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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.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: A, instantaneous leaf net assimilation rate [µmol (CO2) m–2 s–1] • Amax, daily maximum leaf net assimilation rate [µmol (CO2) m–2 s–1] • AN, season-long relative mean leaf net assimilation rate normalized between 0 to1 scale (i.e., AN = A/Amax; 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 • BS, season-long average total shoot biomass (g m–2 ground area) • BS(max) maximum season-long total shoot biomass (g m–2 ground area) • BR, season-long average living root biomass (g m–2 ground area) • BR(max), maximum season-long living root biomass (g m–2 ground area) • BR/BS, season-long living root to total shoot biomass ratio (dimensionless) • CD, Control-Dry • CW, Control-Wet • C, carbon dioxide effect in ANOVA • Ca, atmospheric CO2 concentration [µmol (CO2) mol–1] • Ci, intercellular CO2 concentration [µmol (CO2) mol–1] • Ci/Ca, ratio of Ci to Ca (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) • ETc, season-long cumulative evapotranspiration (mm) • ea, atmospheric water vapor pressure at Ta (Pa) • es, atmospheric saturation water vapor pressure at Ta (Pa) • e*, atmospheric water vapor pressure deficit (i.e., e* = es – ea) at Ta (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) • gs, 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] • Ta, ambient air temperature inside cuvette (°C) • Tl, leaf temperature inside cuvette (°C) • 
Tl, 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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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Wheat is the world's foremost grain source (FAO, 1996). As such, it has been identified as a high priority for global change research (IPCC, 1996, 2001) by the Global Change and Terrestrial Ecosystems–International Geosphere–Biosphere Program (Steffen et al., 1992). Wheat, a cool-season annual monocot C3 grass, is representative of graminaceous species common to temperate zone grassland prairies and savannas that cover a significant proportion of Earth's landmass and that make a substantial contribution to the above- and belowground global carbon balance. An imperative exists, therefore, to elucidate the effects that a future high-CO2 world and potential changes in soil–water supply could have on edaphic, gas exchange, water relations, carbohydrate pool dynamics, growth, and development characteristics of wheat. Dissemination of such information could be of use to those individuals responsible for formulating strategies to mitigate the adverse effects of global change, while optimizing those that are beneficial (i.e., development of new cultivars with higher yields or improved grain end-use quality).
Our objective was to characterize and quantify a wheat plant's response to global change. Pursuant to this aim we elucidated the interdependency of four edaphic, nine gas exchange, four leaf tissue carbohydrate content, four water relations, and five growth parameter responses of spring wheat grown under ample and reduced soil–water content and ambient and elevated Ca levels. Specifically, we hypothesized the following:
S), thereby causing less negative soil matric potential (M). W), and during the nocturnal period greater recovery in plant water status will be observed at predawn and sunset. ), thereby resulting in a decrease in the rate in loss of turgor (P), as evidenced by leaves that maintain higher values of turgor potential (P). To test the validity of these eight hypotheses, we characterized and quantified the effect of tissue dessication (during soil dehydration) and subsequent recovery (after soil rehydration) on a total of 26 parameters of open-field spring wheat grown in ambient air and air enriched with Ca, and under four soil dehydration–rehydration cycles over a 2-yr study.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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S on spring wheat. The experiments were conducted in a 12-ha open field at the University of Arizona's Maricopa Agricultural Center, Maricopa, AZ (33.07° N lat; 111.97° W long; 361 m above sea level), located 50 km south of Phoenix, AZ. The soil was a Trix clay loam [fine-loamy, mixed (calcareous) hyperthermic Typic Torrifluvents]. Details of the field site and biological activity sampling areas (Wall and Kimball, 1993), as well as the irrigation system (Hunsaker et al., 1996), have been described previously. Except for the use of drip rather than flood irrigation, all agronomic practices were in accordance with local cultural practices.
Carbon Dioxide and Irrigation Treatment Description
The FACE approach was employed to create an elevated Ca treatment (C) (Hendrey, 1993). The FACE plots were fumigated with CO2 24 h d–1 at rates required to maintain a concentration of approximately 550 µmol mol–1 at the center point of the plot from 50% emergence until physiological maturity (1 Jan.–16 May 1993, and 28 Dec. 1993–31 May 1994). Control plots contained a plenum and vertical vent pipes, but had no airflow and were at ambient Ca levels of approximately 370 µmol mol–1. Seasonal average values of Ca were within 0.5 µmol mol–1 of the set point and 93% of the 1-min averages were within 10% of the set point (Hendrey, 1993) (i.e., 548 µmol mol–1 during the day and 598 µmol mol–1 at night for FACE and 363 µmol mol–1 during the day and 412 µmol mol–1 at night for Control).
Irrigation water was supplied with a subsurface drip-tape irrigation system (0.5-m tube spacing, 0.3-m emitter spacing, 0.2-m depth) as described by Hunsaker et al. (1996). Each of the main circular C plots were split into semicircular halves, with each half receiving an irrigation (I) treatment of either 100% (Wet, well-watered) or limited 50% (Dry, water-stressed) replacement of potential evapotranspiration (ET). A water balance procedure (Fox et al., 1992) was used to determine both the timing and amount of irrigated water applied to the Wet plots. An irrigation event occurred when the soil–water depletion of plant-available water in the crop root zone for Wet plots was between 30 and 40% of available. The amount of irrigated water applied to the Wet plots was then determined to replenish the root zone to field capacity (0% depletion). Because the Dry plots received only 50% as much irrigation water as the Wet plots, the depletion of water for the Dry plots was between 60 and 80% available. A preexisting crop coefficient curve developed for wheat was used to estimated crop water use that expressed the ratio of daily wheat ET to daily potential ET for a grass-reference crop. The grass-reference ET was calculated with the modified Penman equation as implemented by Doorenbos and Pruitt (1977). All meteorological data required for the modified Penman calculation was obtained from a weather station about 2 km from the field site.
For ease in applying the I treatments, the same irrigation regime was applied to the same split-plot within a C level for each replication: strip-split-plot experimental design. These C and I levels gave four treatment combinations: Control-Dry (CD); FACE-Dry (FD); Control-Wet (CW); FACE-Wet (FW). All treatments were replicated four times for a total of eight rings—four of them were active FACE rings with blowers and four of them were inactive Control rings without blowers (Pinter et al., 2000).
