Yield and quality characteristics of bahiagrass
(
Paspalum notatum
) exposed to ground-level ozone
R.B. Muntifering
a,*, D.D. Crosby
a, M.C. Powell
a,
A.H. Chappelka
baDepartment of Animal and Dairy Sciences, Auburn University, Auburn, AL 36849, USA bSchool of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849, USA
Received 30 September 1999; received in revised form 20 January 2000; accepted 10 February 2000
Abstract
Early and late season-planted bahiagrass (Paspalum notatumFlugge, cultivar `Pensacola') were grown in open-top chambers (OTC) to which added air had been carbon-®ltered (CF), representative of that found at pristine air quality sites; non-®ltered (NF), characteristic of ambient air in Auburn, AL and representative of that found in rural agricultural areas; or enriched with ozone (O3) to twice-ambient O3concentration (2X), representative of that found in the vicinity of
large metropolitan areas. Primary-growth and regrowth forages from each planting were harvested periodically throughout the experiment from each of six OTC (two OTC/air treatment). Mean daytime (09:00±21:00 h) O3concentrations over the entire 24-week experiment (7 May±23 October
1997) were 22, 45 and 91Zl lÿ1
, respectively, for CF, NF and 2X treatments. Mean daytime ambient O3 concentrations peaked in mid-May and again in late August±late September at 50±
60Zl lÿ1
, and highest individual ambient O3concentrations were recorded in late June, late July,
late August and mid-September at90Zl lÿ1
. Dry matter (DM) yield was greater for CF than for NF primary-growth forage, and concentrations of neutral detergent ®ber (NDF) were higher in 2X than in NF primary-growth and regrowth forages from the early-season planting. Concentration of acid detergent ®ber (ADF) tended to be higher in 2X than in NF primary-growth forage and was higher in 2X than in NF regrowth forage, whereas acid detergent lignin (ADL) concentration was higher in 2X than in NF primary-growth forage and tended to be higher in 2X than in NF regrowth forage from the early-season planting. Crude protein (CP) concentrations were lower in CF than in NF regrowth forage from the early-season planting and in CF than in NF primary-growth forage from the initial harvest of the late-season planting. No differences were observed among treatments in DM yield or concentrations of cell wall constituents in primary-growth or regrowth forages from the late-season planting, although concentrations of CP, NDF and ADF tended to be higher in 2X
84 (2000) 243±256
*Corresponding author. Tel.:1-334-844-1533; fax:1-334-844-1519.
E-mail address: rmuntife@acesag.auburn.edu (R.B. Muntifering)
than in NF regrowth forage. No differences were observed among treatments in concentrations of total phenolics in primary-growth or regrowth forages from either planting, although concentrations of total phenolics tended to be higher in CF than in NF primary-growth forage from the late-season planting. Particularly in the case of early-planted bahiagrass, alterations in DM yield and quality of primary-growth and vegetative regrowth forages were of suf®cient magnitude to have nutritional and possibly economic implications to their utilization for ruminant animal production under existing and projected global climate scenarios.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Ozone; Air pollution; Climate change; Bahiagrass; Forage quality
1. Introduction
Tropospheric (i.e. ground-level) ozone (O3) is formed near the Earth's surface from the
photo-oxidation of hydrocarbons and oxides of nitrogen (NOx) from automobiles,
factories, power plants and other sources of high-temperature combustion of fossil fuels (National Research Council, 1991). It is considered the most important phytotoxic gaseous pollutant in the eastern USA (US Environmental Protection Agency, 1996), and it has become ubiquitous in this and other northern mid-latitude regions which collectively dominate global industrial and agricultural productivity; viz. Europe, eastern China and Japan (Chameides et al., 1994). Once thought to be con®ned to large metropolitan areas, air pollutants such as ground-level O3are now known to be transported long distances to
rural areas such that many of the world's most productive agricultural regions are currently exposed to harmful concentrations of O3(Chameides et al., 1994). Computer
models predict that tropospheric O3concentrations will continue to increase globally, on
average, between 0.3 and 1.0% per year for the next 50 years (Thompson, 1992). Global simulation of atmospheric reactive nitrogen compounds, conducted recently by Chameides et al. (1994), estimates that exposure to phytotoxic concentrations of O3
pollution (50Zl lÿ1
for highly sensitive crops such as winter wheat to70Zl lÿ1
for less sensitive crops such as rice) could triple by the year 2025 if rising anthropogenic NOx
emissions are not abated. According to their analysis, by 2025 as much as 30 to 75% of the world's cereals may be grown in regions with O3concentrations exceeding the 50±
70Zl lÿ1
threshold.
