the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Oct 2024
Technical note: Investigating saline water uptake by roots using spectral induced polarization
Abstract. There has been some improvements in the methods available for root investigation in recent years that has enabled many studies to be carried out on the root, which represents the hidden half of the plant. Despite the increased studies on roots, there are still knowledge gaps in our understanding of the electromagnetic processes in plant roots which will be useful to quantify plant properties, and monitor plant physiological responses to dynamic environmental factors amidst climate change. In this study, we evaluated the suitability of spectral induced polarization for non-invasive assessment of root activity. We investigated the electrical properties of the primary roots of Brachypodium distachyon L. and Zea mays L. during the uptake of fresh and saline water using SIP measurements in a frequency range from 1 Hz to 45 kHz. The results show that SIP is able to detect the uptake of water and saline water in both species, and that their electrical signature were influenced by the solute concentration. The resistivity and phase response of both species increased with solute concentration until a certain threshold before it decreased. This concentration threshold was much higher in Maize than in Brachypodium, which implies that tolerance to salinity varies with the species, and that Maize is more tolerant to salinity than Brachypodium. We conclude that spectral induced polarization is a useful tool for monitoring root activity, and could be adapted for early detection of salt stress in plants.
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RC1: 'Comment on egusphere-2024-2628', Imre Cseresnyes, 25 Oct 2024
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General comments: High-quality paper with actual choice of topic and important novelties. Congratulations for the Authors!
Specific comments:
1) L36: Consider to add “electrical impedance” and/or “phase angle” to the keywords.
2) L42–44: Besides low water potential and ion toxicity, salinity provokes an oxidative stress as well by excessive ROS generation. Consider to mention it as a third effect.
3) L55: Among plant physiological processes, diurnal cycles in root uptake activity has recently been monitored by impedance measurement, see: https://doi.org/10.1186/s13007-023-01133-8
4) L59: Basically, there is a composite water pathway inside the root cylinder, including three routes temporally variable: 1) apoplastic pathway (cell walls and extracellulars), 2) symplastic pathway (plasmodesmata), 3) transcellular pathway through (aquaporin channels). The last two are often named “cell-to-cell pathway”.
5) L68–70: It is worth mentioning that living tissues are equivalent to parallel RC circuits, which has a characteristic phase angle depending on AC frequency. It could be important later.
6) L71: There is another work to evaluate salinity effect on impedance phase angle at a single frequency: https://doi.org/10.1016/j.biosystemseng.2018.03.004
7) L74 (caption of Fig. 1): “Low” and “high” frequency is rather relative. I think that a specification of frequency ranges (according to alpha and beta dispersion regions) would give a help for the readers.
8) L87–104: Consider to shorten the description of the previous work by Ben Hamed et al. (2016), focusing the main finding only. I think this long description is not necessary.
9) L113–114: “More studies are still needed to better understand how roots respond to salt stress.” I fully agree with it, as root cells are the first target of soil salinity. I may be emphasized here.
10) L123–125: As maize tolerance to salinity depends on genotype (as you write), specify the cultivar of maize applied in the experiment, and add some information (if available) of its salinity tolerance level.
11) L146: Add terminal (input) voltage of the AC signal used for measurement.
12) L147 and thereafter: In my opinion, it would be better to always use the conventional symbol φ (phí) for phase angle both in the text and in the figures. Likewise, symbol “R” is worth using for the magnitude of resistance.
13) L208–209: “Polarization (phase peak) of Brachypodium showed a decrease and a shift towards lower frequencies while that of Maize first showed an increase followed by a stabilization.” The sentence is difficult to follow. Make clear that changes occurred over root exposition time, and consider to give frequencies at which the phase shift reach a peak.
14) L222: I think “larger canopy transpiration” could be written instead of “larger transpiration pull”. The latter characterize the negative xylem pressure, which was not obviously higher in maize.
