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To determine the likely effects of global warming on field-grown wheat (Triticum aestivum L.), a "Hot Serial Cereal" experiment was conducted—so-called "Cereal" because wheat was the crop, "Serial" because the wheat was planted about every 6 wk for 2 yr, and "Hot" because infrared heaters were deployed on six of the planting dates in a temperature free-air controlled enhancement (T-FACE) system, which warmed the canopies of the Heated plots. During the experiment, measurements of canopy reflectance were made two to five times per week from which values of normalized difference vegetation index (NDVI) were calculated. As expected, curves of NDVI from the Heated plots vs. time and vs. growing degree days (GDD) computed from air temperatures generally were ahead of those from Reference plots. However, when plotted against GDD computed from canopy temperatures the curves coalesced, which gives confidence that the infrared-heater treatment simulates natural warming and will produce plant responses not unlike those expected with future global warming. Biomass and grain yields were correlated with the areas under the NDVI vs. GDD curves for the air-temperature-based GDDs, but high variability prevented such a correlation to be detected using canopy-temperature-based GDD. Large differences existed between the total amounts of air or canopy temperature-based GDDs required for wheat to mature in our irrigated fields in an arid region. This implies that GDD based on air temperatures should be regarded only as a local guide to plant development rates, whereas those based on canopy temperatures would be more universal. [ABSTRACT FROM AUTHOR]

Climate warming may raise wheat (Triticum aestivum L.) yields in cooler climates and lower them in warmer climates. To understand these contrasting effects, infrared heating lamps were used to warm irrigated spring wheat by 1.5° C (day) and 3.0° C (night) above unheated controls during different times of the year at Maricopa, AZ. Changes in wheat growth with warming were used to test hypotheses for temperature effects on crop growth in the process model ecosys. Infrared heating substantially raised phytomass growth and grain yield under lower air temperature (Ta) following plantings from September through December. The same heating, however, lowered growth and yield under higher Ta following plantings from January through March. Gains in wheat yield of as much as 200 g C m-2 with heating under lower Ta were attributed in the model to more rapid CO2 fixation and to reduced chilling effects on seed set. These gains were only partially offset by losses from shortened wheat growth periods. Losses in wheat yield of as much as 100 g C m-2 with heating under higher Ta were attributed in the model to adverse effects of heating on crop water status and on CO2 fixation vs. respiration, to greater heat stress effects on seed set, and to shortened crop growth periods. Model hypotheses thus explained contrasting effects of heating on wheat yields under different Ta found in the field experiment as well as in many earlier studies. Well-constrained tests of these hypotheses are vital for models used to project climate change impacts on agricultural ecosystems. [ABSTRACT FROM AUTHOR]

Summary • The simulation model DEMETER was used here to investigate which mechanisms led to a larger CO2 effect on biomass production and yield of a spring wheat crop under drought compared with unlimited water supply. • Field data of the free-air CO2 enrichment (FACE) wheat experiments in Arizona (1993–94) were used to test the model. The influence of a particular mechanism leading to a higher CO2 effect under drought was investigated by eliminating the influence of the other causes on the simulation results on selected days during the growing seasons. • A larger CO2 effect under drought was caused in the model by the lower potential transpiration rate, higher root biomass and the nonlinear functional dependence of net assimilation rate on leaf internal CO2 concentration. The contribution of the different mechanisms changed in significance during the growing season depending on the degree of soil water limitation. The model successfully described the qualitative and quantitative behaviour of the crop under elevated CO2 . • A well-tested simulation model can be a useful tool in understanding the complex interactions underlying observed ecosystem responses to stress under elevated CO2 . [ABSTRACT FROM AUTHOR]

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 13 C-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 13 CO2 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 13 CO2 tracer induced a SOC δ13 C 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 13 CO2 -tracer result, however, is less reliable because of lower significance, fewer replicates and heterogeneous isotopic distribution within plants. [ABSTRACT FROM AUTHOR]