Ozone can directly injure plant tissue and disrupt normal patterns of resource acquisition and allocation such that chronic exposure over a growing season ultimately reduces crop yield. Generally, root biomass in graminoids is reduced more than shoot biomass (Lechowicz, 1987), and this change in priority of biomass allocation to different plant organs leads to reduced seed size and decreased seed and grain production in O3
-sensitive species (Miller, 1988). Long-term research programs such as the U.S. National Crop Loss Assessment Program (Heck et al., 1988) and the European Open-top Chamber Program (Commission of the European Communities, 1993) have been directed largely toward O3stress in cereal crops, because of their economic importance as sources of food
for human consumption. Forage crops have historically received much less attention in spite of their importance for animal production, and most of the experimental studies involving forage crops have been restricted to cool-season (C3photosynthetic pathway)
grasses and legumes (Fuhrer, 1997). Also, there are few experimental studies involving
interactions between O3 and forage management; e.g. mechanical harvesting, grazing,
etc. (Davison and Barnes, 1998).
Available data suggest that forage yield decreases with increasing O3, but very few
studies have investigated changes in quality characteristics in response to O3 stress.
Fuhrer et al. (1994) reported marginal changes in calcium, crude protein and crude ®ber concentrations of a mixed fescue±clover pasture fumigated with O3. Flagler and
Youngner (1985) reported that O3reduced crude fat, crude ®ber and total nonstructural
carbohydrates, but increased calcium and crude protein concentrations at the expense of yield in tall fescue. Ozone reduced shoot total nonstructural carbohydrate and increased mineral concentrations of ladino clover (Blum et al., 1982), and it decreased digestibility of mixed fescue±clover regrowth forage (Blum et al., 1983). Also, O3exposure has been
shown to increase the activities of select phenylpropanoid and ¯avonoid pathway enzymes, which results in foliar accumulation of phenolic compounds associated with accelerated senescence and death of plant tissue (Runeckles and Krupa, 1994). Recent biochemical and molecular biological evidence suggests that such defense reactions are similar to those induced by other abiotic and certain biotic stressors (KangasjaÈrvi et al., 1994). Whether this general defensive response to stress occurs in O3-exposed forage
with resultant implications to nutritional quality for livestock has not been investigated. Phenolics are also effective protractors of decomposition and nitri®cation by soil-borne micro-organisms, and it is conceivable that O3impacts on these constituents could have
subsequent effects on nutrient cycling in grassland ecosystems. Kim et al. (1998) have reported increased lignin content and reduced rate of litter decomposition in an O3
-exposed blackberry±broomsedge mixture.
As very little is known about O3 effects on yield and quality of warm-season (C4
photosynthetic pathway) forages commonly used for pasture and hay production, the effect of exposing bahiagrass, planted twice in the growing season, to three ground-level O3 scenarios on forage DM yield and concentrations of select chemical constituents
in¯uencing its nutritive quality for ruminant animals was studied.
