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a USDA-ARS Air Quality - Plant Growth and Development Research Unit, 3908 Inwood Road, Raleigh, NC 27603 and Dep. of Plant Pathology, North Carolina State Univ, Raleigh, NC USA
b USDA-ARS Air Quality - Plant Growth and Development Research Unit, 3908 Inwood Road, Raleigh, NC 27603, and Dep. of Crop Science, North Carolina State Univ. Cooperative Investigations of the USDA-ARS Air Quality Research Unit and the North Carolina State University USA
asheagle{at}unity.ncsu.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Abbreviations: DAP, days after planting • CF, open-top field chamber receiving charcoal filtered air • NF, open-top field chamber receiving nonfiltered air • NF+, open-top field chamber receiving nonfiltered air with O3 added for 12 h d-1 • A1.0, A1.4, and A1.9, carbon dioxide treatments at approximate seasonal proportions of the ambient carbon dioxide concentration
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Research on effects of O3 and CO2 on wheat has followed the pattern established for other crops: each gas has been studied singly until recently. Research with O3 singly has shown that yield of winter wheat (Heagle et al., 1979; Kohut et al., 1987; Kress et al., 1985) and spring wheat (Fangmeier et al., 1993; Fuhrer et al., 1989, 1992; Pleijel et al., 1991; Slaughter et al., 1989) is suppressed by seasonal O3 exposure. Unlike more sensitive crop species, however, there is a question as to whether ambient O3 concentrations are high enough to suppress wheat yield. In Europe, yield of spring wheat exposed to near-ambient O3 concentrations in nonfiltered-air (NF) chambers was 5 to 10% less than yield in charcoal filtered-air (CF) chambers at some locations (Fuhrer et al., 1989, 1992; Pleijel et al., 1991). However, near-ambient O3 did not affect yield significantly at another location (Fangmeier et al., 1993). An overview of a 3-yr study at numerous locations in Europe concluded that the spring wheat cultivar Minaret was relatively unaffected by ambient concentrations of O3 (Bender et al., 1999). In the USA, winter wheat yield in NF air was less than in CF air at some locations in the USA during some seasons (Kohut et al., 1987; Kress et al., 1985) but not at other locations (Heagle et al., 1979) or other seasons (Kress et al., 1985). Estimates from dose-response studies indicate that seasonal exposure to O3 levels that occur in some wheat production areas (7 h d-1 seasonal mean of approximately 40 nL L-1) suppress winter wheat yields by approximately 8% (range of 0–26%) (Heagle, 1989). Wheat grain quality has not been significantly affected by ambient O3 concentrations (Anguissola et al., 1994; Fuhrer et al., 1990, 1992; Pleijel et al., 1991).
Wheat responses to CO2 enrichment is similar to response of most other C3 crops, including increased photosynthesis, growth, and yield (Bender et al., 1999; Chaudhuri et al., 1990; Delgado et al., 1994; Manderscheid and Weigel, 1997; Thiobald et al., 1996; Veisz et al., 1996; Weigel et al., 1994). Wheat yield responses to CO2 in five studies in controlled environments prior to 1983 indicate that double-ambient CO2 concentrations will increase yield by 37% (with 95% confidence interval of 22 –53%) (Kimball, 1983). Studies after 1993 also show large differences in wheat yield response to CO2 enrichment (Kimball et al., 1997; Manderscheid and Weigel, 1997; Rawson, 1995; Veisz et al., 1996; Weigel et al., 1994). For example, double-ambient CO2 CO2 enrichment (Kimball et al., 1997; Manderscheid on the cultivar (Manderscheid and Weigel, 1997), and CO2 enrichment at approximately 550 µL L-1 increased yield of water-stressed and well-watered winter wheat by approximately 23 and 10%, respectively (Pinter et al., 1996).
Reports of interactive effects of O3 and CO2 for wheat indicate that elevated CO2 will ameliorate effects of O3 as expressed by photosynthesis, foliar injury, growth, and yield. (Balaquer et al., 1995; Barnes, et al., 1995; McKee et al., 1997; Mortensen, 1990; Mulholland et al., 1997a,b, 1998; Rao, et al., 1995; Rudorf et al., 1996). In spite of this general trend, none of these reports showed statistically significant O3 x CO2 interactions for yield.
