Identification of causal relationships among traits related to drought resistance in Stylosanthes scabra using QTL analysis.

Previous studies have shown that a negative relationship exists between transpiration efficiency (TE) and carbon isotope discrimination (Delta) and between TE and specific leaf area (SLA) in Stylosanthes scabra. A glasshouse experiment was conducted to confirm these relationships in an F(2) population and to study the causal nature of these relationships through quantitative trait loci (QTL) analysis. One hundred and twenty F(2) genotypes from a cross between two genotypes within S. scabra were used. Three replications for each genotype were maintained through vegetative propagation. Water stress was imposed by maintaining plants at 40% of field capacity for about 45 d. To facilitate QTL analysis, a genetic linkage map consisting of 151 RAPD markers was developed. Results from this study show that Delta was significantly and negatively correlated with TE and biomass production. Similarly, SLA showed significant negative correlation with TE and biomass production. Most of the QTL for TE and Delta were present on linkage groups 5 and 11. Similarly, QTL for SLA, transpiration and biomass productivity traits were clustered on linkage groups 13 and 24. One unlinked marker was also associated with these traits. There were several markers coincident between different traits. At all the coincident QTL, the direction of QTL effects was consistent with phenotypic data. At the coincident markers between TE and Delta, high alleles of TE were associated with low alleles of Delta. Similarly, low alleles of SLA were associated with high alleles of biomass productivity traits and transpiration. At the coincident markers between trans-4-hydroxy-N:-methyl proline (MHP) and relative water content (RWC), low alleles of MHP were associated with high alleles of RWC. This study suggests the causal nature of the relationship between TE and Delta. Phenotypic data and QTL data show that SLA was more closely associated with biomass production than with TE. This study also shows that a cause-effect relationship may exist between SLA and biomass production.