2. Materials and methods
2.1. O3exposure system
The O3exposure system comprised six large (4.8-m height4.5-m diameter) open-top
chambers (OTC), each consisting of an aluminum frame surrounded by clear plastic and perforated at the bottom to allow introduction and circulation of air by large fans (Heagle et al., 1989). The chambers were assigned randomly to three air treatments, resulting in two OTC/air treatment: carbon-®ltered (CF), non-®ltered (NF) or enriched with O3 to
twice-ambient O3 concentration (2X). Carbon-®ltered chambers were calibrated to
remove approximately 50% of O3from ambient air, resulting in a range of O3
concentra-tions (20±45Zl lÿ1
) currently found in relatively unpolluted environments around the world (Lefohn et al., 1990). Ambient air in the Auburn, AL area is representative of that found in rural agricultural and forested regions of the southeastern USA, with typical summer daytime O3concentrations of50Zl lÿ1and occasional episodes above 100Zl lÿ1.
The 2X treatment approximated O3concentrations representative of those in the vicinity
of major urban centers and other areas which are frequently violative of the current National Ambient Air Quality Standard for O3of120Zl lÿ1for 1 h (US Environmental
Protection Agency, 1996). Ozone for the 2X treatment was generated by passing pure O2
through a high-intensity electrical discharge source and was added to the chambers daily between 09:00 and 21:00 h. Ozone concentrations in all OTC were monitored continuously using a US EPA-approved monitor with a rapid response time (20 s). Monitoring was time-shared such that the monitoring port for each OTC was read twice per hour. Monitoring instrumentation was calibrated according to US EPA quality assurance guidelines, and both the monitoring system and calibrator were audited by the Alabama Department of Environmental Management prior to initiation of treatments.
2.2. Forage establishment, management and sampling
Bahiagrass (Paspalum notatumFlugge, cultivar `Pensacola') was planted in large (15-l volume, 30.5-cm top diameter) pots and placed into OTC on 7 May 1997 (early-planted) and again on 10 July 1997 (late-planted). Pots were ®lled to capacity with a Norfolk sandy loam soil, and seeds were planted to a depth of 0.6 cm at 50 seeds per pot. Each OTC was delineated into quadrants, and eight pots of each planting were assigned randomly to quadrants with the added restriction that no quadrant could be represented more than once in an air treatment replicate. Pots were arranged in an isosceles triangular con®guration which was right-angled with respect to the center of the OTC. Six weeks postseeding, each pot of early-planted bahiagrass was topdressed with 4 g each of a controlled-release fertilizer (14:14:14 of N:P2O5:K2O) and a conventional pelleted
fertilizer (29:3:4 of N:P2O5:K2O). Late-planted bahiagrass was topdressed with 4 g of
each fertilizer source at the time of planting. Pots were watered as necessary to maintain moisture at near ®eld capacity. Precipitation was allowed to fall into chambers through the open tops, and fans were turned off at night from 22:01 to 06:59 h to permit natural dew formation within the OTC.
Primary-growth forage from early-planted bahiagrass was harvested from pots randomly selected at 12 (four pots), 18 (two pots) and 24 (two pots) weeks postseeding at approximately early-vegetative, late-vegetative and early-bloom stages of maturity, respectively. Vegetative regrowth forage from the ®rst (i.e. 12-week) primary-growth cutting was harvested three times at 4-week intervals. Primary growth forage from late-planted bahiagrass was harvested from pots randomly selected at 9 (four pots) and 15 (four pots) weeks postseeding at approximately early-vegetative and early-bloom stages of maturity, respectively. Vegetative regrowth forage from the ®rst (i.e. 9-week) primary-growth cutting was harvested twice at 3-week intervals. Forages were cut to leave a 2.5-cm aboveground stubble, composited on an OTC basis, dried to constant weight at 508C and ground in a Wiley mill to pass a 1-mm screen.
2.3. Chemical analyses
Forage samples were analyzed for dry matter (DM) and crude protein (CP) according to Association of Of®cial Analytical Chemists (1995) procedures. Forage concentrations
of neutral detergent ®ber (NDF) and acid detergent ®ber (ADF) were determined sequentially, and of acid detergent lignin (ADL) separately according to procedures of Goering and Van Soest (1970). The prussian blue assay (Price and Butler, 1977) as modi®ed by Graham (1992) was used to estimate forage concentration of total phenolics by reference to a gallic acid standard.
