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Genetic enhancement of sorghum for yield-related traits and drought tolerance through induced mutagenes.

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Sorghum (Sorghum bicolor [L.] Moench) is the foundation crop in the world's dry regions, for food, feed, and bioenergy feedstock. There has not been a systematic breeding program and farmers-preferred varieties of the crop in Namibia due to several constraints. There is a need to develop high-yielding and farmer-preferred sorghum varieties with drought-adaptive traits to boost sorghum productivity in the country. The overall goal of this study was to contribute to the national sorghum breeding program aimed at improving sorghum production and productivity through the development and deployment of climate-smart cultivars preferred by farmers and markets in Namibia via induced mutagenesis. The specific objectives of the study were: (1) to assess the present state of sorghum production in northern Namibia and document farmers’ perceived production constraints and trait preferences in new varieties to guide drought-tolerance breeding; (2) to determine the optimum doses of a single and combined use of gamma radiation and ethyl methanesulfonate (EMS) for effective mutation breeding in sorghum; (3) to determine the genetic profile of elite sorghum lines developed via gamma radiation using diagnostic simple sequence repeat (SSR) markers and phenotypic traits for selection; and (4) to determine the Genotype by environment interaction (GEI) of newly-developed mutant and traditional sorghum lines for grain yield and yield related traits for drought-prone areas of Namibia. In the first study, a survey was conducted using a participatory rural appraisal in the following six selected sorghum-growing constituencies: Kapako and Mpungu (Kavango West Region), Eenhana and Endola (Ohangwena Region), and Katima Mulilo Rural and Kongola (Zambezi Region). Data were collected using a structured questionnaire involving 198 farmers in 14 sampled villages across the regions. An equal proportion of male and female respondent farmers cultivate sorghum, suggesting the value of the crop to both genders in Namibia. Most respondent farmers (63.6%) were in productive age groups of <40 years old. In the study areas, low-yielding landrace varieties, namely Ekoko, Okambete, Makonga, Kamburo, Nkutji, Katoma, Fuba, Dommy, Kawumbe, and Okatombo, were widely cultivated, and most of the farmers did not use chemical fertilizers to cultivate sorghum. Farmers’ perceived sorghum production constraints in the study areas included recurrent drought, declining soil fertility, insect pest damage, high cost of production inputs, unavailability of improved seed, lack of alternative improved varieties with farmers’ preferred traits, lack of organic manure, limited access to market and limited extension service. The key farmers’ preferred traits in a new sorghum variety included high grain yield, early maturity, and tolerance to drought and storage pests. The study recommends genetic improvement and new variety deployment of sorghum with the described farmers-preferred traits to increase the sustainable production of the crop in Namibia. In the second study, two concurrent experiments were conducted as follows: in experiment I, the seeds of four sorghum genotypes (Parbhani Moti, Parbhani Shakti, ICSV 15013, and Macia) were treated using five gamma radiation doses (0, 300, 400, 500 and 600 Gray [Gy]), and three EMS doses (0, 0.5 and 1.0%), and gamma radiation followed by EMS (0 and 300 Gy and 0.1% EMS; 400 Gy and 0.05% EMS). In experiment II, the seeds of two sorghum genotypes (Macia and Red sorghum) were treated with only seven doses of gamma radiation (0, 100, 200, 300, 400, 500 and 600 Gy). The combined applied doses of gamma radiation and EMS are not recommended due to poor seedling emergence and seedling survival rate below LD50. The best dosage of gamma radiation for genotypes Red sorghum, Parbhani Moti, Macia, ICSV 15013 and Parbhani Shakti ranged between 392 and 419 Gy, 311 and 354 Gy, 256 and 355 Gy, 273 and 304 Gy, and 266 and 297 Gy, respectively. The optimum dosage ranges of EMS for genotypes Parbhani Shakti, ICSV 15013, Parbhani Moti and Macia were between 0.41% and 0.60%, 0.48% and 0.58%, 0.46% and 0.51%, and 0.36% and 0.45%, respectively. The above dose rates are useful for induced mutagenesis and creating genetic variation in the tested sorghum genotypes for breeding programs. In the third study, 20 mutant lines (which were at mutation generation 7 [M7]) were developed using gamma-irradiation at 350 Gy from the seed of the variety Macia (SDS 3220). Also, five check varieties were used for the comparative study. DNA extraction was carried out on young and fresh leaves samples per test line 20 days after sowing. Seventeen SSR markers amplified a total of 50 alleles, which varied from 2 to 5 (mean = 2.94). The number of effective alleles per locus varied from 1.08 to 2.53, with a mean of 1.96. The observed heterozygosity ranged from 0.00 to 0.21 (mean = 0.09). The mean expected heterozygosity value was 0.45 indicating moderate genetic differentiation of the tested lines for selection and hybridization. Cluster analysis classified the genotypes into three main groups. Moderate to high genetic distance (≥ 0.50) was displayed between drought-tolerant and high-yielding genotypes that aided in selecting mutant lines such as ‘ML2, ML3, ML4, ML7 and ML14’ compared with the check varieties ‘Macia, Kotovara, ICSR 137, and ICSV 17004’. The selected lines are a useful source of genetic variation for breeding high-yielding and drought-tolerant varieties suited for the drought-prone environments of Namibia. In the fourth study, 50 sorghum genotypes, including 10 newly-developed mutant lines (M9), 33 landraces, two sorghum varieties widely grown in Namibia, and five standard check varieties were evaluated under field conditions using a 10 x 5 alpha lattice design with three replications. The experiments were carried out in four environments with two growing seasons in Namibia. Data were collected on grain yield and related traits and subjected to the Additive Main Effects and Multiplicative Interaction (AMMI) model. The AMMI model showed that 93.9% of the total genetic variation was attributed to days to 50% flowering (DF), while 94.04% of the variation was due to plant height (PH), 86.52% to panicle weight (PW), 70.67% to thousand-grain weight (TGW), and 90.68% to grain yield (GY). The larger variations attributed to genotypic effects for PL (36.3%), TGW (33.2%) and PH (20.7%) are useful for genotype selection for yield-related traits. Based on a multi-trait biplot and Best Linear Unbiased Prediction (BLUPs) analyses of the GEI data across all drought-prone testing environments, the medium maturity mutant line designated as L7P9-13 was selected as the best yielding (2 tons/ha) and recommended for the drought-prone areas of Namibia. In summary, the study identified sorghum production systems, key farmers’ perceived production constraints and trait preferences in new varieties in Namibia. Also, the best dosage of gamma radiation and EMS were determined for increasing the genetic diversity in sorghum for genetic enhancement. Newly developed mutant lines ML2, ML3, ML4, ML7 and ML14 displayed moderate to high genetic distance useful for breeding high-yielding and droughttolerant varieties suited for the drought-prone environments of Namibia. The medium maturity and drought-tolerant mutant line designated as L7P9-13 was the best yielding (2 tons/ha) and recommended for large-scale production in the country.


Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.