Screening methods based on damage, or tolerance to
very high salinity levels
Techniques that can handle large numbers of genotypes
include germination or plant survival in high
salinity, leaf injury as measured by membrane damage
(leakage of ions from leaf discs), premature loss of
chlorophyll (using a hand-held meter), or damage to
the photosynthetic apparatus (using chlorophyll fluorescence).
These are listed in Table 1, and are discussed
in some detail at the end of this article. These methods
can identify genotypes able to germinate in, or survive,
very high salinities (over 200 mM NaCl), but do not
discriminate between genotypes in ability to tolerate
the low or moderate salinities typical of many saline
fields (50–100 mM NaCl). This could be why genotypic
variation in germination or survival has rarely
been replicated in the field or in long-term growth
experiments.
A major limitation to the use of injury or survival
to identify salt-tolerant germplasm arises when the
cause of injury is not known. The injury could be due
to water stress, the Na+ or Cl− accumulating within
the leaf, or to K+ or Ca2+ deficiency (Greenway and
Munns, 1980). Alternatively, leaf death could be due
to accelerated senescence due to osmotic effect of the
salt. This becomes particularly relevant when considering
the reproducibility of the responses in the field.