Novel Sources of the Stay Green Trait in Sorghum and Its Introgression Into Farmer Preferred Varieties for Improved Drought Tolerance

Abstract

Drought is an important abiotic stress in the tropics that highly constrains sorghum production. Sorghum landraces and wild relatives have been known to harbor sources of novel genes but there is hardly any information about their drought tolerance performance during the post flowering period based on the stay green trait. There is need to characterize this stay green expressed drought tolerance and transfer the mapped QTLs into drought susceptible farmer preferred varieties. This research aimed at identifying sorghum genotypes that have the stay green trait through phenotypic and molecular characterization and subsequently, introgress the stay green QTL from mapped donor lines into farmer preferred varieties. This characterization was done phenotypically and also with Diversity array technology (DArT) molecular markers in genotypes grown under well irrigated and induced drought stress conditions. The trials in the field were set in an alpha lattice design of 12*8 replicated three times. The backcross progenies were genotyped using DArT markers. The genotypes and water regimes used had effects on various traits and helped to identify stay green genotypes. Nine genotypes, namely OKABIR, LODOKA, IESV92043 DL, IESV21400 DL, IESV23010 DL, IESV23006 DL, AKUOR-ACHOT, GBK 016109, GBK 048156 outperformed the check varieties, B35 and E36-1 and in their relative chlorophyll content, whereas the genotypes namely, IBUSAR, LODOKA, GBK 047293 AKUOR-ACHOT, OKABIR, F6YQ212, GBK 048917 had more green leaves at maturity than B35 and E36-1 in drought induced conditions. Ten genotypes, namely, AKUOR-ACHOT, LODOKA, GBK 045827, GBK 047293, WAHI IESV23010 DL, IESV23006 DL, IESV92043 DL, GBK 016114, OKABIR that outperformed B35 when ranked using Relative chlorophyll content measurements yielded higher than both B35 and E36-1 which were the check varieties. LODOKA a landrace, recorded the highest chlorophyll content, highest number of green leaves at maturity and a yielded 2.2 tons ha-1. The accessions whose yield was higher than B35 and E36-1 and B35 and also had higher GLAM and RCC values were chosen as novel sources of stay green. The results also indicated the possibility of xiii finding stay-green alleles from wild genotypes with five wild genotypes, namely, GBK016114, GBK045827, GBK016109, GBK048922, GBK047293 that also clustered separately from B35 and E36-1 in the Neighbor Joining tree. The high significant positive correlation coefficients observed between the relative chlorophyll content and number of green leaves at maturity confirmed that the stay green trait was exhibited as functional stay green. High broad sense heritability estimates of the relative chlorophyll content (0.61) and the number of green leaves at maturity (0.64), indicated the influence of additive gene effects. The narrow sense heritability estimates for the quantity of green leaves at maturity (0.52) and for the relative chlorophyll content (0.45) also indicated the likelihood of a high positive response to selection. This study also identified 20 informative SNP markers that were highly polymorphic and were well distributed across the genome. The F2 genotypes from parental lines, ICSV 111 IN and LODOKA gave high general combining ability (GCA) for relative chlorophyll content and number of green leaves at maturity. Backcrossing for the stay green trait from mapped donor lines into farmer preferred varieties was successful with over 50% of the genotypes having greater than 75% recovery of the genome of the recurrent parent in the first backcross. These genotypes will form a strong basis for selection of superior drought tolerant sorghum varieties and the potential of improving susceptible sorghum genotypes for drought tolerance through marker assisted breeding. Keywords: Sorghum, drought, stay green, diversity analysis, marker assisted backcrossing