2.4. Statistical analyses
Data were analyzed using general linear model procedures of the Statistical Analysis System Institute Inc. (1989) for a completely randomized split-plot design. Error term (a) for testing main effects of O3treatment (main plots) was OTC within treatments, and
residual mean squares was the error term (b) for testing harvest period (subplots) effects and the harvest periodO3 treatment interaction. Dunnett's procedure was used to
independently compare CF and 2X with NF means in order to evaluate biological response to decreased and increased O3 concentrations, respectively, compared with
ambient conditions. In recognition of the low statistical power characteristic of ®eld studies which employ limited numbers of OTC, treatment differences were considered signi®cant whenp<0.10 (Peterman, 1990). Because the objective of our experiment was to determine effects of O3exposure rather than stage of maturity effects on forage DM
yield and quality characteristics, least-squares means are presented only for main effects of O3across all harvest periods. Harvest period main effects are not presented, and O3
effects within harvest period are presented only when the harvest periodO3interaction
is signi®cant (p<0.10).
3. Results
3.1. Weather data
Average monthly temperatures for May±October 1997 were within 0.88C of 30-year (1960±1990) average monthly temperatures for the same period in the Auburn area, except for May and June average temperatures which were 1.5 and 2.08C lower, respectively (Table 1). Monthly precipitation for May, August and September was within
Table 1
Mean monthly air temperature and precipitation for May±October 1997 and 30-year averages for Auburn, AL
Month Air temperature (8C) Precipitation (cm)
1997 30-year average 1997 30-year average
May 19.7 21.2 9.8 9.7
June 22.9 24.9 15.5 10.3
July 26.9 26.2 9.1 14.9
August 25.4 26.1 7.7 9.2
September 24.5 23.7 10.2 9.6
October 18.3 18.0 1.3 7.4
1.5 cm of 30-year average precipitation for these months, whereas precipitation for June was much greater (5.2 cm) and for July and October much lower (5.8 and 6.1 cm, respectively).
3.2. O3exposure
Mean daytime (09:00±21:00 h) O3concentrations over the entire 24-week experiment
were 22, 45 and 91Zl lÿ1
, respectively, for CF, NF and 2X treatments. Mean daytime ambient O3concentrations peaked in mid-May and again in late August±late September
at 50±60Zl lÿ1
, remained at 40Zl lÿ1
during the intervening period and declined to <40Zl lÿ1
from late September through October (Table 2). Highest individual ambient O3 values were recorded in late June, late July, late August and mid-September at
90Zl lÿ1
.
3.3. Forage DM yield and quality characteristics
Yield of DM was 51% greater (p<0.10) for CF than NF primary-growth forage, and concentrations of NDF and ADL were 2.4 and 0.6 percentage units higher (p<0.10), respectively, in 2X than in NF primary-growth forage from the early-season planting (Table 3). Concentrations of CP, ADF and total phenolics did not differ among treatments, although ADF concentration tended to be higher (p0.17) in 2X than in NF primary-growth forage from the early-season planting. No signi®cant differences were observed among treatments in yield of regrowth forage DM from the early-season planting, although DM yield was 40% higher (p0.26) for CF than NF regrowth forage. Concentrations of NDF and ADF were 2.6 and 1.8 percentage units higher (p<0.10), respectively, in 2X than in NF regrowth forage, and CP concentration was 2.6 percentage units lower (p<0.10) in CF than in NF regrowth forage from the early-season planting. Table 2
Biweekly daytime (09:00±21:00 h) ambient mean and peak O3concentrations from 7 May to 23 October 1997
Biweekly period Ambient O3concentration (Zl l
ÿ1)
Mean Peak
7 May±20 May 52.2 77.5
21 May±3 Jun 37.1 67.5
4 Jun±17 Jun 39.5 67.0
18 Jun±1 Jul 37.6 88.0
2 Jul±15 Jul 44.9 82.5
16 Jul±29 Jul 46.1 92.0
30 Jul±12 Aug 37.7 69.0
13 Aug±26 Aug 43.0 91.0
27 Aug±9 Sep 57.4 85.0
10 Sep±23 Sep 59.7 97.5
24 Sep±7 Oct 38.9 73.0
8 Oct±23 Oct 33.1 71.5
Concentrations of ADL and total phenolics did not differ among treatments, although ADL concentration tended to be higher (p0.14) in 2X than in NF regrowth forage from the early-season planting.
