the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Imaging of the active root current pathway under partial root-zone drying stress: A laboratory study for Vitis vinifera
Veronika Iván
Franco Meggio
Luca Peruzzo
Guillaume Blanchy
Chunwei Chou
Benedetto Ruperti
Giorgio Cassiani
Abstract. Understanding root signals and their consequences on the whole plant physiology is one of the keys to tackling the water-saving challenge in agriculture. The partial root-zone drying (PRD) method is part of an ensemble of irrigation strategies that aim at improving water use efficiency. To reach this goal tools are needed for the evaluation of the root’s and soil water dynamics in time and space. In controlled laboratory conditions, using a rhizotron built for geoelectrical tomography imaging, we monitored the spatio-temporal changes in soil electrical resistivity for more than a month corresponding to six Partial Rootzone Drying (PRD) cycles. Electrical Resistivity Tomography (ERT) was complemented with Electrical Current Imaging (ECI) using plant stem-induced electrical stimulation. We demonstrated that under mild water stress conditions, it is practically impossible to spatially distinguish the PRD effects using ECI. We evidenced that the Current Source leakage depth varied during the course of the experiment but without any significant relationship to the soil water content changes or transpiration demand. On the other hand, ERT showed spatial patterns associated with irrigation and, to a lesser degree, to RWU. The interpretation of the geoelectrical imaging with respect to root activity was strengthened and correlated with indirect observations of the plant transpiration using a weight monitoring lysimeter and direct observation of the plant leaf gas exchanges.
Benjamin Mary et al.
Status: open (until 05 Jul 2023)
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RC1: 'Comment on bg-2023-58', Alexandria S. Kuhl, 28 May 2023
reply
In this paper the authors present a very nice laboratory-based experimental study to assess the utility of novel electrical methods for monitoring and quantifying root water uptake and plant stress under partial root zone drying and irrigation schemes in an effort to study plant phyisology. They use both electrical resistivity tomography and electrical current imaging, two geophysical techniques that through a computational inversion provide visual and quantifiable evidence for the distribution of changes in electrical resistivity as a proxy for water content and current source density as a proxy for active root pathways, respectively. This experiment was conducted using a single vine grown in a small rhizotron over the course of several months in 2022. In addition to repeated cycles of irrigation and geophysical data collections, the transpiration was monitored using the weight of the rhizotron. Stomatal conductance and leaf area were also measured during one cycle to capture plant stress during a partial root drying event. Aside from a few grammatical errors and some issues with figure quality, this paper is an important addition to the biogeosciences literature, in particular for scientists taking advantage of novel geophysical techniques for noninvasive methods.
Major comments:
- There are some major inconsistencies and errors in the labelling of 'left' and 'right' irrigation/PRD that persists in most Tables and Figures which makes it difficult to understand the results. In addition to correcting these issues, interpretation would be easier if the same vertical blue arrows used in Figure 1 was added to the top axis of all the subsequent cross section figures (i.e. Fig. 5, 7, A2-A11). Some examples of the inconsistencies:
- Numbering of irrigation cycles:
- In Fig 1, 'Cycle 1' is depicted as left-side irrigation, yet in Table 1 the cycle numbering begins at 0, so that odd numbered cycles are actually right-side irrigation. I would make the initial wetting through all Holes Cycle 0 and then the first left-side irrigation on 2022-05-19 Cycle 1 etc.
- Color coding of left vs right sided irrigations:
- In Figs 4a and 6a, dark green is 'left' and orange is 'right'
- In Fig A1 there is a legend at the top indicating the opposite of the caption - that dark orange is partial left and dark green is partial right.
- In Table 1, while labelled correctly, it is also color coded the opposite to Figs 4 and 6, so that 'green is right' and 'orange is left'.
- Erroneous Figure captions:
- Table 1 cycle 2 Date column should be 2022-06-01 instead of 2022-05-01
- Fig 5 and Fig A9 are the same - labelled time lapse between cycles 5 and 6 - however Figure 5 is captioned 'following partial right irrigation' while Figure A9 is captioned 'partial left-side irrigation'.
- The date/time formatting on the header row of the time lapse ERT figures (5, A2-A11) are inconsistent and I find the cycle number labels particularly confusing in this context because the 'background' is labelled as the end of the prior cycle and I don't intuitively think of background is the end of the old cycle. I thought 'between cycles 5 and 6' meant the start of 5 to just before the start of 6, as opposed to the very end of 5 to the very end of 6 which I believe is the intention.
- Figures A2-A7 label the second image as the next cycle but A8-11 do not and that makes it seem like the cycle number is incorrectly labelled for that entire set of images. Since the time is shown in at least some of the timelapse figures, it would be helpful to have the time of the irrigation in Table 1 (as it is in Table A1) (and to use the same date formatting throughout all figures/tables). I think a combination of the headers would be best i.e. for Fig 5: Background (-1h) = 2022-06-29 16h20, Just After Irrig. (+1h) = 2022-06-29 17h20, Six days after Irrig. = 2022-07-05 17h20, and simple caption this is cycle 6 and add a title to the whole figure that says Cycle 6.
