A. Intracellular Osmotic Adjustment
Plants rely on inorganic solutes (such as K or Na in vacuoles) to maintain s when subjected to water
deficits or high salts in the root zone [143,144]. Using organic solutes (such as sugars, betaines, or amino
acids) to regulate s in vacuolar volumes is too expensive metabolically as an effective strategy of adap-
tation [141,149]. Equivalent molar concentrations of inorganic solutes, such as NaCl or KCl, have twice
thes compared with GB (Figure 4a–c). For example, NaCl, KCl, and GB at 100 mM reduce s by 0.41,
0.42, and 0.23 MPa, respectively (Figure 4a–c). But, as noted earlier, inorganic solutes are potentially
damaging to the cytoplasm where organic solutes must be used for s maintenance [141,149]. Glycine
betaine is used in this capacity to maintain osmotic balance between the cytoplasm and vacuole [19]. Typ-
ical GB levels are between 50 and 200 mol g^1 dwt and account for 2 to 3% of the s of the leaf sap
[2,86,150,151].
Spinach leaves with a s of 2.0 MPa had a GB content of 320 mol g^1 dwt [127]; these GB
levels could contribute 0.07 MPa to the total s if distributed uniformly throughout the leaf. Simi-
larly, in red beet, leaf GB levels reached 104 mol g^1 dwt under moderately saline conditions at a s
of 1.39 MPa, accounting for 2.5% of the total s (G.V. Subbarao et al., unpublished results). Using
histochemical techniques, Hall et al. [152] demonstrated cytoplasmic localization of GB in leaf cells of
salt-grownSuaeda maritima. In wheat, GB accounted for 4.5% of the s of the leaf sap, but in chloro-
plasts, the GB concentrations were about 20 times higher and accounted for a major portion of the
chloroplast’ss [153]. For beets, concentrations of GB in cytoplasm ranged from 45 to 470 mM [154].
InAtriplex gmelini, nearly 320 mM GB levels were reported in cytoplasm, with only 0.24 mM GB in
the vacuole [155]. For many halophytic chenopods, GB is predominantly localized in the cytoplasm of
the leaf cells [152]. Concentrations of GB up to 300 mM were measured in isolated chloroplasts of
salinized spinach leaves [28,43]. At these concentrations, GB contributes significantly to chloroplast
OA, volume maintenance, and photosynthetic capacity at low leaf water potentials [43]. Many hy-
potheses about GB as an intracellular osmoticum are valid only if it is localized in the cytoplasmic com-
partment of the cell [9,156].
In contrast, Leigh et al. [157] reported that 26 to 84% of the total tissue GB is localized in the vac-
uole for red beet. The physiological significance of this finding is not clear. High vacuolar concentrations
might result from passive diffusion through the tonoplast as cytoplasmic concentrations increase [43], or
GB may be actively, and reversibly, transported to the vacuole in response to the level of osmotic stress.
In highly stressed cells, most of the GB can be located in the cytoplasm, but after removal of stress, GB
could possibly be transferred to the vacuole instead of being degraded.
- Threshold Stress for Glycine Betaine Accumulation
Glycine betaine accumulates in the cytoplasm only after a threshold turgor is reached [2,40,
137,156,158,159], suggesting that cytoplasmic s is initially regulated using inorganic solutes. For ex-
ample, if K salts and other solutes maintain a basal cytoplasmic osmotic pressure of 300 to 400 mOsm
GLYCINE BETAINE IN STRESS RESISTANCE 891
Figure 4 Osmotic potential of NaCl, KCl, and glycine betaine.