Total leaf NaCllevels exceeded 200 mM in all three Chloridoid grasses grown at moderate to
high salinity (Figure 2), necessitating vacuolar ion compartmentation for survival. Glycinebetaine levels
increased under salinity in all grasses, reaching highest levels (62 mM) in desert saltgrass (Table 2). Al-
though proline concentrations also increased under salinity, maximum levels occurred in salt-sensitive
buffalograss, reaching only 6 mM. Assuming that glycinebetaine and proline are located in the cytoplasm
(see earlier), which occupies 10% of the total cell volume, the contributions of glycinebetaine and proline
to cytoplasmic osmotic adjustment can be calculated (Table 3). Glycinebetaine made substantial contri-
butions to cytoplasmic osmotic adjustment in desert saltgrass (73%) and bermudagrass (39%) only. In
contrast, proline contributions were insignificant in all grasses.
Shoot sap glycinebetaine concentrations in grasses grown at 200 mM NaCl (17 dS m^1 ) ranged from
2 mM in centipedegrass [Eremochloa ophiuroides(Munro) Hack.] to 89 mM in bermudagrass [34]. St.
Augustinegrass, seashore paspalum, Zoysia japonica, and Zoysia matrellawere intermediate (in order
from low to high) in shoot glycinebetaine concentrations. Given the same assumptions as before, glycine-
betaine made substantial contributions to cytoplasmic osmotic adjustment in all grasses except cen-
tipedegrass. Centipedegrass stands alone among these grasses as being very salt sensitive. As before, pro-
line contributions were too small to contribute to cytoplasmic osmotic adjustment, with the possible
exception of bermudagrass, although contributions would still be minor. In the halophyte Sporobolus vir-
ginicus, glycinebetaine accumulated to 126 mM in shoots of plants grown at high salinity (450 mM NaCl
or 35 dS m^1 ), possibly contributing 93% of total cytoplasmic osmotic adjustment [40]. In contrast, pro-
line levels were 11 times lower (11 mM) at this salinity, forming an insubstantial contribution to cyto-
plasmic osmotic adjustment. Other studies involving Sporobolus virginicussupport the importance of
glycinebetaine as a compatible solute relative to proline. Quaternary ammonium compounds (predomi-
nately glycinebetaine and possibly other related betaines) accumulated to 48 mol g^1 dry weight in
shoots of Sporobolus virginicusgrown in seawater, while proline levels reached only 1.6 mol g^1 dry
weight [41]. Similarly, proline levels were insufficient to contribute significantly to cytoplasmic adjust-
ment of Sporobolus virginicusgrown in 80% seawater [44]. In lines of tall wheatgrass grown at 20 dS
m^1 total salinity, glycinebetaine accumulated to 45 mol g^1 fresh weight in shoots, compared with
only 1 mol for proline [70].
Whereas glycinebetaine concentrations under salinity were positively correlated with salinity toler-
ance among seven Chloridoid grasses, proline concentrations were negatively correlated, suggesting that
glycinebetaine, but not proline, acts as a compatible solute [17]. Although both compounds have tradi-
tionally been considered compatible solutes, more recent evidence has favored the role of glycinebetaine.
For example, (1) glycinebetaine is excluded from the hydration sphere of enzyme proteins and thus tends
to stabilize their tertiary structure [112], (2) corn (Zea maysL.) mutants lacking a critical enzyme for
glycinebetaine biosynthesis also lack salt tolerance [113], and (3) exogenously applied glycinebetaine has
enhanced the salinity tolerance of glycophytes such as rice (Oryza sativaL.) [114]. In contrast, proline
accumulation has been considered by some investigators merely a result of plant injury because of its uni-
versally rapid appearance following any type of stress [115,116].
IV. SUMMARY
The Poaceae, represented by over 7500 species, show extreme range in salinity tolerance, from salt sen-
sitive to extremely salt tolerant (halophytic). In this chapter, the range of salinity tolerance and physio-
GROWTH AND PHYSIOLOGICAL ADAPTATIONS OF GRASSES TO SALINITY STRESS 631
TABLE 3 EstimatedaContribution to Cytoplasmic Osmotic Adjustment of
Glycinebetaine and Proline, in mosmol kg^1 (Osml) and as a Percentage (%) of Total
Osmolality, of Plants Grown at 300 mM NaCl
Glycinebetaine ProlineGrass Osmol % Osmol %
Buffalograss 209 9.7 59 2.8
Bermudagrass 378 39.2 27 2.7
Desert saltgrass 625 73.7 17 2.0
aEstimate assumes glycinebetaine and proline are located in the cytoplasm, constituting 10% of total
cell volume, with an osmotic coefficient of 1.0 for each compound.