ABSTRACTThe mid-day responses of wheat ear CO2 and water vapour exchange to full-season CO2 enrichment were investigated using a Free-Air CO2 Enrichment (FACE) apparatus. Spring wheat [Triticum aestivum (L). cv. Yecora Rojo] was grown in two experiments under ambient and elevated atmospheric CO2 (Ca) 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 Ca enrichment. Ambient Ca plots were exposed to blower induced winds as well the Ca × N but not in the Ca × Irr experiment. The net photosynthesis for the ears was increased by 58% and stomatal conductance (gs) was decreased by 26% due to elevated Ca under ample water and N supply when blowers were applied to both Ca treatments. The use of blowers in the Ca-enriched plots only during the Ca × Irr experiment (blower effect) and Low N supply restricted the enhancement of net photosynthesis of the ear due to higher Ca. In the latter case, the increase of net photosynthesis of the ear amounted to 26%. The decrease in gs caused by higher Ca was not affected by the blower effect and N treatment. The mid-day enhancement of net photosynthesis due to elevated Ca 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 Ca environments in the future. In the comparison between Wet and Dry, the higher Ca did not alter the response of net photosynthesis of the ear and gs to Irr. However, Ca 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 Ca increases. [ABSTRACT FROM AUTHOR]

AbstractSpring wheat [Triticum aestivum (L). cv. Yecora Rojo] was grown from December 1992 to May 1993 under two atmospheric CO2 concentrations, 550 μmol mol–1 for high-CO2 plots, and 370 μmol mol–1 for control plots, using a Free-Air CO2 Enrichment (FACE) apparatus. In addition to the two levels of atmospheric CO2, there were ample and limiting levels of water supply through a subsurface trip irrigation system in a strip, split-plot design. In order to examine the temporal and spatial root distribution, root cores were extracted at six growth stages during the season at in-row and inter-row positions using a soil core device (86 mm ID, 1.0 m length). Such information would help determine whether and to what extent root morphology is changed by alteration of two important factors, atmospheric CO2 and soil water, in this agricultural ecosystem. Wheat root growth increased under elevated CO2 conditions during all observed developmental stages. A maximum of 37% increase in total root dry mass in the FACE vs. Control plots was observed during the period of stem elongation. Greater root growth rates were calculated due to CO2 enhancement until anthesis. During the early vegetative growth, root dry mass of the inter-row space was significantly higher for FACE than for Control treatments suggesting that elevated CO2 promoted the production of first-order lateral roots per main axis. Then, during the reproductive period of growth, more branching of lateral roots in the FACE treatment occurred due to water stress. Significant higher root dry mass was measured in the inter-row space of the FACE plots where soil water supply was limiting. These sequential responses in root growth and morphology to elevated CO2 and reduced soil water supports the hypothesis that plants grown in a high-CO2 environment may better compensate soil-water-stress conditions. [ABSTRACT FROM AUTHOR]

The effects of elevated carbon dioxide (CO2) concentration on plant water use are best evaluated on plants grown under field water use are best evaluated on plants grown under field conditions and with measurement techniques that do not disturb the natural function of the plant. Heat balance sap flow gauges were used on individual main stems of wheat (Triticum aestivum L. cv Yecora rajo) grown under normal ambient conditions (control) and in a free-air CO2 enrichment (FACE) system in Arizona with either high (control + high H2O=CW; FACE+high H2O=FW) or low (control+low H2O=CD; FACE+low H2O=FD) irrigation regimens. Over a 30 d period (stem elongation to anthesis), combinations of treatments were monitored with 10-40 guages per treatment. The effects of increased CO2 on tiller water used were inconsistent in both the diurnal patterns of sap flow and the statistical analyses of daily sap flow (Ftot). Initial results suggested that the reduction in Ftot from CO2 enrichment were small (0-10%) in relation to the H2O treatment effect (20-30%). For a 3d period, Ftot of FW was 19-26% less than that of CW (P=0.10). Examination of the different sources of variation in the study revealed that the location of gauges within the experimental plots influenced the variance of the sap flow measurements. This variation was probably related to positional variation in subsurface drip lines used to irrigate plots. A sampling design was proposed for use of sap flow gauges in FACE systems with subsurface irrigation that takes into account the main treatment effect of CO2 enrichment and the other sources of variation identified in this study. Despite the small and often statistically non-significant differences in Ftot between the CW and FW treatments, cumulative water use of the FW treatment at the end of the first three test periods ranged from 7 to 23% lower than that of the CW treatment. Differences in sap flow between FW and CW compared well with treatment differences in evapotranspiration. The results of the study, based on the first reported sap flow measurements of wheat, suggest that irrigation requirements for wheat production in the present climatic regimen of the south-western US, may be predicted to decrease slightly because of increasing atmospheric CO2. [ABSTRACT FROM AUTHOR]

