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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

View larger version (57K): [in a new window]   Fig. 1. Soil matric potential (M) for a 0.9-m soil profile of a Trix clay loam vs. day after 50% emergence (DAE) (i.e., calendar date same as DAE) for the (a) 1993 and (c) 1994 growing seasons. Symbols in legend for panels (a) and (c) refer to Control-Dry (CD), FACE-Dry (FD), Control-Wet, and FACE-Wet (FW) treatments (Control at 370 µmol CO2 mol–1 and FACE at 550 µmol CO2 mol–1; Dry at 50% and Wet at 100% replacement of evapotranspiration). Vertical dashed lines in panels (a) and (c) separate four individual soil dehydration cycles by development stages as follows: (1) seedling development, tillering, and stem elongation (DAE 1–71 during 1993; DAE 1–76 during 1994); (2) inflorescence emergence (DAE 72–85 during 1993; DAE 77–88 during 1994); (3) anthesis (DAE 86–103 during 1993; DAE 89–111 during 1994); and (4) grain filling through physiological maturity (DAE 104–136 during 1993; DAE 112–137 during 1994). Symbols in legend given in panel (b) refer to drip-tape irrigation for Dry and Wet treatments, rainfall amounts, and N fertilizer application dates during (b) 1993 and (d) 1994. 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 0.05, P 0.01, P 0.001, and not significant, respectively. (ne for effect not estimated); actual probability of a greater F statistic by chance reported if P 0.10 and P 0.25 P 0.10. Each mean datum was derived from four replications (i.e., means based on n = 4, SE of replication means based on n = 4).

View larger version (46K): [in a new window]   Fig. 2. Dawn to dusk trends in mean stomatal conductance to water vapor (gs) of fully expanded sunlit spring wheat leaves for day after 50% emergence (DAE) and development stages given for 5 d during the (a–e) 1993 and (f–j) 1994 growing seasons. Typical values of photosynthetic photon flux density, air temperature, and vapor pressure deficit for this semiarid desert region, for DAE 75–118 during (a–e) 1993, have been reported elsewhere (Garcia et al., 1998). Symbols in legend and sources of variance and results from ANOVA same as described in Fig. 1. Each mean datum was derived from five leaves across two observations (repeated measures), made within a 60- to 90-min period (U.S. Mountain Standard Time), across two replications for Dry or four replications for Wet during 1993 and three replications for both Dry and Wet during 1994 (i.e., means based on n = 20 for Dry and n = 40 for Wet during 1993, and means based on n = 30 for both Dry and Wet during 1994). Vertical bars are 1 SE of replication means (i.e., based on n = 2 for Dry and n = 4 for Wet during 1993 and n = 3 for both Dry and Wet during 1994). The above illustration was derived from measurements of as many as 3540 leaves.

View larger version (45K): [in a new window]   Fig. 3. Predawn to sunset trends in mean total leaf water potential measured with a pressure chamber (W(PB)) of expanded sunlit spring wheat leaves for 50% day after emergence (DAE) and development stages given for 5 d during the (a–e) 1993 and (f–j) 1994 growing seasons. Symbols in legend, sources of variance, and results from ANOVA same as described in Fig. 1. Each mean datum was derived from three to four subsamples for four replications (i.e., means based on n = 12 or n = 16). Vertical bars are 1 SE of replication means (i.e., n = 4). The above illustration was derived from measurements from as many as 3392 leaves.

View larger version (39K): [in a new window]   Fig. 4. Dawn to dusk trends in mean total leaf water potential measured with a pressure chamber (W(PB)) in fully expanded sunlit spring wheat leaves at (a, b) predawn, (c, d) midday (solar noon), and (e, f) sunset for day after 50% emergence (DAE) 43 to 118 during the (a, c, e) 1993 and DAE 41 to123 during the (b, d, f) 1994 growing seasons. Measurement dates correspond with up-pointing arrows shown in Fig. 1b, d. Each mean datum is the pooled average of three to four leaves collected within a 15-min period in four replicate blocks (i.e., means based on n = 12 or n = 16). Vertical bars are 1 SE of replication means (i.e., n = 4). Symbols in legend, source of variance, and results from ANOVA are the same as described in Fig. 1. Note that the y axis scale for (a, b) predawn and (e, f) sunset are the same, but the scale for (c, d) midday is double that of predawn and sunset. The above illustration was derived from measurements of as many as 2304 leaves.

