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Leaf wax n-alkanes and d13C values of CAM plants from arid southwest Africa

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We analysed leaves from 42 plants from the South African Succulent Karoo. Whole leaf d13C values clearly differentiated 3 different populations, consisting of plants operating under obligate CAM (crassulacean acid metabolism), facultative CAM and C3
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  Note Leaf wax  n -alkanes and  d 13 C values of CAM plants from aridsouthwest Africa A. Boom a, ⇑ , A.S. Carr a , B.M. Chase b,c , H.L. Grimes a , M.E. Meadows d a Department of Geography, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom b Institut des Sciences de l’Evolution de Montpellier, Département Environnements, UMR 5554, Université Montpellier 2, Bat. 22, CC061, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France c Department of Archaeology, History, Culture and Religion, University of Bergen, Postbox 7805, 5020 Bergen, Norway d Department of Environmental and Geographical Sciences, University of Cape Town, Rondebosch, Cape Town 7701, South Africa a r t i c l e i n f o  Article history: Received 5 September 2013Receivedinrevisedform15November2013Accepted 10 December 2013Available online 16 December 2013 a b s t r a c t Weanalysedleavesfrom42plantsfromtheSouthAfricanSucculentKaroo.Wholeleaf  d 13 Cvaluesclearlydifferentiated 3 different populations, consisting of plants operating under obligate CAM (crassulaceanacid metabolism), facultative CAM and C 3  modes. In contrast, the leaf wax  n -alkane  d 13 C data from thesemetabolicgroupsshowedabroaderoverlappingdistribution. CAMplantsoperatingunderfullCAMmodeproduced a wide range of apparent  13 C fractionation. Succulent/CAM plant wax yield was considerable(up to 23mg/g in our plants), so its contribution to soil composition and sedimentary leaf wax compo-sitionshouldnotbedismissed. Thepresenceof CAMplant wax n -alkanesinsedimentary n -alkanes couldtherefore be a problem for accessing ecosystem scale C 3 –C 4  proportion.   2013 Elsevier Ltd. All rights reserved. 1. Introduction n -Alkanes are increasingly being used as palaeoenvironmentalproxies, particularly through the application of compound-specific d 13 C and  d D analyses. The proportion of plants using the C 3  and C 4 photosynthetic pathways can be reconstructed from sedimentary n -alkanes using established end member  d 13 C values from modernC 3 andC 4 plants(e.g.Maslinetal.,2012).Asignificantisotopicfrac-tionation of   13 C during leaf wax lipid synthesis produces a largeoffsetbetweenbulktissueandleafwaxlipids(deNiroandEpstein,1977).For n -alkanes,thisisintheregionof6 ‰ and10 ‰ forC 3  andC 4  plants, respectively(Collisteret al., 1994;Lockheart et al., 1997;Bi et al., 2005; Vogts et al., 2009).The isotopic composition of leaf wax derived from the thirdmajor photosynthetic pathway, crassulacean acid metabolism(CAM) has received much less attention. Up to 33 families and10,000 angiosperms (ca. 7% of vascular plants) utilise CAM metab-olism (Cushman, 2001). CAM plants are particularly common intropical and arid regions (Ting, 1985).CAM plant whole leaf   d 13 C values are known to be highlyvariable, ranging between   10 and   20 ‰  (Mooney et al., 1977;Lüttge, 2004). This reflects flexibility in the CAM photosyntheticmechanism. Some species are obligate CAM plants, while othersarefacultativeandareabletoshiftbetweenC 3  andCAM.Theeffectof CAM plant waxes on biomarker-based palaeoecological records,particularly marine leaf wax records close to arid parts of Africa, isunclear. It is often assumed to be low (e.g. Maslin et al., 2012), butthere is not much basis for this, and it may depend on the geo-graphical region. Only a handful of studies have reported leaf wax d 13 CvaluesforknownCAMplantsandnonewerefromnaturalhabitats (Feakins and Sessions, 2010 and references therein). Withso few data points it is not possible to evaluate the wider signifi-cance of CAM plants for  n -alkane proxy records.In arid western parts of southern Africa CAM plants arecommon and dominate in the Succulent Karoo Biome. Here wepresent  n -alkane  d 13 C data for 42 succulent plants from theirnatural habitat across the Succulent Karoo Biome of South Africa.Thedata areconsideredin thecontext of CAMplant ecophysiologyand the implications for palaeoecological reconstruction in the re-gion.Moredetaileddescriptionsofsamplingandleafwax n -alkanedistributions are presented in an accompanying study (Carr et al.,2014). 