Because of the wide range in reported responses of wheat to O3 and CO2 singly, and because significant O3 x CO2 interactions for wheat yield have not been reported, additional research with wheat seemed warranted. Our objective was to determine if interactions between O3 and CO2 occur for responses of winter wheat exposed to nine combinations of three O3 and three CO2 concentrations.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Plants were grown and exposed to O3 and CO2 in open-top field chambers, 3-m-diam by 2.4 m tall (Heagle et al., 1973). Plants were irrigated with drip tubes as needed to prevent visible symptoms of water stress. Pot temperatures were moderated with an insulating cylinder composed of 0.6 cm-thick bubblewrap coated on both sides with aluminum (Reflectix, Reflectix, Inc., Markleville, IN), fit tightly around each pot and secured with aluminum tape. This method of temperature moderation has proven more effective than grain straw as a mulch (Heagle et al., 1999a).
The whole plot (chamber) design was combinations of three CO2 and three O3 treatments. Ozone was dispensed for 12 h d-1 (0800–2000 h EST), and CO2 was dispensed for 24 h d-1. The O3 treatments were charcoal-filtered air (CF = approximately 0.56 times ambient O3), nonfiltered air (NF = about 0.97 times ambient O3) and NF with O3 added proportionally to the ambient O3 concentration (NF+ = about 1.86 times ambient O3). The CO2 treatments were ambient
, and two treatments with CO2 added for 24 h d-1 to achieve proportions of about 1.4 (A1.4) and 1.9 (A1.9) times ambient. General dispensing and monitoring protocols have been described for O3 (Heagle et al., 1979) and for CO2 (Rogers et al., 1983). Both gases were monitored for 24 h d-1 at canopy height in the center of each chamber. Ozone was monitored with UV analyzers (TECO Model 49, Thermo Environmental Instruments, Inc., Franklin, MA) calibrated biweekly with a TECO Model 49 PS calibrator. Carbon dioxide was monitored with infrared analyzers (LI 6252, LI-COR Inc., Lincoln, NE) calibrated biweekly with pressurized tank CO2 over the range of concentrations used in these experiments.
Ozone and CO2 concentrations and proportions of ambient (Table 1) for a given period differed somewhat from overall mean proportions. To simplify this presentation, the O3 treatments will be referred to as CF, NF, and NF+, and the CO2 treatments will be referred to as A1.0, A1.4, and A1.9. The design required 22 chambers to provide three randomized replicates for the four treatment combinations containing the high and low O3 and high and low CO2 concentrations and two randomized replicates for the five treatment combinations containing the mid-level O3 or mid-level CO2 concentrations.
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Measurements
One plant per pot was harvested on 21 April and one plant per pot was harvested on 12 May. Dry weight and number of tillers were measured for all cultivars at each harvest. Preliminary observations of foliar symptoms indicated that Coker 9904 (C9904) was relatively sensitive to O3, and that Coker 9835 (C9835) was relatively resistant. Therefore, height, leaf area, and dry weight of the mainstem shoot and tiller shoots were also measured for C9904 and C9835 at each harvest. On 12 May, the number and weight of heads were added to measures made on 21 April for all cultivars. Visible foliar injury (chlorosis and necrosis) was estimated in 5% increments (0–100%) for C9904 and C9835 on 12 May.
On 2 June, the four remaining plants per pot were harvested. All cultivars were measured for dry weight of stems + leaves, heads, seeds, 100-seeds, and number of heads. Root dry weights were measured only for C9904 and C9835.
Experiment 2 (1997–1998)
In Exp. 1, C9904 was more sensitive to injury caused by O3, C9835 was relatively resistant, and maturity dates were similar for both cultivars. Therefore, C9904 and C9835 were chosen for Exp. 2. Seeds were planted on 23 October in 15 L pots fit with aluminized bubblewrap cylinders in the open-top chamber plots. Plants were placed in a 2 by 2 Latin square arrangement in each of the four chamber quadrants with the convention that two pots of a given cultivar could not be adjacent within a given row or column. Seedlings emerged on 2 November, and plants were fertilized on 17 November with 1 L per pot of water containing 2.7 g of 20:20:20, N:P:K and 0.31 g of STEM. They were fertilized on 6 March with 1 L per pot of water containing 2.7 g of 10:30:20, N:P:K and on 26 March with 1 L of water per pot containing 4 g of 20:20:20, N:P:K. Insects were controlled with bifenthrin (Talstar F at 3.2 mL L-1 water) on 15 April. Exposures began on 6 November, ended for the winter on 5 December, resumed on 5 March and continued until 26 May when grain kernels of both cultivars were hard in all treatments.
