also indicated that the results of Kemp and Cunningham [46] with Distichlisshowed that the reduced pho-
tosynthesis at increasing salinity is due to changes in stomatal frequency.
Some of the differences may be due to real differences among species, but some are also the result
of differences in experimental conditions and differences in the manner in which the data are expressed
[41,56]. It depends on whether the photosynthetic rate is expressed on a leaf area, leaf weight, or chloro-
phyll basis. Depending on which is used, the photosynthetic rate may be shown to be lower or higher [61].
F. Summary
It should be clear from this brief survey that it currently is no easier to explain the cause for the reduced
growth at suboptimal salinity that it is to explain the reduced growth at supraoptimal salinity. In the for-
mer case, however, the reason is largely due to the fact that not nearly as much research has been directed
at the question as in the latter case. Nevertheless, based on the limited database available, it does not seem
likely that the reduced growth at suboptimal salinity is due to insufficient turgor. Neither does it seem that
there is insufficient production of substrates for growth. In fact, with the few exceptions noted before, the
evidence seems primarily to indicate that the rate of photosynthesis at suboptimal salinity is higher than
at optimal salinity. If that is the case, and yet the plants are significantly smaller than those growing at op-
timal salinity, the obvious question that comes to mind is: Where is all the carbon going? Part of the prob-
lem may be that all of the photosynthetic rate measurements are instantaneous values, whereas the growth
data are integrated values. The total carbon fixed per day, as well as the total carbon lost to respiration
during the ensuing night period, needs to be determined under those salinity levels. A total carbon bal-
ance needs to be determined, in other words.
Partitioning of the photosynthate may be more important than the total amount produced. As de-
scribed here, there is very little information available that bears on that point, particularly as concerns the
plants growing at suboptimal salinity. There is almost no information available on hormone metabolism
at the various salinity levels. Because the reduced growth at suboptimal salinity seems to be due to an ap-
parent overall stunting of the plant, the problem may be one of growth regulation. There is a lack of in-
formation on this topic. Much needs to be done yet. That is clear.
Despite it being no easier to explain the cause of reduced growth at supoptimal salinity than it is to
explain the reduced growth at supraoptimal salinity, it is clear that there are physiological differences be-
tween the plants growing at those salinities. It is likely that the causes for the growth reduction in each
case are different. Thus, increased attention to the response of highly salt tolerant halophytes to subopti-
mal salinity is warranted.
III. CONCLUSIONS AND RECOMMENDATIONS
The decreasing availability of fresh water for agriculture, coupled with the increasing demand for plant-
based agricultural commodities, makes the eventual use of increasingly saline water in agriculture a cer-
tainty. There are abundant reserves of saline water within reasonable pumping distance from the surface
available in many areas of the world, especially in areas where successful crop production depends on ir-
rigation. The limitation to eventual use of that water to irrigate crops is the availability of sufficiently salt
tolerant crops. The long-term survival of agriculture in such areas is dependent on development of such
crops. Even though progress has been made in increasing salt tolerance of some crops over the years, the
pace of continued improvement and the ultimate maximum tolerance that can be achieved through con-
ventional breeding approaches may not be sufficient to fulfill this need. Thus, other approaches that offer
promise are required. The ability now to transfer genetic information between widely different types of
plants makes the possibility of moving traits associated with increased tolerance of environmental stresses
from alien genotypes into crop plants highly likely, ifthe required traits can be identified. Study of the
physiology of highly salt tolerant “wild” plants (halophytes) is a necessary but not sufficient step. For the
reasons described here, a completely different approach to studying salt tolerance in those plants is de-
sirable as an additionalapproach to this important problem. This paradigm shift in our conceptual ap-
proach to analysis of salt tolerance has the strong potential for opening up a new line of research that could
be highly productive. It could provide the means whereby the emerging molecular genetic techniques can
be applied to what heretofore has been viewed as one of the most intractable problems at the whole plant
level—stress resistance.
620 O’LEARY