Crop Culture
Certified wheat seeds (cv. Yecora Rojo) were sown (John Deere Flex-Planter 71, Moline, IL) into flat beds at 0.25-m row spacings on 15 Dec. 1992 at a plant population of 130 plants m–2; 50% emergence occurred on 1 Jan. 1993, and the crop was harvested 25–27 May 1993. During the second year seeds were sown at a higher plant population of 180 plants m–2 on 7–8 Dec. 1993; 50% emergence occurred on 28 Dec. 1993, and the crop was harvested on 1 June 1994. To germinate the seeds a drip irrigation system was used to apply 317 mm of water from 9 to 13 d after they were sown during 1993, whereas a portable sprinkler system was installed temporarily to apply a total of 30 mm of water from 6 to10 d after seeds were sown during 1994 (Fig. 1b
, d). Because day after 50% emergence (DAE) corresponded with the first day of the year during this 2-yr study, hereafter DAE is equivalent to day of year.
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s between Dry and Wet treatments for a comparative study (Fig. 1a, c). Fertilizer amounts were applied so that nutrients were nonlimiting. A preplant application of granular (16–20–0) fertilizer supplied 54 kg N ha–1 and 29 kg P ha–1 in both years. Nitrogen, as urea–ammonium nitrate solution (0.32 kg N kg fertilizer–1, Uran-32), and P, as super-phosphoric acid, fertilizer were applied through the irrigation tubes at tillering, inflorescence emergence, and anthesis (Fig. 1b, d). Total amounts of N applied were 277 and 261 kg N ha–1, and of P were 44 and 29 kg P ha–1 during 1993 (Fig. 1b) and 1994 (Fig. 1d), respectively.
Volumetric Soil–Water Content and Evapotranspiration Rate
Measurements of S were obtained in each subplot using time domain reflectometry (TDR) and neutron scattering equipment (Hydroprobe Model 503 DR, Campbell Pacific Co., Martinez, CA). Access tubes (50 mm diam. and 2.0 m long) were installed in the no-traffic section (Wall and Kimball, 1993) of each of 16 plots, before seeds were sown during 1992. The TDR measurements were taken over the 0- to 0.3-m soil depth, whereas neutron scattering measurements were taken from the 0.4- to 2.0-m depth in 0.2-m increments (Hunsaker et al., 1996). The soil-water retention characteristics for the site were estimated from data obtained from Post et al. (1988). These data were used to develop individual moisture release curves for Control and FACE plots, which were used to derive M over a 0.9-m soil profile for each date S was measured (Fig. 1a, c).
After expected root penetration had occurred to a given depth (Robertson et al., 1993a, 1993b), a stair-step model incorporated values of S at consecutively deeper depths in 0.3-m intervals. By assuming negligible deep percolation, average daily ET was essentially set equal to the temporal changes in S for the active root zone. Whenever irrigation or rain occurred, however, it was estimated by interpolation by using the average ET before and after the wetting event for the active root zone. Hence, S and the total amount of irrigation plus rain were used to calculate ET, and therefore season-long cumulative evapotranspiration (ETC), based on the soil–water balance method (Jensen et al., 1990) as implemented by Hunsaker et al. (1996).
Measurements of Leaf Stomatal Conductance and Net Assimilation and Transpiration Rates
Gas exchange rates were measured on randomly selected leaves with three portable closed-exchange (transient) systems with a 0.25-L transparent Plexiglas cuvette (Model LI-6200, LI-COR, Lincoln, NE). Each infrared gas analyzer was calibrated against a gravimetrically prepared mixture of CO2 in air (±1% Primary Standard, Matheson Gas Products, Cucamonga, CA), and cuvette humidity sensors were calibrated with a dew-point generator (LI-610, LI-COR, Lincoln, NE) immediately before use. Before each measurement, the cuvette was allowed to come into equilibrium with prevailing air temperature and relative humidity. As described previously by Wall et al. (2000), dawn to dusk measurements of gs, and net assimilation (A) and transpiration (TR) rates were made on the central portion of fully expanded (ligule emerged) upper-canopy sunlit leaves, begun at a cuvette CO2 concentration of 370 ± 35 or 550 ± 35 µmol mol–1 for Control and FACE, respectively. One leaf per row from each of five different rows (repeated measures) was randomly selected for measurement. The leaf cuvette was held in the horizontal position and caution was used not to shade any portion of the leaf. Because three separate portable closed-exchange systems were used, three replications could be measured simultaneously within 60 min at 2-h intervals, thereby minimizing variation in gas exchange measurements due to diurnal changes in meteorological conditions, particularly incident photon flux density (PPFD: µmol photons m–2 s–1), air temperature (Ta), and water vapor pressure deficit (e*) (i.e., e* = es – ea). Three observations were recorded, but the first 10-s measurement interval was discarded from the statistical analysis because Ca in the assimilation chamber was unstable during that period.
Each gas exchange system made direct measurements of Ca, atmospheric water vapor pressure (ea), leaf temperature (Tl), Ta, and incident PPFD normal to the leaf surface. We calculated leaf A, gs, TR, saturation water vapor pressure (es), e*, internal CO2 concentration (Ci) and the ratio of Ci to Ca (Ci/Ca) as suggested by LI-COR (1990) following equations of von Caemmerer and Farquhar (1981). A value for water use efficiency (WUE) and intrinsic water use efficiency (IWUE) was derived as the ratio of A to TR and A to gs, respectively,whereas the difference between Tl and Ta was used to derive the leaf minus air temperature (Tl). Phenological development (average numerical decimal code across all treatments) was used to group values of the 26 response parameters by distinctive development stages (Zadoks et al., 1974) as follows; inflorescence emergence until hard dough (DAE 75, 89, 99, 105, and 118; Zadoks 50–88) during 1993; tillering until soft dough (DAE 41, 69, 81, 95, and 123; Zadoks 24–84) during 1994. To investigate the effect of the lack of blowers in the Control plots (Pinter et al., 2000), during 1994 additional midday measurements of gas exchange parameters (i.e., gs, A, TR, Tl, T, Ci, Ci/Ca) were taken on DAE 116, 119, 126, 131, 133, and 137 from grain filling until physiological maturity (Zadoks 84.1–87.8).
The daily accumulation of carbon (A') was derived by integrating the area under each dawn to dusk net carbon uptake curve. Dawn and dusk times in Mountain Standard Time (MST) were obtained from standard astronomical tables for Maricopa, AZ, and served as a zero response point for the purpose of integration. The seasonal accumulation of net carbon uptake (A'') was derived by integrating the area under each daily carbon uptake curve from 50% emergence until canopy senescence reduced fractional absorption of PPFD to 25% using a trapezoidal integration routine (Area.xfm, Sigma Plot, v. 4.01, SPSS, Chicago, IL).