View larger version (52K): [in a new window]   Fig. 5. Mean (b) total leaf water (W(PSY)), and (b) osmotic () potentials determined using standard thermocouple psychometric techniques, and the (a) resultant pressure (P) potential (i.e., P = W(PSY) – ) for fully expanded sunlit flag leaves of spring wheat at midday (MD) (solar noon) and at midafternoon (MA) (2.5 h after solar noon) on day after 50% emergence (DAE) and development stage given during the 1994 growing season. Letters e through j in legend of panel (b) 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 (Fig. 1c). Vertical dashed lines in panels (a) and (b) delineate the third and fourth soil dehydration cycles illustrated in Fig. 1c. Horizontal arrows in panel (a) denote overall trend in values of P across treatments. Each mean datum for W(PSY), , and the resultant P was derived from three repeated measures across four treatments and three replications (3 x 4 x 3) when Model C-30s psychrometers were used at MD on DAE 120 and MA on DAE 124, and four repeated measurements across four treatments and three replications (4 x 4 x 3) when Model 84-3vC J.D.J. Merrill Specialty Equipment, Longan, UT, were used at MD (DAE 96, 106, 111, 113, 124) (i.e., means based on n = 36 or n = 48, respectively). Vertical bars are 1 SE of replication means (i.e., n = 3). Symbols in legend, source of variance, and results from ANOVA same as described in Fig. 1. Regression of on W(PSY) (i.e., rate of change in osmotic potential; ) from anthesis until soft dough for fully expanded sunlit flag leaves of (c) spring wheat. Summary statistics for a linear regression are as follows: N, number of observations; R2, coefficient of determination; Sy/x, SE of y given a value of x. The above illustration was derived from measurements from as many as 312 leaves.

View larger version (45K): [in a new window]   Fig. 6. Dawn to dusk trends in mean leaf net assimilation rate (A) of fully expanded sunlit spring wheat leaves for day after 50% emergence (DAE) and development stages given for 5 d during the (a–e) 1993 and (f–j) 1994 growing seasons. Measurements were simultaneous with and sampled as described for gs in Fig. 2. Symbols in legend, source of variance, and results from ANOVA same as described in Fig. 1. The above illustration was derived from measurements of as many as 3540 leaves.

View larger version (47K): [in a new window]   Fig. 7. Mean maximum midday (MD: solar noon) net assimilation rate (A) (a, b) and stomatal conductance to water vapor (gs) (g, h) for fully expanded sunlit spring wheat leaves for day after 50% emergence (DAE) and development stages given during the (a, g) 1993 and (b, h) 1994 growing seasons, respectively. Histograms shown during 1994 in panel b (inserts c, d, e, and f) illustrate values of A from inflorescence emergence (DAE 81) until physiological maturity for (c, d) Wet (DAE 137) and (e, f) Dry (DAE 123) treatments, a period of accelerated senescence. Treatment inversion in the carbon dioxide (C) effect on values of A are denoted by up-pointing arrows in panel (b), which intersect approximately on DAE 124 (histogram panels c and d) for Wet, and on DAE 103 (histogram panels e and f) for Dry treatments. Histograms shown during 1994 in panel (h) (inserts i and j) illustrate values of gs for (i) Wet and (j) Dry treatments. Unlike the treatment inversion observed in the C effect for A (inserts c, d, e, f), no such effect was observed in gs (i, j). All measurements were simultaneous with, and sampled as described for gs in Fig. 2 and for A in Fig. 6. Symbols in legend, source of variance, and results from ANOVA same as described in Fig. 1. The above illustration was derived from measurements of as many as 960 leaves.

View larger version (42K): [in a new window]   Fig. 8. Dawn to dusk trends in mean leaf total nonstructural carbohydrates (TNC), consisting of water-soluble sugars (glucose, fructose, sucrose), low and high molecular weight fructans, and starch. Uppermost fully expanded sunlit spring wheat leaves were sampled for day after 50% emergence (DAE) and development stages given for 6 d during the 1993 (a–f) and 5 d during the 1994 growing seasons (g–k) (note that during 1993 panels (e) and (f) are denoted by the right-most y axis, which is scaled half that of the left-most y axis). Symbols in legend, source of variance, and results from ANOVA are the same as described in Fig. 1. Each mean datum was derived from four individual assays (repeated measures) performed on a 30-mg sample (i.e., pooled sample taken from the thoroughly mixed powder of 10 freeze-dried leaves) across four replications at 2-h sample intervals (i.e., means based on n = 16). Vertical bars are 1 SE of replication means (i.e., n = 4). The above illustration was derived from measurements of as many as 9120 leaves.

