kg^1 , then GB accumulation is necessary only if vacuolar osmotic pressures rise above this level
[156,157]. Also, the threshold K level for GB may vary depending on the external K concentration, the
type of stress, and genetic factors. In yeast (Saccharomyces cerevisiae), two plasma membrane proteins,
Sln1 and Sho1, operate as sensors for turgor loss under mild osmotic stress and these proteins activate
several defense mechanisms that include Na efflux pumps [160]. It is not known whether such sensor pro-
teins are involved in the activation or acceleration of GB synthesis in plants. Similarly, abscisic acid
(ABA) synthesis, stomatal closure, and subsequent physiological responses do not occur until this thresh-
old is met [158,161].
- What Triggers Glycine Betaine Production? (Osmotic Stress or Internal Sodium
Levels?)
Internal Na levels alone can trigger GB production in red beet when tissue Na levels are increased with-
out subjecting the plants to osmotic stress [86]. In contrast, tissue Na levels alone do not trigger GB pro-
duction in spinach under similar conditions (G.V. Subbarao and R.M. Wheeler, unpublished). Several re-
ports indicate that spinach accumulates GB when subjected to high levels of NaCl salinity, suggesting that
osmotic stress may be the physiological trigger [43]. Glycine betaine concentration and BADH activity
increased severalfold in spinach leaves under salt stress [29,128,162]. Also, GB accumulation appears to
be dependent on the rate at which water stress develops; when water stress develops gradually, GB accu-
mulates, but when water stress develops abruptly, it does not [30].
- The Relationship Between Glycine Betaine and the Solute Potential (s) of the Leaf
Sap
Glycine betaine accumulation is linearly related to s in sugar beet (r^2 0.99), although GB’s contri-
bution to the total leaf s is 5% [27]. In other crops, nearly linear relationships between leaf s and
GB levels were observed when plants were exposed to NaCl salinity or water deficits [21,163–165]. In
barley, isopopulations that differ nearly twofold in unstressed GB content, it was shown that the high-GB
population maintained 0.1 MPa lower s at 300 mM NaCl. For several halophytic species belonging to
the family Chenopodiaceae, GB levels were correlated with sap s under salinity [163]. In red beet,
turgid leaf s is linearly related to GB levels (Figure 5). Collectively, GB levels appear to be a genetic
phenomenon and modulated by the environment. In addition, GB accumulation might be tightly linked to
other loci that regulate the accumulation of other organic and inorganic solutes, which together determine
the water relations of plants [43].
892 SUBBARAO ET AL.
Figure 5 Relationship between glycine betaine and turgid osmotic potential of leaf sap in red beet.