Analysis of Leaf Total Nonstructural Carbohydrate Concentration
Ten uppermost fully expanded (i.e., ligule emerged) sunlit leaves were randomly selected every 2 h (as many as eight sample intervals) from dawn to dusk. Sampling dates corresponded with the development of leaf number five through eight and the flag leaf. At anthesis on DAE 81 during 1994, after the canopy had reached its maximum height, a whole-canopy leaf profile was sampled from the flag (top of the canopy) through the flag-4 leaf (lower photosynthetically active leaf at the bottom of the canopy) on an individual culm for each of 10 culms. All leaf samples for each treatment were pooled and kept on dry ice for up to 10 h until they were transported to the laboratory. Leaves were kept overnight on dry ice in a cold storage facility maintained at 10°C. The following day the leaf samples were freeze-dried, ground through a 0.4-mm mesh in a Wiley Mill, and quickly transferred to tightly stoppered glass vials that were stored at room temperature.
The TNC, comprised of simple sugars such as glucose (Glu), fructose (Fru), and sucrose (Suc), and complex carbohydrates such as low (LMW) and high (HMW) molecular weight fructans and starch (S), were determined using a microplate assay methodology as described elsewhere (Hendrix and Peelen, 1987; Hendrix, 1993).
Measurements of Total Leaf Water Potential with a Pressure Chamber
Blades of uppermost fully expanded sunlit leaves were excised approximately 5 mm apical to the leaf collar and sealed in a plastic bag containing a damp paper towel. Plastic bags containing excised leaves were then stored in an insulated container containing ice. No direct contact occurred between the ice and leaf samples. Values of W were measured with a pressure chamber (bomb) (W(PB)) (Scholander et al., 1965) (Model 3000, Soil Moisture Equipment Corp, Santa Barbara, CA). This protocol has given reliable results as long as particular precautions are taken to minimize error due to rapid water loss (Turner and Long, 1990).
Measurements of Total Leaf Water and Osmotic Potentials with Thermocouple Psychrometers
Measurements of thermocouple psychrometers-based total leaf water (W(PSY)) and osmotic potentials were made using standard psychrometric techniques (Brown and Collins, 1980; Walker et al., 1983; Briscoe, 1984). A leaf sample approximately 50 mm long was rapidly excised from the central section of the uppermost fully expanded sunlit leaf (excluding the midrib), rolled, and inserted into the sample chamber and sealed within 15 s. Surgical gloves were worn to minimize contamination of the sample. Psychrometer chambers containing an excised leaf were placed in an insulated isothermal cooler at the field site and transported to a temperature-controlled room maintained at 25°C within 2 to 4 h. The W(PSY) was measured when thermal and vapor pressure equilibrium had been established (within 4 h). The sample chambers were then frozen in liquid N2 for 3 min and thawed. Measurements of were obtained after re-establishment of thermal and vapor pressure equilibrium (within 2 h). Measurements of W(PSY) and were taken at midday from anthesis through grain filling, up to soft dough, which corresponds with the third (e:96 and f:106) and fourth (g:111, h:113, i:120, and j:124) soil dehydration cycles during 1994 (letters in circles correspond with DAE illustrated for FD treatment in Fig. 1c). Both midday and midafternoon psychrometric measurements were taken on DAE 124. Values of turgor potential (P) were derived from W(PSY) and (i.e., P = W(PSY) – ; Kirkham, 1990). No attempt was made to correct the psychrometric obtained measurements of for apoplastic dilution of the symplast (Campbell et al., 1979) or starch hydrolysis (Bennett et al., 1986).
Two types of leaf in situ thermocouple psychrometers were used in combination with their associated stainless steel sampling chambers; 48 of them were Model 84-3vC psychrometers (J.R.D. Merrill Specialty Equipment, Logan, UT), and 36 of them were Model C-30 psychrometers (Wescor, Logan, UT).
Root and Shoot Growth
The seasonal average (BR) and maximum (BR(max)) living root biomass (excluding crown tissue) in a 1-m soil profile was obtained from two in-row and one inter-row root cores (86 mm, i.d.) using a gas-driven soil core device (Eijkelkamp Agrisearch Equipment, Cobra Model 248, Sweden). Cores were extracted with a portable lever system to minimize mechanical damage to adjacent sampling areas. Following extraction, all individual cores were placed in a –14°C freezer within 4 h of sampling. Root and organic debris material from each core was elutriated (Smucker et al., 1982) from the soil with a modified hydropneumatic elutriation system (Wall, 2000). Live roots were separated from organic debris material manually (intact, white-colored roots). The total shoot biomass (BS) above each root core (i.e., aboveground green and brown leaves, sheaths, culms, and ears and belowground crown tissue) was also collected. From these data the seasonal maximum shoot biomass (BS(max)) was obtained. Both BR and BS material were oven-dried at 68°C for 48 h, desiccator cooled, and weighed. Lastly, the root/shoot ratio (BR/BS) was derived from values of BR and BS.
Statistical Analysis
Data were analyzed as a strip-split-plot design using the SAS Mixed Procedure (Littell et al., 1996) for the ANOVAs with C as the main plot and I as the strip-split plot (Wall and Kimball, 1993). A third factor in the ANOVA was soil dehydration cycles (D). As many as nine and six soil dehydration cycles occurred in the Dry plots during 1993 and 1994, respectively (Fig. 1a, c), whereas the Wet plots remained hydrated throughout the experiment. Nevertheless, to synchronize development stages and soil dehydration cycles between years, both Dry and Wet plots were reclassified into two separate soil dehydration cycles during vegetative and reproductive development, respectively. The D effect was treated as a repeated measure specifying first order, autoregression correlation for the covariance structure. Time of day (T) was treated as a fourth factor in the ANOVA [MM: midmorning (2.5 h before solar noon); MD: midday (solar noon); MA: midafternoon (2.5 h after solar noon)]. A fifth factor in the ANOVA was year (Y). To determine the effect of leaf position (Z) on TNC levels, a three-way ANOVA (i.e., Z, C, I) was performed.
To avoid pseudo-replication, all ANOVAs were performed on replication means. Effects (E) (i.e., Y, D, T, C, I, Z) declared "significant" in the text have P values 
0.05 unless stated otherwise. Tests for homogeneity of variance and subsequent data transformation were performed whenever appropriate before any ANOVA (Box et al., 1978). A covariant analysis (W(PSY) as the covariant) was used to determine the C effect on and P.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Analysis of Variance
Overall, C, I, and C x I interactive effects were consistent across Y, D, and T effects (Table 1). The I effect predominated over the C effect. Significance C x I interaction effects increased in both frequency and level of significance (lower P value) as S became depleted. This occurred mostly because of a proportionately greater disparity in the C effect under Dry compared with that under Wet. Significant Y effects occurred primarily because the low-volume, high-frequency irrigation regime caused only mild water stress during 1993, whereas the high-volume, low-frequency irrigation regime caused more severe water stress during 1994 (Fig. 1a, c). The D effect was insignificant during vegetative development (first and second soil dehydration cycles), but both the frequency and level of significance of the D effect increased during reproductive development (third and fourth soil dehydration cycles) (Fig. 1a, c). Significant T effects occurred primarily because of a greater responsiveness of parameters at MA (a period when drought stress is usually at its diurnal peak) compared with either MM or MD.
