The underlying philosophy is that, although different genotypes may show the same level of toler-
ance to salinity, they could attain this level of tolerance (i.e., phenotype) through different physiological
mechanisms or traits. Lack of sufficient phenotypic variation for salinity tolerance is a serious problem in
many crops, and this is particularly so with rice [149]. Even after screening the entire world collection of
rice germ plasm, the most tolerant genotypes would still suffer about a 50% yield reduction at 5 dS/m
[149–151]. Conceptually, the physiological approach for improving salinity tolerance in crop plants
should be to bring together the relevant traits that would complement each other in a pyramidal manner
(“building block” approach) by their selective incorporation into a single genotype or variety under im-
provement (i.e., optimization of several, probably independent, physiological mechanisms in a single va-
riety) [31].
GENETIC IMPROVEMENT OF SALINITY TOLERANCE IN CROP PLANTS 871
Figure 4 Continued.