| dc.description.abstract | The productivity of rice is affected by various biotic and abiotic stresses; the key among them being drought. Drought has major effects on plant growth as it affects both elongation and expansion growth, hence overall yield. Conventional breeding for drought tolerance is further slowed down by the complex nature of mechanisms underlying this stress although molecular markers offer a promising alternative approach. Therefore, evaluating lines for tolerance to drought under controlled conditions and determining markers associated with drought tolerance traits, will provide useful information that can be used to develop drought-tolerant cultivars and to select the best breeding strategy. In this study, 61 rice genotypes obtained from different breeding backgrounds were examined for drought tolerance under greenhouse conditions. Two separate treatments were established to estimate the effect of drought stress on rice plants under induced water stress regimes for 14 days during the vegetative and reproductive stages and later rewatering, to quantify the level of drought tolerance among rice genotypes studied. The same set of genotypes was screened using 20 SSR and 9 SNP drought-associated markers to quantify genetic diversity among the genotypes and detect possible associations with phenotypic variables estimated under drought stress conditions. The results showed a significant variation in plant production traits among rice genotypes under the induced conditions. Grain weight per panicle was positively correlated with the tillers' number and seed number per panicle but negatively correlated with the number of days to physiological maturity. In performing the Drought Susceptibility Reduction test, on average, a similar effect on yield caused by vegetative and reproductive stresses conditions, averaged 11.1 and 10.6 respectively, however, the severity was very high during the reproductive stage as grain yield under vegetative stress recorded a reduction range between 1 to 53 % while the range was 1 to 93 % under reproductive stress. Overall, the best performing and the highest yielding genotypes under normal watering and the induced stress conditions at vegetative and reproductive stresses were NM-17-72-2, NM-17-72-65, NM-18-58-35, NM-17-72-20, and ARU 1190. The result of the genetic diversity study showed that the SSRs detected a higher number of alleles per locus (3.95) compared to SNP markers (2.89). Also, SSRs provided more information on gene diversity, averaging 0.64 compared to SNPs (0.55) and the average polymorphism information content (PIC) of these two markers was 0.58 and 0.45 respectively. The SSR and SNP markers grouped the 61 genotypes into two major clusters. Cluster 1 comprised 18 genotypes containing 16 belonging to O. sativa and 2 belonging to O. barthi genotypes. Cluster 2 comprised 43 genotypes, 40 of which were O. bathi drought-tolerant genotypes. Single Marker Analysis identified 19 out of 20 SSR and 8 out of 9 SNP markers, distributed across the 12 chromosomes, were associated with at least one of the traits studied. They detected 153 QTLs associated with yield and its attributes under normal watering (60 QTLs) and stressed conditions (42 under vegetative stress and 51 under reproductive stress). Chromosome 10 did not influence the phenotypic traits studied as no QTLs were detected on it, while the most influential chromosome was chromosome 2 under the stressed conditions. Also, the multiple marker-trait association analysis showed that snpOS00412 and snpOS00414 on chromosome 2 followed by snpOS00400 on chromosome 1 and snpOS00483 on chromosome 12, had the highest effect on the studied phenotypic traits. Therefore, the outstanding drought-tolerant genotypes are highly recommended to be used in future drought breeding activities as donors and both SSR and SNP marker sets are promising for marker-assisted breeding in the future molecular breeding for drought tolerance in rice. | en_US |
