Differential Responses In Two Varieties Of Winter Wheat To .
Global Change Biology (2011) 17, 580–591, doi: 10.1111/j.1365-2486.2010.02184.xDifferential responses in two varieties of winter wheat toelevated ozone concentration under fully open-airfield conditionsZ H A O Z H O N G F E N G *, J I N G P A N G *w , K A Z U H I K O K O B AY A S H I *, J I A N G U O Z H U z andDONALD R. ORT§*Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan,wAcademy of Resource and Environment, Hubei University, Wuhan 430064, China, zState Key Laboratory of Soil and SustainableAgriculture, Institute of Soil Sciences, Chinese Academy of Sciences, Nanjing 210008, China, §Photosynthesis Research Unit,USDA/ARS & Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USAAbstractTwo modern cultivars [Yangmai16 (Y16) and Yangfumai 2 (Y2)] of winter wheat (Triticum aestivum L.) with almostidentical phenology were investigated to determine the impacts of elevated ozone concentration (E-O3) onphysiological characters related to photosynthesis under fully open-air field conditions in China. The plants wereexposed from the initiation of tillering to final harvest, with E-O3 of 127% of the ambient ozone concentration (A-O3).Measurements of pigments, gas exchange rates, chlorophyll a fluorescence and lipid oxidation were made in threereplicated plots throughout flag leaf development. In cultivar Y2, E-O3 significantly accelerated leaf senescence, asindicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. Thelower photosynthetic rates were mainly due to nonstomatal factors, e.g. lower maximum carboxylation capacity,electron transport rates and light energy distribution. In cultivar Y16, by contrast, the effects of E-O3 were observedonly at the very last stage of flag leaf ageing. Since the two cultivars had almost identical phenology and very similarleaf stomatal conductance before senescence, the greater impacts of E-O3 on cultivars Y2 than Y16 cannot be explainedby differential ozone uptake. Our findings will be useful for scientists to select O3-tolerant wheat cultivars against therising surface [O3] in East and South Asia.Nomenclature:%D 5 the fraction of photons dissipated in the antenna%P 5 the fraction of photons utilized in PSII photochemistry%X 5 the fraction of absorbed photons by PSII neither used in photochemistrynor dissipated in the PSIIA 5 photosynthetic rateAOT40 5 the cumulative O3 exposure over a threshold of the 1 h average [O3] of40 ppb during daytimeAsat 5 light-saturated photosynthesisCar 5 CarotenoidChl 5 ChlorophyllCi 5 intercellular CO2 concentrationAQY 5 apparent quantum efficiencyDAFE 5 days after full expansion of flag leavesFv 0 /Fm 0 5 actual photochemical efficiency of PSII in the saturated lightgs 5 stomatal conductanceJmax 5 the maximum rate of electron transportl 5 stomatal limitation to photosynthesisM7 5 the mean of the daily 7 h average [O3]MDA 5 malondialdehydeO3 5 ozoneOTC 5 open-top chambersCorrespondence: K. Kobayashi, tel. 1 81 3 5841 1292, fax 1 81 3 5841 5186, e-mail: aclasman@mail.ecc.u-tokyo.ac.jp580r 2010 Blackwell Publishing Ltd
VA R I E T A L D I F F E R E N C E I N W I N T E R W H E A T R E S P O N S E S T O O 3PPFD 5PhiPSII 5qP 5SUM06 5Vcmax 5Y16 5Y2 5581photosynthetic photon flux densitylight-adapted apparent quantum efficiency of PSIIquenching of photochemical efficiency of PSIIsum of hourly average [O3] 0.06 ppm during daytimethe maximum carboxylation efficiencywheat variety Yangmai 16wheat variety Yangfumai 2.Keywords: chlorophyll fluorescence, gas exchange, ozone, ozone uptake, variety, winter wheatReceived 3 November 2009 and accepted 15 December 2009IntroductionOzone (O3) is among the most important air pollutantsin many parts of the world causing significant biologicaland economic damage to crop plants (Wahid et al., 1995;Fuhrer & Booker, 2003; Morgan et al., 2003; Ashmore,2005; Ashmore et al., 2006; Felzer et al., 2007; Wang et al.,2007; Feng et al., 2008, 2009; Fuhrer, 