excretion rates [17]. Table 1 shows ion excretion rates for three of the Chloridoid grasses: buffalograss
(salt sensitive), bermudagrass (moderately salt tolerant), and desert saltgrass (halophytic). Note that
desert saltgrass had Naand Clexcretion rates 32 and 34 times higher, respectively, than buffalograss.
Similar strong correlations between salt gland excretion rates, shoot Naand Clconcentrations, and
salinity tolerance were observed among three Chloridoid grasses in another study [34]. Relative order of
salinity tolerance again followed saline ion excretion rates, with Zoysia matrella(highly salt tolerant)
having an Naexcretion rate of 730 compared with bermudagrass (salt tolerant) at 660 and Zoysia japon-
ica(moderately salt sensitive) at 360 mol/g leaf dry wt/week, respectively. Sodium and Clexcretion
rates were negatively correlated with shoot concentrations but positively correlated with leaf salt gland
density and salinity tolerance among 57 zoysiagrass species accessions [64,74]. Excretions rates of vari-
ousZoysiaspp. reported range from 130 mol Na/g leaf dry wt/week in salt-sensitive Zoysia japonica
to 730 mol Na/g leaf dry wt/week in salt-tolerant Zoysia matrella, with gland densities ranging from
28/mm^2 leaf surface in salt-sensitive Zoysia japonicato 100/mm^2 in salt-tolerant Zoysia macrostachya
Franch. & Sav.
D. Ion Compartmentation and Compatible Solutes
In vitro studies have shown that enzymes of both glycophytes and halophytes have similar sensitivities to
salt, being inhibited at concentrations above 100–200 mM (approximately 8–17 dS m^1 ) [71,105]. There-
fore, salt-tolerant plants growing under saline conditions must restrict the level of ions in the cytoplasm.
As preceding data have illustrated, salt-tolerant grasses utilize inorganic ions for a large part of their os-
motic adjustment under saline growing conditions, as the ability to accumulate organic solutes on a whole
cell basis is metabolically expensive and therefore limited [66,77]. Salt-tolerant plants that successfully
accumulate saline ions for osmotic adjustment above concentrations of 100–200 mM do so by compart-
mentalizing them within the vacuole, which typically makes up 90 to 95% of a mature plant cell’s vol-
ume [106]. Evidence exists for salinity inducing a K/Naexchange across the tonoplast mediated by
Na/Hantiport activity, resulting in saline ion compartmentation in vacuoles [78,79]. Under these con-
ditions, the osmotic potential of the cytoplasm is maintained by the accumulation of organic solutes that
are compatible with enzyme activity, termed “compatible solutes” [107,108]. Under highly saline condi-
tions, relatively few organic solutes, including glycinebetaine, proline, and certain polyols and cyclitols,
can be accumulated in sufficient concentrations to adjust the cytoplasm osmotically without inhibiting en-
zymes [75]. Evidence exists for the cytoplasmic localization of these compounds [107,109,110]. Of these,
glycinebetaine and proline typically accumulate in grasses [111].
630 MARCUM
TABLE 1 Leaf Salt Gland Cland Na
Excretion Ratesaof Three Chloridoid Grasses: Ion
Excretion Measured in Plants Exposed to 200 mM
NaCl
Grass Cl Na
Buffalograss 39 36
Bermudagrass 191 163
Desert saltgrass 1267 1200
LSDb0.05 56 72
aExcretion rates in mol ion/g leaf dry wt/week.
bFishers Protected Least Significant Difference
TABLE 2 Leaf Sap Glycinebetaine and Proline Levels (mM) of grasses exposed to 0 and 300
mM NaCl
Glycinebetaine ProlineGrass 0 mM 300 mM 0 mM 300 mM
Buffalograss 9.0 18.9 1.7 5.9
Bermudagrass 6.1 38.5 0.7 2.7
Desert saltgrass 11.0 62.2 0.6 1.8
LSD0.05 0.6 4.1 0.8 1.0