Measurements
Plants were thinned to four per pot on 11 December (49 DAP), and shoot dry weight and total leaf area were measured for one plant per pot. After the 11 December harvest, plant density was less than four in several pots in some chambers. Therefore, pots with four plants from the south row of the southeast quadrant of some chambers were used to replace pots with less than four plants at other positions within the same treatment. Plants from the south row of southeast quadrants were not used for response measurements.
On 3 March, one plant was harvested from each pot to measure shoot dry weight, total leaf area, and number of tillers. On 21 April, one plant was harvested from each pot, and measures were made as described for the 21 April, 1997 harvest. Visible foliar injury was estimated on 7 May as the percentage chlorosis and necrosis of whole canopies for two plants of each cultivar in all plots.
On 2 June, the two remaining plants per pot were harvested. Measurements were made for seven pots per cultivar in each plot as described for the 2 June 1997 harvest. Root dry weights and 100-seed weight were also measured for three pots per cultivar per plot.
Statistical Analyses
Experiment 1 was designed and analyzed as a sub-sub plot. Because there were no position (north vs south) interactions in Exp. 1, the Latin square design for Exp. 2 was analyzed with a sub-plot model. In both experiments, the whole plot factor was the O3 + CO2 combination. For Exp. 1, the sub-plot factor was the position (north or south half of the chamber) and the sub-sub-plot factor was cultivar. For Exp. 1, separate analyses were performed for the eight cultivar group combined and for C9904 and C9835 combined. For Exp. 2, the sub-plot factor was cultivar. These whole and sub-plot factors were fixed effects. The within-chamber and within-chamber half variations were analyzed as random effects. SAS PROC MIXED software (SAS Institute, 1990) was used for analysis of variance of plot means for each sub and sub-sub plot treatment for all dependent variables at all harvest times.
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Results |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Experiment 1 (1996–1997)
Midseason Growth
On 21 April, 36 d after exposures began, mean shoot weight was 7.8 g per plant and mean number of tillers per plant was 11. Most cultivars were in the boot stage, although Pioneer 2684 and Pioneer 2643 were at early anthesis in some plots. The CO2 effect was significant
for the eight-cultivar analysis but not for the two-cultivar analyses. For the eight cultivars and O3 levels combined, shoot weight in the A1.4 or A1.9 CO2 treatments was 9 or 16% greater, respectively than in the A1.0 CO2 treatment (data not shown). The O3 effect was not significant for weight or number of tillers for either the eight- or two-cultivar analysis.
On 12 May, for the eight cultivars and treatments combined, mean shoot dry weight per plant was 12.8 g, and mean number of heads was 6 (data not shown). Foliar injury caused by O3 was more severe on C9904 than C9835, and CO2 enrichment protected both cultivars from O3 injury. For example, at A1.0 CO2, 7% of leaf area was affected by O3 injury in the NF treatment and 69% was affected in the NF+ treatment for C9904, but only 2 or 4% injury respectively, was found for C9835. However, at A1.9 CO2, no O3-induced injury was found on C9904 in the NF treatment and 16% leaf area was affected in the NF+ treatment. Foliage of C9835 was not injured in either the NF or NF+ treatment (data not shown). All main and interactive effects of O3 and cultivar on foliar injury were statistically significant. For the eight cultivars and O3 levels combined, shoot weight at A1.4 or A1.9 CO2 was 13 or 15% greater respectively, than at A1.0 CO2. Shoot weight was significantly increased by elevated CO2. Tiller and head numbers were higher by approximately 6% at both elevated CO2 levels (O3 levels combined) but differences were not significant. The O3 effect was significant on 12 May for all weight measures in the eight-cultivar and two-cultivar analyses, because plants were smaller in the NF than CF or NF+ treatments.
Final Harvest
At final harvest, mean shoot weight per plant was 14.8 g and mean number of heads was six (cultivars and treatments combined). For the eight cultivars and O3 levels combined, total shoot weight at A1.4 or A1.9 CO2 was 6 and 17% greater, respectively, than at A1.0 CO2. The CO2 effect was significant for shoot weight and several other response measures (Tables 2 and 3)
, but the cause for significance was response to CO2 only in the NF and NF+ treatments. For example, compared with A1.0 CO2, combined seed yield in the NF+ treatment increased by 3 or 15% at A1.4 or A1.9 CO2 respectively, whereas seed yield in the CF treatment was almost identical at all CO2 levels (Table 2, Fig. 1)
. In spite of this general trend for a CO2 effect on the O3 response, the O3 x CO2 interaction was significant only for total stem + leaf weight (Table 2).