2008; Feng &Kobayashi, 2009). It is well documented that chronicexposure to elevated O3 concentration ([O3]) causes arange of adverse effects on plants including reducedphotosynthetic activity, altered carbon allocation, diminished biomass accumulation, reduced yield andaccelerated senescence, with or without visible injury(Kobayashi et al., 1995; Farage & Long, 1999; Morganet al., 2004, 2006; Ashmore, 2005; Pleijel et al., 2006; Fenget al., 2007; Biswas et al., 2008). A meta-analysis of 53peer-reviewed chamber studies showed that elevated[O3] (averaged 73 ppb) decreases leaf photosyntheticrate (A) by 20% and grain yield by 29% in wheat plantscompared with those grown in carbon-filtered air (Fenget al., 2008), which suggested that decreased photosynthesis was a key factor driving yield loss in wheatexposed to elevated [O3]. The meta-analysis also indicated that the negative effects of O3 increased with theduration of leaf exposure to elevated [O3], implyingcumulative effects of O3 during wheat leaf development(Feng et al., 2008).For field-grown plants, photosynthesis in saturatinglight (Asat) is normally co-limited by stomatal factorscontrolling gas exchange along with biochemical andphotochemical processes of photosynthesis. Mostresults indicated that stomatal limitation (l) of A wasnot increased despite significant reduction of stomatalconductance (gs) in response to elevated [O3] (Fiscuset al., 1997; Farage & Long, 1999; Zheng et al., 2002;Morgan et al., 2004). The central biochemical processescontrolling photosynthesis are, the maximum carboxylation efficiency (Vcmax), which reflects in vivo activity ofRubisco activity (Rogers & Humphries, 2000), and themaximum rate of RuBP regeneration (Jmax) (Sage, 1990).Chamber results indicated that reduced Asat fromr 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 580–591growth at elevated [O3] was due primarily to a lowerVcmax (Atkinson et al., 1988; Farage & Long, 1995, 1999;Reid & Fiscus, 1998; Zheng et al., 2002; Morgan et al.,2004; Fiscus et al., 2005). Chlorophyll a fluorescencemeasurements showed decreased photochemical efficiency and quantum yield of PSII in the light as a resultof growth at elevated [O3] that depended on [O3] doseand crop species (Castagna et al., 2001; Calatayud et al.,2002, 2004; Zheng et al., 2002; Biswas et al., 2008).The effects of growth at elevated [O3] on photosynthesis are strongly influenced by genetic background,developmental stage as well as interacting environmental factors. To date, most O3 exposure results are basedon experiments in controlled environment enclosuressuch as greenhouses and closed or open-top chambers(OTC). Enclosures can substantially modify the plantcanopy microclimate (McLeod & Long, 1999) and thereby alter both qualitatively and quantitatively the effectsof O3 exposure (Nussbaum & Fuhrer, 2000; Piikki et al.,2008). Morgan et al. (2004, 2006) reported thatO3-induced inhibition of photosynthesis and losses inyield of soybean under fully open-air field conditionswere even greater than the losses reported in earlierchamber studies. Therefore, it is necessary to investigatethe basis for the negative effects of O3 on wheat insystems where artificial perturbations to the soil–plant–atmosphere continuum are minimal.Ozone-sensitive and tolerant cultivars or clones havebeen investigated for numerous crop species e.g. soybean (Robinson & Britz, 2000), wheat (Heagle et al.,2000; Biswas et al., 2008), and snap bean (Phaseolusvulgaris L.) (Guzy & Heath, 1993). At least in somecrops, O3 tolerance has been shown to be a heritabletrait involving the antioxidant system and high apoplastic ascorbate (ASC) content (Fiscus et al., 2005).Modern wheat cultivars are reported to be more sensitive to O3 than older accessions, which was largelyattributed to higher gs in modern cultivars allowingfor greater O3 uptake (Barnes et al., 1990; Pleijel et al.,2006; Biswas et al., 2008). Since high gs is necessary forhigh production, from an agronomic view point, it ismore important to compare among modern high gs
