The oxygen isotope enrichment of leaf‐exported assimilates – does it always reflect lamina leaf water enrichment?

Abstract

The oxygen stable isotope composition of plant organic matter (OM) (particularly of wood and cellulose in the tree ring archive) is valuable in studies of plant–climate interaction, but there is a lack of information on the transfer of the isotope signal from the leaf to heterotrophic tissues.We studied the oxygen isotopic composition and its enrichment above source water of leaf water over diel courses in five tree species covering a broad range of life forms. We tracked the transfer of the isotopic signal to leaf water-soluble OM and further to phloem-transported OM.Observed leaf water evaporative enrichment was consistent with values predicted from mechanistic models taking into account nonsteady-state conditions. While leaf water-soluble OM showed the expected 18O enrichment in all species, phloem sugars were less enriched than expected from leaf water enrichment in Scots pine (Pinus sylvestris), European larch (Larix decidua) and Alpine ash (Eucalyptus delegatensis).Oxygen atom exchange with nonenriched water during phloem loading and transport, as well as a significant contribution of assimilates from bark photosynthesis, can explain these phloem 18O enrichment patterns. Our results indicate species-specific uncoupling between the leaf water and the OM oxygen isotope signal, which is important for the interpretation of tree ring data.

French summer droughts since 1326 CE: a reconstruction based on tree ring cellulose <i>δ</i><sup>18</sup>O

“…The enrichment depends on the difference in vapor pressure of leaf air and ambient air, and is therefore controlled by relative humidity (e.g. Barbour et al, 2004; Sternberg, 2009;Gessler et al, 2013). Drought conditions therefore lead to an enrichment of 18 O in both the source water and the leaf water, yielding higher δ 18 O c values.…”

“…Recent studies have revealed the sensitivity of cellulose δ 18 O to the combined effects of temperature and humidity, and therefore of drought (Raffalli-Delerce et al, 2004;MassonDelmotte et al, 2005;Etien et al, 2008Etien et al, , 2009Labuhn et al, 2014). This influence can be explained by the physical processes and mechanisms, such as the isotopic enrichment of leaf water, which relate the cellulose δ 18 O signal to climate (Hill et al, 1995;Barbour and Farquhar, 2000;Barbour et al, 2004;Cernusak et al, 2005;Sternberg, 2009;Gessler et al, 2009Gessler et al, , 2013.…”

Seasonal temperature and precipitation recorded in the intra-annual oxygen isotope pattern of meteoric water and tree-ring cellulose

“…Plant characteristics that are known to affect absolute d 18 O cell values, but do not change seasonally, would not affect the applicability of the equations presented above. Such characteristics include tree species (e.g., Wang et al, 1998), stomatal density (Larcher et al, 2015), and differences in leaf-water enrichment and transpiration among individual trees (e.g., Gessler et al, 2013). Likewise, absolute d 18 O MW values have been shown to vary as a function of geographic parameters such as elevation (altitude) or distance from the coast (e.g., Dotsika et al, 2010;Dutton et al, 2005;Garzione et al, 2000;Gonfiantini et al, 2001;Rozanski et al, 1993), but these too, do not change seasonally.…”

“…By studying relative changes in d 18 O, rather than absolute d 18 O values, we avoid issues arising from differences in leaf-water enrichment and transpiration among individual trees (e.g., Gessler et al, 2013), allowing us to also compare diverse species that may otherwise have different intercept values (for example, see Bowen, 2008;Craig, 1961;Dansgaard, 1964;Gat and Matsui, 1991;Njitchoua et al, 1999;Rozanski et al, 1992Rozanski et al, , 1993. In order to quantitatively evaluate the potential for D(d 18 O MW ) to accurately predict these two climate parameters, we compiled the average precipitation (mm), temperature ( C), and d 18 O MW value for each month of the year at the 365 globally distributed GNIP sites pictured in Fig.…”

Multi-isotope labelling of organic matter by diffusion of <sup>2</sup>H/<sup>18</sup>O-H<sub>2</sub>O vapour and <sup>13</sup>C-CO<sub>2</sub> into the leaves and its distribution within the plant

“…O and H exchanges with tissue water during transport and the synthesis of new compounds (as recently discussed for oxygen in phloem sugars and cellulose in Offermann et al, 2011, andGessler et al, 2013). O of carbonyl groups (Barbour, 2007;Sternberg et al, 1986) and H in nucleophilic OH and NH groups or H adjacent to carbonyl groups (Augusti et al, 2006;Garcia-Martin et al, 2001) exchange with water.…”