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Agroclimatology

Interactive Effects of Elevated Carbon Dioxide and Drought on Wheat

^a G.W. Wall, B.A. Kimball, D.J. Hunsaker, P.J. Pinter, Jr., R.L. LaMorte, and S.B. Idso (retired), USDA-ARS, U.S. Water Conservation Lab., 4331 E. Broadway Rd., Phoenix, AZ 85040
^b LI-COR, P.O. Box 4425, Lincoln, NE 68504
^c Univ. of Illinois, Dep. of Crop Science and Plant Biology, 1201 W. Gregory Dr., Urbana-Champaign, IL 61801
^d Dep. of Animal and Plant Sciences, Univ. of Sheffield, Sheffield, S10 2TN, UK
^e D.L. Hendrix (retired), USDA-ARS, Western Cotton Research Lab., 4135 E. Broadway Rd., Phoenix, AZ 85040
^f Potsdam Institute for Climate Impact Research, P.O. Box 601203, D-14412 Potsdam, Germany
^g Dep. of Soil Science, Humboldt Univ. of Berlin, Invalidenstrasse 42, 10115 Berlin, Germany
^h Lab. of Tree-Ring Research, Univ. of Arizona, Tucson, AZ, 85721

^* Corresponding author (gwall{at}uswcl.ars.ag.gov)

Received for publication March 30, 2004. 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 µmol mol^–1) and free-air CO₂ 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 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.

Abbreviations: A, instantaneous leaf net assimilation rate [µmol (CO₂) m^–2 s^–1] • A_max, daily maximum leaf net assimilation rate [µmol (CO₂) m^–2 s^–1] • A_N, season-long relative mean leaf net assimilation rate normalized between 0 to1 scale (i.e., A_N = A/A_max; dimensionless) • A', daily integral of net leaf carbon accumulation [g (C) m^–2 d^–1] • A'', seasonal integral of net leaf carbon accumulation [g (C) m^–2 yr^–1] • ANOVA, analysis of variance • B_S, season-long average total shoot biomass (g m^–2 ground area) • B_S(max) maximum season-long total shoot biomass (g m^–2 ground area) • B_R, season-long average living root biomass (g m^–2 ground area) • B_R(max), maximum season-long living root biomass (g m^–2 ground area) • B_R/B_S, season-long living root to total shoot biomass ratio (dimensionless) • CD, Control-Dry • CW, Control-Wet • C, carbon dioxide effect in ANOVA • C_a, atmospheric CO₂ concentration [µmol (CO₂) mol^–1] • C_i, intercellular CO₂ concentration [µmol (CO₂) mol^–1] • C_i/C_a, ratio of C_i to C_a (dimensionless) • CHO, concentration of simple sugars (sucrose, glucose, fructose) in leaf tissue (g kg^–1) • D, soil dehydration cycle effect in ANOVA • DAE, day after 50% emergence • E, effect in the ANOVA (i.e., Y, D, T, C, I, Z) • ET, season-long average daily evapotranspiration rate (mm d^–1) • ET_c, season-long cumulative evapotranspiration (mm) • e_a, atmospheric water vapor pressure at T_a (Pa) • e_s, atmospheric saturation water vapor pressure at T_a (Pa) • e*, atmospheric water vapor pressure deficit (i.e., e* = e_s – e_a) at T_a (Pa) • F, F statistic • FACE, free-air-CO₂-enrichment • FD, FACE-Dry • FW, FACE-Wet • Fru, concentration of fructose in leaf tissue (g kg^–1) • g_s, stomatal conductance to water vapor [mol (H₂O) m^–2 s^–1] • Glu, concentration of glucose in leaf tissue (g kg^–1) • HMW, concentration of high molecular weight fructans in leaf tissue (g kg^–1) • I, irrigation effect in ANOVA • IWUE (A/g _s) intrinsic water use efficiency [µmol (CO₂) mol (H₂O)^–1] • PPFD, photosynthetic photon flux density [µmol (photons) m^–2 s^–1] • LMW, concentration of low molecular weight fructans in leaf tissue (g kg^–1) • Suc, concentration of sucrose in leaf tissue (g kg^–1) • P, probability of a greater F statistic by chance • S, concentration of starch in leaf tissue (g kg^–1) • T, time of day [mid-morning (MM: 2.5 h prior to solar noon), midday (MD: solar noon), and midafternoon (MA: 2.5 h after solar noon)] effect in ANOVA • TR, leaf transpiration rate [mmol (H₂O) m^–2 s^–1] • T_a, ambient air temperature inside cuvette (°C) • T_l, leaf temperature inside cuvette (°C) • T_l, leaf – air temperature inside cuvette (°C) • TNC, concentration of total nonstructural carbohydrates in leaf tissue (g kg^–1) • WUE, (A/TR) water use efficiency [µmol (CO₂) mmol (H₂O)^–1] • Y, year effect in ANOVA • Z, canopy height effect in ANOVA • _M, soil matric potential (MPa) • _W, total leaf water potential (MPa) • _W(PB), total leaf water potential measured with a pressure chamber (MPa) • _W(PSY), total leaf water potential measured with thermocouple psychrometers (MPa) • , leaf osmotic potential (MPa) • _P, leaf turgor potential (MPa) • , rate of osmotic adjustment (dimensionless) • _P, rate of loss of turgor (dimensionless) • _S, volumetric soil–water content (m³ m^–3)

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