No differences were observed among treatments in DM yield or chemical composition of primary-growth forage from the late-season planting (Table 4), although concentration of total phenolics tended to be greater (p0.15) for the CF than for the NF treatment. A harvest periodO3 treatment interaction (p<0.10) was observed for CP concentration
such that it was lower (p<0.10) in CF than in NF forage (6.6 vs. 13.0%) from the initial primary-growth harvest. Limited DM yield and resulting insuf®cient sample from the initial primary-growth harvest precluded analysis for ADL. No differences were observed among treatments in DM yield or chemical composition of regrowth forage from the late-season planting, although concentrations of CP (p0.18), NDF (p0.12) and ADF (p0.19) tended to be higher for the 2X than for the NF treatment. A harvest periodO3
treatment interaction (p<0.10) was observed for DM yield such that it tended to be greater for CF (92 g mÿ2
Dry matter (DM) yield and chemical composition of primary-growth and regrowth forages from early-planted bahiagrass exposed to carbon-®ltered air (CF), non-®ltered ambient air (NF) or twice-ambient O3concentration (2X) in open-top chambers (OTC)a
Item Air treatment S.E.M.b
CF NF 2X
Primary-growth forage
DM yield (g mÿ2)c 131 87 88 15
Chemical composition:
crude protein (%) 15.6 15.9 16.5 0.7
neutral detergent ®ber (%)d 64.7 64.8 67.2 0.6
acid detergent ®ber (%) 28.1 28.1 30.9 1.5
acid detergent lignin (%)d 0.3 0.3 0.9 0.1
total phenolics (mg gÿ1)e 11.9 10.8 11.9 0.9
Regrowth forage
DM yield (g mÿ2) 84 60 62 12
Chemical composition:
crude protein (%)c 16.2 18.8 18.0 0.5
neutral detergent ®ber (%)d 65.2 64.6 67.2 0.6
acid detergent ®ber (%)d 29.3 29.5 31.3 0.5
acid detergent lignin (%) 0.5 0.5 0.7 0.1
total phenolics (mg gÿ1)e 11.2 11.3 10.3 0.5
aValues are least-squares means of two OTC replicates/air treatment and are expressed on a DM basis. bStandard error of the mean.
cCF and NF differ atp<0.10. d2X and NF differ atp<0.10.
eConcentrations of total phenolics are expressed as gallic acid equivalents.
4. Discussion
Mean daytime ambient O3concentrations over the 24-week experiment approximated
or were only slightly lower than the range of ambient O3concentrations typically found
in central Europe and large areas of the USA (40±60Zl lÿ1
) and which routinely result in forage yield reductions of 5±10% compared with lower cumulative exposures of 20± 30Zl lÿ1
(Chameides et al., 1994; Fuhrer, 1997). Yield of DM was 34% lower for primary growth forage from early-planted bahiagrass grown under NF than under CF air in our OTC exposure system, which is probably a greater forage yield reduction than might be expected under open-®eld conditions for sown pasture. Moreover, caution must be exercised in extrapolating results obtained during the establishment phase of perennial plants. Bungener et al. (1999) grew a number of semi-natural grassland plant species in pots and exposed them to O3 in OTC for two seasons, and effects of O3 were less
pronounced during the second season than in the ®rst year.