2. Methods Samples of leaf and stem succulents were selected from plantsobtained from a wider study that encompassed the Succulent Kar-ooandFynbosBiomesoftheWesternandNorthernCapeProvincesofSouthAfricaduringApril/May2010(TableS1). Theywerestoredin paper bags, air dried at the University of Cape Town and freezedried. Ca. 0.5g powered photosynthetic tissue was extracted usinga Soxhlet system [24h, hexane/CH 2 Cl 2 /MeOH (1:2:2 v/v/v)];5 a -cholestane was added as internal standard. Each extract was 0146-6380/$ - see front matter   2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.orggeochem.2013.12.005 ⇑ Corresponding author. Tel.: +44 (0) 1162523832. E-mail address:  ab269@le.ac.uk (A. Boom).Organic Geochemistry 67 (2014) 99–102 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem  purified and separated using an Al 2 O 3  column. Gas chromatogra-phy–mass spectrometry (GC–MS) analysis of each apolar fractionwas carried out with a Perkin Elmer Clarus-500 (column: CP-Sil5CB MS, 30m  0.25mm). The linearity of the GC–MS systemwas tested using an  n- alkane standard (C 7 - C 40 ) at a concentrationrange from 5 l g/ml to 800 l g/ml. High signal linearity wasachievedoverthefullchainlengthrangeandnotrendsinresponsefactor with  n- alkane chain length were observed. For more detailssee Carr et al. (2014).The  n -alkane  d 13 C values were determined using isotope ratiomonitoring GC–MS (irm-GC–MS), with an Agilent 6890N gas chro-matograph equipped with a PtCu wire combustion reactor, inter-faced to a SerCon GC-CP and 20–20 MS instrument. Isotopereference compounds comprising 5 a -cholestane and C 19  fatty acid Fig. 1.  Frequency distributions showing, (A) whole leaf   d 13 C values, (B) weighted mean average of   n -alkane  d 13 C values and (C)  D d 13 C (whole leaf- WMA alkanes) of 42selected succulent plants from the succulent Karoo in South Africa.100  A. Boom et al./Organic Geochemistry 67 (2014) 99–102  methyl ester were co-injected with every analysis;  n -alkanes weremeasured in triplicate with a precision of 0.2 ‰ . Whole leaf   d 13 Cvalues were determined in triplicate using a SerCon ANCA GSL elemental analyser interfaced to the SerCon 20–20 MS, with a pre-cision better than 0.1 ‰ . 3. Results  3.1.  13 C stable isotope results for whole leaf tissue Whole leaf   d 13 C values showed marked similarities withreported values (von Willert et al., 1977; Rundel et al., 1999;Winter and Holtum, 2002), with clusters (Fig. 1) centred at   13to   18 ‰  ( n  =17) representing obligate CAM plants and   20 to  27 ‰ ( n  =18) representing facultative CAM plants. A cluster of 6C 3  plants was centred at   25 to   29 ‰ . With the exception of  Salsola tuberculata , a C 4  plant (  13 ‰ ), the most enriched plantswere almost certainly obligate CAM on the basis of the measure-ments (Winter and Holtum, 2002). They were from three families:Xanthorrhoeaceae (  Aloë ), Crassulaceae ( Tylecodon ) and Aizoaceae( Cheiridopsis ,  Ruschia  and  Mesembryanthemum ).  Aloë ramosissima is an obligate CAM plant (Rundel et al., 1999), as is  Ruschia stricta (Matimati et al., 2012). The lower  d 13 C cluster (  20 to   27 ‰ )formeda broader andmore complexdistribution, probablyreflect-ingplantscapableofCAMflexibility(Rundeletal., 1999) forwhichthere is a variation in day and dark fixation (Winter and Holtum,2002).  3.2. n-AlkanesThe n -alkane concentrations and compound specific  d 13 C valuesare shown in the Supplementary material (Table S1). A range fromC 21  to C 35  was found, with C 36  and C 37  detected in a few samples,but in low amount (<1%). Total (C 21 –C 35 ) concentration rangedfrom 0.02 to 22mg/g dry wt. (dw). The yield distribution washighly skewed by a small number of outliers and the majority(58%) of plants produced a yield <10mg/g dw (see Carr et al.,2014 for a more detailed discussion). All plants had a strong odd/evenpredominance anda maximumat C 31  or C 33 , althougha max-imum at C 29  was also observed. 4. Discussion The whole leaf   d 13 C values clearly indicated three distinctpopulations, one with an obligate CAM photosynthetic pathway,one reflecting a varied CAM-C 3  flexibility and the third, C 3  plants(Winter and Holtum, 2002). The  n -alkane concentration valueswere markedly higher than reported for southern African grasses(Rommerskirchen et al., 2006 – avg. yield 0.3 to 0.4mg/g dw),savannah vegetation (avg. ca. 0.6mg/g dw; Vogts et al., 2009), aswell as most woody vegetation fromthe Succulent Karoo and Fyn-bosbiomes(woodyshrubavg.1.1±2.6mg/gdw;Carretal., 2014).A high yield from drought adapted plants is perhaps unsurprising.Maffei et al. (1997) reported  n -alkane concentration of 1–10mg/g(  fresh  weight) for various