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W (Fig. 3
, 4,
and 5;
Table 1), A (Fig. 6
and 7;
Table 1), and particularly TNC (Fig. 8
and 9)
, whereas they were less prevalent and less significant for edaphic (Fig. 1; Table 1) and growth (Fig. 13; Table 1) parameters.
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S, thereby causing less negative MS was maintained at a soil–water depletion of between 30 and 40% of available water in the Wet plots, whereas it was reduced to as low as 60 to 80% of available water for the Dry plots (Hunsaker et al., 1996). Over the 2-yr study, rainfall amounts were nominal (Fig. 1b, d); therefore, values of S in the Dry plots became progressively lower with consecutive soil dehydration cycles (Fig. 1a, c). Although the high-volume, low-frequency irrigation strategy enabled greater soil dehydration during 1994 (Fig. 1c, d) than the low-volume, high-frequency one did during 1993 (Fig. 1a, b), these irrigation strategies provided large enough differences in S between Dry and Wet treatments for a comparative study.
Since S was used to derive M, patterns in Y, D, C, and I and their interaction effects were similar (Table 1). For well-watered wheat (Wet), M remained relatively constant at field capacity (i.e., M 
–0.03 MPa) throughout the season (Fig. 1a, c). Notwithstanding, because of a slight reduction in whole-canopy ET (Table 2), presumably because of an elevated Ca–induced reduction in gs (Fig. 2; Table 3), a slight reduction in soil–water depletion was observed for well-watered wheat grown in elevated Ca. The mitigating effect of elevated Ca in conserving S via a reduction in gs, however, can be counterbalanced by the relatively greater stimulation of elevated Ca in increasing root/shoot biomass (Fig. 13) and subsequent leaf area (Kimball et al., 1995; Samarakoon and Gifford, 1995; Pinter et al., 1996). Under ample water supply, therefore, elevated Ca had only a nominal effect on soil–water depletion. Nevertheless, M was not sensitive enough to detect these minor differences in soil–water depletion, whereas the soil–water (Hunsaker et al., 1996) and energy (Kimball et al., 1995) balance methods were. Therefore, under ample water supply we accept Hypothesis 1 for S, but reject it for M.
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M were as negative as –0.91 MPa for FD, but only reached –0.27 MPa for CD (Fig. 1c). The soil–water balance analysis demonstrated that wheat grown under elevated Ca and ample water supply (Wet) had a 5% reduction in seasonal ETc compared with Control. In contrast, under limited water supply (Dry) an increase of 5 and 0.9% in ETc occurred compared with Control under mild (1993) and more severe drought (1994) conditions, respectively (Hunsaker et al., 1996). This resulted in greater soil–water depletion when water was limited compared with when it was abundant, suggesting that the more robust root system, particularly fine roots, under FACE (Fig. 13a, b) (Wechsung et al., 1995, 1999) was able to extract more water from the soil. Under limited water supply, therefore, elevated Ca caused lower S, hence more negative M (Fig. 1a, c), particularly when drought conditions were the most severe during the fourth soil dehydration cycle at grain filling during 1994 (Fig. 1c). Hence, under reduced water supply, we reject Hypothesis 1 for both S and M.
Water Vapor Exchange Rates
Hypothesis 2: Conservation of water via an elevated Ca–based reduction in gs will enable stomata to remain open for a longer period into a drought
The overall treatment response ranking for gs was CW > FW 
CD > FD (Fig. 2). Across most of the sunlit period an approximately 180 µmol mol–1 increase in Ca reduced gs for well-watered wheat by 33%, regardless of time of day (Fig. 2; Tables 1, 3). Furthermore, any differences in gs resulted from a decrease in stomatal aperture, rather than stomatal density (Estiarte et al., 1994). Morison (1993, 1998) reported that a Ca as high as twice (i.e., 700 µmol CO2 mol–1) that in the present atmosphere generally resulted in a reduction in gs of well-watered wheat at midday by about 40% (±5%). The elevated CO2–based reduction in gs across the sunlit period for well-watered wheat reported herein was consistent with a 20% lower TR measured in a leaf cuvette (Table 3), and with companion studies that showed a 7 to 23% lower TR obtained from stem flow gauges (Senock et al., 1996), an 8% lower latent heat flux derived from energy balance measurements (Kimball et al., 1995, 1999), a 5% lower rate of S extraction derived through a soil–water balance technique (Hunsaker et al., 1996), lower apparent root hydraulic conductivity (Wall, unpublished data, 1994), a 1.5°C increase in individual leaf temperature measured in a cuvette (Table 3), a 0.6°C increase in daytime canopy surface temperatures (Kimball et al., 1995), and improved (less negative) W(PB) of 0.04 MPa (3%) (Fig. 3
–5; Tables 1, 3).
Under water stress, gs was reduced by as much as 70% (Fig. 2; Tables 1, 3). Furthermore, under drought conditions, the effect of elevated Ca on gs was dependent on T (Fig. 2; Tables 1, 3) because gs was reduced by an insignificant 2% at MM and MD, but it was reduced by a significant 25% at MA (Fig. 2). This elevated Ca–based reduction in gs across the sunlit period for water-stressed wheat lowered TR measured in a leaf cuvette by 29% (Table 3) and improved (less negative) W(PB) by 0.16 MPa (9%) (Fig. 3–5; Tables 1, 3). Significant C x I interaction effects occurred because of a greater disparity in TR between FD and CD than between FW and CW (Tables 1, 3). Under drought conditions and ambient Ca levels, the more negative values of W(PB) induced a reduction in stomatal aperture. But, under elevated Ca, less negative values of W(PB) enabled less of a proportionate midafternoon depression in gs by as much as 25% (Fig. 2). Therefore, we accept Hypothesis 2.
Plant Water Relations
Hypothesis 3: During the diurnal period, when transpiration rates are at their maximum, elevated Ca will improve (less negative) midday W, and during the nocturnal period greater recovery in plant water status will be observed at predawn and sunset
More negative M usually cause more negative W and , and lower P in field-grown wheat in both irrigated and dryland soil moisture regimes (Denmead and Millar, 1976; Millar and Denmead, 1976). Our results were consistent with this trend because as S became depleted throughout the ontogeny of the crop, M became more negative (Fig. 1a, c). And as M became more negative, season-long values of W(PB) also became progressively more negative (Fig. 3 and 4). Furthermore,W(PB) was significantly more negative in Dry than Wet and for Control compared with FACE (Fig. 3; Tables 1, 3), thereby resulting in a treatment response ranking (i.e., less negative) for W(PB) of FW > CW > FD > CD (Fig. 3). During 1993, the first significant C x I interaction effect for W(PB) was observed at MA during inflorescence emergence on DAE 75 (Fig. 3a), but during 1994 it occurred one soil dehydration cycle earlier at stem-elongation on DAE 69 (Fig. 3g). These significant C x I interaction effects occurred because, on a relative basis, a greater disparity in W(PB) was observed between FD and CD than between FW and CW.