Doubling (2X) of ambient O3concentration did not result in further reduction in DM
yield of primary-growth forage from the early-season planting. Barbo et al. (1998) observed decreased density of bahiagrass in an early-successional plant community fumigated from April through August with NF compared with CF air, and fumigation Table 4
Dry matter (DM) yield and chemical composition of primary-growth and regrowth forages from late-planted bahiagrass exposed to carbon-®ltered air (CF), non-®ltered ambient air (NF) or twice-ambient O3concentration (2X) in open-top chambers (OTC)a
Item Air treatment S.E.M.b
CF NF 2X
Primary-growth forage
DM yield (g mÿ2) 78 83 76 14
Chemical composition:
crude protein (%)c 10.5 13.4 13.4 1.2
neutral detergent ®ber (%) 62.5 64.1 64.2 1.1
acid detergent ®ber (%) 28.4 28.7 28.3 1.7
total phenolics (mg gÿ1)d 10.4 8.8 10.1 0.9
Regrowth forage
DM yield (g mÿ2)c 66 60 50 10
Chemical composition:
crude protein (%) 18.2 18.9 19.9 0.4
neutral detergent ®ber (%) 61.8 61.2 63.9 0.9
acid detergent ®ber (%) 26.6 26.2 27.6 0.6
acid detergent lignin (%) 0.5 0.5 0.4 0.1
total phenolics (mg gÿ1)d 10.0 10.2 10.5 0.8
aValues are least-squares means of two OTC replicates/air treatment and are expressed on a DM basis. bSandard error of the mean.
cHarvest periodair treatment interaction signi®cant atp<0.10.
dConcentrations of total phenolics are expressed as gallic acid equivalents.
with twice-ambient O3 resulted in further growth reduction; other C4 grasses such as
Panicum spp. increased in density, whereas density of Andropogon virginicus was unaffected by elevated O3in their study. Volin et al. (1998) observed slight reductions in
growth rate of two C4 grasses, Bouteloua tremuloides and Schizachyrium scoparium,
when grown under an O3concentration of 95Zmol molÿ1for a 101-day period. Plant
sensitivity to O3has commonly been found to be correlated with stomatal conductance,
and species with intrinsically higher stomatal conductance have a greater potential O3
uptake and thus greater potential growth reduction in response to a given level of O3than
do species with lower stomatal conductance (Reich and Amundson, 1985). While C4
plants might be expected to be less sensitive than C3 plants to O3 because of their
intrinsically lower stomatal conductance (Volin et al., 1998), other physical and biochemical leaf characteristics certainly play a role in determining sensitivity to O3
injury (Davison and Barnes, 1998). Because high stomatal conductance is found under well-watered conditions involving high air humidity (low vapor pressure de®cit) and high light, it is possible that our watering regime caused plants to maintain higher stomatal conductance and sensitivity to O3injury than might be expected under warm, dry ®eld
conditions. Also, it is conceivable that our fertilization regime accentuated response to O3
insofar that nutrients, especially N, affect resource allocation, canopy structure and leaf anatomy, and rates of leaf development, maturation and senescence. There has been comparatively little interest in the effects of mineral nutrition on O3 in herbaceous
species, presumably because crop nutrition is optimized by fertilizers, and there are few reports in support of a generalized, mechanistic explanation of O3-mineral interactions
beyond the broad conclusion that mineral nutrients can alter O3response (Davison and
Barnes, 1998).
