UUnniivveerrssiittyy ooff WWoolllloonnggoonngg RReesseeaarrcchh OOnnlliinnee Faculty of Science - Papers (Archive) Faculty of Science, Medicine and Health 1-1-2002 SSuurrffaaccee rreeflfleeccttaannccee pprrooppeerrttiieess ooff AAnnttaarrccttiicc mmoossss aanndd tthheeiirr rreellaattiioonnsshhiipp ttoo ppllaanntt ssppeecciieess,, ppiiggmmeenntt ccoommppoossiittiioonn aanndd pphhoottoossyynntthheettiicc ffuunnccttiioonn C. E. Lovelock University of Queensland Sharon A. Robinson University of Wollongong, [email protected] Follow this and additional works at: https://ro.uow.edu.au/scipapers Part of the Life Sciences Commons, Physical Sciences and Mathematics Commons, and the Social and Behavioral Sciences Commons RReeccoommmmeennddeedd CCiittaattiioonn Lovelock, C. E. and Robinson, Sharon A.: Surface reflectance properties of Antarctic moss and their relationship to plant species, pigment composition and photosynthetic function 2002. https://ro.uow.edu.au/scipapers/48 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] SSuurrffaaccee rreeflfleeccttaannccee pprrooppeerrttiieess ooff AAnnttaarrccttiicc mmoossss aanndd tthheeiirr rreellaattiioonnsshhiipp ttoo ppllaanntt ssppeecciieess,, ppiiggmmeenntt ccoommppoossiittiioonn aanndd pphhoottoossyynntthheettiicc ffuunnccttiioonn AAbbssttrraacctt We investigated how surface reflectance properties and pigment concentrations of Antarctic moss varied over species, sites, microtopography, and with water content. We found that species had significantly different surface reflectance properties, particularly in the region of the red edge (approximately 700 nm), but this did not correlate strongly with pigment concentrations. Surface reflectance of moss also varied in the visible region and in the characteristics of the red edge over different sites. Reflectance parameters, such as the Photochemical Reflectance Index (PRI) and Cold Hard Band (CBH) were useful discriminators of site, microtopographic position and water content. The PRI was correlated both with the concentrations of active xanthophyll-cycle pigments and the photosynthetic light use efficiency, Fv/Fm, measured using chlorophyll fluorescence. Water content of moss strongly influenced the amplitude and position of the red-edge as well as the PRI, and may be responsible for observed differences in reflectance properties for different species and sites. All moss showed sustained high levels of photoprotective xanthophyll pigments, especially at exposed sites, indicating moss is experiencing continual high levels of photochemical stress. KKeeyywwoorrddss Bryum pseudotriquetrum, Ceratodon purpureus, chlorophyll, chlorophyll fluorescence, Grimmia antarctici, Photochemical Reflectance Index, xanthophyll cycle pigments DDiisscciipplliinneess Life Sciences | Physical Sciences and Mathematics | Social and Behavioral Sciences PPuubblliiccaattiioonn DDeettaaiillss This article was originally published as: Lovelock, CE & Robinson, SA, Surface reflectance properties of Antarctic moss and their relationship to plant species, pigment composition and photosynthetic function, Plant Cell and Environment, 2002, 25(10), 1239-1250. Copyright 2002 Blackwell Publishing. The definitive version can be found here through Blackwell Publishing. This journal article is available at Research Online: https://ro.uow.edu.au/scipapers/48 1 Surface reflectance properties of Antarctic moss and their relationship to plant species, pigment composition and photosynthetic function 5 RUNNINGTITLE: Surfacereflectanceofmoss CatherineE.LovelockandSharonA.Robinson* SmithsonianEnvironmental ResearchCenter,POBox28,EdgewaterMD21037,USA. 10 *InstituteforConservationBiology,UniversityofWollongong,Northfields Avenue, Wollongong,NSW2522,Australia Correspondenceabout this manuscript to: 15 DrCatherineLovelock SmithsonianEnvironmental ResearchCenter POBox28 EdgewaterMD21037,USA Tel 4434822264,FAX4434822380 