- The abstract is too technical and abbreviated in my opinion. It will probably be unclear to general readers why either ERT or ECI could be useful for understanding root water dynamics so I would advise adding a sentence that introduces the concept of Archie's law. Further, the term 'current source leakage' is only used twice in the paper, once in the abstract and once in the conclusion although much of the paper focuses on current source density/current density. Making the usage of the electrical methods terminology more consistent throughout the paper would be helpful to the reader.
- Similar to the abstract, the title has an emphasis on specifically imaging 'the active root current pathway' but after reading the paper my takeaway was that the focus of the paper is more of an assessment of electrical methods for assessing root water uptake and observing the patterns of the PRD. To justify this in the title I think the introduction would need more background on the signifcance and meaning of the active root current pathway.
Minor comments:
Fig 1: the horizontal flux arrows are a bit confusing since I would expect at least some of the flux coming from the surface to be vertical. I would consider simply removing those arrows or replacing them with something more realistic.
Fig 5 (and A2-A11): the axes labels and legend are too small and low resolution to read clearly
Ln 20: Based on the other figures/tables it seems like there are only 4 or 5 PRD cycle pairs, not six as stated here
Ln 23: 'Current Source Leakage Depth' is a very technical term that is not going to be understood by most readers without some explanation. I would consider rephrasing or adding some clarification.
Ln 88: The concept of 'active roots' is very important for the paper but there is not really a definition of what makes roots 'active' vs inactive. The sentence on this line explains how water moves in active roots but doesn't make it clear whether inactive roots are those that will never take up water, or those that aren't taking up water at the moment.
Lns 109-113: consider rephrasing this sentence to make it more clear
Ln 122: this is the first use of term 'electrical current leakage' which has not yet been explained, particularly as it relates to understanding root water dynamics or the electrical methods being used.
Ln 132: this is the first use of EC abbreviation which has not previously been described but is used through page 6
Ln 207: Rephase, i.e. For each irrigation event we regulated the amount of water supplied based on the information obtained from the scale data. The plant received 75% of the measured transpiration since the last irrigation cycle.
Ln 214: in Table 1 the cycles go from May 13th to July 12th
Ln 219: Cycle 9 in Table 1 says it uses holes H1-H8 and is not colored green or orange which conflicts with the statement here that 'From cycle number 3 to 9, we restricted the water input to the two lateral holes'. Also, although lateral does mean coming from the sides, it is not often used i in that context so I would consider changing it to something more descriptive, like left-most and right-most.
Table 1: see Major comment 1 above
Ln 233: an abbreviation for electrical resistivity is defined on Ln 50 but then only used intermittently - make this more consistent throughout
Ln 301: the abbreviation given here is ICSD, although subsequently only CSD is used
Ln 341: 'were' is typed twice in this sentence
Fig 2b: the time series has a datetime mix up. Labelled August 6 (8/6) instead of June 8 (6/8). It would be helpful to remind the reader in this paragraph (~Ln 351) that the measurements shown come from the 26 leaves which is described back in the methods.
Fig 3: this transpiration data is really nice to see
Ln 380: I would change this to say 'Fig 4 shows the trend for the irrigation cycles (-1 - 8) since cycle -1 was not PRD and cycles 0 and 1 were not the same as cycles 3-8. Also, here and in Fig 4, Cycle 9 is dropped, which conflicts with Ln 219 and Table 1.
Ln 405: Reword to specify which side is the 'irrigated side'. Also, when you say the ER of the irrigated side had dropped by 20% how is that being calculated?
Fig 5: see Major comment 1
Ln 426-427: It's not immediately clear why this confirms the quality of the estimated background values. Can you elaborate on what would be expected here and the underlying mechanism?
Fig 6: Rescaling the y axis on the center of mass to be narrower would help accentuate any slight variations. This graph should also have a unit (cm?). The caption mentions cycles 5 and 6 were used in Fig 7 but based on the dates it is actually cycles 6-7 (see my point in Major comment 1.3.3 above).
Ln 457: 'in' is typed twice
Lns 460-47: it would be helpful to explicitly state how mean SWC was calculated for each side (i.e. all nodes left of center averaged). Were nodes on the edges excluded?
Ln 499: The dates in Fig 3 and Table 1 suggest that the decrease in the rate of uptake is happening between July 5th and July 11th between cycles 7/8
Ln 537:539: consider rewording to use less colloquial language.
Ln 540:549: I like this discussion point regarding the potential impact of root growth over the course of the experiment
Ln 573: I would rephrase 'really good' to something more specific or to simply 'good'
Ln 575-577: I'm finding this sentence unclear, please rephrase
Ln 581: 'the' is typed twice in this sentence
Citation: https://doi.org/10.5194/bg-2023-58-RC1
Benjamin Mary et al.
Benjamin Mary et al.
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