At predawn and sunset, the dependency of W(PB) on M was more pronounced in Dry than Wet (Tables 1, 3). Although FACE fostered greater recovery in values of W(PB) to almost predawn (Fig. 4a, b) levels by sunset (Fig. 4e, f), over the nocturnal period, enough time had elapsed to enable recovery of predawn value of W(PB) to that of M, regardless of Ca level. Nevertheless, FACE caused slightly less negative values of W(PB) at both predawn and sunset.
The S became the most depleted during the third and fourth soil dehydration cycles (see circled letters e–j; Fig. 1c). Although water stress was mild during the third soil dehydration cycle (DAE 96, 106) (Fig. 1c), W(PSY) was less negative and was more negative in FACE compared with Control (Fig. 5b). But, little difference in P was observed (Fig. 5a). In contrast, during the fourth soil dehydration cycle, a parabolic trend reversal in P occurred as S became depleted from fully rehydrated (P: ranking of CD > FD > CW > FW) at mid milk ripe on DAE 111 to dehydrated (P: ranking of FW > CW > FD > CD) at soft dough on DAE 124, as evidenced by a significant C x I interaction during the fourth soil dehydration cycle on DAE 120 (Fig. 5a). Following soil rehydration on DAE 111 (Fig. 1c), greater recovery in P occurred between CD and FD than between CW and FW. As the number of days since the last irrigation in the Dry treatments increased from 1 on DAE 111 to 14 on DAE 124 (Fig. 1c), compared with CD, W(PSY) for FD remained less negative and became more negative (Fig. 5b). This resulted in more positive P (Fig. 5a). This parabolic trend reversal in P between FACE and Control occurred despite the fact that at soft dough on DAE 124 M became as negative as –0.91 MPa in FD but was only –0.27 MPa in Control (Fig. 1c).
Regardless of C level, W(PSY) and became more negative with increasingly negative M. But, as W(PSY) became increasingly negative, became negative 11% slower for Control than for FACE (P = 0.20) (Fig. 5c). When M was at its most negative at –0.91 MPa at soft dough on DAE 124 (Fig. 1c), was –3.2 MPa in Control compared with –3.7 MPa in FACE (Fig. 5c). Conversely, as W(PSY) became increasingly negative, P became less positive 11% faster for Control than for FACE (P = 0.20) (data not shown). When M was at its most negative at –0.91 MPa at soft dough on DAE 124, P was 0.33 MPa less in Control (0.50 MPa) than FACE (0.83 MPa).
These results clearly demonstrate that lower gs in FACE compared with Control (Fig. 2) lowered TR (Table 3) in both Wet and Dry, thereby reducing internal plant water deficits, which resulted in less negative W(PB), particularly during MD and MA (Fig. 3–5). Ultimately, improved water relations during the diurnal period occurred because elevated Ca reduced water stress–induced midafternoon depression in A (Fig. 6, 7a, b), thereby enabling an increase in carbon gain for both A' (Fig. 11) and A'' (Fig. 12). A similar elevated Ca–based improvement in water relations occurred during the nocturnal period because less recovery in W(PB) in response to more negative M was required at predawn (Fig. 4a, b) and sunset (Fig. 4e, f). Hence, we accept Hypothesis 3.
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, thereby resulting in a decrease in P, as evidenced by leaves that maintain higher values of PW than those grown in elevated Ca. Results reported herein support this interpretation because, as M became more negative with depletion of S (Fig. 1a, c), gs was greater for elevated Ca plants. The higher gs occurred because stomata of plants grown in elevated Ca did not close until M was –1.1 MPa, whereas those of plants grown in ambient Ca closed at a M of –0.24 MPa (Wall, 2001). For a given W, elevated Ca caused more negative that improved the ability of wheat leaves to maintain greater P, and thus higher gs. Maintaining higher gs for a longer period into a drought cycle resulted in greater overall carbon gain. In a companion study of wheat ears, Wechsung et al. (2000) reported that elevated Ca caused an even greater reduction in gs of ears to water deficits than that reported herein for leaves. Since ears accumulate even more TNC than leaves for osmotic adjustment (Morgan, 1980a, 1980b), the reduction in gs of ears in response to water deficits may have also resulted from an elevated Ca–induced enhancement in accumulation of osmotic solutes.
Under more severe water stress, leaves lose turgor pressure because, as W becomes more negative (with more negative M), becomes more negative at a slower rate, which results in less positive P in wheat leaves (Johnson et al., 1984; Morgan and Condon, 1986; Kameli and Losel, 1994). However, osmoregulatory mechanisms of plants enable them to maintain more positive P for a longer period into a drought (Kirkham, 1990). In our study, elevated Ca affected the relative change in W with respect to because, as M became more negative, was 11% greater resulting in an 11% decrease in P (Fig. 5c). This Ca–based enhancement in osmoregulation was most evident in the fourth soil dehydration cycle during 1994 because, over a 14-d interval (DAE 111–DAE 124; see circled letters g–j in Fig. 1c), following an irrigation on DAE 110 that rehydrated the soil profile, a parabolic trend reversal in P occurred (Fig. 5a). Since elevated Ca is known to alleviate drought stress (Wall, 2001; Wall et al., 2001a; Wullschleger et al., 2002), leaves grown under elevated Ca require less recovery following soil rehydration. In contrast, leaves grown under current ambient Ca levels were the most stressed and required much greater recovery. As many as three soil dehydration cycles had already occurred for the Dry treatments (Fig. 1c), so that wheat plants had already become acclimated to drought. Apparently, elevated Ca enhanced this acclimation response because an increase in osmotic adjustment decreased the plasticity in response of wheat leaves to environmental constraints such as drought. This was also observed in spring wheat grown in controlled-environment growth cabinets maintained at 350 and 1000 µmol mol–1 CO2 by Sionit et al. (1981a, 1981b), who reported that elevated Ca consistently caused less negative W, more negative , and more positive p. They reported that at the end of the second soil dehydration cycle during grain filling, after leaves had become preconditioned to drought, was –1.8 MPa compared with –2.1 MPa for 350 and 1000 µmol (CO2) mol–1, respectively. Results reported herein were similar because on the last sampling date on DAE 124 1994 when M was as negative as –0.91 MPa (Fig. 1c), was –3.2 MPa compared with –3.7 MPa for ambient and elevated Ca, respectively (Fig. 5). Interpolating results from Sionit et al. (1981a, 1981b) during their second dehydration cycle (between anthesis and grain filling), was 0.50 at ambient Ca (350 µmol mol–1) and 0.75 at elevated Ca (1000 µmol mol–1). A 650 µmol mol–1 increase in Ca, therefore, induced a 50% increase in , whereas herein we report that a 180 µmol mol–1 increase in Ca induced an 11% increase in (Fig. 5). Between these two wheat experiments, a 4.5-fold difference in may have occurred, in part, because of the 3.6-fold difference between Ca treatment levels. Allen et al. (1998) also reported that, as S was depleted over a 4-d period during pod-fill for soybean grown in controlled-environment sunlit cabinets maintained at 330 and 660 µmol (CO2) mol–1, elevated Ca also enabled water-stressed soybean leaves to maintain higher levels of P.