20 email [email protected] 2 Abstract Weinvestigatedhowsurfacereflectanceproperties and pigment concentrations ofAntarctic moss variedoverspecies,sites,microtopography,and withwatercontent. Wefoundthat species hadsignificantlydifferent surfacereflectanceproperties,particularlyintheregionoftherededge 5 (approximately700nm),but this didnot correlatestronglywithpigment concentrations. Surface reflectanceofmoss alsovariedinthevisibleregionandinthecharacteristics oftherededgeover different sites. Reflectanceparameters,suchas thePhotochemical ReflectanceIndex(PRI)and ColdHardBand(CBH)wereuseful discriminators ofsite,microtopographicpositionandwater content. ThePRIwas correlatedbothwiththeconcentrations ofactivexanthophyll-cycle 10 pigments andthephotosyntheticlight useefficiency,F /F ,measuredusingchlorophyll v m fluorescence. Watercontent ofmoss stronglyinfluencedtheamplitudeandpositionofthered- edgeas well as thePRI,andmayberesponsibleforobserveddifferences inreflectanceproperties fordifferent species andsites. All moss showedsustainedhighlevels ofphotoprotective xanthophyll pigments,especiallyat exposedsites,indicatingmoss is experiencingcontinual high 15 levels ofphotochemical stress. Keywords: Bryumpseudotriquetrum,Ceratodonpurpureus,chlorophyll,chlorophyll fluorescence,Grimmiaantarctici,Photochemical ReflectanceIndex,xanthophyll cyclepigments 3 Introduction Solarradiationis essential forphotosynthesis andgrowthofplants.Thesurfacereflectance characteristics ofleaves is keyindeterminingboththequantityandqualityofsolarradiationthat 5 is incident onleafinternal organs (Vogelmann1993).Althoughsolarradiationinthevisible regiondrives photosynthesis,absorptanceofhighlevels ofvisibleradiationandradiationat other wavelengths canbedamaging. At shorterwavelengths,absorbedUV-B (280-320nm)radiation cancauselesions tonucleicacidandproteins. Excess levels ofvisibleradiation(400-750nm) cancausephotoinhibitionofphotosynthesis (Cornic,Woo& Osmond1982). Highabsorptance 10 at longerwavelengths (>750nm)leads toincreases intemperaturethat canbedetrimental inarid environments whereit is important forplants toconservewaterbylimitingtranspirationandto maintainleaftemperatures closetothat ofairtemperatures (Gates et al.1965; Ehleringer1981). Variationinabsorptanceofleaves has beenobserved(Ehleringer& Björkman1978; Vogelmann 1993)andspecializedadaptationofsurfacereflectanceproperties inplants is knowntofacilitate 15 photosyntheticcarbongain. Forexample,indesert ecosystems pubescent orwaxyleafsurfaces that increasereflectancearecommon(Billings & Morris 1951,Ehleringer1981)and havebeen demonstratedtobeimportant inprotectionfrom photoinhibitionofphotosynthesis (e.g. Robinson,Lovelock & Osmond1993).Moreover,epidermal tissues oftenaccumulate compounds that areeffectivescreens against UV-B radiation,therebydecreasingtheirreflectance 20 intheUVregion(e.g.Tevini 1993). In this studywesurveyedthenatural variabilityofsurfacereflectanceinmoss in continental Antarctica.At ourstudysiteinWilkes Land,easternAntarctic,conditions arecold andusuallyveryarid,resultinginlichens dominatingtherocky,ice-freeareas. Themoss 4 communityis well developedfortheregion,but it is restrictedtomelt lakes,melt streams and otherlocations wherefreewateris availableforat least someofthesummer(November-March). Inwinterthemoss is coveredwithsnowand iceandis subject todisturbancefrom cryo- perturbation,whichcauses highlycomplexmicroreliefofridges andvalleys withinthemoss 5 turves. Comparedtolowerlatitudes,levels ofUVradiationarelowinAntarctica. But, Antarcticais experiencing largeincreases inincident UV-B radiationduetoreductions in concentrations ofstratosphericozone(Frederick& Snell 1988).Increases inUVradiationare havinglargeeffects ontheprimaryproductionoftheoceans (Neale,Davis & Cullen1998),and couldalso beaffectingterrestrial vegetation.Theheat balanceofmoss,photosyntheticprocesses 10 andtheimpacts