the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Does back-flow of leaf water introduce a discrepancy in plant water source tracing through stable isotopes?
Jeroen D. M. Schreel
Kathy Steppe
Adam B. Roddy
María Poca
Abstract. Plant water source tracing studies often rely on differences in stable isotope composition of different water sources. However, an increasing number of studies has indicated a discrepancy between the isotopic signature of plant xylem water and the water sources assumed to be used by plants. Based on a meta-analysis we have reconfirmed this discrepancy between plant xylem water and groundwater and suggest back-flow of leaf water (BFLW), defined as a combination of (i) the Péclet effect, (ii) foliar water uptake (FWU) and (iii) hydraulic redistribution of leaf water, as a possible explanation for these observations. Using the average 2.21 ‰ 18O enrichment of xylem water compared to groundwater in our meta-analysis, we modelled the potential of BFLW to result in this observed isotopic discrepancy. With a low flow velocity of 0.052 m.h−1 and an effective path length of 2 m, the Péclet effect alone was able to account for the average offset between xylem water and groundwater. When including a realistic fraction of 5–10 % xylem water originating from FWU and tissue dehydration, 60–100 % of the average observed enrichment can be explained. By combining the Péclet effect with FWU and tissue dehydration, some of the more extreme offsets in our meta-analysis can be elucidated. These large effects are more probable during dry conditions when drought stress lowers transpiration rates, leading to a larger Péclet effect, more tissue dehydration, and a potential greater contribution of FWU.
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Jeroen D. M. Schreel et al.
Status: open (until 06 Apr 2023)
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CC1: 'Comment on hess-2023-13', Carel Windt, 15 Feb 2023
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Dear Jeroen,
Interesting manuscript! One comment: I'm surprised that phloem flow is not mentioned as a contributor to the back flow of water from leaves. Can it be neglected in this context, in your opinion? In older work, measuring water fluxes in the main stem of plants by means of MRI, I found that a significant fraction of xylem sap returns to the roots by means of the phloem (up to ~10% during the day, up to ~50% at night. Windt et al, PCE, 2006). This was measured in the main stem. This means, water used for expansion growth of shoots and young leaves did not even show up in this number (in the back flow to the roots). I therefore would expect, for individual leaves, the potential fraction of phloem backflow to be even larger than measured in the main stem.
Best, Carel
Citation: https://doi.org/10.5194/hess-2023-13-CC1 -
AC1: 'Reply on CC1', Jeroen Schreel, 16 Feb 2023
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Dear Carel,
Thank you for this nice comment! Prior to publishing this preprint, we discussed the option of including phloem flow in our manuscript and had chosen to exclude this as we are considering the bias introduced in water source tracing which solely focusses on xylem water. In the current version of our manuscript, we have alluded to xylem-phloem water exchange to not leave this topic completely untouched (Line 34-35, 178, 224-225). However, based on your comment it does seem beneficial to include some more information on backflow through the phloem and we will do so in our revised manuscript.
Thanks again!Kind regards,
JeroenCitation: https://doi.org/10.5194/hess-2023-13-AC1 -
CC2: 'Reply on AC1', Carel Windt, 17 Feb 2023
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Citation: https://doi.org/
10.5194/hess-2023-13-CC2
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CC2: 'Reply on AC1', Carel Windt, 17 Feb 2023
reply
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AC1: 'Reply on CC1', Jeroen Schreel, 16 Feb 2023
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Jeroen D. M. Schreel et al.
Data sets
Isotopic data of xylem water and groundwater, extracted from literature. Jeroen D. M. Schreel, Kathy Steppe, Adam B. Roddy, and María Poca http://doi.org/10.6084/m9.figshare.21940955
Jeroen D. M. Schreel et al.
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