ABSTRACT
I. INTRODUCTION Plants have evolved two very different strategies in adapting to high levels of sodium salts in their environments. 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 former 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 following discussion, these two broad categories of plants will be referred to as excluders and includers, respectively.
