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.
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Benjamin Mary et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-58', Alexandria S. Kuhl, 28 May 2023
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 -
AC1: 'Reply on RC1', Benjamin Mary, 13 Aug 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2023-58/bg-2023-58-AC1-supplement.pdf
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RC2: 'Comment on bg-2023-58', Anonymous Referee #2, 09 Jul 2023
In this paper, electrical resistivity tomography and electrical current imaging are used to monitor water content distributions and the distribution of electrical current from the root system into the soil. The experiment is carried in a rhizobox, so that transpiration rates and total soil water contents could be monitored carefully. The water application is alternated between the two different sides of the box and is changed over time to generate a certain stress level in the second part of the experiment. To my opinion, the most interesting outcome of the study is that the electrical source when an electrical current is injected in the plant shoot, is apparently more homogeneously distributed (one could assume homogeneously distributed along the root length) when stress occurs. Another important result is that with ERT, the water content changes over time could be imaged quite accurately.
A first main comment on the paper is that the authors describe their experiment as a partial root zone drying experiment. But, in such type of experiments, a part of the root zone is continuously kept wet by nearly continuous application of water to a part of the root zone using for instance drippers, whereas the other part is left to dry out. In their experiments, water is also applied to a part of the root zone and the application is alternated between the two sides. But the duration between the applications, is quite long so that both parts of the root zone dry out to the same level and water can flow from one part to the other. These conditions do not really generate a spatially variable root water uptake. The authors correctly recognize that their experimental setup did not exactly reproduce partial root zone drying experiments. To avoid confusion, I would propose not to call the experiments PRD experiments.
A second main comment is on the relation between soil water content changes and root water uptake and the interpretation of current source distribution images. Due to water redistribution in soil, local soil water content changes must not be interpreted as local root water uptake. In the text, the authors seem to allude to this although they do not use this approach when interpreting the electrical resistivity images. Concerning the current source distributions, there it would be helpful if the authors could make the analogy between water and electrical current flow in the soil root system. The distribution of both depends on the distribution of the water and electrical conductivities in the soil and the plant/root system. In unsaturated soil, both conductivities change with the water content and they also change very close to soil-root interface so that the conductances close to the soil-root interfaces may differ considerably from the bulk soil conductances, The latter changes depend strongly on the local flow densities near the root surfaces and hence on the transpirational demand. To my understanding, the results that are presented (how the current density distribution changes when water stress occurs, which is mainly the result of a higher transpirational demand and not of drier soil conditions in this experiment) suggest that these changes in conductances of both water and electrical conductances near soil-root interfaces may explain these observations. Along the same lines of reasoning, I do not think that under conditions when soil electrical conductances are high near the soil root interface and when there is good electrical contact between soil and roots, current source density distributions are related to water uptake distributions.
A third main comment is that the interpretation presentation of the results is often not clear to me. For instance, figure 8 seems to suggest that resistivity increases with increasing water content. This is opposite to what is generally known and opposite to Archie’s Law. At several points, I could not follow the reasoning of the authors and the text should be proofread carefully. I noted a few spelling errors but these are not exhaustive.
Below are detailed comments that were written during a first read of the paper. They reflect my confusion that sometimes occurred when reading the paper.
Introduction: The introduction part on the electrical capacitance and electrical current imaging should be clearer by better stating which assumptions are made in these methods and which root traits and soil properties could (potentially) be derived from these methods. For instance, in capacitance imaging, a lumped property of the root system is derived. But it was not immediately clear to me how that is done and what the underlying assumptions are. Which assumptions about the axial and radial electrical conductances of the root system are made and how do these properties determine the total root system capacitance? It would be helpful if an analogy to root system properties that relate to water flow could be made, like root system conductance (root system capacitance for water flow is generally not considered). Another issue is that is not clear to which electrical properties of the root system is referred to. I think both capacitance and resistance should be considered. Finally, the abbreviations used are confusing: ECroot stands for capacitance of the root system, ECI for electrical current imaging. Note that EC is often used for electrical conductivity.
Ln 32 ‘The partial root zone drying (PRD) method is part of an ensemble of irrigation strategies that aim at improving water use efficiency. It consists of irrigating only one part of the root system of the same plant using a certain percentage of the potential evapotranspiration (ETp), usually inferior to the total water needed.’ I would be great if you could include some explanation about the difference between partial root zone drying versus deficit irrigation and why a partial drying of the root zone would lead to a better result than drying of the entire root zone. In fact, as it turns out later, in the experiment that was conducted, the whole root zone dried out during a drying cycle.
Ln 35 ‘Application of PRD triggers a physiological response in the plant via a hormone called Abscisic acid (ABA), which is produced in the roots and transmitted to the leaves to regulate the stomata closure and thus reducing water transpiration while keeping photosynthesis active and finally leading to increased water use efficiency.’ What is different compared to entire root zone drying? Why is it important to dry out only a part of the root zone?