Presumably, an increase in A because of elevated Ca (Fig. 6) resulted in higher leaf TNC content (Fig. 8 and 9), as reported elsewhere (Hendrix, 1992; Hendrix et al., 1994; Drake et al., 1997). In elongating and fully expanded zones of leaves of water-stressed wheat seedlings grown at ambient Ca and high light intensities (i.e., high availability of organic solutes), an increase in osmotic adjustment was positively correlated with higher TNC pools (Munns et al., 1979; Munns and Weir, 1981). Blum et al. (1988) showed a positive correlation between the accumulation of TNC and stomatal aperture in wheat, presumably because osmoregulation enables wheat leaves to tolerate drought. Hence, an increase in the TNC pool because of an elevated Ca–based stimulation of A (Fig. 3) may have contributed to an increase in the accumulation of osmotica in vacuoles (Acevedo et al., 1979; Pollock, 1986; Chatterton et al., 1987), thereby explaining an increase in (Fig. 5c). Therefore, we accept Hypothesis 4.
Stomatal and Nonstomatal Limitations to Carbon Gain
Hypothesis 5: Under ample water supply any reduction in carbon gain will occur mostly because of nonstomatal limitations, whereas under drought conditions stomatal limitations will reduce carbon gain
The treatment response ranking for A was FW > FD CW > CD (Fig. 6)—nearly the inverse of that for gs (Fig. 2). Compared with Wet, Dry significantly reduced A, whereas compared with Control, FACE significantly increased A (Fig. 6; Tables 1, 3). Significant C x I interaction effects occurred because elevated Ca reduced water stress–induced depressions in carbongain, particularly at MD (Fig. 6; Tables 1, 3). This resulted in a hysteresis effect when A vs. PPFD was plotted from dawn until MD compared with that from MD to dusk (Fig. 10a
). Across years and development stages, this hysteresis effect appeared to be greater at inflorescence emergence on DAE 81 (Fig. 6h). Although a minor hysteresis effect occurred for CW on DAE 81, it was more pronounced in CD (Fig. 10a). In contrast, under elevated Ca, hysteresis patterns were more similar between Wet and Dry plots. The hysteresis effect on A became more pronounced as S became more depleted later in the ontogeny of the crop. Relationships between A and PPFD from dawn to MD and from MD to dusk were normalized (An) by dividing each observation of A within a day by the maximum value of A (Amax) for that day (Fig. 10b). All values of An were pooled across years and development stages, except during senescence at hard-dough (DAE 118) during 1993 and soft dough (DAE 123) during 1994. Clearly, no hysteresis effect occurred under CW, a slight hysteresis effect was observed under FW, but only a modest increase in this effect was observed for FD. In contrast, a large hysteresis effect was observed for CD. These hysteresis effects occurred primarily because of a midafternoon depression in leaf A (Fig. 6). The degree of hysteresis was proportional to any stomatal and/or nonstomatal limitations to carbon gain, whereas the amount of potential carbon loss was directly proportional to the integrated area within the hysteresis loop (Fig. 10).
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At midday during inflorescence emergence (DAE 75), a period of rapid growth when sink limitations usually does not limit carbon uptake, leaf starch levels were 67% greater at elevated (10 g kg–1) compared with ambient (6 g kg–1) Ca, and by dusk they more than doubled, reaching 27 g kg–1 in elevated compared with 12 g kg–1 in ambient Ca (Nie et al., 1995a). Furthermore, by dusk the TNC content reached 250 g kg–1 in elevated compared with 195 g kg–1 in ambient Ca (Fig. 8c). And at anthesis (DAE 90, 1993), when sink demand for the developing kernels during reproductive development was even greater than that at inflorescence emergence during vegetative development, elevated Ca more than doubled these aforementioned effects (Fig. 8d). Further evidence for this phenomenon was observed by comparing the TNC content in the flag leaf of the whole-canopy profile during 1994 (Fig. 9a) with midday values of TNC during anthesis in 1993 (Fig. 8d). Clearly, throughout the diurnal period export of carbon from the leaves lagged behind production. A similar elevated Ca–based increase in TNC pools occurred for all development stages except during the onset of senescence at late grain filling (Fig. 8f, k). Only a minor hysteresis effect was observed for well-watered ambient Ca Control leaves (Fig. 10a) (presumably, export of carbon from the leaves kept pace with synthesis), whereas a much greater hysteresis effect was observed for well-watered elevated Ca leaves (presumably, export of carbon from the leaves lagged behind synthesis). Therefore, the observed hysteresis effect for well-watered leaves grown under elevated Ca (Fig. 10b) can be explained by accumulation of carbon that probably caused nonstomatal limitation such as photoinhibition (Azcón-Bieto, 1983; DeLucia et al., 1985).
Compared with the substantial reduction in gs due to FACE under well-watered conditions, FACE caused only a nominal decrease in gs of the water-stressed plants (Table 2, Fig. 2), and only a slightly greater hysteresis loop than that of well-watered wheat was observed (Fig. 10b). Consequently, we believe that the bulk of the potential loss in daily carbon gain within the hysteresis loop (Fig. 10b) resulted from nonstomatal rather than stomatal factors, and we accept Hypothesis 5.
Hypothesis 6: On a relative basis, the net gain in carbon uptake in response to elevated Ca will be proportionately greater under dry than wet
Essentially no hysteresis occurred in well-watered, ambient Ca grown leaves (Fig. 10b) where gs was nonlimiting, and TNC pools were much lower than in elevated Ca leaves (Fig. 9). However, a large hysteresis effect was observed for leaves grown under water stress and ambient Ca (Fig. 10b). Undoubtably, stomatal factors were responsible for the hysteresis effect observed in water-stressed, ambient Ca leaves because mild water stress should not damage chloroplast reactions (Sharkey and Seemann, 1989). Furthermore, water stress–induced reductions in gs were observed (Fig. 2 and 7g, h), and TNC pools were much lower in ambient than in elevated Ca leaves (Fig. 8). Apparently, these stomatal limitations were severe enough to limit substrate availability at the site of carboxylation, thereby resulting in midafternoon depressions in carbon gain for water-stressed, ambient Ca leaves (Boyer, 1971; Sionit et al., 1981a, 1981b; Tenhunen et al., 1980; Tenhunen, 1984). Under water stress conditions, therefore, elevated Ca minimized stomatal limitation, which enabled greater carbon gain for a longer period into the drought. These results also demonstrate, however, that although leaves grown under elevated Ca will have less stomatal limitation, regardless of water supply, they will still experience some midafternoon depression in A because of nonstomatal limitation. Therefore, we accept Hypothesis 6.
