Genotype by environment interaction effect and fresh root yield stability of cassava genotypes under contrasting nitrogen regimes.

Nitrogen (N) is an important nutrient element needed by cassava for optimum yield and it is a vital component of nucleotides (nucleic acids), enzymes, amino acids (proteins), chlorophyll molecules and hormones, among other essential compounds required for growth and development of cassava. Nitrogen stress is a major cassava production constraint, the study aimed to examine genotype by environment interaction (GEI) effects and fresh root yield stability of 203 diverse cassava clones to identify genotypes with stable performance under low and optimum nitrogen regimes across environments using AMMI and GGE biplot analysis. Experiments were conducted using an augmented block design with three replications for two years in three locations in Nigeria. There were significant differences (p < 0.001) in the genotype's mean performances as well as significant differences (p < 0.001) in the environment's mean performances for all the traits measured in both nitrogen regimes. The AMMI analysis of variance showed significant effects (p < 0.001) for genotypes, environments and the interactions for fresh root yield in both nitrogen regimes. The biplot analysis showed that for fresh root yield in the optimum nitrogen regime, the principal component accounted for 81.54% of the G + GE (Genotype plus and Genotype by Environment) variation. The G + GE for fresh root yield in the low nitrogen regime accounted for a total of 71.64% of the variation. Ten genotypes were identified as the best genotypes under the optimum nitrogen regime, while eleven genotypes were the best under the low nitrogen regime. Three genotypes under optimum nitrogen regimes were high-yielding. Still, they were unstable in their fresh root yield performance across the environments and can be recommended as specifically adapted to the environments they performed best. Three other genotypes were high-yielding genotypes under low nitrogen but were highly unstable in their fresh root yield mean performance across the environments. The environments Otobi_YR1, Igbariam_YR2, and Umudike_YR1 were identified as the most discriminatory among the test environments. The environments Umudike_YR2 and Igbariam_YR1 were identified as the most representative of the test environments and can represent a mega-environment. The best 21 genotypes that performed above the grand mean for fresh root yield in both nitrogen regimes can be further evaluated on the farmer's field for possible advancement.

Novel SNP markers and other stress-related genomic regions associated with nitrogen use efficiency in cassava.

Cassava productivity is constrained by low soil nitrogen, which is predominant in most cassava-growing regions in the tropics and subtropical agroecology. Improving the low nitrogen tolerance of cassava has become an important breeding objective. The current study aimed to develop cassava varieties with improved nitrogen use efficiency by identifying genomic regions and candidate genes linked to nitrogen use efficiency in cassava. A genome-wide association study (GWAS) was performed using the Genome Association and Prediction Integrated Tool (GAPIT). A panel of 265 diverse cassava genotypes was phenotyped for 10 physiological and agronomic traits under optimum and low-nitrogen regimes. Whole-genome genotyping of these cassava cloneswas performed using the Diversity Arrays Technology (DArTseq) sequencing platform. A total of 68,814 single nucleotide polymorphisms (SNPs) were identified, which were spread across the entire 18 chromosomes of the cassava genome, of which 52 SNPs at various densities were found to be associated with nitrogen use efficiency in cassava and other yield-related traits. The putative genes identified through GWAS, especially those with significant associated SNP markers for NUE and related traits have the potential, if deployed appropriately, to develop cassava varieties with improved nitrogen use efficiency, which would translate to a reduction in the economic and environmental cost of cassava production.

Genome-Wide Association Study of Root Mealiness and Other Texture-Associated Traits in Cassava.

