Various steps are involved in this kind of approach:- Define the various physiological traits having functional significance in determining the toler-
ance and productivity of a given crop in saline environments. - Establish genetic variability and locate sources of high efficiency for each physiological trait in
the germ plasm. Selection should be directed toward the individual components of salinity tol-
erance on a trait-by-trait basis irrespective of phenotype. - Establish the genetic basis for each physiological trait under consideration by studying its in-
heritance pattern and estimating its heritability, which would determine the feasibility of using
that particular trait in a breeding program. - Develop restriction fragment length polymorphism (RFLP) markers if easily identifiable mor-
phological, physiological, or other markers are not readily available for each physiological trait
as this would streamline the selection process of segregating materials in a breeding program. - Identify genotypes for each physiological trait that have good combining ability.
- Incorporate relevant traits into an agronomically acceptable background basis.
Information generated through this exercise could be stored in a database system that would be made
available to breeders interested in incorporating salinity tolerance in their breeding programs. This is sim-
ilar to information databases that are available for morphological traits from the germ plasm evaluation
exercises at CGIAR (Consultative Group for International Agricultural Research) centers.
Selection of traits to be introduced into a given genotype or variety under improvement depends on
the target environment in which it will be grown and the specific traits a particular variety may be lack-
ing. For instance, a variety may be very efficient in Na and Cl compartmentation in the root as well as in
the shoot but may be lacking effective Na or Cl regulation at the plasmalemma. There is evidence of geno-
typic variation within crop species in Na compartmentation in shoots [31,152,153] and tolerance to high
internal Na and Cl levels [80]. In this case, only the trait that is lacking needs to be introduced. Similarly,
a given variety may be very efficient in ion regulation but lack the genetic means necessary to produce
organic solutes.
Development of RFLP markers for each of these physiological components of salinity tolerance
could play a crucial role in the incorporation of these physiological traits into a genotype or variety under
improvement. Salinity tolerance traits are controlled by a number of genes located throughout the chro-
GENETIC IMPROVEMENT OF SALINITY TOLERANCE IN CROP PLANTS 873
Figure 5 Effect of salinity on tissue Na concentration (g/kg dry weight) of Atylosia albicans,Cajanus ca-
jan(ICP 3783), and their reciprocal F 1 hybrids (a and b), 75 days after transplanting. Data are means of two
replications. (From Ref. 48.)