ofenhancedUVradiationarelikelytobepartiallydependent onmoss surface reflectancecharacteristics andpigment composition. Thus,thefirst aim ofthis studywas totest thehypothesis that moss species andmoss inhabitingdifferent environments havedifferent surfacereflectancecharacteristics. Wemeasuredsurfacereflectanceat approximatelyonenm intervals from 200-900nm ofthreedominant moss species overthreesites withdifferingwater 15 availabilities,andoverdifferences inmicrotopography. Surfacereflectanceproperties ofvegetationareoftenusedas indicators ofphotosynthetic functioninremotesensingofvegetation(reviewedinField,Gamon& Penuelas 1994). Variationinreflectancehas beenobservedamongplant taxa(e.g.Gates et al.1965; Penuelas et al.1993; Gamonet al.1995),duetodifferences inwaterandnutrient availability(e.g.Penuelas 20 et al.1994; Gamon,Serrano& Surfus 1997),environmental stress (e.g.Rock,Hoshizaki & Millar1988),andbothseasonally(Running& Nemani 1988; Gamonet al.1995)anddiurnally (Gamonet al.1990; Penuelas et al.1994; Gamonet al 1997). Changes insurfacereflectanceof intact leaves havebeendirectlycorrelatedwithleafchemical composition(Jacquemoudet al. 5 1996)andchanges inphotosyntheticprocesses insomehigherplant taxa(Gamonet al.1990; Gamonet al.1997,Gilmore& Ball 2000). But it is oftendifficult toreconcilephysiological measurements,whichareoftenmadeat thescaleofindividual leaves and remotelysensed measurements,whichareoftenmadeat thescaleofwholecanopies (Williams 1991; Fieldet al. 5 1994). Someofthedifficultyarises becausecanopies arecomplex. Canopies arecomposedof layers ofleaves ofdifferent species that canhavearangeofproperties. Leaves within canopies canhavevaryingdistributions,orientations,morphologies,internal structures,pigment compositions,andepidermal characteristics,all properties that couldinfluencethesurface reflectanceofthecanopy(Rocket al.1988; Fieldet al.1994;Voseet al.1995). At theleaflevel, 10 mosses areverysimpleconsistingofafewcell layers. But at ahigherlevel oforganization, moss turves,althoughsmall incomparisontocanopies oftrees,arerelativelycomplex, composedofmanyleafygametophytes packed togethertoform what canbethought ofas a ‘micro-canopy’. Inmosses,measurement ofphotosyntheticprocesses andsurfacereflectance canbemadeat thesamescale. Thus,thesecondaim ofourstudywas totest thehypothesis that 15 spectral properties ofmoss aredirectlyrelatedtophotosyntheticprocesses and pigment composition. Wewishedtounderstandwhethernon-destructivehyperspectral measurements couldpotentiallybeusedtomapmoss species distributions andphysiological activityboth spatiallyandtemporally.Herewedescribetests ofcorrelations amongsurfacereflectance properties andcommonlyusedreflectanceindices andpigment concentrations,watercontents 20 andphotosyntheticfunctionofAntarcticmoss. 6 Materialsand methods Moss was obtained from threesites aroundtheAustralianbaseCasey,Wilkes Land,continental Antarctica(66º17’S,110º32’E).Twosites weredirectlyadjacent tothebase(RedShed and 5 Science),whiletheothersitewas 20km west at RobinsonRidge. Wateravailabilityvaries across thesites. TheRedShedsiteis directlyadjacent toalargesummermelt lake,the RobinsonRidgesiteis adjacent toamelt stream that flows intermittentlyoverthesummer months,whileat theSciencesitewateris onlyavailablewhenoverlyingsnowmelts earlyinthe summer. Temperaturealsovaries overthesites. RobinsonRidgeis cooler,andmoreexposed, 10 thantheRedShedandSciencesites (Melick, Hovendon& Seppelt 1997).Intheearly1990s the Sciencesitewas exposed tocontaminationduetodepositionofcement dust duringconstruction (Adamson,Adamson& Seppelt 1994). At eachsiterepresentativesamples,approximately2cm2werecut from theturves over3 summerseasons (OctobertoFebruary)from 1996to1999. Thethreemost commonspecies, 15 Grimmiaantarctici Card.,Ceratodonpurpureus (Hedw.)Brid.,andBryumpseudotriquetrum (Hedw.)Gaertn.,MeyerandScherb.weresampled.Thenecessity