Cactaceae, further suggesting that lipidconcentration from CAM plants may be substantial.Average chain length (ACL) values were similar to thosereported for CAM plants (Feakins and Sessions, 2010). In generalthe distributions are comparable to reported values for naturalsouthern African vegetation (Vogts et al., 2009), although therewas an overall tendency for succulent plant functional types toproduce longer and less dispersed distributions in the study area(Carr et al., 2014).In terms of   d 13 C, the plants showed some interesting patterns.The CAM-C 3  flexible plants produced  n -alkanes depleted in  13 Cvs. whole leaf tissue by on average 6.1 ‰ . This is consistent withearlier studies, which reported  D d 13 C as being near 6 ‰  for C 3 and some CAM plants (Collister et al., 1994; Feakins and Sessions,2010). However, the obligate CAM plants were highly variable;although their mean  D d 13 C value was similar at 7.1 ‰ , there wasa greater spread, ranging from 0.5 ‰ to 12.8 ‰ . This implies largedifferences in C allocation to lipid biosynthesis, presumably toaid water conservation.As a result of this highly varied fractionation during lipid syn-thesis, the resulting distribution of the  n -alkane  d 13 C valuesshowed far less separation of the two modes of photosynthesisthantheplot ofbulktissuedata(Fig. 1). Thishaspotentialimplica-tions for attempts at quantitative C 3 /C 4  vegetation reconstructionin regions where CAM/succulent plants are a component of thelandscape. This may be particularly important in certain regions,suchas the west coast of southernAfrica, wherethese plantsdom-inate. Our data also demonstrate  n -alkane yield similar to, or evenmarkedly higher than, that reported for ordinary C 3  and C 4  plants(see also Carr et al., 2014). 5. Conclusions Whole leaf   d 13 C analysis of naturally occurring succulent plantsclearlydifferentiatedobligateCAMandC 3 /CAMflexibleplants.The n -alkane  d 13 C values were less discriminatory, with CAM plantsoperating under full CAM exhibiting a wide range of  D d 13 C values,obscuring the pattern observed for whole leaf tissue. This impliesthat  n -alkane  d 13 C measurement is not a goodtool for determiningthe mode of CAM, nor for differentiating some CAMplants fromC 4 plants. Succulent/CAM plant wax yield can be substantial.Although these plants are not important in most biomes, thereare some regions, such as southern Africa, where this is not thecase and their influence on soil and sedimentary leaf wax compo-sition should not be dismissed.  Acknowledgements The study was funded by the Leverhulme Trust (Grant # F/00212/AF). B.M.C. is also supported by FP7 ERC Starting Grant‘‘HYRAX’’ (Agreement #258657). We would like to thank twoanonymous reviewers for valuable comments.  Appendix A. Supplementary material Supplementarydataassociatedwiththisarticlecanbefound,inthe online version, at http://dx.doi.org/10.1016/j.orggeochem.2013.12.005.  Associate Editor   –  I.D. BullReferences Bi, X.H., Sheng, G.Y., Liu, X.H., Li, C., Fu, J.M., 2005. Molecular and carbon andhydrogen isotopic composition of   n -alkanes in plant leaf waxes. OrganicGeochemistry 36, 1405–1417.Carr, A.S., Boom, A., Grimes, H.L., Chase,B.M., Meadows, M.E., Harris, A, 2014. Leaf wax  n -alkane distributions in arid zone South African flora: environmentalcontrols, chemotaxonomy and palaeoecological implications, OrganicGeochemistry 67, 72–84.Collister, J.W., Rieley, G., Stern, B., Eglinton, G., Fry, B., Adeline, M.T., 1994.Compound-specific d 13 Canalysesofleaflipidsfromplantswithdifferingcarbondioxide metabolisms. Organic Geochemistry 21, 619–627.Cushman, J.C., 2001. Crassulacean acid metabolism. A plastic photosyntheticadaptation to arid environments. Plant Physiology 127, 1439–1448.de Niro, M.J., Epstein, S., 1977. Mechanism of carbon isotope fractionationassociated with lipid synthesis. Science 197, 261–263.Feakins, S.J., Sessions, A.L., 2010. Crassulaceanacid metabolisminfluences D/Hratioof leaf wax in succulent plants. Organic Geochemistry 41, 1269–1276.  A. Boom et al./Organic Geochemistry 67 (2014) 99–102  101  Lockheart,M.J.,vanBergen, P.F.,Evershed,R.P., 1997.Variationsinthestablecarbonisotope compositions of individual lipids from the leaves of modernangiosperms: implications for the study of higher land plant-derivedsedimentary organic matter. Organic Geochemistry 26, 137–153.Lüttge, U., 2004. Ecophysiology of crassulacean acid metabolism (CAM). Annals of Botany 93, 629–652.Maffei, M., Meregalli, M., Scannerini, S., 1997. Chemotaxonomic significance of surface wax  n -alkanes in the Cactaceae. 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