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Sorghum is grown in semi-arid to arid regions of the world and serves as the staple food for about half a billion people in sub-Saharan Africa and Asia. The adaptation of grain sorghum to a wide range of environmental conditions has led to the evolution and existence of extensive genetic variation for drought tolerance. Consequently, sorghum is expected to play an increasingly important role in agriculture and meeting world food demand in the face of climate change, land degradation and increasing water scarcity. Drought is a complex phenomenon, and is considered one of the most significant factors limiting crop yields around the world and continues to be a challenge to plant breeders, despite many decades of research. Underestimating the genetics and the physiological mechanisms underlying drought tolerance is vital for the breeding to alleviate adverse effects of drought in order to boost productivity. In this literature review, research findings from the 1970s up to present are included. Most of the basic researches on the mechanism of drought tolerance were done in the early 1980s, and most of the current researches focus on verification and fine-tuning of methodologies. The paper outlines the main effects of drought on crop growth and development, and yield. It then examines the basic information on physiological mechanisms of drought in crops. Subsequent discussion is given on the genetic control of drought tolerance, and breeding methods in sorghum.

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Bottle gourd [Lagenaria siceraria (Molina) Standl.] landraces are widely grown in South Africa, and genetic diversity analysis is necessary to identify promising genotypes for breeding or systematic conservation. Sixty-seven diverse bottle gourd landraces were genotyped using 14 selected simple sequence repeat (SSR) markers. The number of alleles detected per marker ranged from 4 to 11, with a total of 86 putative alleles being amplified. Allele sizes ranged from 145 to 330 base pair (bp). Number of effective alleles (Ne) ranged from 1.58 to 6.14 with a mean of 3.10. Allelic richness varied from 3.00 to 8.90 with a mean of 5.23. Expected heterozygosity (He) values ranged from 0.37 to 0.84 with a mean of 0.65. The mean polymorphic information content (PIC) was 0.57. Jaccard’s coefficient of similarity values ranged from 0.00 to 1.00, with a mean of 0.63. Analysis of molecular variance (AMOVA) revealed that 79%, 17%, and 4% of the variation in bottle gourd landraces was attributable to among landraces, within landraces, and between populations, respectively. The study established the existence of considerable genetic diversity among South African bottle gourd landraces. Unique landraces such as BG-4, BG-6, BG-8, BG-9, and BG-15 from cluster I; BG-55, BG-42, BG-57, and BG-58 from cluster II; BG-28, BG-23, BG-29, and BG-34 from cluster III were selected based on their highest dissimilarity index. These could be useful for bottle gourd breeding and systematic conservation.

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Yam (Dioscorea spp.) is an important root crop widely used for food, feed and industrial raw material. Knowledge on the genetic diversity present among yam genetic resources is fundamental for variety development and conservation strategies. The objectives of this study were to determine the magnitude and genetic relationship present among yam landrace collections using simple sequence repeat (SSR) markers and to identify genetically unique genotypes for efficient breeding and conservation. Thirty-three yam landraces collected from various regions of Ethiopia were genotyped using 10 selected polymorphic SSR markers. The markers amplified a total of 30 alleles from the population sampled, of which 80% was polymorphic. The number of alleles detected per locus ranged from 1 to 5, with a mean of 3. Number of effective alleles ranged from 1.00 to 3.57 with a mean of 1.71. Gene diversity ranged from 0.00 to 0.80 with a mean of 0.53. The mean polymorphic information content was 0.30. Genetic distance values ranged from 0.0 to 1.0, with a mean of 0.39. Analysis of molecular variance revealed that 79% of the variation detected was found within collection sites, while collection sites accounted for only 17% of the total variation. The study established the existence of considerable genetic diversity among yam landrace collections from Ethiopia. Distinct landraces such as 32/83 and 46/83 from Cluster I and 6/02, 2/87, 3/87, 45/03, 76/02, 21/02 and 34/87 from Cluster II were selected based on their highest dissimilarity index. The selected genetic resources are useful as a source of genes of novelty for yam breeding and variety development.

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