Evaluation of maize hybrids for low-nitrogen stress tolerance, yield stability and genetic purity.
Josia, Chimwemwe Chabulika.
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Nitrogen stress is among the major abiotic constraints that impede maize production in Africa. Therefore, development of maize varieties that are tolerant to low N stress conditions and stable across environments is needed. Assessment of genetic purity of inbred lines and their F1 hybrids is among the quality control measures in hybrid breeding, seed production, variety release as well as intellectual property protection (IP). The objectives of this research were, therefore: a) to assess the grain yield performance, genetic parameter estimates, correlations and conduct path coefficient analysis for grain yield and related traits under low N and optimum conditions, b) to assess the magnitude of genotype by environmental interaction (GEI) and hybrid yield stability under low N and optimum conditions and c) to assess the genetic purity of maize parental lines and their F1 hybrids. To achieve these objectives, 170 single cross maize hybrids were evaluated across low N and optimum environments at three locations in South Africa (SA) during 2017/18 summer season viz. Potchefstroom, Vaalharts and Cedara. The experimental setup comprised of five-production conditions across these three locations. The collected data was subjected to analyses using Genstat software 18th edition, SPSS version 25 and SAS version 9.3. For genetic purity analysis, 158 single-cross maize hybrids along with 30 elite parental inbred lines were genotyped using 92 SNPs markers and the molecular data was analysed using GenAlex software. Results revealed that variance due to environment, genotype and GEI were highly significant (P<0.001) for all the traits under low N and optimum conditions. Lower heritability values were observed for grain yield (0.29) compared to secondary traits including days to anthesis, plant height, ear height and anthesis-silking interval, which had heritability estimates of 0.85, 0.43, 0.38 and 0.52, respectively. Higher phenotypic coefficient of variation (PCV) as compared to genotypic coefficient of variation (GCV) were observed under low N and optimum environments, respectively. Under low N, grain yield was positively correlated with field weight, plant height and ear height, but negatively correlated with days to silking, anthesis-silking interval and leaf senescence. Under optimum environment, grain yield was positively correlated with field weight and ears per plant, and negatively correlated with days to anthesis, days to silking, anthesis- silking interval, plant height and ear height. Highest positive direct effect on grain yield was observed for days to silking and field weight under low N, while under optimum, field weight and days to anthesis exhibited the highest direct effects. AMMI and GGE biplot analyses revealed high yielding hybrids in each specific environment and high yielding and stable hybrids across the environments. Five high yielding and stable hybrids across environments; G134 (I-42/CKDHL0295), G12 (CB399/CML442), G24 (CK21/CML216), G33 (CKDHL0089/CML442) and G102 (CML544/I-42) are recommended for further evaluation and release. Using SNP markers, 66.7% of maize parental lines genotyped were considered pure with residual heterozygosity of <5%, while the remaining 33.3% had residual heterozygosity levels of > 5% hence not pure. Cluster analysis effectively discriminated the parental lines into three distinct genetic clusters. Parent-offspring test conducted on 158 hybrids resulted to the elimination of 38% of the hybrids due to genetic contamination of their parental inbred lines. Of the 68 hybrids that passed the parent-offspring test, seven hybrids, including SCHP29, SCHP95, SCHP94, SCHP134, SCHP44, SCHP114 and SCHP126, were selected as potential candidates for further evaluation and possible release in South Africa due to their outstanding yield performance.