Thus, it should not be surprising that there is not much information available about the cause for the
growthreductionat low salinity. Munns et al. [14] did acknowledge that maybe we should think in terms
of a growth reduction at zero NaCl rather than a growth stimulation at 50 to 200 mM. Their suggestion
for the cause of the difference in growth is increased water deficit at the lower salinity. They feel that “the
improvement in growth above 1 mM NaCl is most likely related to improved water relations of the leaves,
due to accumulation of Cl and Na.” This feeling that less growth of halophytes at suboptimal levels of
salinity is due to lack of sufficient solutes for generating turgor or even possibly to reduced root hydraulic
conductivity is widely shared [10,20,22].
C. Water Relations at Low Salinity
As already mentioned, there is a paucity of data on water relation parameters that allow one to compare
the water status of plants at suboptimal versus optimal salinity. As one might expect, the osmotic poten-
tial in leaves typically declines with increasing salinity of the growth medium (e.g., Refs. 23 and 24), al-
though Matoh et al. [25] found there to be no difference in osmotic potential of Phragmitesat 0 and 100
mM NaCl. The calculated turgor pressure in Atriplexfell to almost 0 by 9:00 AMin control plants, but in
the salinized plants it stayed positive all day long, albeit dropping slightly at midday [23]. However, when
Clipson et al. [26] measured turgor directly with a pressure probe in Suaeda,they found it to be about the
same at all salinities. Downton [27] measured osmotic and water potentials in Avicenniaand calculated
turgor pressure. In all salt treatments (10 to 100% seawater), turgor was about 0.8 MPa, but at zero salin-
ity, it was only about 0.2 MPa. Growth was less at zero than at all salinities. In young seedlings of Sal-
icorniagrown at low light in the laboratory, Stumpf et al. [28] found the turgor to be almost zero (0.02
MPa) when grown on nonsaline conditions, but at salinities of 170 and 340 mM, the turgor pressures were
0.53 and 0.99 MPa, respectively. On the other hand, Weeks [29] found that in greenhouse-grown Sal-
icorniathe osmotic and water potentials both parallelled the decline in salinity, with the result being al-
most no difference in calculated turgor pressure across the entire range from 17 to 1020 mM salinity. The
turgor pressure was close to 1.0 MPa at all treatment levels, but the growth at the lowest salinity level was
very poor, and the growth was high and no different between 170 and 1020 mM salinity. Some of the dis-
crepancies are undoubtedly due to methodology problems. It is difficult enough to obtain reasonably ac-
curate measures for water and osmotic potentials in any plants, so that the calculated turgor pressures can
be accepted with reasonable confidence, but with halophytes it is even more problematic, especially suc-
culent halophytes. Nevertheless, based at least partly on the direct measure of turgor with a pressure probe
inSuaedaby Clipson et al. [26] mentioned before, Munns [30] acknowledged that inadequate turgor is
probably not a likely cause for the lower growth of halophytes at suboptimal salinity.
Hydraulic conductivity of roots typically falls with salinity in both halophytes [23,31] and nonhalo-
phytes [32–34]. Thus it would not be expected to find that the hydraulic conductivity of halophytes at sub-
optimal salinity is less than at optimal salinity. However, there really have not been enough measurements
of conductivity at the appropriate salinity levels to allow any conclusive statements at this point. Never-
theless, Munns et al. [14] speculated that the relatively low values for root hydraulic conductivity in halo-
phytes, in general, coupled with the low root/shoot ratio in halophytes, in general, could account for the
low turgor, if it in fact occurs, in the plants at suboptimal salinity. The difficulty with such a scenario is
that at least in the few observations cited above, the root hydraulic conductivity in halophytes is usually
higher in the plants at suboptimal salinity, and also the root/shoot ratio is usually higher in those plants
[35,36]. Thus the plants at optimal salinity would be more likely to be at a disadvantage in this context.
It is clear that there is a need for measurement of all of these parameters in the same plants at optimal and
suboptimal salinity levels.
D. Growth Component Analysis
Most of the difference in dry weight production between optimal and suboptimal salinities is accounted
for by difference in shoot growth (e.g., Refs. 35 and 36), but there are some reports of root growth being
affected as well [27,37]. In addition to leaf size being reduced at suboptimal salinity, the leaf number can
be less [38]. However, Longstreth and Strain [39] found no differences in leaf area or specific leaf weight
inSpartinaat 10 versus 0.5 ppt salinity. The difference in total leaf area between the two salinities is
thought to be a major cause of the growth difference between the two by Munns et al. [14]. Osmond et al.
618 O’LEARY