582 Z . F E N G et al.varieties to identify sources of tolerance other thanrestricting ozone flux into leaves by low gs.In this work, we studied two modern cultivars ofwinter wheat with nearly identical phenology in termsof photosynthetic response to elevated [O3] in a fieldexperiment by using open-air O3 fumigation systemsbuilt in China. Our objectives were (1) to compare thevarieties in response to elevated [O3]; (2) and to test ifstomatal O3 uptake contributed to the differentialresponse between the varieties.Materials and methodsExperiment siteThe experiment was conducted in Xiaoji town, Jiangdu county,Jiangsu province, China (119142 0 E, 32135 0 N). This site has beenin continuous cultivation for more than 1000 years with rice–wheat or rice–rapeseed rotations. The soil is Shajiang AquicCambosols with a sandy-loamy texture. The region has asubtropical marine climate with mean annual precipitationof 1100–1200 mm, mean annual temperature of 16 1C, a totalannual sunshine duration of 42000 hours, and a frost-freeperiod of 4230 days.Fumigation treatmentThree 240 m2 plots were treated with elevated [O3] (hereinaftercalled E-O3 plots) and three equal size plots were maintainedat ambient [O3] (hereinafter called A-O3 plots). The target [O3]for E-O3 plots was 50% higher than the A-O3. All the E-O3plots were separated from the other plots by at least 70 m toavoid cross-contamination. The experimental design wasbased on completely randomized plots allocated to either AO3 or E-O3, and split into subplots of wheat cultivars. In the EO3 plots, crops were grown within 14 m in diameter octagonswith a perimeter of eight 6 m ABS pipes. A mixed gas consisting of about 5% O3 and 95% O2 was produced by an O3generator (KCF-BT0.2; Jiangsu Koner Ozone Co. Ltd., Yangzhong, China). Using a mass flow controller, the O3/O2mixture was released in a stream of compressed air into theplots through ABS pipes positioned 50 cm above the canopyheight. [O3] at the middle point of each plot was measuredevery 20 s by O3 analyzer (model 49C; Thermo EnvironmentalInstruments, Franklin, MA, USA). Based on the wind directionand wind speed, O3 achieved a concentration within 15% ofthe set point maintained 90% of the time, and within 20% ofthe set point for 95% of the time. Mean [O3] throughout the O3fumigation period had the coefficient of variation of only 2.5%across 13 locations within an E-O3 ring. Owing to the dewin the earlier morning, E-O3 treatment was added from 9:00hours to sunset except when raining or when the background[O3] lower than 20 ppb or higher than 170 ppb. In the ambientplots, plants were grown under A-O3 without perimeterpipes. Owing to low temperature and A-O3 o30 ppb beforetillering stage, ozone fumigation began on March 5, 2008 at theinitiation of tillering stage of wheat and continued untilharvest.Plant materialThe winter wheat varieties cv. Yangmai 16 (medium glutencultivar, hereafter called Y16) and Yangfumai 2 (weak glutencultivar, hereafter called Y2) were selected due to similarphenology, as shown in Table 1. Standard cultivation practicescommon to the region were followed in all experimental plots.The seeds of two cultivars were sown in two of five subplots(each about 11 m2) which were distributed randomly in eachplot of A-O3 and E-O3 treatments on November 15, 2007, at adensity of 210 plants m 2. Healthy flag leaves fully unfoldedon the same day were marked and used to make the followingmeasurements.Gas exchange and fluorescence measurementsThe marked plants were excised predawn, as described byMorgan et al. (2004), placed in water and quickly taken to alaboratory where they were kept in low light(o20 mmol m 2 s 1) until 30 min before the measurement, atwhich time they were light acclimated at 400 mmol m 2 s 1.Gas exchange and fluorescence measurements were madeusing a LI-6400 photosynthesis system (LICOR, Lincoln, NE,USA) fitted