excluders to some degree, but the plants that have been selected by nature to tolerate the most saline habi-
tats are the includers, not the excluders. Why has increased ability to tolerate high salt concentration in
leaf cells not been a target for plant breeders? The answer to that question and my feelings about the
prospects of using that approach successfully to increase salt tolerance in crop plants will also be ad-
dressed in the following discussion.
The typical approach to studying salt tolerance is to compare plants (both sensitive and tolerant
plants) subjected to excess salinity with plants not subjected to salinity, looking for responses to the added
salt. Examples of such responses are production of unique proteins or large amounts of presumed com-
patible osmotic solutes, such as proline and glycinebetaine. The difficulty with such an approach is that
it is difficult to distinguish the responses that are truly adaptive from those that are reflections of metabolic
lesions.
For example, even though there has been a substantial amount of research over the years devoted to
comparison of plant responses to growth-inhibiting salinity and nonsaline conditions, and the production
and accumulation of putative compatible osmotic solutes such as proline and glycinebetaine have been
investigated almost exhaustively in numerous plant species [1–4], the role of those solutes in salt toler-
ance has not yet been clearly demonstrated. The pathways, and control points therein, of synthesis and
degradation of such solutes have been studied in great detail, yet there is now growing concern whether
production of those solutes for osmotic adjustment is of any adaptive or other beneficial value [5,6]. That
is, they may be produced in large quantities as a result of disruptions in metabolism (metabolic lesions)
in response to stress, or they may simply accumulate as a result of a lower utilization of photosynthate in
stressed plants.
It seems appropriate to suggest that it is time to take a fresh look at the salt tolerance question and
consider some new approaches. For example, rather than continuing to focus on the plants that are not es-
pecially salt tolerant and trying to decide how to make them more tolerant, it may be more productive to
devote more effort to trying to find out what makes the highly salt tolerant plants so tolerant. The plants
to which I refer are halophytes, and one advantage they provide is the opportunity to compare growth at
suboptimal salinity with optimal salinity, an approach that is not possible with present crop plants or other
glycophytes. The optimum salinity for growth in crop plants and other glycophytes is zero, with decreased
growth as salinity increases beyond a few mol/m^3. In contrast, in many (but not all) halophytes, the opti-
mum salinity for growth has shifted to 50 to 200 mol/m^3 , with decreased growth occurring at both higher
and lower salinities. Thus, if one compares the responses of such plants to less than optimum salinity with
responses in plants grown at optimum salinity, it might be possible to distinguish the responses that are
truly adaptive from those that are the result of lesions or other types of damage. The hypothesis is that
adaptation in these halophytes involves some processes having optimum performance at a salinity level
well above zero, while at lower salinities these processes do not function as well. A process that does not
function as well in a plant growing at 50 mol/m^3 as it does in a plant growing at 200 mol/m^3 certainly is
not being altered by excess salinity and a priori would seem to be involved in the better growth of that
plant at the higher salinity. The challenge, then, is to identify those processes.
Unfortunately, most research so far has focused on the effects of excess salinity rather than inade-
quate salinity, but there are a few examples in which those studies did include the suboptimal salinity lev-
els as well. A quick survey of some of those studies may give us a hint about where the attention should
be concentrated in studies involving comparison of suboptimal and optimal salinity levels.
II. PLANT RESPONSES TO SALINITY
A. Effect of Salinity Level on Growth
That some halophytes grow better at an appreciable salinity level than they do in fresh water was ac-
knowledged by Chapman [7] and Waisel [8] in their comprehensive reviews of halophytes, although they
implied that this represented a minority of halophytes. Barbour’s [9] thorough review of the older litera-
ture specifically relating growth to salinity, however, revealed that such observations were fairly numer-
ous, and the classic review by Flowers et al. [10] listed several species that showed greater growth at salin-
ity levels equivalent to 50 to 200 mM NaCl than in nonsaline conditions. In fact, the general feeling now
that this is such a common response among halophytes is reflected in a subsequent review by Flowers et
616 O’LEARY