Combining Ability of Quality Protein Maize Inbred Lines for Yield and Morpho-Agronomic Traits under Optimum as Well as Combined Drought and Heat-Stressed Conditions
Round 1
Reviewer 1 Report
The manuscript has been improved
Author Response
Reviewer 1
Comments
Reviewer 1 was happy with improvements to the article.
Reviewer 2 Report
Here is my only comment on the revised manuscript: - The authors seem to have not followed up on the point of reviewer 1's comment (not part of my critique). All they need to do is state, on lines 104-105, ...' incomplete block design with three replications where each replication is a separate experiment' - simple and clear. This is what is needed for clarity if I have interpreted the structure accurately.
Author Response
Reviewer 2
Comment
Here is my only comment on the revised manuscript: - The authors seem to have not followed up on the point of reviewer 1's comment (not part of my critique). All they need to do is state, on lines 104-105, ...' incomplete block design with three replications where each replication is a separate experiment' - simple and clear. This is what is needed for clarity if I have interpreted the structure accurately.
Response
We thank the reviewer for the suggestion. The suggested addition was included in lines 104 -105.
Reviewer 3 Report
Although the authors resolved some issues, there is still room for improvement.
- I’m not convinced that the conditions in the field trial can be considered CDHS: the authors claim that “heat stress at flowering and grain filling occurs when plants are exposed to temperatures above 35oC for two or more days successively”. The longest heat period was three days, which much correspond to the “broadest” peak in Figure 1 around January 16th. Flowering took place between 5-19 December, during which there is only one narrow (so 1 day) peak that barely goes over 35˚C. In addition, early grain filling (or until early January) was not characterized by heat spells. The heat peak of three days only occurred one month after flowering was finished. In the manuscript the authors already refer to the drought as “mild drought” (line 131) and a s the heat stress is also not very pronounced, I am not convinced that these data can be described in light of CDHS.
- several abbreviations are still not properly explained: GCAm is not explained in Table 1 while it is in Table 5 and 6. In the results the discrimination between GCAm and GCAf is made, but there are no conclusions, so it is not clear to me why it should be determined for males and females. Not being an expert in the field of combining ability, I wonder why only few lines are female and others are male: should this not be assessed from reciprocal crosses?
Author Response
Reviewer 3
Comment
I’m not convinced that the conditions in the field trial can be considered CDHS: the authors claim that “heat stress at flowering and grain filling occurs when plants are exposed to temperatures above 35oC for two or more days successively”. The longest heat period was three days, which much correspond to the “broadest” peak in Figure 1 around January 16th. Flowering took place between 5-19 December, during which there is only one narrow (so 1 day) peak that barely goes over 35˚C. In addition, early grain filling (or until early January) was not characterized by heat spells. The heat peak of three days only occurred one month after flowering was finished. In the manuscript the authors already refer to the drought as “mild drought” (line 131) and a s the heat stress is also not very pronounced, I am not convinced that these data can be described in light of CDHS.
Response
We agree with the Reviewer that the successive heat stress period did not coincide with flowering. However, based on the above statement quoted by the Reviewer, heat stress coincided with the grain filling period. The grain filling period begins with successful pollination and initiation of kernel development, and ends approximately 60 days later when the kernels are physiologically mature (Daynard and Kannenberg, 1975; Nielsen. 2019). Heat stress during this period has adverse effects on the final yield. Given that the plants were exposed to “mild” drought stress, we are still convinced that conditions that existed during the experimental period can be considered to constitute CDHS conditions. Lines 132 to 134 state the following “As such, this trial was considered to have been exposed to mild drought stress in combination with random heat stress”.
R.L. (Bob) Nielsen. 2019. Grain Fill Stages in Corn. Corny News Network. Agronomy Dept., Purdue Univ. URL: http://www.king.corn/news/timeless/GrainFill.html. https://www.agry.purdue.edu/ext/corn/news/timeless/GrainFill.html. Accessed 21 January 2020.01.21
Daynard, T.B. and Kannenberg, L.W. 1975. Relationships between length of the actual and
effective grain filling periods and the grain yield of corn. Can. J. Plant Sci. 55:237-242
Comment
- several abbreviations are still not properly explained: GCAm is not explained in Table 1 while it is in Table 5 and 6.
Response
Indeed, GCAm and GCAf were not explained in Table 3, but not Table 1. The explanations were added to Table 3, to correct the omission.
Comment
In the results the discrimination between GCAm and GCAf is made, but there are no conclusions, so it is not clear to me why it should be determined for males and females.
Response
The North Carolina designs II (NCDII) is normally used when a large number of parents have to be investigated in that it generates fewer progenies than would be by the same parents using Diallel designs. The NCDII stipulates that the parents be randomly divided into males and females so as to generate a reasonably sized number of relatives for estimation of the additive as well as other genetic variance components. The main aim of using these mating designs is to determine the gene action controlling given traits through estimation of combining ability (GCA & SCA). Male and female main effects, and the male × female interaction effects in a NCD II mating design are equivalent to the GCA and the SCA effects in a diallel (Hallauer and Miranda, 1988 – cited below). Conclusions were therefore stated with respect to objectives of the study. Parental lines with superior GCA and hybrid combinations with exceptional performance based on their SCA effects were identified. Additional commentary was added in the discussion in lines 395 to 397, and reference [31] was cited. In the conclusions, the breeding strategy for improvement of yield under CDHS was stated in lines 439 to 440.
