Exploring the Sensitivity of Solar-Induced Chlorophyll Fluorescence at Different Wavelengths in Response to Drought
, Zhigang Liu 1,2,*, Shuai Han 1,2, Zhuang Chen 1,2, Xue He 1,2, Huarong Zhao 4,5 and Sanxue Ren 4,5Round 1
Reviewer 1 Report
The paper by Dr. Xu and co-authors presents intereting results based on well-designed leaf-level fluorescence experiments. Some detailed experimental setup, figures, facts, and results are provided, with reasonable analyses and discussions. The paper constributes the community with this interesting case study. The manuscript is very well written and easy to follow. I suggest publication after considering some minor comments:
specific comments:
lines 64-66, this is only ture for ground-based measurements.
line 152, remove 'PAM-2500'?
line 208, better provide the bands used for calculating NDVI.
Fig. 2, the figure should be self-explantory. C, D1, and D2 should be explained in the caption.
Figs. 3A and 3B are nicely plotted! but it does not make sense to me why R2 in Figs. 3C and 3D behaves differently around the red peak?
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript introduced their studies of SIF bands responses to drought. They assessed the performance of leaf scale SIF measurements in predicting photosynthesis efficiency. The writing needs to be improved, and eq 7 and 8 are not well explained.
You get the conclusion at the leaf scale. If others want to apply your findings to the regional scale, what difference between your findings at the leaf level and canopy level?
Line 39. I don't understand this sentence, please rewrite it.
Line 78. "fluoresces" should be fluorescence.
Line 85. Please give examples of physical and biochemical factors
Line 112. You can't say "we use a water stress experiment".
Line 120. You can't improve the ability of red SIF. You could get a better understanding of red SIF responses to drought.
Line 126. Add the year of the date
Line 208. Has the eq. 7 been validated? I'm still uncertain how you got this equation.
Line 209. I don't understand why you converted the measured SIF687. SIF687 was measured at the leaf scale, and fesc was used to convert SIF between canopy scale and the leaf scale. I think you should re-define the fesc. As my understanding, fesc = SIFtoc/SIFtot * pi, so why you apply fesc for leaf-scale experiment?
Some papers may help you define fesc.
Lu, X., Liu, Z., Zhao, F., & Tang, J. (2020). Comparison of total emitted solar-induced chlorophyll fluorescence (SIF) and top-of-canopy (TOC) SIF in estimating photosynthesis. Remote Sensing of Environment, 251, 112083. https://doi.org/10.1016/j.rse.2020.112083
Gao, S., Huete, A., Kobayashi, H., Doody, T. M., Liu, W., Wang, Y., Zhang, Y., & Lu, X. (2022). Simulation of solar-induced chlorophyll fluorescence in a heterogeneous forest using 3-D radiative transfer modelling and airborne LiDAR. ISPRS Journal of Photogrammetry and Remote Sensing, 191, 1-17. https://doi.org/10.1016/j.isprsjprs.2022.07.004
Line 250. Please define SIFtot
Line 252. Why did you add upward and downward SIF? what's your purpose? Do upward and downward SIF response to drought differently?
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 3 Report
My main criticism with this manuscript is the limitation of the dataset. Authors conducted a very short experiment in just one year, with no temporal repetition in subsequent years/seasons to confirm or rule out the consistency of the results. One-year repetition is very limited, both for field conditions and pot studies. I am therefore concerned that conclusions can be presented as general because your study just span a couple of months of drought of a particular year. In addition, conclusions are just based on two days of measurements, in a few replicated leaf samples. I seriously think that it is not possible to draw sound conclusions with these limitations.
In the attached pdf file I provide some editorial comments for authors.
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 4 Report
The paper Exploring the Sensitivity of Solar-induced Chlorophyll Fluorescence in Response to Drought at Different Wavelengthsis devoted to field studies of the reaction of chlorophyll fluorescence in maize leaves to water stress conditions.
The study of plant behavior in a variety of environmental conditions requires adequate methods for assessing the processes occurring in the plant. A reliable source of information about the state of the plant is the photosynthetic apparatus and its response to stress. One of the most informative monitoring methods based on measuring the biophysical characteristics of plants is the measurement and analysis of chlorophyll fluorescence light emitted by a plant.
The authors used standard sets of measurements for such studies, and the correct interpretation of the measurement results is crucial to obtain the necessary information about the state of the plant under study. The paper established various (linear) correlations between the characteristics of the fluorescence spectra and the biophysical and physiological parameters of the leave state. Unfortunately, the general conclusion from the results obtained is that they have very limited applicability to remote monitoring of plant stress. This conclusion, however, cannot be blamed on the authors, since the studies were carried out mainly methodically correctly, and this conclusion is mainly explained by the complexity of the system under study and the difficulties of separating the influences of various processes inside and outside the leaf. Further research is required to find informative parameters for optical remote monitoring the plants.
The results of the work are undoubtedly of interest to specialists in the field of remote diagnostics of the state of plant objects in natural conditions. The article may be published after some revision, taking into account the comments below.
Comments:
1. The title of the article should be corrected because it reads as if the drought occurs at different wavelengths.
2. In methodological terms, very few samples were taken for research (gradations of water stress), as well as measurement dates to obtain statistics and available correlations.