View larger version (58K): [in a new window]   Fig. 9. Midday (MD: solar noon) trends in leaf tissue total nonstructural carbohydrate concentration (TNC) (i.e., water-soluble sugars (glucose, fructose, sucrose), low and high molecular weight fructans, and starch). (a) At anthesis (day after emergence, DAE) 89 during 1994, the uppermost fully expanded sunlit flag leaf and all physiologically active leaves beneath the flag leaf (i.e., flag-1, flag-2, flag-3, and flag-4) were sampled in sequential order from the same culm, for each of 10 culms, from the top of the canopy height (Z) denoted by the left-most y axis. (b) The whole-canopy TNC profile represents the sum of the flag through flag-4 leaves denoted by right-most y axis. Each mean datum was derived from four individual assays (repeated measures) performed on a 30-mg sample (i.e., pooled sample taken from the thoroughly mixed powder of 10 freeze-dried leaves) across four replications sampled at MD (i.e., means based on n = 16). Symbols in legend and results from ANOVA same as described in Fig. 1. Vertical bars are 1 SE of replication means (i.e., n = 4). Percentages in parentheses above each mean histogram datum indicate relative CO2 enhancement at Dry and Wet, respectively. Source of variance in ANOVA are as follows: replication (R); CO2 (C); irrigation (I); and, canopy height (Z). The corresponding error term for each effect is given in parenthesis as follows: C (R x C); I (R x I); C x I (R x C x I); Z (R x Z); Z x C (R x Z x C); Z x I (R x Z x I); Z x C x I (R x Z x C x I). The above illustration was derived from measurements of as many as 800 leaves.

View larger version (31K): [in a new window]   Fig. 10. Absolute mean leaf net assimilation rate (A) (left-most y axis) in response to diurnal course of incident photosynthetic photon flux density (PPFD) during inflorescence emergence on day after 50% emergence (DAE) 81 during the 1994 growing season (replotted from Fig. 6h). The season-long mean leaf net assimilation rate (AN) was normalized to a relative scale between 0 and 1 by dividing each value of A, obtained per sample interval, by the daily maximum (Amax) leaf net assimilation rate (i.e., AN = A/Amax) across all treatments, for both years, and all development stages except for soft and hard-dough (replotted from Fig. 6a–d and f–i). On the second y axis (right-most y axis), the resultant values of AN were plotted in response to the diurnal course of (b) PPFD. Arrows denote direction of hysteresis loop from dawn (DW), after dawn (AD), early morning (EM), midmorning (MM), midday (MD), midafternoon (MA), late-afternoon (LA), predusk (PD), and dusk (DK). Symbol legend same as given in Fig. 1a. Open-circle symbol with two-way error bars denote the pooled SE across all sample intervals for the A (vertical) and PPFD (horizontal) parameters. The single diurnal course illustrated above in panel (a) was derived from measurements of as many as 360 leaves, whereas the normalized response was derived from measurements of as many as 3180 leaves.

View larger version (58K): [in a new window]   Fig. 11. Daily integrals of carbon accumulated (A') for fully expanded sunlit spring wheat leaves at development stages given during the (a) 1993 and (b) 1994 growing seasons. (a) During 1993, eight measurement dates are shown for Wet (Garcia et al., 1998) and five for Dry. (b) During 1994, five sampling dates are shown for both Wet and Dry. Measurements were simultaneous with, and sampled as described for, A in Fig. 6. Symbols in legend, SE, percentages in parentheses, source of variance, and results from ANOVA same as described in Fig. 9. The above illustration was derived from measurements from as many as 3540 leaves.

View larger version (63K): [in a new window]   Fig. 12. Seasonal integral of total carbon accumulated (A'') (integration performed from 50% emergence until canopy senescence reduced fractional absorption of incident photosynthetically active radiation to 25%) for fully expanded sunlit spring wheat leaves during the 1993 and 1994 growing seasons. During 1993, A'' is based on eight measurement dates for Wet and Dry (i.e., n = 8). During 1994, A'' is based on five sampling dates for Wet and Dry (i.e., n = 5). Measurements were simultaneous with, and sampled as described for, A in Fig. 6. The SE, percentages in parentheses, symbols in legend, source of variance, and results from ANOVA same as described in Fig. 9. The above illustration was derived from as many as 3540 leaves.

View larger version (39K): [in a new window]   Fig. 13. Seasonal trend in the (a, b) total root biomass (BR) in a 1-m soil profile and (c, d) shoot biomass (BS), and (e, f) the root to shoot ratio (BR/BS) for spring wheat during the (a, c, e) 1993 and (b, d, f) 1994 growing seasons. Sampling occurred on day after emergence (DAE) 16 and DAE 3 at the 3-leaf, DAE 36 and DAE 26 at tillering, DAE 63 and DAE 40 at stem-elongation, DAE 92 and DAE 102 at anthesis, DAE 113 and 118 at soft dough, and DAE 159 and DAE 154 at hard dough developmental stages during the 1993 and 1994 growing seasons, respectively. Each mean datum for BR was derived from four replications during vegetative development (i.e., 3-leaf, tillering, stem-elongation) and two replications during reproductive development (i.e., anthesis, soft dough, hard dough) (i.e., means and SE of replication means based on n = 4 and n = 2, respectively). During 1994, the BR was collected for four replications during vegetative and three replications during reproductive development (i.e., means and SE of replication means based on n = 4 and n = 3, respectively). The total BS (i.e., aboveground green and brown leaves, sheaths, culms and ears, and belowground crown tissue) was collected for four replications both seasons (i.e., means based on n = 4, SE of replication means based on n = 4). Symbols in legend, source of variance, and results from ANOVA same as described in Fig. 1.