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For the two-cultivar analyses, trends for O3-induced suppression of plant size and yield were greater at A1.0 than at elevated CO2 (Table 3). This effect of CO2 on the O3 response was greater for the relatively O3-sensitive C9904 than for C9835, so the differences in plant size and yield between the CF, NF, and NF+ treatments at A1.0 CO2 were greater for C9904 than C9835. Carbon dioxide at A1.9 completely protected both cultivars from effects of O3 on seed yield (Table 3, Fig. 1). Differences in seed yield resulted from combined differences in number of heads per plant (and thereby number of seeds) and weight per seed. The cultivar x O3 x CO2 interaction was statistically significant
only for weight per 100 seeds, but similar trends were found for all other measures
. There was no consistent trend for CO2 enrichment to cause increased yield of C9904 or C9835 plants grown in CF air (Table 3, Fig. 1).
Experiment 2 (1997–1998)
Midseason Growth
On 11 December, 6 d after fall exposures ended, mean dry weight of C9904 and C9835 was 0.26 and 0.22 g per plant, respectively (gas treatments combined). Visible foliar injury was not found for any treatment combination. On 3 March, 2 d before spring exposures began, mean dry weight of C9904 and C9835 was 4.0 and 3.5 g per plant, respectively. C9904 weighed significantly more and had more leaf area than C9835 on 11 December and 3 March, but neither O3 nor CO2 showed significant effects for either harvest.
On 21 April, mean shoot dry weight per plant was 24.9 g, mean number of tillers was 16, and mean number of heads was seven (cultivars and treatments combined). C9904 was larger than C9835 as measured by all growth parameters. The O3 and CO2 main effects were significant for some but not all measures (data not shown). For example, total shoot weight of plants exposed to NF+ (cultivars and CO2 levels combined) was 9% less than plants in the CF treatment, and total shoot weight of plants at A1.9 CO2 (cultivars and O3 levels combined) was 12% greater than for plants at A1.0 CO2. Ozone appeared to cause more stress to C9904 than to C9835 at A1.0 CO2, whereas the effects of O3 were negligible for both cultivars at A1.9 CO2 (data not shown). This differential amount of O3 stress between the cultivars, and the differential increase in plant size between the cultivars as the level of CO2 increased, was the apparent cause for significant cultivar x O3 x CO2 interactions for stem + leaf weight, stem height, and a nearly significant cultivar x O3 x CO2 interaction
for total shoot weight and number of heads (data not shown).
Foliar injury estimates on May 7 confirmed that C9904 was more sensitive to O3 than C9835 and that elevated CO2 protected plants from O3 injury, as occurred in Exp. 1. For example, at A1.0 CO2, leaf area injured on C9804 was 11% in the NF and 46% in the NF+ treatment, whereas comparable values for C9835 were 1 or 29%. At A1.9 CO2, maximum injury was 4% on C9904 in the NF+ treatment (data not shown).
Final Harvest
Elevated CO2 slightly increased partitioning of biomass to vegetative tissues at the expense of reproductive tissues (Table 4)
. Most of the increase in the vegetative partition with elevated CO2 component was due to effects on roots. All main and interaction effects were significant for most growth and yield response measures (Table 4). Elevated O3 suppressed growth and yield at A1.0 CO2, whereas elevated CO2 protected plants from O3-effects, thereby increasing growth and yield at higher O3 concentrations. Two-way interactions (O3 x CO2, cultivar x O3, and cultivar x CO2) were significant for most response measures. The O3 x CO2 interaction was caused by a greater response to O3 at low than at high CO2. The cultivar x O3 interaction occurred because C9904 was more sensitive to O3 than C9835. Similarly, C9904 was enhanced more by CO2 enrichment than was C9835 because of differential cultivar sensitivity to O3 (Table 4). This differential cultivar sensitivity to O3, and resulting differential cultivar response to CO2, caused significant cultivar x O3 x CO2 interactions for all measures. For example, the NF+ treatment suppressed seed yield of C9904 by 48% at A1.0 CO2, but by only 8% at A1.9 CO2 (Table 4, Figure 2)
. Conversely, the NF+ treatment suppressed seed yield of C9835 by only 15% at A1.0 CO2 and by 14% at A1.9 CO2. Differences in seed yield were caused by combined differences in number of heads and weight per seed. For instance, the 48% decrease in seed yield for C9904 caused by the NF+ treatment at A1.0 CO2 was accompanied by a 14% decrease in number of heads and a 25% decrease in weight per 100 seeds (Table 4). At A1.9 CO2, the 8% loss of C9904 seed yield in the NF+ treatment was accompanied by a 5% decrease in number of heads and a 7% decrease in weight per seed.
