Phenotypic assessment and molecular diversity evaluation using simple sequence repeats of Kenyan sorghum germplasm under drought stress conditions

Abstract

In terms of tonnage, sorghum is Africa's second most important cereal. The continent produces about 20 million tonnes of sorghum per annum, about onethird of the world crop. The crop is widely grown in the semi arid tropics where local farmers depend on the natural rainfall and the crop is subject to unpredictable drought stress factors. The objective of this study was to assess the genetic diversity and the wide range of sorghum accessions grown in Kenya under drought stress conditions. A total of 139 accessions were characterized and evaluated for drought tolerance and stay green during the dry seasons of 2007 and 2008 at the University of Nairobi, Kibwezi farm. All accessions were grown in 2 m entry rows using an alpha lattice design. Both qualitative and quantitative traits were used to assess the phenotypic variability. There was significant phenotypic variability in the diverse traits of sorghum measured. There was a clear indication of the suitability of the genotypes to specific environments. The early maturing accessions exhibiting an adaptation to drought escape, originated mainly from dry and marginal Northeastern and eastern provinces of Kenya. In both seasons, water stress caused decrease in plant height. Under drought stressed conditions, grain yield was significantly correlated to days to 50% flowering, and also to the number of productive tillers. Grain yield, Days to flowering, rates of senescence (15, 30 and 45 DAF) showed high and significant broad-senescence heritabilities under drought conditions indicating that these traits are highly heritable and can be selected for under drought stress conditions. Green leaf area or 'stay-green' levels was higher in the season with less drought stress than in the more stressed one. Correlations for stay-green measured either at 15, 30 or 45 days after flowering were highly significant. Accessions with high green leaf area yielded higher than those with lower green leaf areas in both seasons. Accessions clustered in groups C and A gave useful higher grain yields, seed mass and also stay green than those clustered in groups Band D. Principal Component Analysis hi-plots identified a total of three similar groups. One group was mainly influenced by days to 50% flowering, the other one was mainly attributed to the green leaf area (stay green) and the last group was mainly influenced by yield, 100 seed mass, plant height and lodging. Kenyan sorghum germ-plasm can therefore be categorized as having been selected for early maturing drought escaping types, drought tolerant stay-green types and high yielding drought susceptible genetic materials. Molecular data on diversity demonstrates that the Kenyan sorghum accessions contain a great deal of genetic diversity as indicated by the the Nei's genetic diversity. The high genetic diversity value among the sorghum accessions indicates that the level of genetic diversity was not influenced by selection. The FST values observed in this study are relatively low, indicating a reduced degree in allelic fixation. The cluster analysis based on genetic similarity among the bulked sorghum accessions examined showed a clear demarcation of the germplasm mainly according to their breeding origin.