ofminimizingimpacts ofthe study,theextent ofthemoss beds (smallerat Science)and thenatural distributionofthethree moss species resultedindifferent representations ofeachspecies at eachsite(Table1). We testedfordifferences betweenmoss species andtheinfluenceofsiteonpigment andsurface 20 reflectanceparameters usingsamples from all sites. Weusedasubset oftheRobinsonRidgeand RedShedGrimmiaantarctici samples totest foreffects ofmicrotopography. Additionallywetestedtheinfluenceofmoss watercontent onsurfacereflectance. We used4samples ofG.antarctici from microtopographicridges at RobinsonRidge. Wemeasured 7 thesurfacereflectanceofthesamples whentheyweredry,andafterthey hadbeenrehydrated withafinemist ofwaterfor5minutes. Excess waterwas blottedfrom themoss before measurements.Moss re-hydraterapidly,regainingfull physiological activityinonetotwo minutes (Robinsonet al.2000). 5 Measuringsurfacereflectance Weusedanintegratingspherefittedtoascanningspectrophotometer(GBC UV-Vis 918, GBC Australia)tomeasurespectral reflectanceofmoss between200–900nm at approximately 10 1nm intervals on2cm2areas ofmoss canopy. Themoss turfsamples werelargeenoughto completelyfill thesampleapertureoftheintegratingsphere. Tomaintainmoss canopy architecture(gametophytearrangement)thesamples wereheldinplacebywithina1cm deep highlyreflectivesampleholder.Forsomesamples this requiredshavingtissuefrom thebasal portionofthesampleusingarazorbladeuntil theturfwas 1cm thick.Thedepthand packingof 15 thesamples resultedintransmittanceofthroughthesamplebeingnegligible.Reflectance measuredthereforerepresents themoss canopywithgreenphotosynthetictissueofbetween2-5 mm depthand8-5mm ofnon-photosynthetictissuebelow.From theentirereflectancespectra, usingsubsets ofthetotal 87spectra(Table1),wepresent meanreflectanceat keywavelengths andalsocalculatereportedindices usedinremotesensingvegetation(Table2).TheColdHard 20 Band(CHB,Gilmore& Ball 2000)has beenfoundtocorrelatewiththeformationofa chlorophyll-proteincomplexinleaves that protects against freezingdamage.ThePhotosynthetic ReflectanceIndex(PRI,Gamonet al.1990,1997)was developedtoreflect changes in concentrations ofthexanthophyll cyclepigments that areformedwhenplants arestressed. The amplitudeofthereflectancechangeat thered-edge( )andthepositionofthered-edge( ) (cid:1)RE RE 8 werecalculatedfrom thefirst derivativeofthespectra(Horler,Dockray& Barber1983).Other reflectanceindices usedbyarangeofresearchers werealsocalculated(e.g.Vogelmann,Rock& Moss 1993; Gitelson& Merzylak1997; Lichtenthaler,Gitelson& Lang1996; Carter1991, 1993). 5 Pigment,water andphotosyntheticcharacteristics Priortomeasurement ofthereflectancespectra,asubset oftheG.antarctici samples weredark adaptedfor20minutes afterwhichthechlorophyll fluorescenceparameterF /F was measured v m 10 usingaPAM 2000(H.Walz,Effeltrech,Germany). Afterthereflectancespectraweremeasured, thephotosyntheticallyactiveapices ofthemoss wereremovedusingarazorbladeand thetissue was frozeninliquidnitrogen. Samples werereturnedtoAustraliainliquidnitrogenand then storedat –80˚C inafreezerpriortopigment analysis. Forchlorophyll and carotenoid determination,samples (50-100mgfreshweight)weregroundwithliquidnitrogenandsandina 15 mortarandpestleandthenextractedin(1.5ml)100%acetone.Thesamples weretransferredtoa micro-centrifugetubecontaining1mgsodium bicarbonateandkept oniceinthedarkfor20 minutes. Aftercentrifugation(13,000rpm for5minutes)thepellet was re-extractedin0.5ml 80%acetoneusingapolypropylenetissuegrinder(CrownScientific,Australia). Afterafurther 20minutes onice,thesamplewas centrifugedas aboveandthesupernatants from each 20 extractioncombinedandmadeupto3ml with100%acetone. Chlorophylls and carotenoids werethenquantifiedbyhigh-pressureliquidchromatography(HPLC)usingamethodadapted from Gilmore& Yamamoto(1991).Thesamples (30-70µl)wereinjectedintoaShimadzu HPLC system (Shimadzu,Australia)at aflowrateof2ml min-1. Solvent A