3. There are somewhat contradictory statements in the article, for example, in the Abstract: “red SIF is thought to be more responsive to environmental stress than far-red SIF” and “SIF yield in the far-red region has a strong and stable correlation with NPQ”, and since NPQ is closely related to stress, the latter should mean that SIF yield in the far-red region should better respond to drought. These statements also do not agree well with the statement in section 4.1: "that SIF in red bands, as opposed to far-red SIF, cannot better indicate NPQ variation". The Abstract also states that “the red:far-red ratio of downward SIF and the ratio between the downward SIF and upward SIF at the red peak can be good indicators of chlorophyll content. These findings can help to interpret SIF variations in remote sensing techniques and fully exploit SIF information in red and far-red regions when monitoring plant water stress.”
These results may not be applicable to remote monitoring because it is not possible to measure downward SIF in remote (aerospace) measurements.
4. In addition, if the authors set the task of finding informative SIF parameters applicable in remote sensing, they should have used only a measurement system with constant excitation (continuous-excitation type chlorophyll fluorescence system) for research, as is the case with solar illumination, and not the PAM system. (Pulse Amplitude Modulation), to excite chlorophyll fluorescence.
5. Lines 62-64: "Recent advances in remote sensing of SIF show promise for mapping the drought status of plants across a continuum of spatial scales from the perspective of photosynthesis".
Here one should briefly indicate what recent advances in remote sensing of SIF are meant, especially since there are no references to the relevant works in this place.
6. Lines 68-71: "In addition, by comprising SIF measurements in far-red and normalized difference vegetation index (NDVI) data, studies have found that SIF is a more sensitive indicator for early warning of drought than the NDVI."
This statement does not take into account the complexity and associated uncertainties of remote sensing SIF signal extraction compared to NDVI acquisition.
7. Lines 130-136: It is not clear what role the measurement of the soil water potential played in the study, which is not used anywhere else in the work.
"To maintain the soil water potential of the C plot, the C plot was again irrigated with 0.4 m3 on 24 August during the measurements".
Judging by the values in Table 1, the maintenance of a constant water potential of the soil in plot C did not take place, the soil potential of plot C changed, although it was re-irrigated on August 24?
It is not clear what considerations determine the time interval between measurements on August 21 and 25. It seems that more measurements of all plots should be made (more measurement dates), as well as the number of control plots with different levels of drought.
8. Line 143: Why are there only three leaves, which ones? Further on the pictures we see 16-17 measurement samples?
9. Lines 152-153: "Then PAM-2500, a shortpass filter that cuts off light above 650 nm was inserted into the open aperture."
PAM-2500 is a filter or a fluorometer?
10. Lines 171-172: "leaf temperature were measured at the sun-exposed position"
How was leaf temperature used next?
11. Line 179: "Therefore, assuming that r and t are constant with or without a filter".
A dubious assumption, given that reflection and transmission are wavelength dependent.
12. Line 182: There is no explicit definition of the APAR value (although it is clear from the equation).
13. Line 184: Also, there is no definition of Cab value (chlorophyll concentration?)
14. Lines 232-233: "The values of ?ν/?m decreased slightly under moderate drought (D1) and severe drought (D2) conditions on Aug. 21 and Aug. 25, respectively".
What justifies such an interval of measurements (4 days)?
15. Lines 382-384: "Additionally, current remote sensing instruments are not capable of retrieving the full SIF spectrum and can only measure emitted photons within narrow atmospheric windows."
SIF signals, as is known, can be extracted from remote sensing data in oxygen absorption bands, and not in atmospheric transparency windows. However, the problem is that remote sensing only measures upward SIF, and according to your data, the relationship between red:far red upward SIF and NPQ is weak, and stronger between physiological changes and downward SIF, which is unmeasurable.
16. Lines 428-430 “An alternative method is to use the SIF signal at low light (early morning) as a proxy for ?0 based on geostationary satellites”.
It is not clear how geostationary satellites, which have a completely insufficient spatial resolution, are related to this problem, and the SIF signal can hardly be extracted from their data due to complex atmospheric correction, which does not provide the required accuracy of spectra measurements.
17. Lines 439-441. “To correct the effect of reabsorption on red SIF, we proposed a simple reflectance-based approach to estimate the escape probability, which can improve the performance in indicating NPQ for red SIF».
The accuracy of this approach (approximation) has not been evaluated or commented on in any way.
18. The conclusions in the "Discussion" section are not formulated very clearly, are overloaded with details (the section can be shortened), which makes them difficult to understand.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Thanks for feedback, no further comments.
Author Response
Thanks for your positive reception of our work.
Reviewer 3 Report
Dear authors/editors,
Unfortunately, I still think that the present dataset is too limited and that no solid conclusions can be drawn from this limited experimentation. The responses given by the authors do not convince me that the complementary data collected during "five days" of experimentation provide a solid basis for the conclusions, as the lack of temporal replication of the study or repeated temporal measurements, prevent authors to integrate variability over their dataset. I am not saying that the data does not allow drawing some conclusions, but that the limitation of the dataset prevent from drawing solid conclusions that can be applicable in a variable environment.
Author Response
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Author Response File: Author Response.pdf
