Genetic analysis of maize hybrids derived from temperate by tropical germplasm under low and high plant population density stress.

Abstract

Maize is a major staple food crop in Sub-Saharan Africa and it plays a vital role in the livelihoods of small-scale and resource-limited farmers. The demand for maize is high and is also expected to increase due to the increasing population. Grain yield per unit area can be increased by increasing the plant population density. In this regard, the new improved maize varieties should be richly endowed with high and enhanced frequency of genes that confers high yield under varying plant population density stress conditions. The objective of this study was to conduct a genetic analysis of maize hybrids derived from temperate by tropical germplasm under low and high plant population density stress in order to identify hybrids that combine high yield, earliness and tolerance to high plant population density stress as well as the breeding strategy for these essential traits. New maize inbred lines derived from tropical by temperate populations were selected based on observation trials for yield potential and prolificacy to ensure adaptation. These were used to generate hybrids using two different testers with different genetic attributes. The hybrids were planted in four environments; Ukulinga1 (Env-1), Cedara (Env-2), Dundee (Env-4) and Ukulinga 2 (Env-3) in two replications. One of these environments (Ukulinga 2), had a high plant population density. Data was collected on various agronomic traits that include grain yield, plant height, ear height, days to anthesis, ear position, number of ears per plant, anthesis-silking interval, grain moisture content, root and stem lodging, number of tassel branches, number of leaves above the cob, days to cob dryness and number of plants per row. Analysis of variance for single sites showed that hybrids were significantly different on the traits studied, and across environments (low and high plant population densities). This enabled the genotype plus genotype by environment biplots to be used in identifying varieties suitable for given environments as well as stable and high yielding varieties. The line by tester analysis of variance showed that the general combining ability effects of the lines were significant (P<0.05) and that narrow sense heritability was low for grain yield but higher for other traits. The results of this study identified hybrids (and their inbred lines) that performed better under high plant population density stress and the traits associated with high yield under high and low plant population densities across different genetic backgrounds. Superior, stable and high yielding hybrids were selected and hybrids 15XH214, 15XH215 and 15XH121 were the most adaptable genotypes across environments, out-competing the highest yielding commercial hybrids such as PAN6Q345BC and BG5285 under high stress conditions. With regards to the genetic gain, the study revealed 16.70 % and 22.70 % genetic gain of grain yield under both Tester 9 (Testers A) and DTAB32 (Tester B), respectively, which was displayed by the high-yielding experimental hybrids. The studies also revealed high genetic variability of traits among hybrids, which can be exploited to obtain further breeding gains. The high genetic gains and stress tolerance indices of these hybrids over the checks were related to resistance to stem lodging and increased ears per plant. Most of these hybrids were derived from the tester DTAB32 which is associated with a huge contribution to stress resistance, including lodging. Based on the combining ability analysis, inbred lines with resistance to stem lodging and high ear prolificacy were identified as, 15XH214 and 15XH215 under tester B and 15XH121 under tester A. In producing better hybrids, such inbred lines were complimented by Tester DTAB32 that has been shown to have resistance to lodging and other abiotic stresses. The identification of the best genotypes based on the increase of plant population density stress tolerance was achieved through the selection of the hybrids which possessed good standability, yield stability and high grain yielding ability. The genetic coefficient of variation (GCV) and narrow sense heritability values estimated were moderate for all traits but low for grain yield thus calling for a need to identify the traits which could be targeted for improving the grain yield of the hybrids based on indirect selection of traits highly correlated with grain yield, easy to measure and have higher heritability. Generally, the results of this study identified inbred lines with good general combining ability (GCA). This shows the possibility of developing desirable cross combinations and synthetic varieties through crossing of inbred lines with desirable traits of interest. Furthermore, promising cross combinations identified in this study could be used for future breeding work as well as for direct release after confirming the stability of their performances observed in this study. Hence, the information on combining ability from this study may be useful for researchers to develop high yielding varieties of maize under high and low plant population densities as well as assisting in defining genetic advance; which will enable effective and efficient selection of the germplasm lines to produce new maize hybrids. From the study it was revealed that ears per plant and stem lodging were highly correlated with grain yield and had high positive direct effects on grain yield under high plant population density. These traits did not only have high correlations with grain yield, but also had high narrow sense heritabilities as well and were easy to select for, thus making them ideal candidates for indirect selection for improved grain yield under high plant population density stress. This study concluded that high plant population density reduces ears per plant and increases stem lodging which result in reduced grain yield. Development of ideal breeding strategies that can improve grain yield under high plant population density is desired. Advance testing of these maize hybrids in more seasons could enhance good and desired breeding productivity with reference to cultivar stability and adaptation across environments. This suggested that this selected hybrids exhibits progressive stability in different environments, which is a desirable attribute for the smallholder farming conditions, where management conditions are variable. These hybrids have the potential to respond positively to improved environmental conditions, since they were able to obtain high yields under high plant population density. Therefore, they can be recommended for advancement in the following season. Further tests on these experimental hybrids for commercial use could be done to enable their release given the need for increased maize production and productivity in South Africa, to prevent recurrent food shortages that result food insecurity.