Germplasm Diversity and Genetics of Quality and Agronomic Traits in Ethiopian Mustard (Brassica carinata A. Braun)

Abstract

Ethiopian or Abyssinian mustard (Brassica carinata A. Braun) has a number of agronomic advantages over other oilseed crops of similar ecological adaptation in Ethiopia. Very' high contents o f erucic acid in its oil and glucosinolates itl the meal are, however, undesirable. Therefore, there is a pressing need of developing cultivars of B. carinata with fatty acid profile of nutritional worth and better meal quality. The present study was thus undertaken with the objectives to: (1) assess the extent of genetic divergence within a germplasm material of B. carinata from Ethiopia; (2) identify accessions from which genes could be sought for improving agronomic and quality traits; (3) identify genotypes with higher levels of oleic acid and lower levels o f erucic acid; (4) establish the inheritance o f erucic acid in the germplasm; (5) identify genotypes with low levels of total glucosinolate and high levels o f oil and protein contents; and (6) study the inheritances o f total glucosinolate, oil and protein contents. Four separate experiments were conductcd to meet these objectives. In the first experiment, genetic divergence of 36 germplasm collections from Ethiopia was studied based on multivariate analyses of data from multi-location trials. The second experiment included ten crosses, which were made between three low- and three higherucic acid lines. The F? seed was used to identify genotypes with high oleic acid and low erucic acid contents and also to study the inheritance o f erucic acid in B. carinata. In the third experiment, a full diallel cross of 6 lines was made to study the genetics of total glucosinolate content whereby the crosses and parental lines were tested at two locations. In the fourth experiment, genetic characteristics of oil, protein and seed weight and size were determined from 81 incomplete partial diallel crosses of 14 parental lines tested at two locations. The inbred lines used to make the three groups o f crosses (one each for fatty acids, total glucosinolate, and oil and protein contents) were developed from the germplasm material by continuous selection over several generations. The results have generally revealed that there was an enormous amount of genetic variability among the B. carinata accessions collected from Ethiopia. The level o f heterogeneity was the same for all geographical regions, but there were some traits, which occurred more often in a certain geographical region than they did in others. Accessions, which could serve as a potential source o f genes for oil and protein contents, earliness, higher number o f siliquae and secondary branches and seed weight and size, were identified. Two problems were evident with glucosinolate. Firstly, the level of variation was not only very narrow, bul also the minimum value was relatively high. Secondly, an increase in the level o f protein with genotypes of current glucosinolate level intensified the latter even further but reduced oil accumulation. Therefore, improvement o f protein content has to be made on cultivars o f low-glucosinolate background. Wide variations in fatty acid contents were obtained after one generation of recombination. Oleic acid varied from 5 to 34% and erucic acid from 6 to 51%. The high-oleic genotypes exhibited not only lower erucic (20%) but also higher linoleic (25%) and considerably lower (8%) linolenic acids. The F2 seeds o f the cross, which resulted in the lowest erucic acid content, segregated into three distinct classes o f low (6-12%), medium- low and/or high (18-32%) and high (>36%) erucic acid with a statistically acceptable digenic inheritance ratio o f 1:14:1. There was also a range of 10% (42-51%) erucic acid content between the maximum values o f the crosses, which suggested the existence o f multiple alleles. The variation within the germplasm of Ethiopian mustard will provide an opportunity for undertaking various purposeful recombinations including the objective of developing low- or zero-erucic genotypes without necessarily going for inter-specific crosses. Results from the two diallel crosses revealed that all the genetic components o f variation, including the cytoplasmic and their interactions with the environment, were important for oil, protein and total glucosinolate contents and also for seed weight and size. This indicates that, in addition to the nuclear genes, components of the cytoplasm could also play equally important role in the inheritance of quantitative traits in B. carinata. In all the trails, the additive components constituted relatively the largest part o f the variations, implying that genotypes with lower levels of glucosinolate or higher levels of oil and/or protein could be fixed by continuous selection from the segregating populations, but due consideration should be given to the type o f seed-bearing plant as well as the growing environment. There were several genes involved in the control of total glucosinolate content and the proportion of dominant genes, which appeared to have decreasing effects on the trait was at least twice as much as the recessive genes of counter effects. The genetic relationships between oil and protein contents were antithetical that lines with genetic effects of increasing oil content had decreasing effects on protein or vice versa. Not withstanding this, there were lines identified with positive genetic effects on oil and protein contents, and thus a possibility for simultaneous improvement of both traits. Unlike with protein content, the genetic relationship between oil content and 1000-seed weight was positive implying the possibility of selecting high-oil genotypes based on iheir seed sine.