Cultivars from the genus Stylosanthes are the most important pasture legumes grown in the semi-arid tropics of northern Australia. In the semi-arid tropics, where Stylosanthes spp. have their most significant role as pasture plants, water is the major environmental constraint to plant growth. Stylosanthes scabra cv. Seca is the most widely sown cultivar and is well adapted to drought conditions in the semi-arid tropics of northern Australia. Seca has been used as one of the parents in breeding programmes to increase anthracnose resistance. However, it has been found that lines selected for disease resistance from these programmes are not as drought resistant as Seca.
The main objective of this work is to identify the traits which contribute to the drought resistance of Seca, so that these traits can be incorporated into the potential cultivars selected from the breeding program for disease resistance.
Results from the experiment conducted with ten diverse accessions of Stylosanthes under well-watered conditions have shown that there was a significant variation for transpiration efficiency (biomass produced per unit of water transpired; TE) among the accessions. A significant negative correlation was observed between TE and carbon isotope discrimination (Δ) and between TE and specific leaf area (SLA). Seca maintained high TE (low Δ). In a different experiment conducted under terminal water-stress conditions, Seca used the available water more conservatively than the rest of the accessions. The relationships observed among the different traits related to TE and biomass production and conservative water use of Seca were confirmed in further and independent experiments conducted under well-watered and water-stressed conditions. In S. scabra, TE appears to be more influenced by biomass production than transpiration.
Trans-4-hydroxy-N-methyl proline (MHP) was identified as the osmoprotectant accumulating in S. scabra under water-stress conditions. MHP values significantly correlated with TE under prolonged and moderately water-stressed conditions, and with leaf survival measured in the terminal water stress treatment.
An F2 population consisting of 120 plants from a cross between cv. Fitzroy and CPI93116 was used for studying the heritability, the genetic relationships among the different traits of drought resistance, for developing the genetic linkage map and for the analysis of the quantitative traits loci (QTL). Three replications for each plant were maintained through vegetative propagation.
Results from the genetic studies indicate that broad sense heritability measured on genotypic mean basis was high for Δ. transpiration and traits related to biomass production, intermediate for TE, SLA, and MHP and low for relative water content (RWC) and osmotic potential at full turgor. Genetic correlations were stronger than the phenotypic correlations but the direction of genetic and phenotypic correlations was similar among the various traits. SLA appears to be closely associated with biomass production. High broad-sense heritability of Δ and moderately high broad-sense heritability of SLA indicate that A and SLA may resporui to selection and selection may be done in early generations. Low genotype X treatment interaction and the consistent negative relationship between TE and Δ and /or SLA together with the negative relationship between biomass production and SLA and/or Δ indicate that Δ or SLA may be used to select for TE and biomass production indirectly. As the measurement of Δ is expensive, SLA could be used for screening in the early generations, while Δ could be used in advanced generations.
A genetic linkage map containing 120 random amplified polymorphic DNA (RAPD) markers was developed to identify the QTLs of different traits associated with drought resistance. The map consisted of 25 linkage groups. All the markers except for three followed the expected segregation ratio of 3:1 in F2 generation. Total genome length covered by markers in the present map was about 1406 cM with an average interval of 12 cM.
The QTL analysis of the different traits has shown that most of the QTLs for TE and Δ were present on linkage groups 5 and 11. Similarly the QTLs 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 A the high alleles of TE were associated with the low alleles of Δ Similarly, the low alleles of SLA were associated with the high alleles of biomass productivity traits and transpiration. The QTL analysis of TE and Δ suggests the causal nature of the relationship between Δ and TE. The phenotypic data and the QTL data show that low SLA leads to high biomass production and consequently high TE. The QTL analysis also indicates that a cause-effect relationship may exist between SLA and the biomass production. There is also evidence that RWC is negatively associated with MHP genes. At the coincident markers between MHP and RWC, the low alleles of MHP were associated with the high alleles of RWC. A low correlation coefficient between TE and MHP may be partly due to the contrasting QTL effect of MHP with respect to increase in TE.
It has been concluded that high TE coupled with conservative water use under water stress conditions may be the significant factors contributing to drought resistance of Seca. Δ or SLA may be used to indirectly select for TE. All the traits related to TE, MHP and biomass production have shown moderate to high broad sense heritability. The phenotypic relationships between the different traits are consistent with the QTL analysis. The QTL detected in the present study may be used for pyramiding different traits together to improve the drought resistance.