A spring wheat crop was grown at ambient and elevated (550 μmol mol-1) CO2 concentrations under free-air CO2 enrichment (FACE) in the field. Four experimental blocks, each comprising 21-m-diameter FACE and control experimental areas, were used. CO2 elevation was maintained day and night from crop emergence to final grain harvest. This experiment provided a unique opportunity to examine the hypothesis that CO2 elevation in the field would lead to acclimatory changes within the photosynthetic apparatus under open field conditions and to assess whether acclimation was affected by crop developmental stage, leaf ontogeny and leaf age. Change in the photosynthetic apparatus was assessed by measuring changes in the composition of total leaf and thylakoid polypeptides separated by SDS-PAGE. For leaves at completion of emergence of the blade, growth at the elevated CO2 concentration had no apparent effect on the amount of any of the major proteins of the photosynthetic apparatus regardless of the leaf examined. Leaf 5 on the main stem was in full sunlight at emergence, but then became shaded progressively as 3-4 further leaves formed above with continued development of the crop. By 35 d following completion of blade emergence, leaf 5 was in shade. At this point, the chlorophyll a/b ratio had declined by 26% both in plants grown at the control CO2 concentration and in those grown at the elevated CO2 concentration, which is indicative of shade acclimation. The ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content declined by 45% in the control leaves, but by 60% in the leaves grown at the elevated CO2 concentration. The light- harvesting complex of photosystem II (LHCII) and the chlorophyll content showed no decrease and no difference between treatments, indicating that the decrease in Rubisco was not an effect of earlier senescence in the leaves at the elevated CO2 concentration. Following completion of the emergence of the flag-leaf blade, the elevated-CO2 treatment inhibited the further accumulation of Rubisco which was apparent in control leaves over the subsequent 14d. From this point onwards, the flag leaves from both treatments showed a loss of Rubisco, which was far more pronounced in the elevated-CO2 treatment, so that by 36 d the Rubisco content of these leaves was just 70% of that of the controls and by 52d it was only 20%. At 36d, there was no decline in chlorophyll, LHC!I or the chloroplast ATPase coupling factor (CFI) in the elevated CO2 concentration treatment relative to the control. By 52 d, all of these proteins showed a significant decline relative to the control. This indicates that the decreased concentration of Rubisco at this final stage probably reflected earlier senescence in the elevated-CO2 treatment, but that this was preceded by a CO2-concentration-dependent decline in Rubisco. [ABSTRACT FROM AUTHOR]

Scope The recent publication by Yin (2013; Annals of Botany 112: 465–475) referred to in the title above provides an excellent review of modelling approaches to predict the impact of elevated CO2 on crop productivity, as well as on the controversy regarding whether yield responses observed in free-air CO2 enrichment (FACE) experiments are indeed lower than those from chamber-based experiments. However, the wheat experiments in the example of fig. 1 in Yin's paper had a flaw as the control plots lacked blowers that were in the FACE plots, which warmed the FACE plots at night and hastened plant development. This Viewpoint seeks to highlight this fact, and to comment on the relative merits of FACE and enclosure experiments. [ABSTRACT FROM PUBLISHER]