Kahmen Et Al Resubmission

Originally published as: Kahmen, A., Dawson, T. E., Vieth, A., Sachse, D. (2011): Leaf wax n‐alkane δ,{delta} D values are determined early in the ontogeny of Populus trichocarpa leaves when grown under controlled environmental conditions. ‐ Plant, Cell & Environment, 34, 10, 1639‐1651 DOI: 10.1111/j.1365‐3040.2011.02360.x 1 2 Leaf wax n-alkane δD values are determined early in the ontogeny of 3 Populus trichocarpa leaves when grown under controlled environmental 4 conditions 5 6 7 Ansgar Kahmen1,*, Todd E. Dawson1, Andrea Vieth2 and Dirk Sachse3 8 9 10 1 Center for Stable Isotope Biogeochemistry, Department of Integrative Biology, 11 University of California – Berkeley, USA 12 2 Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, 13 Germany 14 3 DFG-Leibniz Center for Surface Process and Climate Studies, Institute of Earth and 15 Environmental Sciences, University of Potsdam, Germany 16 17 *Current address and correspondence: 18 Ansgar Kahmen, ETH Zürich – Institute of Agricultural Sciences 19 Universitätsstrasse 2, LFW C55.2 20 CH-8092 Zürich 21 email: [email protected] 22 Tel: +41-44-6328515 23 1 23 Abstract 24 The stable hydrogen isotope ratios (δD) of leaf-wax n-alkanes record valuable 25 information on plant and ecosystem water relations. It remains, however, unknown if leaf 26 wax n-alkane δD values record only environmental variation during the brief period of 27 time of leaf growth or if leaf wax n-alkane δD values are affected by environmental 28 variability throughout the entire life of a leaf. To resolve these uncertainties, we irrigated 29 Populus trichocarpa trees with a pulse of deuterium-enriched water and used compound 30 specific stable hydrogen isotope analyses to test, if the applied tracer can be recovered 31 from leaf wax n-alkanes of leaves that were at different stages of their development 32 during the tracer application. Our experiment revealed that only leaf wax n-alkanes from 33 leaves that had developed during the time of the tracer application were affected, while 34 fully mature leaves were not. We conclude from our study that under controlled 35 environmental conditions leaf wax n-alkanes are synthesized only early in the ontogeny 36 of a leaf. Our experiment has important implications for the interpretation of leaf wax n- 37 alkane δD values in an environmental context as it suggests that these compounds record 38 only a brief period of the environmental variability that a leaf experiences throughout its 39 life. 40 2 40 Introduction 41 The oxygen and hydrogen isotope composition (δ18O and δD, respectively) of organic 42 plant materials can be used as valuable indicators of environmental and physiological 43 processes (Dawson & Siegwolf, 2007). Over the last two decades plant physiological 44 research has provided a detailed understanding of the mechanisms that determine the 45 δ18O and δD values in plant cellulose and it is now becoming well established as an 46 integrative recorder of environmental and physiological processes (Yakir, 1992, Roden et 47 al., 2000, Brooks & Coulombe, 2009, Sternberg, 2009, Kahmen et al., 2011). The 48 development of new analytical instrumentation and methodologies over the last decade 49 offers now the opportunity to use the δ18O and δD values obtained from other plant 50 compounds such as lignin or leaf waxes as additional recorders of environmental signals 51 that are complementary in the sort of information they can provide to plant cellulose 52 (Burgoyne & Hayes, 1998, Hilkert et al., 1999a, Keppler et al., 2007, Greule et al., 53 2008). In particular compound-specific stable hydrogen isotope analysis of leaf wax n- 54 alkanes has now been shown to be a powerful tool for investigating present and past 55 hydrological processes as well as an indicator of plant and ecosystem water relations (Xie 56 et al., 2000, Sauer et al., 2001, Huang et al., 2004, Sachse et al., 2004, Schefuss et al., 57 2005, Tierney et al., 2008). 58 Leaf wax n-alkanes are long-chained alkyl lipids with 25 to 33 carbon atoms that are 59 vital components of higher plant cuticles (Jetter et al., 2006). Several chemical and 60 biological characteristics make leaf wax n-alkanes ideal biomarkers for the investigation 61 of modern or past environments. This is because n-alkanes are relatively easy to extract 62 from plant leaves or from sediment samples where these waxes have accumulated in. 3 63 Also, the analytical tools for determining δD values of individual n-alkanes are now well 64 developed (Burgoyne & Hayes, 1998, Hilkert et al., 1999a). Additionally, n-alkanes can 65 persist in the sedimentary record over geological time scales, which is an important 66 prerequisite for paleoclimatic reconstructions (Radke et al., 2005). Finally, n-alkanes are 67 composed of only carbon and hydrogen atoms and the hydrogen atoms are covalently 68 bound to the carbon atoms in the molecule. Therefore the original hydrogen isotope 69 composition can be preserved in the molecules over geologic time scales (Schimmelmann 70 et al., 2006). 