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Adaptive Components of Salt Tolerance
James W. O’Leary
The University of Arizona, Tucson, Arizona
615I. INTRODUCTION
Plants have evolved two very different strategies in adapting to high levels of sodium salts in their envi-
ronments. One strategy is to exclude the salts from the interior of the leaf cells, and the other includes the
salts within the leaf cells but sequesters most of them in the vacuoles of those cells. In both cases, the end
result is to maintain the cytoplasmic sodium concentration relatively low. This is accomplished in the for-
mer case by either preventing entry of the ions into the plant at the root surface or preventing them from
being transported in the xylem from the roots to the leaves. In the latter case, entry and transport to the
leaves are not prevented or severely restricted, and the problem is handled primarily at the tonoplast level
of the leaf cells themselves. The latter strategy seems to have been more effective when adapting to the
most extreme saline habitats, but the former seems to have been manipulated more successfully during
directed selection by plant breeders. Even though the exclusion process is not perfect in most plants, and
some might argue that there is no sharp line separating the two categories, for simplicity’s sake in the fol-
lowing discussion, these two broad categories of plants will be referred to as excludersandincluders,re-
spectively.
Although both strategies are effective, there are some important differences between the two types
of plants. Those that exclude salts from the leaf cells are able to tolerate high levels of those salts in the
root environment but at the expense of reduced growth. That is, as cultivars or ecotypes within a species
are developed with increasing ability to exclude the sodium salts and thereby survive at increasingly
higher concentrations of those salts in their environment, growth is reduced to well below what it is in the
absence of those salts, even within the range of the relatively low salt concentrations characteristic of ir-
rigated agriculture. On the other hand, the plants that not only allow the salts to reach the leaves but con-
tain them at relatively high concentrations typically show increased growth with increasing level of ex-
ternal salinity within this range of salinities. It seems to me that this difference provides a unique
opportunity to address the question of what constitutes salt tolerance, a philosophy developed further later
in this discussion.
The excluders include virtually all crop plants and most, if not all, monocotyledonous halophytes,
plus many dicotyledonous halophytes. The includers are limited to a relatively small number of dicotyle-
donous halophytes. It is somewhat surprising that the domesticated plants whose salt tolerance has been
increased most successfully by selection and breeding are the monocotyledonous species, all of which are