269its own range of variation. Manipulating a single trait, disregarding all others, is
relatively straightforward; however, this is unlikely to result in a useful variety. The
vast diversity of breeding methods can be simplified into three categories: (i) plant
breeding based on observed variation by selection of plants based on natural vari-
ants appearing in nature or within traditional varieties; (ii) plant breeding based on
controlled mating by selection of plants presenting recombination of desirable
genes from different parents; and (iii) plant breeding based on monitored recombi-
nation by selection of specific genes or marker profiles, using molecular tools for
tracking within-genome variation. The continuous application of traditional breed-
ing methods in a given species could lead to the narrowing of the gene pool from
which cultivars are drawn, rendering crops vulnerable to biotic and abiotic stresses
and hampering future progress (Breseghello 2013 ).
7.1 Screening for Biotic and Abiotic Stresses
Biotic and abiotic stresses are the major limiting factors for high crop productivity
in drylands worldwide. In response to abiotic stresses, plants undergo a variety of
changes at the molecular level (gene expression) leading to physiological adapta-
tion. Drought, cold and salinity are the major abiotic stresses which severely affect
yield and quality in many regions of the world endangering the food security. The
situation has become more serious with the global climate change. Therefore, stud-
ies on abiotic stress tolerance have become one of the main areas of research world-
wide (Patade et al. 2011 ; Mohammadi et al. 2014 , 2015 ). Indirect selection for
stress tolerance may be performed at the laboratory level or in open field conditions.
Chlorophyll fluorescence and thermal imaging are well-established, powerful, non-
destructive, and rapid techniques for screening plants against abiotic stresses in crop
plants (West et al. 2005 ).
7.2 Genotype × Environment (GE) Interaction
Selection for greater tolerance to abiotic stresses such as drought and heat stresses,
via the identification of high-yielding genotypes, is an important element in the
development of sustainable agriculture systems in drylands. However, when a geno-
type is grown in different environments, it frequently shows significant fluctuation
in yield performance in comparison to other genotypes. These fluctuations are influ-
enced by environmental conditions (i.e. water availability, temperature) experienced
at any given location or year and are often referred to as genotype × environment
(GE) interactions (Allard and Bradshaw 1964 ). GE interaction affects breeding
progress because it makes difficult the evaluation and selection of superior geno-
types. On the other hand, GE interaction may also offer opportunities, especially in
the selection of genotypes showing positive interaction with a specific location and
Breeding and Genetic Enhancement of Dryland Crops