Phenology-Adjusted Stress Severity Index to Assess Genotypic Responses to Terminal Drought in Field Grown Potato

Round 1

Reviewer 1 Report

Authors present a phenology-adjusted stress severity index for the evaluation of potato yield response to water deficit. The subject of the study is of high relevance due to agricultural production vulnerability to climate change posed risks. Timely selection of the most resistant genotypes is paramount for the development of drought-resistant potato cultivars. Authors describe the conducted study in great detail and flawless English. The results obtained support the conclusions drawn and validity of the stress severity index.

Author Response

Thank you very much for this positive feedback.

Reviewer 2 Report

Authors of the study "Phenology-adjusted stress severity index to assess genotypic responses to terminal drought in field grown potato " propose a new method for evaluation of drought sensitivity of potato cultivars. The aim of the study is important as the response to drought is difficult as due to complexity of various aspects of the methods and "plant response". Moreover as indicated by authors the shift in growth phases and their variable susceptibility do stress complicate the task. Authors used soil water potential as a measure of soil available water, what is generally good idea, however the values used were not directly measured but recalculated from the measurements of volumetric soil moisture using van-Genuchten-Mualem function, of which the measurements and modelling adds its own error. Some limitations of the calculations were probably resulting from limited soil data like for example one value of bulk density for whole depth range (Table 2) what in correspondence with fast drying of topsoil layer and errors of FDR probe at low soil moisture.

It seems that proposed approach is valuable and will be interesting as for breeders as scientists, however I would like to point out some minor issues:

1. The SSI is described in multiple places as:

"based on the soil tension the genotype was subjected to for the duration of the stress modified by the development stage of the genotype. The sum of the daily values was combined in a stress severity index (SSI)."

or

"takes into account the changes in the soil matrix potential during water deficit, the sensitivity of the respective phenological stage, and the duration of the stress"

or

“comprises of the soil water tension experienced by the plant weighted by the relative genotypic sensitivity of the respective phenological stage (Figure 2), cumulated for the duration of the water deficit treatment.”

or using different way, do not allow to easily get information of precise values of the index. Please provide the equation.

2. Authors point out that the relationship soil moisture (tuber fresh weight) presented on figure 3 do not allow for clear evaluation of genotype sensitivity to drought. The figure showing “clear differentiation of genotypes” by SSI is presented on figure with % of tuber yield reduction on vertical axis. It would be good to show the improvement in proposed approach by comparing similar figures (for impact of soil available water or SSI on the reduction of tuber yield (%))

According to the statement on p7 appendix A should contain data on soil bulk density used in modelling. There are no such data, table 2 contains limited to only one, for whole depth range, soil bulk density.

Table 3 is not clear due to small size and overlapping numbers.

Author Response

Thank you for your valuable comments.

1.„Authors used soil water potential as a measure of soil available water, what is generally good idea, however the values used were not directly measured but recalculated from the measurements of volumetric soil moisture using van-Genuchten-Mualem function, of which the measurements and modelling adds its own error.“

Yes, this is correct. We would suggest in general to measure soil tensions directly if possible. However, our data show that you can get meaningful results with the soil moisture method and the transformation calculation even if the error increases.

2. „Some limitations of the calculations were probably resulting from limited soil data like for example one value of bulk density for whole depth range (Table 2) what in correspondence with fast drying of topsoil layer and errors of FDR probe at low soil moisture.“

• Due to the relatively high sand content (80 to 96% sand) the soil bulk density is not likely to change significantly with depth. The interesting depth for potato rooting down to 30-40 cm is covered with presented soil analysis. Nevertheless, a validation study of the stress severity index on heavier soils might be very interesting.
• We do agree that the error of the frequency-domain-reflectometry measurements increased with lower soil water content. Water content lower than 5 Vol.% was poorly measured by the PR2 Soil Moisture Profile Probe, Delta T-Device.
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3. Please provide the equation

We included the equation for the SSI calculation in the Materials and Methods section.

4. It would be good to show the improvement in proposed approach by comparing similar figures (for impact of soil available water or SSI on the reduction of tuber yield (%)).

• we feel that the figures provided - Figure 3 to 6 - already show how phenology weighing of the stressor improves the power of the tool. Additional graphs would not elucidate the process more.
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5. According to the statement on p7 appendix A should contain data on soil bulk density used in modelling. There are no such data, table 2 contains limited to only one, for whole depth range, soil bulk density.

• The sentence „The soil texture and soil density used in “RosettaLite” is given in appendix A.“ has been changed in „The soil texture and soil density used in “RosettaLite” is given in Table 2.“

6. Table 3 should now be fine.