Genetic variation, heritability estimates, and yield relationship of pre-flowering and post-flowering drought resistant traits in grain sorghum
Mkhabela, Sipho Milton
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Breeding for drought-prone environments is constrained by lack of suitable selection indices of drought stress resistance. Genetic improvement in drought stress resistance is dependent on the availability of genetic variation and effectiveness of selection within the variation. The objectives of the study were to determine the genetic variation and estimate heritability of pre-flowering and post-flowering drought resistance traits, and determine the relationships among these traits and between the traits and grain yield in a cross involving two sorghum cultivars with contrasting response to drought stress. Three experiments were conducted over two years. Experiment 1 consisted of 100 F3 families and their corresponding bulk F4 families evaluated for pre-flowering drought response at Lubbock in 1991. The families were developed from the cross of B35 with RTx7000. Panicle exsertion, leaf-rolling, desirability (based on panicle development) and grain yield were used as indicators of pre-flowering drought response. Panicle exsertion showed significant genetic variation in both F3 and F4 families. The estimate of narrow-sense heritability of the panicle exsertion was 28%. This estimate plus a high coefficient of variation for this trait indicate that it may not always be a reliable trait on which to base selection. Leaf-rolling showed no significant difference among families in either generation. Desirability scores were significantly different among F3 families but not among the F4 families. Grain yield was not significantly different among F3 families but significantly different in the F4 families. The lack of significant genetic variation for some of the pre-flowering drought resistance traits was probably due to high variability in moisture stress over the test site which resulted in large experimental errors. However, leaf-rolling, panicle exsertion, and desirability (based on floral abortion and sterility) were significantly correlated with grain yield and among themselves. The simple correlation coefficients were significant (P<0.01) and negative for leaf-rolling (lower=best) and panicle exsertion (r=-0.31), leaf-rolling and grain yield (r=-0.55), panicle exsertion and desirability (r=-0.46), desirability (lower=best) and grain yield (r=-0.73) among the F3 families, and positive for panicle exsertion and grain yield (r=0.41) and leaf rolling and desirability (r=0.51) in these families. The simple correlation coefficients of the F4 families had the same response direction and of relative similar magnitude. These findings indicate that any or all these traits may be very useful as selection indices of pre-flowering drought resistance because all are related to each other and all are related to grain yield and the simple correlation coefficients are in the medium to high range. Experiment 2 consisted of the same 100 bulk F4 family progenies used in experiment 1 along with their corresponding bulk F5 families. The bulk families were evaluated for post-flowering drought response under three different stress environments (dryland, limited and full irrigation ) at Lubbock, Texas, in 1992. The rainfall pattern resulted in only post-flowering stress in all three tests. Significant genetic variation was observed for the stay-green trait and grain yield among the families in both populations in all three tests. The stay-green (high=poorer) trait was positively correlated with grain yield in the limited and dryland test environments. The estimate of narrow-sense heritability of the stay -green trait were 39% for limited, 36% for dryland and 32% for full irrigation environment. These regression estimates were deemed sufficient to justify a breeding and selection program to develop post-flowering drought resistant grain sorghum. Experiment 3 consisted of 88 Fg recombinant inbred lines, one developed per F4 family. The 88 FQ lines were evaluated for post-flowering drought response under five environments in West Texas in 1992. Significant differences were observed among lines for the stay-green trait in each environment. Significant differences were observed for line by environment interaction. The stay-green trait was significantly and positively correlated with grain yield (r=0.15) only in the dryland test. Despite the significant correlation of the stay-green trait with grain yield under dryland environment, the size of the correlation was small and probably of little biological value. The stay-green trait appears to be quite independent of grain yield. Stay-green should be a valuable post-flowering drought resistance trait. It appears from the studies here that the stay-green trait can be successfully manipulated and incorporated into high yielding genetic backgrounds.