**7. Conclusion**

Among the abiotic stresses that limit wheat crop productivity, drought stress alone is by all means one of the most devastating factors. In the past, breeding efforts to improve drought tolerance response have been hindered primarily by its quantitative nature as well as our poor understanding of the physiological basis of yield in water-deficient conditions [16]. So far, most QTLs for drought tolerance in wheat have been identified through yield and yield component measurements under water-limited conditions. No doubt, yield is the most desirable trait to breeders; still, it is very difficult to relate water use efficiency and identify potential target regions for positional cloning [15]. Only few studies have associated QTLs with specific components of drought response. Although the development of gene-based molecular markers and genome sequencing should accelerate positional cloning, the genomic regions associated with individual QTL are still very large and are usually unsuitable for breeding programme [51–55]. From an application point of view, it is imperative to select genotypes that are able to optimize water use efficiency while maximizing yield in response to drought. Improving the competence of root systems to extract water from the soil is highly desirable, and any extra water extracted during grain filling definitely remarkably increases the yield in wheat. Thus, identification of markers or genes associated with root growth and architecture would be particularly useful for breeding programmes to improve root traits by molecular marker-assisted selection.