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Discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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It is well established that plant growth and yield suppression caused by O3 is greater at high than at low O3 concentrations. In Exp. 1, however, growth and yield was less in the NF than in the CF or NF+ treatments. The cause for this apparent anomaly is not known. It was not caused by large differences in just one or two plots, but occurred in four of the six plots containing the NF treatment. We don't believe that differences in the rooting medium were involved because care was taken to ensure uniform volume and quality of rooting medium among the pots. Chambers containing the NF treatment were distributed randomly throughout the field, and there was no obvious field position relationship that might affect yield. We assume that the relatively low yield in the NF treatment in Exp. 1 was random. That wheat yield loss increases with increased O3 concentration has been confirmed many times for winter wheat (Heagle et al., 1979; Kohut et al., 1987; Kress, et al., 1985) and occurred for Exp. 2 of the present report.
The present experiments were performed with plants grown in 15-L pots to ensure uniform edaphic conditions. Use of pot-grown as opposed to soil-grown plants can be viewed as limiting extrapolation to results that may occur in the field with normal agronomic practice. However, several lines of circumstantial evidence suggest that plant response to O3 or CO2 is not necessarily determined by edaphic factors when adequate nutrition, moisture, and rooting volume are supplied. For example, soybean (Heagle et al., 1983), winter wheat (Heagle et al., 1979b), and field corn (Zea mays L., Heagle et al., 1979a) response to chronic O3 exposure was similar for plants grown in pots or in the ground. Soybean response to CO2 enrichment was similar for plants grown in pots and in the ground (Heagle et al., 1999a). A recent experiment at our field site showed that the degree of CO2-induced prevention of O3-induced growth and yield suppression was similar for soybean grown in pots and in the ground (A.S. Heagle and J.E. Miller, 2000, personal communication). Moreover, the present experiments also suggest that plant growth per se does not affect the CO2 x O3 interaction. Plant growth and yield in Exp. 2 was greater than for Exp. 1, probably because plants in Exp. 2 had more time to grow during the fall and were fertilized more than plants in Exp. 1. Also, spring weather conditions during Exp. 2 were cooler and more conducive to wheat growth and reproduction than during Exp. 1. In spite of these large differences in growing conditions, plants grown in the high O3 treatment were protected by elevated CO2 in both experiments.
The yield increase in response to elevated CO2 (O3 treatments combined) in the present study was much less than that reported previously. Although the present report shows that the level of stress caused by O3 may be one reason for these differences, interactions between elevated CO2 and other factors such as moisture stress and nutrient imbalance may also be involved.
The degree of O3 stress response of plants is determined by relative sensitivity to O3, by O3 concentrations, and by duration of exposure. The present report shows that the amount of O3 stress determined the growth response to elevated CO2. Wheat is less sensitive to O3 than some crops, and it is uncertain whether ambient O3 concentrations in wheat production areas routinely decrease wheat yield. Therefore, unless future wheat cultivars are more sensitive to O3 than present cultivars or unless O3 levels rise, we suspect that O3 x CO2 interactions will be of lesser importance for wheat production than for crops such as soybean (Heagle et al., 1998) and cotton (Gossypium hirsutum L.), (Heagle et al., 1999b) which are relatively sensitive to O3.
Results of the present research are consistent with previous reports for other crops showing that CO2 enrichment can prevent O3-induced yield loss. The degree that this will occur in a future CO2 enriched atmosphere will depend on highly interactive relationships between CO2 concentrations, relative O3 sensitivity of plants and O3 concentrations. More research is needed to better estimate how the complex interactions between elevated CO2, O3, and other factors will affect future crop production.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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1 The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service or the USDA of the products named, nor criticism of similar ones not mentioned. 
Received for publication November 5, 1999.
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