On the use of phloem sap  13C as an indicator of canopy carbon discrimination

Abstract

In this study we measured δ¹³C in various carbon pools along the basipetal transport pathway in co-occurring Pinus pinaster and Acacia longifolia trees under Mediterranean climate conditions in the field. Overall, species differences in photosynthetic discrimination resulted in more enriched δ¹³C values in the water-conserving overstory P. pinaster relative to the water-spending understory invasive A. longifolia. Post-photosynthetic fractionation effects resulted in differences in δ¹³C of water-soluble organic matter pools along the plant axis with progressive depletion in δ¹³C from the canopy to the trunk (∼6.5‰ depletion in A. longifolia and ∼0.8‰ depletion in P. pinaster). Regardless of these fractionation effects, phloem sap δ¹³C in both terminal branches and the main stem correlated well with environmental parameters driving photosynthesis for both species, indicating that phloem sap δ¹³C has potential as an integrative tracer of changes in canopy carbon discrimination (Δ¹³C). Furthermore, we illustrate that a simple model based on sap flow estimated canopy stomatal conductance (G(S)) and phloem sap δ¹³C measurements has significant potential as a tool for estimating canopy-level carbon assimilation rates.

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

“…In Acacia longifolia and Pinus pinaster a slight increase at the end of the light period of ca. 2 ‰ was observed in the field also under drought conditions (Rascher et al, 2010).…”

“…A significant increase in δ 13 C res during the photoperiod and a subsequent decrease in the dark were found in a variety of drought-adapted trees and shrubs (e.g. Hymus et al, 2005;Prater et al, 2006;Sun et al, 2009Sun et al, , 2010Werner et al, 2009;Unger et al, 2010a;Rascher et al, 2010) and in wheat Fig. 1b).…”

“…3a. Others found no significant diel variations in leaf soluble sugars or water soluble organic matter (WSOM) (Hymus et al, 2005;Sun et al, 2009;Werner et al, 2009;Wegener et al, 2010;Rascher et al, 2010) but still strong variations in δ 13 C res (Fig. 3b-c), indicating that diel variations in leaf δ 13 C res cannot be solely explained by changes in the isotopic signature of the substrate.…”

“…Consequently, the diel cycle of δ 13 C in organic matter in trunks and stems of trees is mainly dependent on the position along the trunk and dampening as well as time lags have been observed (Keitel et al, 2003;Brandes et al, 2006Brandes et al, , 2007. At the tree trunk base often no diel variation in phloem sugar δ 13 C was present Kodama et al, 2008;Betson et al, 2007;Rascher et al, 2010) whereas strong diel variations in δ 13 C res are generally observed (Kodama et al, 2008;Maunoury et al, 2007). They thus cannot be explained by variation in substrate δ 13 C (Fig.…”

Progress and challenges in using stable isotopes to trace plant carbon and water relations across scales

“…Keitel et al, 2003;Gessler et al, 2004;Scartazza et al, 2004;Barbour et al, 2005;Rascher et al, 2010;Ubierna and Marshall, 2011;Dubbert et al, 2012). A dual-isotope approach (δ 13 C and δ 18 O see Sect.…”