Ln 48: ‘soil moisture patterns determined by PRD are visible from the ERT perspective and can be attributed to the root system distribution.’ What do you mean by this sentence? What is meant specially by ‘attributed to the root system distribution’? Do you mean that the pattern of drying in the part of the root zone that does not receive water can be related to the distribution of the roots in this zone?
Ln 50 ‘Roots induce changes in the soil structure in terms of porosity and hydraulic conductivity which ultimately modify the water pathways and fluxes and thus the ER itself.’ This is certainly correct but isn’t the question to what extent this is a secondary effect compared to the primary effect that is caused by the uptake of water by the roots?
Ln 90: ‘appoplastic’ should be ‘apoplastic’
Ln 97 ‘complex balance between reducing radial flow (as a consequence of ABA signaling sent by the roots) to conserve water in the soil but keeping the axial flow active.’ I am not following the reasoning here. How can axial flow be kept when radial flow is blocked? The reason for reducing the radial flow in dry soil is not to conserve water but to avoid too strong water potential drops between the soil and the plant. By reducing radial conductivity in dry regions, plants shift the uptake towards wetter regions where the soil conductivity is higher so that plants can take up water at the same rate but keeping higher plant water potentials.
Ln 114: ‘Without being able yet to give hints about the electrical current pathway, recent advancements in the development of explicit RWU models, based on plant hydraulics, provide insights into how robust capacitance models hold and under which conditions. We learnt, for instance, that at the root level, RWU models account for the anisotropy by separating the root hydraulic conductance into two terms (longitudinal and radial).’ I think the authors should give references here to explicit RWU models and also refer to work that used these models to simulate electrical currents (and polarization) in root systems (see for instance work by Mathieu Javaux and colleagues and Nimrod Schwartz and colleagues).
Ln 119: ‘Up to now the relationship between root water content and root hydraulic conductivity with electrical resistivity has not been firmly established. Many other parameters can affect the water flow as well as the current pathway of stem-based methods.’ This is quite vague, especially in view of work done by others previously. Which ‘other parameters’ are you referring to?
Ln 132: I suppose you are referring here with EC to electrical capacitance. This may be confusing for many readers since EC is typically associated with electrical conductivity. I am also wondering why you use capacitance and not impedance, which combines both capacitance and resistance. I suppose the signal that is measured is also related to the electrical resistivity or conductance of the root tissue and of the soil and not only by the capacitance.
Ln 151: ‘we aim at showing that the current path through the root system is linked to the active root zones.’ Doesn’t this imply that it is assumed that soil and root hydraulic hydraulic conductances are positively correlated to electrical conductances?
Ln 158 ‘changes in soil water content measured by ERT are a relevant spatial proxy of root activity’ It has been discussed in several papers that changes in soil water content do not map to distributions of root water uptake or root activity. Local root water uptake can be compensated by water redistribution in the soil and decouple local water content changes from root water uptake. Maybe it is better to write that root water uptake and soil water content changes that are averaged over a spatial scale that is larger than the scale over which water redistribution in the soil can compensate soil moisture changes due to local root activity, can be correlated. These spatial scales depend on the soil hydraulic properties and the local extraction rates.
Ln 162: ‘during the application of PRD, only one part of the root system would be active and the current injected in the stem would preferably spread to the side where the root system is irrigated.’ This assumption seems to be contradictory in itself. Partial root zone drying only occurs when the part of the root system in the region that does not receive extra water can remain active for a while. So I think it is better to write, ‘ When during the application of PRD, the part of the root system in the dry zone is deactivated, current injected in the stem would preferably spread to the side where the root system is irrigated.’ This assumption hinges on the assumption that a deactivation of the root system part in the dried out zone corresponds with an increase in root and/or soil electrical resistivities. That electrical resistivities of soil increase with soil drying is trivial. But, the question is whether electrical resistivities of the coupled soil-root system increase to the same extent with soil drying as the hydraulic resistances and decreasing soil-plant hydraulic potential differences. I suppose this hypothesis will hold true in coarser soils but for clay soils, this can be questioned.
Figure 1: give indications of the height, width and depth of the rhizobox in figure 1.
Ln 180: ‘An outlet point was placed on the bottom right side (z=5cm) and the rhizotron was always saturated below this point. In the course of the experiment (after the growing period) no water discharge was observed through the outlet point.’ How was the bottom of the rhizobox kept saturated? Was the outlet connected to a Mariotte system? Was regularly water added? If no water was added at the bottom, then I wonder why the bottom remained saturated.
Ln 295: ‘(2) time-lapse inversion (difference inversion) where the difference in resistivity is inverted between a given survey and a background survey (in this case, the background survey is the previous one).’ Here it is important to give the time difference between the two measurements. In order to be interpretable, the time difference should always be the same. Were daily measurements taken always at the same time of the day?
Ln 417: ‘Time steps correspond to measurements before (a), after one hour (b) and after 6 days (c).’ I do not understand well how the differences are calculated and what the time step of the differences are. Was every day measured at the same time or at different times? Was the difference calculated between the measurement at a certain day and the day before it or was it calculated from the difference between the measurement and the measurement at the start of the irrigation cycle.