with a 6400-40 leaf chamber fluorometer (LCF).Two detached flag leaves per subplot for either cultivar withthree replicates were selected to measure A vs. intercellularCO2 concentration (Ci) and A vs. photosynthetic photon fluxdensity (PPFD) curves every 3–7 days from their initial fullyexpanded state until senescence was visible ( 80% yellow).The automatic program in the LI-6400 photosynthesis system was used to generate the response of A to Ci. Netphotosynthesis and chlorophyll fluorescence characteristicswere determined simultaneously. Measurements were takenby changing [CO2] in LCF in 11 steps (380, 300, 200, 100, 50,400, 400, 600, 800, 1000 and 1200 mmol mol 1) under a constantPPFD of 1500 mmol m 2 s 1, block temperature of 25 1C andrelative humility of 50%–70%. Both steady-state (Fs) andmaximal (Fm 0 ) fluorescence were logged along with standardphotosynthetic parameters. The time allowed for the instrument to reach steady state at each [CO2] was 240 s. Theinstrument logged values when the stability or steady statehad been reached as indicated by total coefficient of variation 3%. Vcmax, Jmax and l were determined following themethod of Farquhar & Sharkey (1982), as described previously(Long & Bernacchi, 2003). The software in the instrumentprovided data on the fluorescence parameters including actualphotochemical efficiency of PSII in the saturated light (Fv 0 /Fm 0 ), quenching of photochemical efficiency of PSII (qP), andthe quantum yield of noncyclic electron transport (PhiPSII). gs,Ci and the fluorescence parameters were extracted from measurements at [CO2] 380 mmol mol 1. The fraction of radiationabsorbed that was dissipated in the antenna (%D) and utilizedin PSII photochemistry (%P) were estimated as 1 (Fv 0 /Fm 0 ) 100 and (Fv 0 /Fm 0 ) qP 100, respectively (DemmigAdams et al., 1996). The fraction of absorbed radiation by PSIIr 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 580–591
VA R I E T A L D I F F E R E N C E I N W I N T E R W H E A T R E S P O N S E S T O O 3583Table 1 Phenology of the two cultivars of winter wheat Yangmai16 (Y16) and Yangfumai2 (Y2) in the ambient (A-O3) and elevated(E-O3) [O3] plotsCultivarTreatmentSowingElongationFlag leaf full 6/2008Y2neither used in photochemistry nor dissipated in the PS2antennae (%X) was estimated as (Fv 0 /Fm 0 ) (1 qP) 100(Demmig-Adams et al., 1996).To determine the response of A to PPFD, chamber [CO2] wasset at 380 mmol mol 1 and block temperature to 25 1C. PPFDwas gradually reduced from 2000 to 0 mmol m 2 s 1 in ninesteps (2000, 1500, 1000, 500, 250, 200, 150, 75 and 0). The initialslopes of the A-PPFD curves were fit with a linear function toestimate the maximum apparent quantum yield (AQY). Thewhole curves were fit to a nonrectangular hyperbola with aleast square curve fitting procedure to derive Asat.Leaf pigment contentAfter measurements of gas exchange and fluorescence, themiddle part of flag leaves was punched and then extracted in95% ethanol in the dark for 72 h at 4 1C. The extract was thenassayed for chlorophyll (Chl) and carotenoid (Car) by usingthe specific absorption coefficients of Lichtenthaler (1987).Malondialdehyde (MDA) contentFlag leaves were sampled at noon on the same day that gasexchange measurements were conducted. MDA was analyzedas a 2-thiobarbituric acid-reactive metabolite (TBA) followingthe method of Heath & Parker (1968). Two punches of themiddle part of leaf samples ( 0.06 g) were homogenized in aprechilled mortar and pestle in 1 mL ice-cold 6% (w/v) trichloroacetic acid (TCA) and centrifuged at 6000 g for 10 min at4 1C. Assay mixture containing 0.1 mL aliquot of supernatantand 0.2 mL of 0.6% (w/v) TBA was incubated in a water bathat 95 1C for 15 min and then rapidly cooled in an ice-bath. Aftercentrifugation (10 000 g for 10 min at 4 1C), the supernatantabsorbance at 532 nm was determined. The values corresponding to nonspecific absorption (600 nm) and sugar disturbanceabsorption (450 nm) were