Assessing the Potential of Extra-Early Maturing Landraces for Improving Tolerance to Drought, Heat, and Both Combined Stresses in Maize

Round 1

Reviewer 1 Report

Nelimor and colleagues have examined the drought and heat tolerance of a set of maize landraces from West Africa.  They selected an initial set of landraces from the IITA and Ghanan National Genebanks, and then further narrowed their screen to those lines that are super-early flowering. They then screened for grain yield and other traits at three sites, where they were able to either maintain control conditions, or implement drought, heat stress, or both.

The authors present the rationale that breeding tends to narrow genetic diversity.  Landraces can possess higher levels of diversity.  In this case, landraces were selected from three countries in West Africa.  Many are consecutive accessions from the genebank, which can indicate they were collected at the same time, and potentially the same or nearby places.  Specific geographic information is not provided.  My most significant concern is that the selection of landraces is in fact quite genetically narrow.  Although African maize landraces can be quite diverse, this method of sampling them would greatly erode that diversity.  This would be further exacerbated by their next step, which is to sub-sample those that flower early.  Consequently, I worry that the premise is not met here, and the subsequent work is not particularly interesting.

The field trials themselves are described clearly enough, and the analyses are mostly those one would expect. But if they are performed on a rather narrow set of lines, they may not be particularly useful.  The writing is not as clear as one would hope, particularly in the abstract and introduction.  

In revisions, I would want the authors to clarify that their selection of accessions , to show that they are not from a very narrow geographic area or genetic base.

If possible, I would also want the authors to look at the genetic correlation of early flowering with the traits they have measured.  I understand that early flowering is likely to be essential to providing drought and heat tolerance.  However, to not narrow the genetic base, ideally one would also want to pull alleles for DS and HS from longer duration, higher-yielding lines as part of a breeding scheme.

Finally, revisions to account for missing grammatical articles (ie, the, a) and other language issues would be helpful.

In addition, 

Author Response

Thank you for taking time to review our manuscript and for the invaluable suggestions to help improve the quality of the manuscript.

The landraces used for this study were originally collected from three countries (Burkina Faso, Ghana and Togo) at different periods and randomly sampled from international and national gene banks where they were conserved. The agro-ecologies/environments in these countries differ in a number of ways in terms of  temperature, precipitation, and the adaptation of the accessions to these contrasting agro-ecologies may have been different. An initial diversity assessment of these landraces revealed marked diversity in the phenotypic traits (https://doi.org/10.1080/15427528.2019.1674760.) Therefore, we are  convinced that the accessions used for the present study were  not from a very narrow geographic area and do not have a narrow genetic base but rather they are invaluable genetic resource for maize improvement programs in sub-Saharan Africa. We could not add records on the collection dates and geographical co-ordinates of the collection sites of the landraces because this  information was   not available in the gene banks from which they were retrieved. Nevertheless, the abstract and introduction have been modified based on the comments of the reviewer. Additionally, the following information have been included in the manuscript based on the comments of reviewer 1:

Genetic correlations between the early flowering trait and  the other secondary traits of the maize accessions have been provided in Table 4 and discussed in the revised manuscript. We do agree with you that it would be desirable to pull alleles for DS and HS from the late maturing  and  higher-yielding local accessions  as part of a breeding scheme. However, please note that the focus of this study was on improvement of earliness and DS and HS tolerance. Beside high yield potential, an appropriate cultivar for large areas of West and Central Africa (WCA), especially the Sudan savanna agro-ecological zone where the growing season  is short, must be DS and/or HS-tolerant in the  extra-early maturity group ( 80-85 days to maturity) or in the early maturity group (90-95 days to maturity). Thus, identifying landrace accessions of maize that can contribute to improvement of both earliness and drought and heat stress tolerance is a prerequisite for adapting maize to climate change in WCA, which was the main focus of this study.

Moreover, it has been shown that large variations in  the flowering period of the genotypes’ can bias the interpretation of the effect of drought-adaptive traits on yield under drought stress (Zaman-Allah et al., 2012). Therefore, it was important for us to carry out the drought and heat phenotyping trials using phenologically homogeneous materials or accessions.

The manuscript has been carefully reviewed and attention paid to the grammatical errors.

Reviewer 2 Report

Thank you for presenting this nice work in a very clear way. I was very pleased to read about your research. I only have a few comments/questions. Since you have a lot of abbreviations do you think you could provide a table or a section in the supplementary section that contains the abbreviations and the corresponding explanations? 

Also, when you describe the statistical model on page 6 you don't specify the response. I am assuming it is all of the traits you considered and not just yield. The model suggests that you assume a normal distribution for the y but you didn't specify, and also, if the response is for example between 0 and 1 or categorical it is not a good idea to assume normal distribution. Could you please address this issue?

Thank you for submitting an organized paper with nice plots and explanations.

Author Response

Thank you for the invaluable comments and suggestions. Based on your comments and suggestions, we have done the following:

A table that contains the abbreviations and the corresponding explanations has now been  included in the supplementary section. The statistical model presented in page 6 has  been specified. The model was employed to estimate variance components and the broad-sense heritability of each environment following the procedure described by Vargas et al. (2013; doi:10.2134/agronj2012.0016) and not to show the distribution.  

Reviewer 3 Report

The manuscript assessed 72 maize genotypes under drought stress at Ikenne during November in 2017 and 2018, heat stress and combined drought and heat stress at Kadawa Nigeria between February and June in 2018 and 2019 as well as under non-stress (NS) conditions as control in 2017 and 2018. Accordingly, notwithstanding, the authors used two replications at all experiments, this work provides a fairly robust dataset. I think it needs major English editing and also the results should be discussed better.

Author Response

Thank you for the invaluable comments and suggestions. Based on your comments and suggestions, we have amended the English and accordingly to the best of our knowledge, discussed the results.

Round 2

Reviewer 3 Report

See the attached file.

Comments for author File: Comments.pdf

Author Response

Thank you very much for your invaluable comments and suggestions. 

Based on your comments and suggestions, the manuscript has been carefully reviewed and attentions paid to the abbreviations in the references. 

Thank you.