δ13C of organic matter transported from the leaves to the roots in Eucalyptus delegatensis: short-term variations and relation to respired CO2

Abstract

Post-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (δ 13 C) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in δ 13 C of up to 2.5 and 2‰, respectively, with 13 C enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. δ 13 C of nocturnal phloem exudate OM corresponded with the δ 13 C of carbon released from starch. There was no change in δ 13 C of phloem carbon along the trunk. CO 2 emitted from trunks and roots was 13 C enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO 2 . The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of δ 13 C in phloem-transported OM and fragmentation fractionation occurring during respiration. When using δ 13 C of OM or CO 2 for assessing ecosystem processes or plant reactions towards environmental constraints, post-photosynthetic discrimination should be considered.

13C isotope fractionation during rhizosphere respiration of C3 and C4 plants

“…Both 13 C values of respiratory substrates and the relative contribution of different metabolic pathways to CO 2 evolution may change within a day, therefore 13 C fractionation during plant respiration can change significantly on a diurnal basis, as shown in previous studies on 13 C fractionation during leaf respiration (Gessler et al 2007;Sun et al 2009;Wegener et al 2010). Diurnal variations in 13 C fractionation during root respiration and rhizosphere respiration are probable, but direct experimental evidence is not yet available.…”

“…Most published studies on 13 C fractionation during root respiration were based on snapshot measurements (minutes to hours) of the δ 13 C value of respiratory CO 2 from excised roots (Gessler et al 2007;Wegener et al 2010) or roots grown in sand or nutrient solution without the presence of soil microorganisms (Bathellier et al 2008;Klumpp et al 2005), and thus did not include the rhizomicrobial respiration by rhizosphere microorganisms utilizing materials released from live roots. In order to partition total soil respiration into rhizosphere respiration that includes both root respiration and rhizomicrobial respiration (root-derived, autotrophic respiration) and microbial decomposition of soil organic matter (soilderived, heterotrophic respiration) using a two endmember isotope mixing model (Cheng 1996), we need to know δ 13 C values of rhizosphere respiration integrated over days or seasons.…”

Diel variations in the carbon isotope composition of respired CO<sub>2</sub> and associated carbon sources: a review of dynamics and mechanisms

“…However, these variations in the fast-turnover organic matter pool in leaves had a much lower day-night amplitude than the observed diel changes in respired δ 13 CO 2 (Brandes et al, 2006(Brandes et al, , 2007Gessler et al, 2007Gessler et al, , 2008Kodama et al, 2008;Werner et al, 2009, see Table 1) and were also phase-shifted compared to δ 13 C res (Kodama et al, 2008, see also Fig. 1).…”

“…(Farquhar et al, 1982), which determines the δ 13 C of primary respiratory substrate, varies over the diurnal course (e.g. Gessler et al, 2007;Wingate et al, 2010) as a result of changes in light intensity, air temperature, vapour pressure deficit (VPD) and other environmental factors, which affect assimilation, stomatal (g s ) and mesophyll conductance (g m ) as well as photorespiration and dark respiration (see Fig. 2 (M1.1), reviewed by Brugnoli and Farquhar, 2000).…”

An analytical model of non‐photorespiratory CO2 release in the light and dark in leaves of C3 species based on stoichiometric flux balance

“…Steady-state respiratory CO2 release in the dark is typically correlated with sucrose concentrations (e.g. Gessler et al 2007) and is greater after a period of illumination than after an extended period of darkness, both of which have been taken as evidence of dependence on carbohydrate concentrations (Azcón-Bieto & Osmond 1983; Atkin et al 1997). This may result at least partly from the ATP costs of sucrose export.…”