As atmospheric carbon dioxide concentrations, [CO2Air], continue their uncontrolled rise, the capacity of soils to accumulate or retain carbon is uncertain. Free-air CO2 enrichment (FACE) experiments have been conducted to better understand the plant, soil and ecosystem response to elevated [CO2], frequently employing commercial CO2 that imparts a distinct isotopic signal to the system for tracing carbon. We conducted a FACE experiment in 1998 and 1999, whereby sorghum (C4 photosynthetic pathway) was grown in four replicates of four treatments using a split-strip plot design: (i) ambient CO2/ample water (365 μmol mol−1, “Control–Wet”), (ii) ambient CO2/water stress (“Control–Dry”), (iii) CO2-enriched (560 μmol mol−1, “FACE–Wet”), and (iv) CO2-enriched/water stressed (“FACE–Dry”). The stable-carbon isotope composition of the added CO2 (in FACE treatments) was close to that of free atmosphere background values, so the subsequent similar 13C-enriched carbon signal photosynthetically fixed by C4 sorghum plants could be used to trace the fate of carbon in both FACE and control treatments. Measurement of soil organic carbon content (SOC (%) = gC/gdry soil × 100%) and δ13C at three depths (0–15, 15–30, and 30–60 cm) were made on soils from the beginning and end of the two experimental growing seasons. A progressive ca. 0.5‰–1.0‰ δ13C increase in the upper soil SOC in all treatments over the course of the experiment indicated common entry of new sorghum carbon into the SOC pools. The 0–15 cm SOC in FACE treatments was 13C-enriched relative to the Control by ca. 1‰, and according to isotopic mass balance, the fraction of the new sorghum-derived SOC in the Control–Wet treatment at the end of the second season was 8.4%, 14.2% in FACE–Wet, 6.5% in Control–Dry, and 14.2% in FACE–Dry. The net SOC enhancement resulting from CO2 enrichment was therefore 5.8% (or 2.9% y−1 of experiment) under ample water and 7.7% (3.8% y−1 of experiment) under limited water, which matches the pattern of greater aboveground biomass increase with elevated [CO2Air] under the Dry treatment, but no parallel isotopic shifts were found in deeper soils. However, these increased fractions of new carbon in SOC at the end of the experiment do not necessarily mean an increase in total SOC content, because gravimetric measurements of SOC did not reveal a significant increase under elevated [CO2Air], at least within the limits of SOC-content error bars. Thus, new carbon gains might be offset by pre-experiment carbon losses. The results demonstrate successful isotopic tracing of carbon from plants to soils in this sorghum FACE experiment showing differences between FACE and Control treatments, which suggest more dynamic cycling of SOC under elevated [CO2Air] than in the Control treatment.

Several cotton experiments have been conducted at the University of Arizona's Maricopa Agricultural Center from which datasets have been obtained documenting cotton responses to elevated CO2 concentrations, water supply, nitrogen fertilizer, and planting density. In particular, these experiments included FACE (free-air CO2 enrichment; CO2, water; 10 treatment-years), AgIIS (Agricultural Irrigation Imaging System, pronounced Ag Eyes; nitrogen fertilizer, water supply; 4 treatment-years), and FISE (FAO-56 Irrigation Scheduling Experiments; irrigation scheduling method, planting density, nitrogen fertilizer; 24 treatment-years). Besides achieving the experimental objectives of determining cotton's response to the several variables, as well as testing remote sensing techniques, the comprehensive datasets are suitable for validating plant growth models because they include weather, soils, management, growth, yield and other data.

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.

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.

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 CO 2 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 CO 2 , 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 CO 2 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-CO 2 world.

Reductions in the protein and nitrogen content of plants grown under enhanced atmospheric CO 2 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 CO 2 under field conditions finds that wheat nitrate assimilation was slower for elevated CO2 than for ambient CO 2 .

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 (similar to 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. (C) 2010 Elsevier B.V. All rights reserved.

Canopy spectral reflectances were measured over six cultivars of spring wheat (Trticum aestivum L.) grown at Phoenix, Arizona. Data were collected at 30-45 min intervals on 9 March 1983 using two ground-based radiometers with bandpass characteristics similar to those of the Muttispectral Scanner and Thematic Mapper on LANDSAT-4 and -5. Major differences in reflectance were observed among cultivars at every time period despite their apparent similarities in green leaf area and green biomass. Single-leaf spectra measured in the laboratory with a spectrophotometer revealed no cultivar-related differences and supported the contention that the reflectances were strongly influenced by canopy architectural features. The diurnal patterns of reflectance reinforced this conclusion with planophile canopies exhibiting the least amount of variability due to changes in Sun angle and erectophile canopies showing the most. These data underscore the complexities of interpreting remotely sensed multispectral data and suggest that multiple Sun-angle data acquisitions may be required to extract desired information.