In contrast to the bene®cial effect of O3 reduction observed for early-planted
bahiagrass, DM yield of primary-growth forage from late-planted bahiagrass was not affected by O3concentration, even though NF and 2X forages were exposed to potentially
phytotoxic concentrations of O3for at least 4 weeks following emergence until time of
®rst harvest. Also, as was observed for early-planted bahiagrass, doubling (2X) of ambient O3concentration did not result in decreased DM yield of primary-growth forage
from the late-season planting. Age-dependent O3 injury is observed for some types of
plants such that effects depend on co-occurrence of high O3episodes and sensitive growth
stages (Fuhrer, 1997). For example, a study with turfgrass species revealed that seedlings between 9 and 14 days of age exhibited greater sensitivity and uniformity in their responses to short-term acute O3 exposure than did seedlings 66 to 71 days of age
(Richards et al., 1980). However, in clover and many other species, younger leaves are more O3-tolerant than are older leaves (Karlsson et al., 1995). Conceivably, the DM yield
reduction observed for O3-exposed primary-growth forage from early-planted, but not
late-planted bahiagrass, is a chronic effect which resulted from accumulated exposure to phytotoxic concentrations of this pollutant over a longer period of time. Length of exposure might also explain the differential response between early- and late-planted bahiagrass in yield of regrowth forage DM. Regrowth forage from the early-season planting was harvested three times at 4-week intervals and was exposed to increasing daytime ambient O3 concentrations from late August (®rst regrowth harvest) to late
September (second regrowth harvest). Regrowth forage from the late-season planting was
harvested twice at 3-week intervals in early and late October after mean ambient daytime O3 concentrations had already subsided to <40Zl lÿ1. The trends toward greater DM
yield for CF and lower DM yield for 2X than for NF forage from the initial regrowth harvest in early October may be related to timing of the harvest immediately following a major O3episode in mid-September. Wilbourn et al. (1995) used different cutting regimes
in an experiment in which a ryegrass±clover mixture was fumigated with O3in an
open-®eld system, and cutting forage immediately after an O3episode had a greater negative
effect on biomass yield than later cuttings. Our observations and those of others (Ashmore and Ainsworth, 1995; Wilbourn et al., 1995) illustrate the potential effects introduced by defoliation, and further suggest that effects of cutting or grazing are likely to vary with timing and frequency of harvest. Such effects could have practical implications to forage DM yield under management systems which utilize high harvesting frequency (e.g. for hay production, rotational grazing) or low harvesting frequency (e.g. for silage production).
While the 40% greater DM yield of CF regrowth forage than NF regrowth forage from early-planted bahiagrass was not signi®cant, it is suf®cient to partly explain its lower CP concentration as a result of dilution by carbohydrate. A similar conjecture may be made in the comparison of NF with 2X regrowth forage from the late-season planting. However, examination of CP yields across all air treatments suggests that O3reduction
might have altered plant N metabolism in a manner which cannot be explained solely on the basis of dilution by carbohydrate. Greater DM yield observed for CF than for NF primary-growth forage from the early-season planting was not accompanied by a decrease in CP concentration, nor was decreased CP concentration in CF primary-growth forage accompanied by an increase in DM yield compared with NF primary-growth forage from the late-season planting. Our observations are in general agreement with ®ndings of other research (Blum et al., 1982; Flagler and Youngner, 1985) and reinforce the conclusion drawn by Davison and Barnes (1998) that interpretation of O3effects can
vary in accordance with which response criteria are selected for this purpose. To illustrate further, Flagler and Youngner (1985) reported that CP concentration (g CP kgÿ1
DM) in tall fescue increased with increasing O3 concentration. This apparent improvement in
forage quality re¯ected a reduction in growth of plants which occurred at a more rapid rate than reduction in plant protein synthesis. However, CP expressed on a mass (g) per plant basis decreased, also re¯ecting growth reduction but suggesting that protein yield per unit area would be lower in an O3-stressed pasture.
Occurrence of peak O3concentrations is associated with acute leaf injury in sensitive
grass species, and altered cellular metabolism and membrane structural components result in accelerated leaf senescence which could impact forage quality (Fuhrer, 1997). Peak O3episodes with daytime ambient concentrations of90Zl lÿ1were recorded in
late June and late July preceding the ®rst primary-growth harvest, in late August preceding the second primary-growth and ®rst regrowth harvests, and in mid-September preceding the second regrowth harvest from early-planted bahiagrass. In contrast to the pattern observed for forage DM yield from the early-season planting, differences in forage quality were observed between 2X and NF but not between CF and NF forages. Forage concentrations of NDF and ADF are inversely related to in vivo voluntary intake and digestibility, respectively, and ADL concentration is inversely related to digestibility
of plant cell wall and total plant DM in ruminant animals (Van Soest, 1994). Differences in concentrations of these constituents between NF and 2X forages from the early-season planting were of suf®cient magnitude to predict modest decreases in in vivo voluntary intake and digestive utilization of 2X primary-growth and regrowth forages. It is dif®cult to ascertain under what speci®c circumstances this decrease in forage quality would constitute a signi®cant economic impact to ruminant animal production, as economic impact would ultimately depend on the summative effect of O3on forage DM yield and
quality. Few studies have investigated changes in quality traits in response to O3stress
(Blum et al., 1982, 1983; Flagler and Youngner, 1985; Fuhrer et al., 1994), and minor changes with no consistent trends typically have been observed. Overall, the available data for C3grasses and legumes suggest that forage quality decreases with increasing O3,
but they do not suggest major changes in quality characteristics, even in the presence of pronounced changes in species composition of mixed pasture. To our knowledge, our study is the ®rst to report signi®cant effects of elevated O3on quality characteristics in an
economically important C4grass.