Hallauer AR, Miranda JB, 1988. Testers and combining ability, in Quantitative genetics in maize breeding. In: Hallauer AR, Miranda JB, editors. Iowa State University Press, Ames, USA,. p. 267–298.
31. Awata, L.A.O., Ifie, B., Tongoona, P., Danquah, E., and Marchelo-Dragga, P.W. Common Mating Designs in Agricultural Research and Their Reliability in Estimation of Genetic Parameters.. IOSR-JAVS, 2018. 11 (7): 16-36. DOI: 10.9790/2380-1107021636
Comment
Not being an expert in the field of combining ability, I wonder why only few lines are female and others are male: should this not be assessed from reciprocal crosses?
Response
As stated earlier, the NDCD II is normally used when there is a very large number of parental genotypes, thus allowing generation of fewer progenies relative to use of diallel designs. There are generally no guidelines as to the numbers of males and females to include in the different groups, and numbers of males and females do not have to be similar. Once the groups have been formed, the important thing is that each member of a group of male is mated to each member of a group of females, which was done in this study. Reciprocal crosses are only used where maternal effects are known to be involved in the inheritance of characteristics under study. In this case, none of the traits, especially tolerance to drought and heat, have been reported to be influenced by maternal factors.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
The study of Chiuta and Mutengwa investigates the combined heat and drought tolerance of Quality protein maize inbred lines. The authors examined a number of various traits to characterize the germplasm and identified hybrids with superior characteristics in the combination of stresses. The manuscript is well written and the findings are novel and of particular interest for breeders. The only point that needs to be clarified is what exactly the authors mean by 3 replicates? Three independent experiments? This is very important and should be addressed
Reviewer 2 Report
Maize production in the semi-arid areas of the world is important, and conditions are challenging. The authors set the stage for this, and the value of publishing this work is clear. However, the presentation is ponderous and difficult to interpret. Rather than excessive text about comparisons to lines and hybrid GCA/SCA trait effect data in tables (where the data is not associated with any indication of significance), adoption of different presentation style that is more readily interpretable at a glance is needed.
Colored 'heat map' style presentations would more readily support interpretation, either instead of or in addition to be tables presented.
Table 8, actual grain yield comparisons of hybrids under CDHS conditions, should be presented earlier, perhaps as Table 2, as it sets context for interpretation. A table of yield levels would be of value as a supplemental table.
The discussion of results has multiple errors that require a great deal of time to identify; one example, line 320 states L6*L18 hybrid was a good combiner for both grain yield and chlorophyll content across environments. This hybrid was not, while other hybrids were, according to the values in Table 9. Line 293 incidates that L6*L18 (2.42) had the highest negative SCA value for number of ears/row. The data seems to indicate that L14xL17 had a higher negative SCA value.
Rainfall is cited in the paper, and the authors clearly indicate that combined drought and heat stress events were limited. There is no general discussion of yield levels experienced under optimum versus CDHS conditions in this area to set the stage for performance expectations in this trial, or whether there is an expectation of yield differential for QPM lines. If possible, discussion of the pedigree backgrounds of the L lines shared would be interesting.
Figure 1 color legend for rainfall events needs to be corrected; color box is green, lines are blue. Not clear how ASI was calculated; if ASI= DS-DT was actually used, then a negative GCA effect is desirable, as the authors state. How it was actually calculated was not explained; line 215 addresses both anthesis and silking, stating -GCA effects are desirable for both. Lines 221-222 address this also. Line 138 should be used to define how the calculation was actually made in this study to avoid confused interpretation Table 2: omits all hybrids made using line L30, but L30 hybrid comparisons are discussed extensively and data for them is in all GCA/SCA effect tables . Lines 336-339 - statement is not supported by explanation. Is GCA:SCA ratio sufficient to differentiate between influence or lack thereof of additive genetic effects on a trait. Lines 394-395 - while I agree with the conclusion presented in the discussion on lines 378-39, that 'stress breeding for improved maize yield in semi-arid regions is needed,' more supporting explanation needed.
Reviewer 3 Report
combined drought and heat stress: there was rainfall but irrigation was stopped: this makes it very difficult to assess the level of drought. Temperature seems the same as normally, so I don't understand the claim for CDHS. Do the selected lines typically grow in the climate, where the field trials were organized? several abbreviations are not explained: e.g. (but not exhaustively) in Figure 1Tx and ET0? and what is the difference between GCAm and GCAf the text is a mere enumeration of line names, traits and positive or negative GCA of SCA, which makes it very boring to read. Some more text should be added to guide the reader through the different combinations, and I'm not convinced that all possibilities should be mentioned. the authors should think of a narrative: what do they want the readers to remember, what is the take-home message of their paper?