Ln 454: ‘Figure 8 shows the relationship between the variation between two consecutive measurements of the weights with the variations of average electrical resistivity (Fig.8a, R2=0.76, p-value=6.5 x 10-5) and those of resistivity-derived average water content (from Archie’s law - Fig.8b, R2=0.815, p-value=6.8 x 10-6). An increase in weight over time is positively correlated with an increase in in resistivity and water content meaning that the changes in resistivity are mainly associated with transpiration (rather than changes in soil structure or other parameters).’ I do not understand these results. You are plotting changes in weight over time versus resistivities or versus water contents. Shouldn’t changes in weight be plotted versus changes in water content or changes in resistivity? Although I am not sure whether the latter makes sense since water content and resistivity are non-linearly related in Archie’s Equation. But, what amazes me the most is that resistivity increases when the weight and hence the water content increases. This cannot be correct. Finally, from the changes in weight, a change in water content can be calculated. These changes in water content calculated from weight changes should be one-to-one related to the changes in water content calculated from the ERT measurements. I propose comparing those.
Ln 484: ‘For cycles where stressed was not applied (i.e. < cycle 3), for the stem injection, Ns1 is distributed between 5 and 25%.’ What is remarkable is that the distribution of sources is not related to the soil water content. At low water contents, when one would expect more stress, the distribution can be like the distribution for the soil injection or like the distribution when water stress is supposed. But also at high water contents, the distribution can be like the distribution when water stress is supposed. Can the grey triangles that are falling in the range of the black triangles be explained? Could they correspond with higher transpiration rates. It seems to me that the stress is related to the transpiration rate or transpiration demand which increases over time due to an increase in leaf area. At high transpiration demands or rates, stress may occur at higher soil water contents because then the soil becomes limiting for the root water uptake.
Ln 501: Garre et al. 2011 is not in the reference list.
Ln 505: ‘Our observation is in line with the literature i.e. in general, low soil water content (SWC) can lead to drought stress in plants, which can result in decreased leaf stomatal conductance and less transpiration, and vice versa.’ Actually, I think that your observations in fact show the opposite. The water content changes during the different cycles with or without stress were very similar and water contents dropped to the same low levels in cycles where no stress was observed as in cycles where stress was observed. Therefore, water content does not seem to be explaining variable for water stress. It rather seems to be the transpiration demand which increased over time.
Ln 518: ‘This is a hint that the hydraulically stressed plant tends to have a wider and deeper active root system, even not necessarily active only on the side where the PRD is temporarily applied. Possibly the reaction of the plant to the changing side is too slow to show up in our measurements, but the reaction to general stress is apparent.’ I am sorry but you lost me here.
Ln 532: ‘tend to show that mixed soil-root pedophysical relationships are preferable (e.g. Rao et al., 2018).’ I think that rather than considering mixed soil-root pedophysical relations, it would be important to consider small scale variations around single root segments in water content and /or soil hydraulic properties.
Ln 552: ‘Additionally, capillary rise may have taken place due to the presence of a saturated zone at the bottom of the rhizotron’ Yes, but then the water content at the bottom of the rhizobox should decline over time since no water was added at the bottom of the box?
Ln 559: ‘Given the stress applied, the ER changes highlighted that root played a major role in the wine plant survival and evidenced strategies of adaptation. Indeed, the plant was able to change its water uptake zones depending on the water availability, from all places, not only from the alternate irrigated areas.’ I don’t think this conclusion can be drawn from this experiment. Plant adaptation means an active adaptation of the plant to redistribute the water uptake. First, I am not sure whether uptake distributions were directly observed. Second, also without any adaption, the uptake distribution changes when the soil water content distribution changes, but also when the water uptake rate changes. The impact of water potential distribution on uptake distribution is trivial. The water uptake rate or transpiration rate may impact the uptake distribution since the soil water potentials near the soil-root interface will drop which leads to a drop in soil conductivity. This dependency on water potentials and flow rates make that the conductivity distributions in the soil-plant continuum, and hence the uptake distributions change with flow rate. I suppose that the change in water potential and water content close to the soil-root interface also had an effect on the electrical conductivity of the soil just around the roots. This means that later during the experiment, the electrical conductances in soil around roots were lower and might have become limiting the current between the root system and the soil. This would have generated a more homogeneous distribution of the electrical current source along the total length of the root system.
Ln 612: ‘We only 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 evaporative demand.’ The current source leakage depth did not vary with transpirational demand or water content. But, didn’t the spatial distribution of the leakage vary with transpiration demand and didn’t you show that this was related to the occurrence of stress?
Citation: https://doi.org/10.5194/bg-2023-58-RC2 -
AC2: 'Reply on RC2', Benjamin Mary, 13 Aug 2023
The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2023-58/bg-2023-58-AC2-supplement.pdf
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AC2: 'Reply on RC2', Benjamin Mary, 13 Aug 2023
Benjamin Mary et al.
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