Carbon Gain during Senescence
Earlier senescence in Dry compared with Wet reduced gs and A, but this reduction was most pronounced in FACE compared with Control at MD during 1994 (Fig. 7b). For Dry, the C effect on A inverted on DAE 103 for both years (Fig. 7a, b, e, f), whereas for Wet this treatment inversion occurred 20 d later on DAE 123, 1994 (Fig. 7b, c, d). The absence of positive carbon gain marked the onset of physiological maturity, which occurred 14 d earlier for Dry (DAE 123) compared with Wet (DAE 137) during 1994 (Fig. 7b). Although gs decreased throughout senescence (Fig. 7g, h), no C treatment inversion was observed (Fig. 7i, j). The precipitous drop in A that occurred during senescence (Fig. 7c, d, e, f) was greater for leaves grown at elevated compared with ambient Ca. In a companion study, Nie et al. (1995a, 1995b) showed a similar precipitous drop in total flag leaf and thylakoid polypeptide content, which can explain the rapid decline in A reported herein during senescence for leaves grown in elevated compared with ambient Ca (Fig. 7c, d, e, f). A rapid reduction in TNC also occurred during soft-dough (Fig. 8k) compared with that observed from tillering through milk-ripe (Fig. 8g, h, i, j). In open-top chambers maintained at 350, 525, and 750 µmol CO2 mol–1, Sicher and Bunce (1997) demonstrated that the effects of elevated Ca on total soluble and Rubisco protein content, chlorophyll content, and Rubisco activity of flag leaves of wheat and penultimate leaves of barley (Hordeum vulgare L.) were consistent with an acceleration of senescence. They concluded that the primary factor affecting Rubisco activity in response to elevated Ca was an acceleration in ontogeny that resulted in premature senescence. During the final 8 d of a 33-d experiment, that began before anthesis in wheat grown in open-top chambers maintained at 350 and 750 µmol CO2 mol–1, evidence of a premature senescence in flag leaf constituents (i.e., lower soluble protein, -amino N, and chlorophyll a and b concentrations and higher acid proteinase activity) was observed in flag leaves grown in elevated compared with ambient Ca (Sicher and Bunce, 1998). Regardless of Ca treatment, plants grown in open-top-chamber studies had similar air turbulence and thermal regimes (±0.5°C). Consequently, the accelerated senescence observed by Sicher and Bunce (1998) was determined to be a direct consequence of elevated Ca rather than any artifact imposed by the open-top chamber itself. In a companion study, Pinter et al. (2000) attributed the accelerated senescence in flag leaves that reduced thylakoid polypeptide content for leaves reported by Nie et al. (1995a) and values of A reported herein (Fig. 6e, j; 7c, d, e, f) to the additional thermal energy caused by nighttime blower-induced turbulence in the Ca–enriched plots above and beyond that available in the ambient plots that had no blowers. Nevertheless, despite the presence or absence of blowers in the ambient plots, preanthesis photoassimilate was greater in elevated Ca plots because they had significant reductions in water stress–induced midafternoon depressions in carbon gain (Fig. 6). During senescence, however, a treatment inversion for A between elevated Ca and ambient plots occurred. An accelerated decline in photosynthetic capacity that reduced production of postanthesis photoassimilates in elevated Ca compared with ambient plants was first observed for water-stressed (Fig. 7e, f) compared with well-watered (Fig. 7c, d) plants. Based on values of A reported herein (Fig. 7c, d, e, f) and trends in green leaf area (Pinter et al., 1996), the accelerated senescence in leaves grown in elevated Ca is consistent with accelerated development and premature senescence reported elsewhere (Sionit et al., 1980a, 1980b; Morison and Gifford, 1984a, 1984b; Sicher and Bunce, 1998).
Carbohydrate Pool Dynamics
Hypothesis 7: Improved water relations that increase carbon gain will be evidenced by an increase in the TNC in source leaves
Throughout most of the growing season, the treatment response ranking for TNC was FW > CW FD > CD (Fig. 8a–d, g–j), but just before physiological maturity this treatment ranking nearly inverted to CD > FD > FW > CW (Fig. 8e–f, k). Overall, TNC levels were significantly greater during a year of mild water stress during 1993 (Fig. 8a–f), compared with a year of more severe water stress during 1994 (Fig. 8g–k). Although the effect of FACE on TNC was nominal under Dry, it significantly increased TNC under Wet conditions (Fig. 8; Tables 1, 3).
During anthesis on DAE 89, 1994, when canopy height (Z) reached its maximum, TNC was measured for the flag through flag-4 leaves on individual culms (Fig. 9a). A significant Z effect (P 0.0001) occurred because TNC was significantly higher at the top of the canopy in the flag leaf and diminished thereafter from the flag-1 to flag-4 leaves (Fig. 9a). The TNC was higher in FACE compared with Control (P = 0.24) for the flag through flag-3 leaves, but a significant Z x C interaction effect (P = 0.0001) was observed because of a treatment inversion that occurred for the flag-4 leaf. Despite the overall seasonal and temporal trend for TNC levels to be higher in the upper-most sunlit leaf for Wet compared with Dry (Fig. 8), at anthesis on DAE 89 during 1994, significantly higher TNC levels were observed in Dry than Wet (P = 0.0009) across the entire crop canopy. Nevertheless, a significant Z x I interaction effect (P = 0.0004) occurred because this effect also diminished from the flag to the flag-4 leaves. Although the C x I interaction was insignificant (P = 0.82), a pronounced Z x C x I interaction effect (P = 0.15) occurred because on a relative basis, a greater disparity in TNC levels occurred in leaves between CW and FW than between CD and FD, but this trend also diminished from the flag to the flag-4 leaves (Fig. 9b).
