5 Tolerance to Combined Stress of Drought and Salinity in Barley 113
Although tremendous efforts have been applied to breed drought- and salt-tolerant
barley by conventional and molecular approaches, truly drought and salt-tolerant
barley cultivars have not been produced that can go to farmer’s field. The promising
drought- and/or salt-tolerant genotypes are still in the laboratory and experimental
fields. To overcome this bottleneck from the laboratory to the farmer’s field, breed-
ing programs should target specific environments and pyramid tolerance genes be-
cause drought and salt stresses are complex and variable in different environments
and in different years.
5.10 Conclusions and Future Perspectives
Crop production under field conditions can be decreased by several abiotic stresses
and the studies on multifactor interactions are of greater importance than analyses
of only one stress. A combination of drought and salinity stress affects the plants to
a larger degree and plant reaction cannot be directly extrapolated from the response
of plants to individual effect of these two stresses. In the case of drought toler-
ance, plants potentiate to maintain the metabolic activities even at lower level of
tissue water potential by accumulating intracellular osmoprotectants such as Pro,
GB, amino acids, and soluble sugars. Besides, scavenging of ROS by enzymatic
and nonenzymatic antioxidants, cell membrane stability, expression of aquaporin,
and stress-related proteins such as LEA (late embryogenesis abundant) are also the
vital mechanisms of drought and salinity stress tolerance.
Marker-trait associations are being identified by the development of a high den-
sity SNP assay platform that provides sufficient marker density for genome-wide
scans and LD-led gene identification (Waugh et al. 2009 ). Projects are aiming to
exploit the discriminatory LD observed in landrace and wild barley populations for
fine mapping and gene identification (e.g., ExBarDiv: http://pgrc.ipk-gatersleben.
de/barleynet/projects_exbardiv.php). Highly significant associations can be identi-
fied between genome-wide SNPs and drought and salt tolerances in wild progeni-
tors, landraces, and varieties. These approaches offer the possibility of identifying
novel allelic variation that may be of considerable value to future crop improvement
(Waugh et al. 2009 ).
Advances are still needed to efficiently explore the extensive reservoir of drought
and salt-tolerant alleles within wild germplasm deciphering: (1) the molecular net-
works those lost during domestication and modern breeding (Fu and Somers 2009 );
(2) the high-throughput screening of wild germplasm for drought/salt tolerance and
their regulation of fitness components; (3) the molecular basis of chromosomal re-
combination; and (4) the potential regulatory relationship between coding and non-
coding regions. This will increase the availability of sequence information and will
encourage new breeding strategies by transferring single and multiple interacting
networked loci/QTLs from wild relatives to commercial varieties via marker-assisted
selection. The International Triticeae Mapping Initiative and the Barley Genome Se-
quencing Consortia are serving as platforms for international collaborative projects