Salinity tolerance in halophytes Halophytes, plants that survive
to reproduce in environments where the salt concentration is
around 200 mm NaCl or more, constitute about 1% of the world's
flora. Some halophytes show optimal growth in saline conditions;
others grow optimally in the absence of salt. However, the
tolerance of all halophytes to salinity relies on controlled
uptake and compartmentalization of Na+, K+ and Cl- and the
synthesis of organic 'compatible' solutes, even where salt
glands are operative. Although there is evidence that different
species may utilize different transporters in their accumulation
of Na+, in general little is known of the proteins and
regulatory networks involved. Consequently, it is not yet
possible to assign molecular mechanisms to apparent differences
in rates of Na+ and Cl- uptake, in root-to-shoot transport
(xylem loading and retrieval), or in net selectivity for K+ over
Na+. At the cellular level, H+-ATPases in the plasma membrane
and tonoplast, as well as the tonoplast