elongatum(Host) D.R. Dewey] [33], seashore paspalum (Paspalum vaginatumSwartz) [22,34,35], and
Kallar (brown beetle) grass [Diplachne fusca(L.) Beauv.] (syn. Leptochloa fuscaL. Kunth.) [36,37]. In
contrast, desert saltgrass is a halophyte [13,15]. Various studies have revealed other halophytic grasses,
includingSpartinaspp. [38,39], Sporobolus virginicus(L.) Kunth [40,41], and Sporobolus airoides
(Torr.) Torr. [17].
Salt-sensitive plants (glycophytes) and moderately salt-tolerant plants (mesophytes) generally have
a flat yield response to salinity prior to a threshold salinity level beyond which shoot growth declines. In
contrast, highly salt-tolerant plants often display stimulated shoot and root growth at moderate salinity
levels, followed by yield decline [13,16,18]. Increased shoot growth (relative to control) at moderate
salinity (100 mM NaCl, or 8 dS m^1 ) was evident in desert saltgrass (Figure 1). However, bermudagrass
and buffalograss displayed progressive shoot growth reductions at all salinity levels. Salt-stimulated
shoot growth has been observed in other salt-tolerant or halophytic grasses. Shoot growth peaked at 90
mM NaCl (8 dS m^1 ), then declined in Halopyrum mucronatum(L.) Stapf., a perennial grass found on
coastal dunes of Pakistan [42]. Shoot growth was stimulated with increasing salinity up to 25 mM NaCl
(2.5 dS m^1 ), then declined, in two of six Sporobolus speciesstudied (S. stapfianusandS. pellucidus)
[43]. Shoot growth of Sporobolus virginicusincreased up to 150 mM NaCl (12 dS m^1 ), then declined
[40]. However, Naidoo and Naidoo [44] reported no shoot increase with increasing salinity for this
species. In addition, shoot stimulation at low to moderate salinity has sometimes been reported in certain
salt-tolerant (although not halophytic) grasses, such as bermudagrass [45], seashore paspalum [18,22,34],
and St. Augustinegrass [34].
B. Root Growth Responses
Root growth stimulation (increased root mass, rooting depth, or both) in salt-tolerant grasses is typically
a more common, accentuated response to moderate salinity stress than shoot growth stimulation [16]. The
net result is generally an increase in root/shoot ratios, which may be a salinity tolerance mechanism to
counter low external water potential by increasing plant absorptive area [46,47]. Increased rooting depth,
relative to control plants, was observed in bermudagrass and desert saltgrass under salinity stress (Figure
1). However, relative rooting depth declined at high salinity for bermudagrass but not desert saltgrass.
Root weight (data not shown) also increased, being highly correlated with rooting depth (r0.83). In
contrast, rooting of buffalograss progressively declined with increasing salinity stress. Rooting decline
under salinity stress has been previously reported in buffalograss [48], and in other moderate to salt-sen-
sitive grasses, such as Kentucky bluegrass [49], bahiagrass [22], chewings fescue [50], and sideoats
grama [Bouteloua curtipendula(Michx.) Torr.] [17].
Root stimulation has been observed in a number of salt-tolerant as well as halophytic grasses. Root
dry weights increased linearly with increasing salinity up to 450 mM NaCl (35 dS m^1 ) in Sporobolus
virginicus, resulting in a root/shoot ratio of 2.2, relative to 0.5 (control) [40]. Blits and Gallagher [41] re-
ported a doubling in root mass of S. virginicusgrown in seawater relative to fresh water. Although root
growth (length) increased under moderate salinity stress, relative to control, shoot growth declined in
rhodesgrass (Chloris gayanaL.) [51], bermudagrass [52], and zoysiagrasses (Zoysia japonicaSteud. and
Z. matrella[L.] Merr.) [31].
III. PHYSIOLOGICAL ADAPTATIONS TO SALINITY
A. Ion Exclusion
It has long been accepted that the major causes of plant growth inhibition under salinity stress are osmotic
stress (osmotic inhibition of plant water absorption) and specific ion effects, including toxicities and im-
balances [53–55]. In comparison with salt-tolerant or halophytic dicotyledonous plants, monocots (in-
cluding Poaceae) tend to exclude saline ions from shoots, thereby minimizing toxic effects [56–58].
Saline ion exclusion from shoots was strongly associated with salinity tolerance among three grasses rep-
resenting the range of salinity tolerance present in the Poaceae (Figure 2). Chloride and Naaccumulated
to high levels in buffalograss shoots but was maintained at concentrations similar to those in the growth
media in bermudagrass and halophytic desert saltgrass shoots, particularly at high salinity. Salinity toler-
ance of other grasses has been related to saline ion exclusion. Salinity tolerance in Sorghum halepense
GROWTH AND PHYSIOLOGICAL ADAPTATIONS OF GRASSES TO SALINITY STRESS 625