Agronomy JournalVolume 99, Issue 1 p. 238-239 Special Submission Introduction to the Symposium “Progress in Radiation and Energy Balance Measurement Systems” Thomas J. Sauer, Corresponding Author Thomas J. Sauer sauer@nstl.gov USDA-ARS, National Soil Tilth Lab., 2150 Pammel Drive, Ames, IA, 50011Corresponding author (sauer@nstl.gov)Search for more papers by this authorPaul J. Pinter Jr., Paul J. Pinter Jr. USDA-ARS, U.S. Arid Lands Agricultural Res. Ctr., 21881 North Cardon Lane, Maricopa, AZ, 85239Search for more papers by this author Thomas J. Sauer, Corresponding Author Thomas J. Sauer sauer@nstl.gov USDA-ARS, National Soil Tilth Lab., 2150 Pammel Drive, Ames, IA, 50011Corresponding author (sauer@nstl.gov)Search for more papers by this authorPaul J. Pinter Jr., Paul J. Pinter Jr. USDA-ARS, U.S. Arid Lands Agricultural Res. Ctr., 21881 North Cardon Lane, Maricopa, AZ, 85239Search for more papers by this author First published: 01 January 2007 https://doi.org/10.2134/agronj2006.0001SRead the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume99, Issue1January 2007Pages 238-239 RelatedInformation

Spatially distributed evapotranspiration (ET) estimation is important for crop stress and assessment and irrigation scheduling. Using remotely sensed thermal infrared data in combination with visible-near infrared data, accurate instantaneous ET estimates are feasible. These estimates, however, need to be extended to daily time steps to have practical value at farm scales. One way to accomplish this extension is to combine hourly image observations of soil and vegetation temperatures. This approach is tested using a one-source energy balance model and data obtained from a series of morning remote sensing flights on 10 March 2005 over a wheat crop planted in Maricopa, Arizona. Aggregate remote sensing ET results from this particular day agreed well with estimates derived from soil moisture observations, but not on a plot-by-plot basis, where modeled ET variability was much less than soil moisture variability. This result is inconsistent with remotely sensed surface temperatures which do show significant spatial and temporal changes. Additional data sets will be investigated to determine how to improve ET modeling with surface temperatures.

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.

Techniques to more accurately quantify crop evapotranspiration (ETc) are needed for determining crop water needs and appropriate irrigation scheduling. In this study, remotely sensed observations of the normalized difference vegetation index (NDVI) were used to estimate cotton basal crop coefficients (Kcb), which were then applied within the dual crop coefficient procedures of the Food and Agricultural Organization (FAO), Paper 56 (FAO-56) to calculate daily ETc. An experiment in central Arizona during 2003 compared irrigation scheduling using a remotely sensed Kcb technique (NDVI treatment) with the FAO-56 Kcb curve (FAO treatment). The FAO curve was locally developed for optimum crop conditions and standard cotton density. Final lint yield means were not significantly different between the two irrigation methods, which included sub-treatments of two levels of nitrogen and three plant densities. However, NDVI attained higher yields under low N input, whereas FAO generally had higher yields under high N. The ETc estimated using the NDVI-Kcb method was in closer agreement with measured cumulative ETc than the FAO Kcb. For high N treatments, the mean absolute differences between measured and estimated cumulative ETc during the growing season for typical, dense, and sparse populations (10, 20, and 5 plants m-2, respectively) were 4, 17, and 4 mm, respectively, for NDVI, whereas they were10, 32, and 13 mm, respectively, for FAO. Although additional research is needed for improving our remote sensing technique, it potentially offers an improvement over the FAO Kcb curve for quantifying actual ETc.

Hyperspectral imagery is capable of providing detailed spectral reflectance information of agricultural fields for potential use in site-specific management operations. Analysis of these data are complicated by the large number of spectral bands, the many different components or endmembers (e.g. plant and soil), and the presence of shadows. Unlike simple unmixing approaches which compute the fraction of a fixed number of components, multiple endmember spectral mixture analysis (MESMA) also determines which components are present in each pixel. This study compared whether using different shadow endmembers (EM) in a 4-EM model (sunlit green leaf, sunlit soil, shadowed leaf, shadowed soil) would improve estimates of scene components compared to a 3-EM model (sunlit green leaf, sunlit soil, photometric shade). Results revealed that correlations with percent cover and height were improved when shadow or shade endmembers were included for both models compared to the green leaf fraction alone. The 3-EM model was superior for developing a direct relationship for estimating cover and height but was not able to estimate SPAD or chlorophyll a. The 4-EM model showed the best results for SPAD and chlorophyll a, with r(2) values of 0.84 and 0.77, respectively.