subtracted. Concentration of MDAwas calculated using the following equation: CMDA (mmol L 1) 56.45 (A532 A600) 0.56 A450 (Heath & Parker, 1968).Statistical analysisThe experiment was laid out according to a split plot designwith either of the [O3] levels assigned to a ring, i.e. the mainplot, which was split into subplots of cultivars. Datasets werebased on the mean values for each subplot. The data for eachdependent variable was subjected to the analysis of variancewith mixed linear model, in which the ring is assigned to ther 2010 Blackwell Publishing Ltd, Global Change Biology, 17, 580–591random variable and other variables to fixed variables, usingsoftware (SAS Institute, USA). Comparison of meansbetween A-O3 and E-O3 on individual dates of measurementwas done on Student’s-t statistics for each cultivar. A difference between the means was considered significant ifP 0.05.JMPResultsOzone exposureWheat plants were exposed to ozone fumigation in thefield from March 5, 2008 through to May 28, 2008, with atotal of 60 days effective fumigation. No fumigation wasgiven on 25 days during this period due to 24 rainy orcloudy days and 1 day that power was lost. The consistent phenology of Y16 and Y2 ensured that the O3exposure dose was same in both cultivars. During thefull 85 days fumigation, the mean of the daily 7 haverage [O3] (M7), accumulated O3 exposure over athreshold of 40 ppb (AOT40) and sum of hourly average[O3] 60 ppb (SUM06) in the E-O3 were 27%, 110% and152% higher than those in A-O3, respectively (Table 2).As shown in Fig. 1, the M7 in A-O3 in May was muchhigher than that before May.During flag leaf development, M7, AOT40 andSUM06 in the E-O3 treatment were 26.7%, 88.9% and110% higher than those in ambient air, respectively. Themean [O3] during flag leaf development was muchhigher than that for the overall fumigation period (Table2), since the higher [O3] occurred more frequently in thelater growth stage (Fig. 1). M7 in A-O3 averaged 52 ppbwith a maximum of 110 ppb. The observed maximumambient 1 h mean [O3] was 140 ppb in the afternoon,suggesting a serious O3 pollution in this region ofChina.Leaf senescenceE-O3 significantly accelerated the senescence of flagleaves as indicated by a significant interaction betweenO3 and leaf age in Chl, Car and MDA contents (Table 3),and by progressively increasing difference betweenA-O3 and E-O3 in the contents (Fig. 2a–d, g and h).
584 Z . F E N G et al.Table 2 O3 exposure indices in the ambient (A-O3) and elevated (E-O3) [O3] plots and the percentage of actual elevation (1 %)during the whole fumigation (85 days) and during flag leaf development (49 days), respectivelyWhole fumigationA-O3E-O31%Flag leaves developmentM7 (ppb)AOT40 (ppm.h)SUM06 (ppm.h)M7 (ppb)AOT40 (ppm.h)SUM06 .026.77.213.688.910.822.7110.2Mean [O3] is calculated based on daily average concentration of 7 h period from 09:00 to 16:00 hours (M7). The accumulatedexposure over a threshold of 40 ppb (AOT40) and sum of hourly average [O3] X60 ppb (SUM06) for the season are calculated fromthe 1 h average of daytime concentrations in A-O3 and E-O3 plots.Fig. 1 Daily 7 h average [O3] for ambient (A-O3) and elevated (E-O3)[O3] treatments from tillering stage of winter wheat to final harvestunder fully open-air field conditions.When compared with A-O3, a statistically significantdifference first emerged in cultivar Y2 at 30 days afterfull expansion of flag leaves (DAFE) and persistedthroughout the remainder of the O3 fumigation period(Fig. 2a, c and g). The effects of O3 elevation appearedmore than 10 days later for the cultivar Y16 than Y2(Fig. 2b, d and h). The varietal difference in the accelerated
Differential responses in two varieties of winter wheat to elevated ozone concentration under fully open-air field conditions ZHAOZHONG FENG*,JINGPANG*w, KAZUHIKO KOBAYASHI*, JIANGUO ZHUz and DONALD R. ORT§ *Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan,
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