al. [11], in which they emphasized a few species that do not show such a response rather than listing those
that do.
In general, monocotyledonous halophytes do not have growth optima at substantial salinity levels
(i.e., they show a steady decline in growth with any increase in salinity). There are a few reports in the lit-
erature (e.g., Refs 12 and 13) indicating that some monocots do have greater growth at salinity levels
greater than 50 mM NaCl, but the overwhelming body of experimental evidence supports the generaliza-
tion that monocotyledonous halophytes do not require substantial salinity levels for optimum growth
[11,14].
Thus this review and discussion are concerned primarily with dicotyledonous halophytes. There are
some problems with interpreting results of previous studies, however. In most cases, the intent was not to
determine optimum salinity levels for growth, so the intervals between imposed salinity treatments were
often large. Some halophytes have been reported to respond to extremely small amounts of Na, with
growth increases two- or threefold in response to 1 mM NaCl [15,16]. So if the lowest treatment level is
50 or 100 mM NaCl, for example, the response to this relatively high level cannot be distinguished from
the response due to satisfying the “need” for the trace amount of Na. However, unless specific measures
are taken to exclude Na, it is usually present in most nutrient solutions at such low levels due to contam-
ination. This should be verified by analysis of the base nutrient solution used, and if there is no Na pre-
sent, NaCl should be added to give 1 to 2 mM Na in the control solution. Further complicating interpre-
tation is the failure, in many cases, to correct for the weight of salt in the tissue. Halophytes
characteristically accumulate substantial quantities of salt in their shoots, easily 30 to 50% of the total dry
weight [17], so much of the difference in dry weight between plants grown in nonsaline conditions ver-
sus those grown in some substantial salinity can be due to the increased salt content in the latter. Never-
theless, when care is taken to account for the weight of salt in the tissue and other factors, it is clear that
there are many halophytes for which growth is maximum at salinity levels on the order of 50 to 200 mM
NaCl or equivalent (ca. 3000 to 12,000 ppm). As part of an intensive halophyte domestication program
[18], we screened 150 diverse species, in several families, and 57 (38%) of them had greater growth at
170 mM NaCl (ca. 10,000 ppm) than they did on nonsaline nutrient solution. We termed those euhalo-
phytes, and the others miohalophytes. A detailed report of 10 of each type, representing 19 genera and 10
families, is given in Glenn and O’Leary [19].
B. Cause of the Growth Reduction at Low Salinity
The emphasis in the reviews by Barbour [9] and Flowers et al. [10,11] was on growth stimulationbe-
tween 1–2 mM and 50–200 mM NaCl. Even in the later review by Rozema [20], the difference is still
viewed as a stimulation effect. This is not surprising because that focus is emphasized in virtually all
graphical comparisons of growth at various salinities. Growth is usually plotted as a percentage, with
growth at zero salinity equal to 100%. When this is done, the extreme halophytes always show growth
greater than 100% over the salinity range between 1–2 or zero NaCl and 50–200 mM NaCl. However,
when actual growth (as either rate or final biomass) is plotted, it is clear that what has happened is a
shift of the response curve, similar to what occurs in plants adapted to extremes of other environmen-
tal parameters, such as light or temperature. It is difficult, if not impossible, to generate a pair of curves
comparing adapted and nonadapted plants for salinity as has been done for light and temperature [21].
However, a comparison of relative growth rates (RGRs) for 10 euhalophytes and 10 miohalophytes (as
defined above) [19] showed that the average of the maximum RGR for the miohalophytes was 0.43 g/g
per week, and it occurred at zero salinity. The maximum RGR for the euhalophytes occurred at 180
mM salinity, and it was 0.42 g/g per week, almost exactly equal to the maximum RGR for the mio-
halophytes. The average RGR for the euhalophytes at zero salinity was 0.33 g/g per week. If these data
were presented by comparing them on a percentage basis, setting growth at zero salinity equal to 100%,
the miohalophytes would show a steady decline in RGR with increasing salinity, but the euhalophytes
would show a “stimulation” effect, having growth at 180 mM salinity equal to 127% of their RGR at
zero salinity. What actually happened is that the euhalophytes still had about the same maximum RGR
as the miohalophytes, but they were able to achieve it at a substantial salinity level (180 mM). The
“price” paid by the euhalophytes, however, is loss of the ability to maintain the same RGR at lower
salinity. That is, the entire response curve has shifted.
ADAPTIVE COMPONENTS OF SALT TOLERANCE 617