•Sorghum accumulates dhurrin (cyanide) and nitrate; forage can be toxic.•Partitioning of N to cyanide and nitrate was measured in FACE studies.•Cyanide, nitrate accumulation depended on tissue type, ...plant age and irrigation.•Drought effected increases in cyanide and nitrate were not moderated at high CO2.•Risk of toxicity likely to increase with climate change but not directly from rising CO2.
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. 550ppm) 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.
Atmospheric CO2 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 micromol mol(-1)) and free-air CO2 enrichment (FACE: ambient + 180 micromol 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 midafternoon 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, C3 cereal monocot grass (i.e., wheat) are anticipated in a future high-CO2 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.
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.
Scientists 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 monitoring of plant growth and development and for the detection of many environmental stresses which limit plant productivity. Coupled with rapid advances in computing
and positionlocating 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 emphasizes contributions by ARS researchers, reviews the biophysical basis of remote sensing; examines approaches that have been developed, refined, and tested for management of water, nutrients, and pests in agricultural crops; and assesses the role of remote sensing in yield prediction.
It concludes with a discussion of challenges facing remote sensing in the future.
Increasing atmospheric carbon dioxide (CO2) likely will affect future water requirements of most plants, including agricultural crops. This research quantifies such effects on the energy balance and ...evapotranspiration (ET) of sorghum (Sorghum bicolor (L.) Moench, a C4 grain crop) using a residual energy balance approach. During the summer and autumn of 1998 and 1999, sorghum was grown under free-air CO2 enrichment (FACE) conditions near Maricopa, Arizona. Latent heat flux (λET) was determined by subtracting soil heat flux (G0) and sensible heat flux (H) from net radiation (Rn) values in both Control CO2 plots (about 370 μmol mol-1) and FACE plots (Control + 200 μmol mol-1). Rn was observed using net radiometers. G0 was measured with soil heat flux plates at a depth of 10 mm, then corrected for heat storage above the plates. H was determined using measurements of air temperature from aspirated psychrometers, leaf temperature from infrared thermometers, and wind data from a three-cup anemometer. Both FACE and Control plots were divided into semicircular halves to allow a well-watered (Wet) treatment and a drought-stressed (Dry) treatment. This allowed comparisons of the FACE effect on ET in normal and water-stressed conditions. Under Wet conditions, FACE decreased λET by 13.8±1.8% in 1998, and 11.8±1.9% in 1999. Drought-stress resulted in a reduction in λET of 8.5±3.7% for the FACE treatments in 1998, but an increase in λET of 10.5±5.1% in 1999. When soil water was readily available, midday canopy temperatures in the FACE plots were increased by 1.47±0.09 °C in 1998, and 1.85±0.20 °C in 1999, indicative of increased stomatal resistance due to CO2 enrichment. These data suggest that soil water availability is a determining factor for the FACE effect. Water use efficiency (WUE) increased about 28% due to elevated CO2 under Wet conditions due to a savings of water for about the same growth, whereas under Dry conditions it increased about 16% due to much greater relative growth on only a slightly higher amount of water.
Documenting crop senescence rates is often difficult because of the need for frequent sampling during periods of rapid change and the subjective nature of human visual observations. The purpose of ...this study was to determine the feasibility of using images produced by a digital camera to measure the senescence rate of wheat and to compare the results with changes in greenness determined by two established methods. Measurements were made as part of an experiment to determine the effects of elevated CO(2) and limited soil nitrogen on spring wheat (Triticum aestivum L.) at the University of Arizona's Maricopa Agricultural Center, near Phoenix, AZ. "Greenness" measurements were made during senescence of the crop with a color digital camera, a hand-held radiometer, and a SPAD chlorophyll meter. The green to red (G/R) for each pixel in an image was calculated and the average G/R computed for cropped images from a digital camera representing 1 m(2) for each treatment and sample date. The normalized difference vegetation index (NDVI) was calculated from the red and near-infrared canopy reflectances measured with a hand held radiometer. A SPAD reading was obtained from randomly selected flag leaves. All three methods of measuring plant greenness showed similar temporal trends. The relationships between G/R with NDVI and SPAD were linear over most of the range of G/R. However, NDVI was more sensitive at low values than G/R. G/R was more sensitive above G/R values of 1.2 than SPAD because the upper limits of SPAD measurements were constrained by the amount of chlorophyll in the leaf, while G/R responded to both chlorophyll concentration in the leaves as well as the number of leaves present. Color digital imaging appears useful for quantifying the senescence of crop canopies. The cost of color digital cameras is expected to decrease and the quality and convenience of use to improve.
Summary
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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.
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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.
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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.
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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.
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.
Leaf N concentration is important because it is associated with the CO
2 assimilatory capacity of crops, and in grasslands, it is an important determinant of forage nutritive value. Consequently, the ...productivity of both domestic and native animals in future global environments may be closely linked to possible changes in leaf N concentration of grasses. Since grasslands are frequently subjected to water-deficit or N-deficit conditions, it is important to investigate the interactive responses between elevated CO
2 and these stress conditions. Therefore, this 4-year research program was undertaken with wheat (
Triticum aestivum L.) as a model system for forage grasses, to document the potential changes in leaf N concentration in response to global environment changes. Wheat crops grown under field conditions near Phoenix, AZ, USA, were subjected to elevated CO
2 and either water-deficit or N-deficit treatments using large Free Air Carbon dioxide Enrichment (FACE) arrays. Surprisingly, the elevated CO
2 treatment under optimum conditions resulted in little change in leaf N concentration. Therefore, no change in the nutritive value of forage from highly managed pastures would be expected. Further, water-deficit treatment had little influence on leaf N concentration. To some extent, the lack of response to the water-deficit treatment resulted because severe deficits did not develop until late in the growing seasons. Only on one date late in the season was the water-deficit treatment found to result in decreased leaf N concentration. The low N treatment in combination with elevated CO
2, however, had a large influence on leaf N concentration. Low levels of applied N resulted in decreased leaf N concentration under both CO
2 treatments, but the lowest levels of leaf N concentration were obtained under elevated CO
2 through much of the growing season. These results point to a potential problem with grasslands in that the nutritive value of the forage consumed by animals will be decreased under future global environment changes.
In order to determine the likely effects of the increasing atmospheric CO2 concentration on future evapotranspiration, ET, plots of field-grown wheat were exposed to concentrations of 550 ...micromol/mol CO2 (or 200 micromol/mol above current ambient levels of about 360 micromol/mol) using a free-air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, Rn; soil heat flux; air and soil temperatures; canopy temperature, Ts; and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime Ts about 0.6 degrees and 1.1 degrees C at high and low N, respectively. Daily total Rn was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively.
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