Molecular genetic analysis of stay-green, a post-flowering drought resistance trait in grain sorghum (Sorghum bicolor L. Moench)

Drought resistance has been a prime breeding objective in sorghum [Sorghum bicolor (L.) Moench] improvement programs. However, selection for drought resistance traits has been difficult under field conditions because of the large environmental influence and lack of control over intensity and timing of stress. The main objectives were to identify the quantitative trait loci (QTLs) influencing the stay-green trait and yield characteristics, and to study the relationship between the two. Two genotypes, B35 and Tx430, and 96 F7 recombinant inbred lines (RILs) from the cross, B35 X Tx430, were evaluated for the stay-green trait, and yield characteristics under post-flowering drought stress (stress) at two locations during 1993 and 1994. Fully irrigated (control) trials were also conducted at two locations during 1994. The phenotypic evaluation for the stay-green trait showed a heritability of 0.68. The high correlation between stay-green rating and chlorophyll index values (r=-0.86 N=94 P<0.0001) confirmed the validity of visual rating of the stay-green response. The restriction fragment length polymorphism (RFLP) linkage map developed using RILs spanned 1455 cM in 12 linkage associations. Regression analysis identified seven genomic regions significantly (P<0.05) influencing the stay-green rating, explaining 58.5% of phenotypic variability. Five putative QTLs influencing the pooled values of the stay-green trait were identified using the interval mapping method, which explained 58.4% of phenotypic variability. Twelve QTLs influencing grain yield were identified across two locations. Five QTLs influencing yield reduction index (YRI) were identified in each of the two stress environments. At Halfway in 1994, the stay-green trait was positively correlated to YRI. All the stay-green QTLs positively contributed to minimizing YRI (r^=0.75 P<0.0001). Stay-green rating was not correlated with yield at phenotypic level (r=0.036 P<0.7303), but some stay-green markers were associated with significant increase in grain yield. The relationship between stay-green and yield could not be established at Lubbock in 1993, due to the mild stress, which resulted in narrow range of allelic differences in the stay-green rating. The complementary nature of stay-green with high yielding ability provides strong evidence for its role as a drought resistance trait.