Because the export of simple sugars can lag behind production, accumulation of TNC in source leaves can cause physical manifestations such as discoloration, chlorosis, necrotic spots, brittleness, leaf curling, and even the onset of early senescence (Neales and Incoll, 1969; Mauney et al., 1979). Many of these visual signs of TNC accumulation, especially the onset of early senescence, were observed in the Ca–enriched plots (Fig. 7c, d, e, f). An abundance of preanthesis TNC production (Fig. 8) and decreased carboxylation capacity during senescence (Nie et al., 1995a), which diminished postanthesis TNC pools, suggest that during grain filling wheat plants grown in elevated Ca were more dependent on preanthesis and less reliant on postanthesis photoassimilate supply to reach physiological maturity. In a field study in Mexico, Bidinger et al. (1977) reported that preanthesis photosynthate supplied 0.12 and 0.22 of grain dry matter under well-watered and drought-stressed conditions, respectively. Gent (1994) reported that preanthesis TNC reserves contributed 0.20 to 0.50 of grain dry weight of winter wheat. More importantly, however, he reported that the contribution of TNC reserves to grain weight could be greater than 0.50 under stressful climates such as that reported herein for water-stressed wheat. Perhaps the amount of carbon accumulated in the kernels each day during grain filling was not as dependent on daily assimilation of carbon because preanthesis TNC reserves were greater in leaves grown in elevated compared with ambient Ca. Wechsung et al. (2000) demonstrated that under elevated Ca, the relative contribution of postanthesis photoassimilate of ears (i.e., awns and glumes) to grain filling was also reduced, presumably because higher levels of preanthesis TNC may have been translocated to the developing kernels.
Establishment of higher TNC pools during the sunlit period ensured that an adequate nocturnal carbon supply was available for growth and maintenance processes at night. During a year of mild water stress in 1993, the initial levels of TNC at dawn increased from the eighth leaf at stem elongation to flag leaf at anthesis (Fig. 8a–f), whereas during a year of more severe water stress during 1994 this trend was less prevalent (Fig. 8g–k). Nevertheless, this suggest that during 1993 (Fig. 5a–f), nocturnal consumption of TNC lagged behind the previous day's accumulation even during a period of rapid growth when TNC demand was high because of developing sinks such as shoots (Pinter et al., 1996) and roots (Wechsung et al., 1995, 1999) (Fig. 13). Since reserve TNCs in C3 cool-season annual grasses, such as wheat, are normally stored in crown and sheath tissue (Weinmann, 1948), the presence of residual TNC during predawn in leaves following nocturnal consumption suggest that higher levels of reserves must have existed in organs known to store TNC (i.e., crown and leaf sheath). This suggests that throughout the season, elevated Ca may have caused daily net increases in TNC reserve pools. Consequently, we accept Hypothesis 7.
Growth and Yield Response
Hypothesis 8: Elevated Ca will foster a positive feedback relationship between aboveground source capacity (shoots) and belowground sink demand (roots), and this effect will be proportionately greater under dry than wet conditions
An increase in both WUE and IWUE, ample TNC pools and as much as a 1.5°C warmer uppermost canopy sunlit leaf temperature reported herein (Table 3), and a0.6°C increase in whole-canopy temperature reported in a companion study (Kimball et al., 1995; Pinter et al., 2000), can explain the elevated Ca–induced increase in the initiation and growth of organs during seedling development (Li et al., 1997), an advantage that persisted throughout the remainder of the growing season (Kimball et al., 1995; Pinter et al., 1996). Elevated Ca caused an increase in A' (Fig. 11
) and A'' (Fig. 12
) carbon gain for uppermost sunlit leaves and also for those lower in the canopy (Fig. 9), which explains the 19 and 44% stimulation of whole-canopy net assimilation rate reported for well-watered and water-stressed wheat canopies, respectively (Kimball et al., 1995). Any Ca–induced enhancement in TNC reserves ensured that actual growth approximated that of potential (Li et al., 1997, 1999). It also ensured that any adaptation to drought (Turner and Long, 1980) that would require additional carbon supply was more likely to occur in water-stressed leaves grown in elevated Ca. An enhancement in drought adaptations because of elevated Ca (Wall, 2001; Wall et al., 2001a) minimized water loss and resulted in less negative W (Fig. 3, 4, and 5), as observed in Brassica species (Uprety et al., 1995). An increase in TNC pools also enhanced morphological characteristics of roots that increased root mass (Fig. 13
), length, density, and average thickness, thereby increasing root cylindrical surface area (Wall et al., 1996, 2001b; Wechsung et al., 1995, 1999)—particularly, for fine roots that are primarily responsible for absorption of water and nutrients. Therefore, it appears that the stimulation of carbon gain because of elevated Ca, mediated by an increase in TNC reserves, enabled a positive rather than a null or negative feedback between source capacity of aboveground (shoots) and belowground (roots) sinks (Diaz et al., 1993; Zak et al., 1993), and this effect was proportionately greater under Dry compared with Wet conditions. Hence, we accept Hypothesis 8.
Because there was an acceleration in senescence under elevated Ca—presumably because of ample supply of preanthesis photoassimilate-based TNC reserves and/or additional thermal energy provided by the blower-induced turbulence in the elevated Ca plots—the duration of grain filling was shortened in the FACE compared with Control plots (Li et al., 2000). Hence, the longer duration of the grain filling period for Control compared with FACE reduced the difference in grain yield between FACE and Control. Nevertheless, even though the postanthesis photoassimilate supply was reduced under elevated Ca, a consistently greater preanthesis TNC supply probably enhanced grain-filling rates (Li et al., 2000). Ultimately, elevated Ca resulted in an increased grain yield by 10 and 23% for well-watered and water-stressed wheat, respectively (Kimball et al., 1995; Pinter et al., 1996).
In view of these results, the accumulation of photoassimilates as TNC pools during the sunlit period, their utilization for growth and maintenance during the nocturnal period, and the quantity and location of long-term TNC storage reserves in plant organs is a subject of vital importance in global change research today, one that requires additional investigation.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Amount of fertilizer (kg N ha–1) applied is given in parenthesis above irrigation histograms at tillering, inflorescence emergence, and anthesis for (b) 1993 and (d) 1994. Cumulative distribution curves (Cum.) denoting the amount of water (drip-tape irrigation + rainfall) given by right-most y intercept for Dry and Wet treatments in panels (b) and (d). Onset of the differential irrigation denoted by separation in Cum. curves on 1 Mar. 1993 (DAE 60) in panel (b) and 20 Jan. 1994 (DAE 20) in panel (d). Up-pointing arrows in panels (b) and (d) denote DAE when gas exchange, water relation, and leaf tissue carbohydrate concentration measurements were measured, whereas parenthesis below up-pointing arrows denote number of days between measurement dates and the last irrigated for (Dry:Wet) treatments. 
Letters e through j in legend of panel (c) denote DAE and number of days between measurement dates and last irrigated for (Dry:Wet) treatments during the third (e, f) and fourth (g, h, i, and j) soil dehydration cycles when standard thermocouple psychometric techniques were employed to measure leaf total water and osmotic potentials during 1994. Given on the right-most y axis are the sources of variance in ANOVA, which include carbon dioxide (C) for main plot, irrigation (I) for split-plot, and C x I interaction effects in a strip-split-plot experimental design. Significance effects given within soil dehydration cycle as *, **, ***, and ns for P
P
