Higher Sensitivity of NIR_v,Rad in Detecting Net Primary Productivity of C4 Than that of C3: Evidence from Ground Measurements of Wheat and Maize

Round 1

Reviewer 1 Report

In this study the potential of near-infrared radiance of vegetation (NIRv,Rad) to track hourly and daily changes in C3 and C4 plants were explored and the impacts of their photosynthetic pathways on NPP/NIRv,Rad - relationship were investigated. The study is interesting and timely, but there are several questions relating to experimental design, data source and statistics, that are not sufficiently explained in the present form of MS. My major concern is, that the two study species are not compared in a proper way due to differences in experimental setups.

 

My specific comments:

 

L95: Please explain why it is important to study differences between C3 and C4 plants in NPP. For more efficient agriculture, climate change mitigation/carbon retention, food security or something else?

 

L120: Please explain the selection criteria for the plants/cultivars, or were they randomly selected?

 

L120-131: Why water stress treatments were done only with wheat experiments, not with maize. The experimental designs of the two study species are different. It is unclear, which data is from winter wheat are used. Also, the statistical testing is not sufficiently explained, regarding the replicates. Are there differences between the different study plots?

 

Figure 2: It is unclear, if the wheat data herein is covering all moisture levels or just well-watered plants.

 

L410-L422: This part could be moved to Introduction.

 

L436 “ In contrast, photosynthesis takes place between mesophyll cells and bundle sheath cells (BSCs) in C4 plants” -  This is unclear sentence. The carbon is first fixed at the mesophyll cells by PEP carboxylase (which is not sensitive to photorespiration), and then moved as oxaloacetate > malate to bundle sheath cells where the Calvin cycle is running. 

 

L470: “As the stomata of C4 plants open and close faster than those of C3 plants..” – did you measure this?

 

Figure 8 and 9 (and related text in P14-15) belongs to Results part or Supplementary materials.

 

Author Response

Please see the attachment.

Author Response File: Author Response.doc

Reviewer 2 Report

The manuscript “Higher sensitivity of NIR_{v,Rad} in detecting net primary productivity of C4 than that of C3: evidence from ground measurements of wheat and maize” authored by Siru Chen, Wenhui Zhao et al. present a study on the capacity of NIR_{v,Rad} to capture the differences between the photosynthetic pathways of C3 and C4 plants and their subsequent relationships to NPP. The manuscript has provided a reference for the rapid and accurate detection of NPP in vegetation ecosystems, and for the establishment of NIR_{v,Rad}-based models for different photosynthetic pathways. The study has the potential to yield insights for the commmunity, however I do feel current manuscript is not so complete that need more in-depth analyses and discussions. Some part of the manuscript is not clear and concise enough.

 

First, it is well known that NPP of C4 should be higher than that of C3, thus it should be normal to find higher sensitivity of NIRv,rad. I don't think it is good for the study to "investigate whether their photosynthetic pathways affect the relationship between NPP and NIRv,Rad." It might be better to explore how the environmental stresses affect the relationship or their difference between C4 and C3. I assume the authors should have more data to enrich their analyses based on their experiment setup. More analyses with additional data might be needed to improve the novelty of this study.

 

Second, this manuscript focuses on differences between C3 and C4 photosynthetic pathways in carbon cycle then C3 winter wheat and C4 summer maize were selected as research organisms. However, the different growing seasons of winter wheat and summer maize may lead to errors. For example, whether the temperatures during growing seasons were suitable for both two species? To me, the authors could present more discussion whether winter wheat and summer maize were free from environmental stress.

 

Third, one of the aims of this manuscript is to assess the potential of NIR_{v,Rad}, NIR_{v,Ref}, NDVI, and EVI2 to track NPP changes in C3 and C4 plants on hourly and daily scales.

However, measurements of photosynthetic data were conducted once every hour from 07:00 to 18:00 on one day per week selected based on weather conditions. The data on daily scales were not continuous. Besides, the numbers of daily data for winter wheat and summer maize are different. To me, it was not credible enough to compare the two species with different amounts of data.

 

Four, the accuracy of expression and spelling is crucial. Some numbers are not subscripted or superscripted. Moreover, there are some errors in descriptions of figures.

 

Other comments:

L47: [2] in [CO2] needs be subscripted.

L65: LUE model has a long history. It is okay to say "recently"?

L75: The definition is needed for NIRv,Ref at the first usage in the ms.

L100: More related literature reviews are needed here before "we thus hypothesized ..."

L139-L140: Seasonal variations affect the solar zenith angle, as well as its sunrise and sunset times. Whether collecting spectral data in same period of every day result in errors?

L180: reference for "fPAR"?

L221-L222: Why did the NPP, NIR_{v,Rad}, and APAR fluctuate abnormally on August 18, August 22, and September 22?

L261: [2] in [R2] needs be superscripted.

L310-L312: On the hourly scale, the intercept of winter wheat is positive so that the respiration volume could not be represented by the intercept. Moreover, the intercept of summer maize was [-14.151].

L338-L341: Whether more amount of data cause the hourly-scale NIR_{v,Rad}/APAR–LUE correlation was weak in summer maize?

Figure 4: The title should be [Changes in vegetation and physiological indices on the daily scale]

Figure 7: In subfigure(d), [-4] in formula for regression between the LUE and NIR_{v,Rad} should be subscripted.

Author Response

Please see the attachment.

Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

The authors have responded to my comments very well. I do not have any further concerns.

Reviewer 2 Report

The authors have addressed most of my concerns and I would like to recommend it for publication.