The Use of Remote Sensing to Determine Nitrogen Status in Perennial Ryegrass (Lolium perenne L.) for Seed Production

Round 1

Reviewer 1 Report

This is an interesting and rather useful article testing possibility of using measurements of canopy reflectance for predicting plant yield depending on the level of supplied nitrogen. This was performed with perennial ryegrass used as an as an object. Experiments were carried out by competent researchers. However there are some drawbacks of the MS that should be rectified before I may recommend the MS for publication.

1. Term lodging is frequently used in the text and nowhere its meaning is clarified. It may be found in the literature that lodging means displacement of crop stems from their upright position. If this is so, I would like to know how lodging was measured in the present experiments.
2. Figure 2 is missing.
3. Line 284. “Nup increases with GDD”. - I failed to see this on fig. 3. Cannot regression line be drawn and help to see this?
4. Lines 282-283. “Nup remained at low levels in N40 and N80, had an upturn from N80 to N120, but there were only slight differences between N120, N160 and N200.” – Again, it is not easy to compare the variants and to notice the difference between the treatments. Cannot average values of Nup for each variant be calculated and provided to simplify comparison?
5. It is not explained which symbol corresponds to results obtained in certain year in Figure 4 (unlike previous figures). Furthermore, not a word is said about the part of figure 4 showing predicted values for NNI, while details of measured values are described
6. Line 305. “NNI only exceeds the line at 900 GDD” – it seemed to me from the fig. 5 that the range of GDD values with corresponding NII values above 1 was broader.
7. I failed to find equations used for calculating predicted values of NII and seed yield on the base of measured vegetation indices. If I just missed them, cannot these equations be emphasized, since I think it important to understand which of numerous indices shown in table 2 were integrated to predict NII and seed yield values.
8. “. The direct method predicts NNI by developing a regression model between NNI and the VI, while the indirect method predicts the Na and the aboveground dry matter DM, and it uses them to calculate NNI” – I think that this should be made clearer.

Author Response

Dear Reviewer 1

We appreciate the time and effort you have used to review our manuscript. Your comments were very usefull and we have corrected the manuscript accordingly. We have responded to your specific comments below:

Comment #1: Term lodging is frequently used in the text and nowhere its meaning is clarified. It may be found in the literature that lodging means displacement of crop stems from their upright position. If this is so, I would like to know how lodging was measured in the present experiments.

Respond to #1:

We agree that lodging was not properly described and we have added the following sentence in Materials and Methods "Stem lodging is a displacement of the crop from its upright position and early and severe lodging can be detrimental for the seed yield. Severe lodging can be a derived effect from excess N application and a lodging scores was obtained to secure that this was not the case in the present study ".

Lodging was measured as described in Materials and Methods using these senteces: Visual lodging scores of the full plots were obtained at flowering and at harvest using a visual score ranging from 0 to 100, where 0 is no lodging, and 100 is a fully lodged plot.

Comment #2: Figure 2 is missing.

Respond to #2:

We appoligice for this and figure 2 is now included in the manuscript

Comment #3: Line 284. “Nup increases with GDD”. - I failed to see this on fig. 3. Cannot regression line be drawn and help to see this?

Respond to #3:

We have changed figure 3 (see #4 for an additional answer)

Comment #4: Lines 282-283. “Nup remained at low levels in N40 and N80, had an upturn from N80 to N120, but there were only slight differences between N120, N160 and N200.” – Again, it is not easy to compare the variants and to notice the difference between the treatments. Cannot average values of Nup for each variant be calculated and provided to simplify comparison?

Respond to #4:

We agree that this was difficult to see the differences in figure 3. We have changed the values to average values as suggested and it is now easier to compare results. 

Comment #5: It is not explained which symbol corresponds to results obtained in certain year in Figure 4 (unlike previous figures). Furthermore, not a word is said about the part of figure 4 showing predicted values for NNI, while details of measured values are described

Respond to #5:

We assumed that this comment was for figure 5 and we have included description of the symbols in all figures. 

Comment #6: Line 305. “NNI only exceeds the line at 900 GDD” – it seemed to me from the fig. 5 that the range of GDD values with corresponding NII values above 1 was broader.

Respond to #6:

We agree and have changed the text to: Measured and predicted NNI values of split N applications are in Figure 5. As in Figure 4, predicted values are calculated using the direct method. Measured NNI increases with increasing GDD until approximately 900 to 1000 GDD after which the values seems to drop. In general, most of the measured NNI values are above one. Results for the predicted NNI are scattered around NNI=1.

Comment #7: I failed to find equations used for calculating predicted values of NII and seed yield on the base of measured vegetation indices. If I just missed them, cannot these equations be emphasized, since I think it important to understand which of numerous indices shown in table 2 were integrated to predict NII and seed yield values.

Respond to #7:

We agree that it would have been interesting if we could show a few variable importance in prediction (VIP), in this case single VI that were important for prediction of NNI and seed yield. However, this was not the case. This is most probably due to the fact that many VI are close to similar and our measurements were not taken on the same dates/GDD in the individual years. In this way we were not able to show a few VIPs. We therefore decided not to show a figure with a lot of VIPs for all years as it would be difficult to see and furthermor to discuss why one VI on a specific GDD was more important in one year while it was less important in another year. 

Comment #8: The direct method predicts NNI by developing a regression model between NNI and the VI, while the indirect method predicts the Na and the aboveground dry matter DM, and it uses them to calculate NNI” – I think that this should be made clearer.

Respond to #8:

We agree that this could be explained more clear and have changed the sentence to: The direct method is founded on a regression model from measured NNI and VI. The indirect method is based on two regression models; one regression model developed for measured N_a and VI and one regression model developed for measured DM and VI, NNI is then calculated based on predictions from these two models [26].

Reviewer 2 Report

This research paper appears to describe in an extensive form, a methodology to predict N requirements for Lolium using reflectance canopy spectra in the visible range. The design of the experiment and the data analysis is well documented. I would suggest to authors proceed further using spectral bands in the NIR range. Some proposals for corrections in spelling are mentioned in the attached file.

Comments for author File: Comments.docx

Author Response

Dear Reviewer 2,

We appreciate the time and effort you have spend on our manuscript.

To your comment "I would suggest to authors proceed further using spectral bands in the NIR range" we would like to refer to the vegetation indices where we actually use NIR like NDVI, RVI1, MCARI etc. see table 2.

We have incorporated your corrections in spelling in the new version of our manuscript, thanks.

Round 2

Reviewer 1 Report

Authors have followed all my recommendationa and their responses are detailed and clear. I think that this article is interesting and may be published in its present form