Cassava breeders have made significant progress in developing new genotypes with improved agronomic characteristics such as improved root yield and resistance against biotic and abiotic stresses. However, these new and improved cassava (Manihot esculenta Crantz) varieties in cultivation in Nigeria have undergone little or no improvement in their culinary qualities; hence, there is a paucity of genetic information regarding the texture of boiled cassava, particularly with respect to its mealiness, the principal sensory quality attribute of boiled cassava roots. The current study aimed at identifying genomic regions and polymorphisms associated with natural variation for root mealiness and other texture-related attributes of boiled cassava roots, which includes fibre, adhesiveness (ADH), taste, aroma, colour, and firmness. We performed a genome-wide association (GWAS) analysis using phenotypic data from a panel of 142 accessions obtained from the National Root Crops Research Institute (NRCRI), Umudike, Nigeria, and a set of 59,792 high-quality single nucleotide polymorphisms (SNPs) distributed across the cassava genome. Through genome-wide association mapping, we identified 80 SNPs that were significantly associated with root mealiness, fibre, adhesiveness, taste, aroma, colour and firmness on chromosomes 1, 4, 5, 6, 10, 13, 17 and 18. We also identified relevant candidate genes that are co-located with peak SNPs linked to these traits in M. esculenta. A survey of the cassava reference genome v6.1 positioned the SNPs on chromosome 13 in the vicinity of Manes.13G026900, a gene recognized as being responsible for cell adhesion and for the mealiness or crispness of vegetables and fruits, and also known to play an important role in cooked potato texture. This study provides the first insights into understanding the underlying genetic basis of boiled cassava root texture. After validation, the markers and candidate genes identified in this novel work could provide important genomic resources for use in marker-assisted selection (MAS) and genomic selection (GS) to accelerate genetic improvement of root mealiness and other culinary qualities in cassava breeding programmes in West Africa, especially in Nigeria, where the consumption of boiled and pounded cassava is low.

Genotypic evaluation of twenty-eight high- and low-cyanide cassava in low-land tropics, southeast Nigeria.

A two-year field experiment was carried out in a randomized complete block design with two replications in 2015/16 and 2016/17 cropping seasons at the National Root Crops Research Institute, Umudike (05° 29'N; 07° 33'E; 122 m above sea level) in Nigeria. The objectives of the study were to assess growth, disease status and yield responses of twenty-eight (28) newly developed high- and low-cyanide cassava genotypes in low-land humid tropics of Umudike, Nigeria. Plant height, stem girth, canopy diameter, number of leaves/plant, cassava mosaic disease (CMD) and cassava bacterial blight (CBB) incidence and severity as well as bulking rate and fresh root yield varied significantly (P < 0.05) amongst the high- and low-cyanide cassava genotypes in both cropping seasons. Also, the results showed that bitter cassava genotypes exhibited greater tolerance to CMD than sweet cassava. However, there was no significant (P > 0.05) difference in bulking rate and fresh root yield between the two groups. The Pearson's and Spearman's ranked associations between fresh root yield of the cassava genotypes and other variables analysed across the two cropping seasons were highly significant (P ≤ 0.01) and positive contrary to the other variables. However, they exhibited different degrees of associations amongst themselves, especially CMD incidence that indicated highly significant and positive association with severity. The principal component analysis across the two cropping seasons indicated eigen-values of the four axes > unity with cumulative variance of 68.98 %. Most of the characters that contributed to the 22.35 % observed variability in principal component (PC1) were CMD incidence and severity, and number of leaves/plant while PC2 also exhibited high vector load from plant attributes such as number of leaves/plant, bulking rate ha-1 and canopy diameter. The bi-plot clustering indicated that genotypes (BI-56, NR110439 and B1-29) exhibited strong similarity amongst themselves across the tested variables. The combined fresh root yield sequence of the first ten high yielder genotypes was in the order: NR110439 > TMS010354 > NR110315 > NR 110238 > NR 110228 > NR 060169 > BI-117 > BI-50 > NR110084 > NR 110181. These cassava genotypes were considered to be better endowed genetically, hence their improvement can be encouraged to ensure high and sustainable root yield. A poly-linear and positive regression was recorded between CMD and root yield as well as between CBB and root yield indicating that they affected fresh root yield of high- and low-cyanide cassava genotypes and demands attention also in cassava improvement studies.