|dc.description.abstract||Sorghum (Sorghum bicolor [L.] Moench) is the fifth most economically
important cereal grown worldwide and is a source of food, feed, fiber and fuel.
Sorghum, a C4 grass and a close relative to sugarcane, is adapted to hot, dry adverse
environments. Some genotypes of sorghum called stay-green have delayed leaf
senescence during grain ripening under drought stress conditions which allows normal
grain filling whereas most sorghum lines senesce early under post-anthesis drought.
Eight sources of stay-green have been identified in the sorghum germplasm collection,
most originating from Sudan and Ethiopia. The diversity of the eight sources of staygreen
was analyzed using 55 simple sequence repeats (SSR) markers with genome
coverage. This analysis showed that the sources of stay-green are quite diverse and can
be divided into five groups based on race or working group. Three sources of stay-green
have been used to identify 12 major quantitative trait loci (QTL) that modulate this trait.
The origin of favorable alleles for stay-green was traced backward to ancestral lines and
forward into breeding materials derived from stay-green germplasm. The analysis of the
origin of favorable alleles for stay-green helped explain why subsets of stay-green QTL
were identified in different studies and provided evidence that there may be more than one favorable allele in the sorghum germplasm for several of the stay-green QTL.
Analysis of stay-green breeding lines from three public sorghum-breeding programs
revealed that one of the main QTL identified in mapping studies was not being used in
the breeding programs (0/13), most likely due to its association with an allele for lemon
yellow seeds. In addition, a subset of the regions containing favorable alleles for staygreen
from the genotype BTx642 were over represented in stay-green breeding lines.
Nearly isogenic lines containing favorable alleles from BTx642 for Stg1, Stg2, Stg3, and
Stg4 in a RTx7000 (senescent) background were characterized and each NIL was shown
to exhibit a stay-green phenotype. Based in part on this information, fine-mapping of
Stg1 was undertaken by crossing the Stg1 NIL to RTx7000. Overall, these results
revealed the origin of favorable alleles for stay-green and the current utilization of alleles
for stay-green in public breeding programs. In addition, this study identified additional
stay-green sources that could be used for further QTL analysis and highlighted the
genetic complexity of the stay-green trait.||