In contrast to the effect of elevated O3 on concentrations of cell wall constituents
observed for early-planted bahiagrass, concentrations of cell wall constituents in primary-growth forage from late-planted bahiagrass were not affected by O3concentration, even
though the ®rst harvest was preceded by peak concentrations of O3 which could be
expected to result in acute leaf injury based on the response observed for early-planted bahiagrass. While differences were not signi®cant, NDF and ADF but not ADL concentrations tended to be higher in 2X than in NF regrowth forage as was observed for primary-growth and regrowth forages from the early-season planting. The highest peak O3 episode of the growing season was recorded in mid-September and immediately
preceded the ®rst regrowth harvest. Presumably, quality of second-regrowth 2X forage could have been impaired by exposure to O3during the ®rst regrowth period. Evidence of
a lag period in which exposure in one growth period affected growth in a subsequent period has been observed by other investigators (Nussbaum et al., 1995; Wilbourn et al., 1995), but whether a similar relationship exists for quality characteristics has not been con®rmed experimentally.
Phenolic compounds are the most widely distributed group of allelochemicals encountered in nature by ruminants. Because of their role in the nutritional ecology of ruminants as negative modi®ers of intake and digestibility, we were interested in determining whether phenolic compounds accumulated in O3-exposed bahiagrass as has
been shown for a number of other agricultural crops. With the exception of a trend for greater concentration of total phenolics in CF than in NF primary-growth forage from the late-season planting, there were no differences among treatments in forage concentrations of these constituents. Lack of a consistent relationship with forage concentrations of ADL may be due to differential response of various plant phenolic compounds as a function of their redox potential and the reaction stoichiometry of their phenolic functional groups. Powell et al. (1999) observed no consistent relationship among concentrations of total phenolics, condensed tannins and ADL in Lespedeza cuneata which had been exposed chronically to CF and NF air. Although redox assays, such as the prussian blue assay, are often standardized with simple phenolics like gallic acid, complex polyphenolics may have a very different response on a molar or mass basis than the simple standard
(Hagerman, 1998). It is possible that use of a gallic acid standard introduces an analytical bias toward simple phenolics characteristic of young, immature forage, and such an artifact as would be problematic in the assessment of O3injury for which accelerated leaf
senescence is a visible symptom.
5. Conclusion
Chronic exposure of early-planted bahiagrass to ambient concentrations of ground-level O3decreased DM yield of primary-growth forage and tended to decrease DM yield
of regrowth forage. Doubling of ambient O3concentrations resulted in decreased quality
of primary-growth and regrowth forages from early-planted bahiagrass and tended to decrease quality of regrowth forage harvested from late-planted bahiagrass. These alterations in DM yield and quality of O3-exposed bahiagrass were of suf®cient
magnitude to predict nutritional and possibly economic consequences to its utilization for ruminant animal production under existing and projected global climate scenarios. Results also suggest that impacts of O3 might be modi®ed by forage management
practices such as timing and frequency of harvesting in relation to O3episodes during the
growing season.
Acknowledgements
The authors thank Efrem Robbins for his assistance with data collection and analysis. Financial support for this project was provided by the Auburn University Competitive Research Program and Alabama Agricultural Experiment Station.
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