71 Much of the variability in leaf wax n-alkane δD values originates from precipitation, 72 which serves as the plants’ principal source of hydrogen when the plant takes this water 73 up from the soil (Dawson et al., 2002). Several studies have now shown that δD values of 74 leaf wax n-alkanes from terrestrial plants, sediments and soils record the isotope 75 composition of precipitation along environmental gradients (Sauer et al., 2001, Huang et 76 al., 2004, Sachse et al., 2004, Sachse et al., 2006, Smith & Freeman, 2006, Hou et al., 77 2008, Feakins & Sessions, 2010, McInerney et al., 2011). Since the isotope composition 78 of precipitation is influenced by a number of hydrological processes (Craig & Gordon, 79 1965, Gat, 1996), leaf wax n-alkane δD values in sediments have been suggested to 80 indicate, for example, the intensity or origin of precipitation (Schefuss et al., 2005, 81 Tierney et al., 2008). 82 In addition to the δD values of precipitation leaf wax n-alkanes δD values can also be 83 influenced by soil water and/or leaf water, which is typically enriched in deuterium when 84 compared to precipitation (Sachse et al., 2006, Smith & Freeman, 2006, Sachse et al., 85 2009, Feakins & Sessions, 2010, McInerney et al., 2011). Further, substantial seasonal or 4 86 cross-species variability has been observed for δD values of leaf wax n-alkanes from 87 temperate, tropical or boreal ecosystems (Liu et al., 2006, Sachse et al., 2006, Smith & 88 Freeman, 2006, Hou et al., 2007, Liu & Yang, 2008, Pederitchouk et al., 2008, Sachse et 89 al., 2009, Feakins & Sessions, 2010). Interestingly, this high interspecific and seasonal 90 variability cannot be fully explained by the influence of precipitation δD values or by leaf 91 water evaporative enrichment in deuterium. This suggests that additional and perhaps 92 fundamentally important plant physiological, biochemical and/or plant ecological 93 processes influence the δD values of leaf wax n-alkanes. These processes are not yet 94 understood and can thus complicate the interpretation of δD values of leaf wax n-alkanes 95 (Zhou et al., 2011). 96 One particularly important plant ecophysiological characteristic that is critical for the 97 robust interpretation of leaf wax n-alkane δD values is the temporal integration with 98 which environmental or physiological signals are recorded in leaf wax n-alkane δD 99 values. The temporal integration of leaf wax n-alkane δD values depends of course on the 100 duration of time over which the leaf wax n-alkanes are synthesized for a particular leaf. 101 The cuticle of plant leaves is typically synthesized early in the ontogeny of a leaf 102 (Kolattukudy, 1970, Jenks et al., 1996, Riederer & Markstaedter, 1996, Hauke & 103 Schreiber, 1998). If n-alkanes are made at the same time then we would expect the δD 104 values of leaf wax n-alkanes to be “locked-in” early in the development of a leaf and to 105 record therefore only a brief period of the environmental or physiological variability that 106 a leaf experiences. The abundance and chemical composition of leaf waxes has, however, 107 been shown to undergo substantial changes in mature and fully expanded leaves (Jetter et 108 al., 2006, Shepherd & Griffiths, 2006). These changes can occur either during the natural 5 109 course of leaf ontogeny (Hauke & Schreiber, 1998, van Maarseveen et al., 2009) or as a 110 response of the leaf to environmental stressors (Baker, 1974, Bengtson et al., 1978, Jetter 111 & Schaffer, 2001, Cameron et al., 2006). Given these observed post-maturation changes 112 in leaf wax abundance and composition it remains unclear if environmental and 113 physiological information in n-alkane δD values is solely recorded and “locked-in” early 114 in the life of a leaf or if the continuous de-novo synthesis of leaf waxes integrates 115 environmental or physiological information in leaf wax n-alkane δD values over the 116 entire lifespan of a leaf. 117 Very few observational studies have investigated the variability of leaf wax n-alkane 118 δD values during the live of a leaf and published studies report contrasting results. Sachse 119 and co-workers (2010) for example have observed that leaf wax n-alkane δD values of 120 barley leaves (Horduem vulgare, Poaceae) are established early in the life of a leaf and 121 show little seasonal variation thereafter. In contrast, Pedentchouk et al. (2008) and Sachse 122 et al. (2009) have shown large seasonal variations in leaf wax n-alkane δD signals in the 123 foliage of deciduous tree species that can reach up to 40‰. In summary, no general 124 pattern for the temporal integration of environmental signals in leaf wax δD values has 125 yet emerged.

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