[21] feel the same way. They analyzed the data from the studies of Gale et al. [40] and Kaplan and Gale
[23] and concluded that even though photosynthetic rate per unit leaf area was less at 72 mM NaCl than
at zero NaCl, the leaf area was greater enough that the total photosynthetic capacity per plant was in-
creased. Plotting of photosynthetic capacity per plant as a function of salinity over the range from 0 to 360
mM NaCl yielded a curve that closely paralleled the growth response curve over the same salinity range.
It should be noted, however, that Clipson [41] found that leaf area in Suaedadecreased with all additions
of salinity. Nevertheless, this may indicate the importance of photosynthate partitioning in determining
the growth response. It may be that an important difference between the plants at the two salinity levels
is how much photosynthate is reinvested in new photosynthetic surface. Unfortunately, there are not many
data available that bear on that question. A thorough growth component analysis, similar to that done by
Aslam et al. [42] in which they compared growth at optimal with growth at supraoptimal salinities, is
needed.
E. Photosynthesis at Low Salinity
Because growth depends on substrate availability as well as sufficient turgor, it is reasonable to ques-
tion the effect of the less optimum salinity on photosynthesis. The effect of salinity on photosynthesis
in halophytes has been investigated, and despite the fact that the focus usually was on comparing opti-
mum versus excess salinity, in some cases data were obtained for a range of salinity levels that enable
one to compare photosynthesis rates at less than optimum salinities with those at optimum levels. In
Salicornia,arguably the most salt-tolerant C 3 vascular plant, photosynthesis was higher at 3.2 MPa
osmotic potential [43] or 342 mM salinity [35] when measurements were made at several salinity lev-
els. However, Kuramoto and Brest [44] found that photosynthesis decreased at all levels of salinity, and
Pearcy and Ustin [45] found no differences from 0 to 450 mM. Kuramoto and Brest [44] found the
same response with Batis maritima, Spartina foliosa,andDistichlis spicata.Kemp and Cunningham
[46] also found a steady decrease in photosynthesis with increasing salinity in Distichlis spicata.
Longstreth and Strain [39] found no difference in photosynthesis at different salinity levels in Spartina
alterniflora.InAtriplex nummularia,photosynthesis was higher at leaf water potentials of 1.5 to
2.0 MPa than at either higher or lower water potentials [47], and in Sporobolus airoidesphotosyn-
thesis was higher at 1.0 MPa than at zero salinity [48]. In Lepochloa fusca,grown in the absence of
NaCl or with NaCl at 250 mM, photosynthesis was higher in the presence of added NaCl than when it
was absent at 32 or 39°C, but the reverse was true when the temperature was 19°C [49]. The data of
Hajibagheri et al. [50] showed that photosynthesis in Suaedawas lower at 170 mM than at either 340
or 680 mM salinity.
In those few cases where photosynthesis was found to be lower at the lower salinity, there was in-
sufficient information to determine whether the lower photosynthetic rates at the lower salinities (or
higher water potentials) were due to stomatal or nonstomatal effects. In fact, even in the cases where in-
vestigators have demonstrated reduced photosynthetic rates at excessive salinity levels in halophytes, the
picture is unclear. Some have attributed the reduced photosynthesis to reduced leaf conductance [51–54],
while others have concluded that photosynthesis was reduced independently of changes in stomatal con-
ductance [45,55,56]. The results of Schwartz and Gale [57] in which Atriplex halimusgrowing at 170 mM
NaCl had a much greater increase in growth in response to increasing CO 2 than that of plants growing in
nonsaline conditions are often cited as support for the view that the stomatal effect predominates at the
higher salinity. On the other hand, Demming and Winter [58] found that even isolated chloroplasts from
Mesembryanthemum crystallinumgrowing at different salinites showed reduced CO 2 fixation with in-
creasing salinity. They found that electron transport was much less sensitive than CO 2 fixation, suggest-
ing a direct effect of salinity on biochemical processes. Pearcy and Ustin [45] found that salinity did not
affect the initial slope of the CO 2 response curve, but it did affect the CO 2 -saturated photosynthetic ca-
pacity in Spartina.However,Salicorniaphotosynthetic capacity seemed to be relatively independent of
salinity.
Furthermore, in some cases it has been concluded that growth was reduced by some factors other than
photosynthesis, and the net effect was due to reduced photosynthetic surface rather than reduced photo-
synthetic rate per unit leaf surface [59–61]. Flowers [62] concluded that the general interpretation of the
available evidence is that photosynthetic rates per unit leaf area are decreased or little affected by in-
creases in salinity, and the rates per unit of chlorophyll appear to be either unaffected or to increase.He
ADAPTIVE COMPONENTS OF SALT TOLERANCE 619