15) L230–231: “Polarization (phase peak) of Brachypodium showed no clear trend while that of Maize remained mostly constant after an initial increase for a broad range of frequencies” For clarity, supplement the sentence that there was a temporal trend, according to the absorption time of DM water.
16) L258–259: “Drought is also known to cause wilting of leaves (e.g. UCANR, 2021; Ji et al. 2022; PlantDitech 2023; Bayer 2024)…” This is evident, references are not necessary, and should be deleted from here.
17) L266–268: “The consistent decrease in resistivity magnitude and phase for both species suggests excessive accumulation of ions in the cytoplasm and apoplast, which makes the roots more conductive” Additionally, salinity can lead to membrane damage with increased permeability (https://doi.org/10.1016/j.biosystemseng.2018.03.004). I think this also contributed to the changes observed in the present study.
18) L300–302: Add literature, if available, to show the salinity thresholds tolerated by some maize genotypes.
19) Fig. 9: For maize, one data seems to be an outlier. Have you tested the correlation without it? Perhaps it would be improved.
Technical corrections:
1) Begin a new paragraph from L55.
2) Write Brachypodium in italics.
3) Write “maize” with lowercase letter, not capital.
4) Fig 2–7: Using more contrasting colors for the curves may improve the visibility of the results.
5) Consider to merge Table 2 and 3.
6) Fig. 7–9: It is confusing that the ranking of the two species (maize a-b, Brachypodium c-d) is the opposite to those of the previous figures. It is worth changing them.
Citation: https://doi.org/10.5194/egusphere-2024-2628-RC1 -
RC2: 'Comment on egusphere-2024-2628', Anonymous Referee #2, 05 Dec 2024
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This manuscript addresses the suitability of Spectral Induced Polarization (SIP) for detecting salt stress in plant roots, using Brachypodium and Maize as model species. The topic is innovative, exploring a less-studied method for assessing root responses to salinity stress. While the study presents promising results, there are significant methodological and interpretative limitations that need to be addressed to enhance the robustness of the conclusions.
Main Comments
- The preliminary SIP measurements were performed on single plants for each species. Given the variability in biological systems, triplicate measurements are necessary to provide statistically meaningful baseline data.
- Fig. 3: If I understand correctly, you suggest that the initial increase in resistivity (from the baseline to 5 minutes) is attributed to water loss through evaporation from the root. However, the changes in resistivity between 5 and 20 minutes are much smaller. Could you elaborate on the factors contributing to the decrease in the evaporation rate during this period?
In addition, I assume that the SIP measurement duration is on the same order of magnitude as the times reported here. Can you clarify?
The Mettler PM 2000 balance has a weighing capacity of 2,100 grams with a readability of 10 mg, a reproducibility of 5 mg, and a linearity of ±20 mg. Given these specifications, measuring changes as small as 20 mg approaches the balance's linearity limit, potentially compromising the accuracy of such measurements. Therefore, the accuracy of the 20 mg changes reported in Tables 2 and 3 is questionable when using this balance.
According to your data, the water uptake by the plant appears to be constant over time. How do you explain the nonconstant change in resistivity over time in light of this observation?It is also challenging to understand how water uptake by the plant is equal or higher than evaporation. Typically, transpiration represents a fraction of the evaporation from a free surface. In this case, the free surface is the water in the tube, and the plant extracts water only from its root tip. Given the relatively small contact area between the root and the water, could you clarify
I am concerned about interpreting physiological mechanisms based solely on SIP measurements without supporting data, such as direct measurements of root salt levels. Without corroborating evidence, these claims remain speculative and weaken the study's impact.
I recommend reporting such an experiment with repetitions, which is far more important than studying various salt levels at this stage.
L203: Could you clarify what you mean by "Maize roots were observed to be more saturated than Brachypodium"? How was the saturation level of the different plants assessed, and what criteria or methods were used to draw this comparison?
L212-214: The statements made here are quite strong. Can you provide supporting evidence or references from the literature to substantiate these claims?Citation: https://doi.org/10.5194/egusphere-2024-2628-RC2
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