give high groundnut equivalent yield at Dharwad (Karnataka) and Junagadh (Gujarat) and at a row relatively tolerant to soil acidity, aluminium-toxicity, and Al-induced P-and Ca-deficiencies with a yield potential of 1,500-4,000 kg/ha and have been recommended for cultivation in NEH region. Rapeseed-Mustard : Sesbania green manuring along with soil incorporation of mustard waste @ 2.5tonnes/ha in kharif season has shown beneficial effect on soil health as well as mustard yield. The beneficial effect of Sesbania + mustard straw incorporation was further enhanced when recommended dose of fertilizer (80 kg N + 40 kg P 2 O 5 + 40 kg K 2 O/ha) was applied to mustard crop. Mustard hybrids produced more yield at a 45 cm × 15 cm spacing. The wider spacing opens avenues to reduce the recommended seed rates of hybrid mustard by 33%, augmenting hybrid seed availability for more acreage. Soybean: Soybean-wheat-maize-wheat rotation system was proved to be the best for productivity, profitability and energy efficiency. Ridge tillage and broad bed furrow significantly increased soil microbial biomass, soil enzyme activities and seed yield as compared to minimum tillage and flat bed planting under soybean-wheat and soybean-chickpea system. Thirteen thermo-tolerant rhizobia surviving at 45°C have been identified. were found promising for solubilization of zinc salts. Sunflower: The highest seed yield of kharif sorghum was obtained with 150% RDF application, while rabi sunflower yield was highest with RDF application with preceding sorghum receiving RDF PRODUCTION Wheat: The long-term effect of five tillage options, i.e. conventional tillage, zero tillage, rotary tillage, strip tillage and bed planting were evaluated. The mean yield was 3.04% higher in rotary tillage, whereas 7.55 and 12.81% lower, respectively in strip tillage and bed planting, options compared to conventional field preparation. However, yield under zero and conventional tillage was similar. In six out of eight years and on mean basis, the yield recorded was highest in rotary tillage. Cost savings in zero tillage varied from Rs 2,500 to 3,000/ha and Rs 2,000 to 2,500/ha in rotary tillage. Millets:Intercropping of 40 – 45 day old pigeonpea seedlings with finger millet (2 : 8) was found to be promising and remunerative in light red soils of southern Karnataka.Application of composted poultry manure @ 1.5 tonnes/ha in Uttarakhand and 2.0 tonnes/ha in red soils of Karnataka is a better option for organic cultivation of finger millet.Finger millet variety, Indaf-7, is a better choice for planting in rabi (second …

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.

We evaluated the influences of CO2 [Control, similar to 370 mumol mol(-1); 200 mumol mol(-1) above ambient applied by free-air CO2 enrichment (FACE)] and soil water (Wet, Dry) on above- and below-ground responses of C-3 (cotton, Gossypium hirsutum ) and C-4 (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 C-3 growth may have on subsequent vegetation change.

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.

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 present study was carried out to test the hypothesis that elevated atmospheric CO2 (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 free-air carbon-dioxide enrichment (FACE) by water-stress (Dry) experiment. Under Dry conditions elevated Ca increased the quantum yield of photosystem II (phiPSII) 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 phiPSII only at midday and was entirely attributed to changes in F-v'/F-m'. Under field conditions, decreases in phiPSII 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 phiPSII 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 FAO Irrigation and Drainage Paper No. 56 (FAO-56) is expected to provide a universally consistent methodology for obtaining reliable estimates of crop evapotranspiration (ETc) from standard weather data and crop coefficient (K-c) information. Yet, for many areas, including the southwestern desert region of the U.S., evaluation of the FAO-56 crop coefficient methods for local conditions has been limited. In this study, measurements of daily alfalfa ETc were used to evaluate the dual crop coefficient approach of FAO-56, which separates the single K-c into two coefficients, a basal coefficient, K-cb (primarily transpiration), and a wet soil evaporation coefficient, K-e. A second objective was to calibrate alfalfa K-cb baselines for constructing localized, climate-adjusted FAO-56 K-cb curves for growth cycles under the seasonal climatic variations encountered in the semiarid, southwestern U.S. Daily ETc was measured for alfalfa in three weighing lysimeters subjected to varying irrigation regimes for eight cutting cycles in 1985 at a field site in Phoenix, Arizona. Daily K, data were determined from the measured ETc and calculations of the FAO-56 grass-reference evapotranspiration (ETo) using meteorological data obtained at the site. Daily K-c values were partitioned into the dual crop coefficients (K-cb and K-e) using back-calculations of the FAO-56 dual procedures. Examples are given to illustrate how the magnitudes Of K-cb, K-e and Kc varied with respect to crop development and wet and dry surface soil conditions. The effects of water stress on ETc were considered in the back-calculations using the FAO-56 water stress reduction coefficient (K-s). Linear-style FAO-56 K-cb curves were then constructed separately for each lysimeter for each of the eight cutting cycles based on the back-calculated K-cb data. Daily ETc based on the constructed curves and FAO-56 calculation procedures were highly correlated to the daily measured lysimeter ETc (r(2) = 0.98), and the mean daily difference between calculated and measured ETc (0.03 mm) was not significant (p > 0.7). Comparison of measured ETc for water-stressed and well-watered alfalfa indicated that calculated K-s described ETc reductions due, to soil water stress adequately when atmospheric evaporative demand was considered in the K-cb. Baseline K-cb values for constructing local climate-adjusted FAO-56 alfalfa K 0.30, 1.22, and 1.05 for the initial, mid-season, and end of late season, respectively.