclimation of photosynthesis to elevated CO2 and nutrient stress.

The response of whole-canopy net CO2 exchange rate (CER) and canopy architecture to CO2 enrichment and N stress during 1996 and 1997 for open-field-grown wheat ecosystem (Triticum aestivum L. cv. Yecora Rojo) are described. Every Control (C) and FACE (F) CO2 treatment (defined as ambient and ambient +200 μmol mol−1, respectively) contained a Low- and High-N treatment. Low-N treatments constituted initial soil content amended with supplemental nitrogen applied at a rate of 70 kg N ha−1 (1996) and 15 kg N ha−1 (1997), whereas High-N treatments were supplemented with 350 kg N ha−1 (1996 and 1997). Elevated CO2 enhanced season-long carbon accumulation by 8% and 16% under Low-N and High-N, respectively. N-stress reduced season-long carbon accumulation 14% under ambient CO2, but by as much as 22% under CO2 enrichment. Averaging both years, green plant area index (GPAI) peaked approximately 76 days after planting at 7.13 for FH, 6.00 for CH, 3.89 for FL, and 3.89 for CL treatments. Leaf tip angle distribution (LTA) indicated that Low-N canopies were more erectophile than those of High-N canopies: 48° for FH, 52° for CH, and 58° for both FL and CL treatments. Temporal trends in canopy greenness indicated a decrease in leaf chlorophyll content from the flag to flag-2 leaves of 25% for FH, 28% for CH, 17% for CL, and 33% for FL during 1997. These results indicate that significant modifications of canopy architecture occurs in response to both CO2 and N-stress. Optimization of canopy architecture may serve as a mechanism to diminish CO2 and N-stress effects on CER.