C. Protection of Chloroplasts and Photosystem II from Na Damage
Glycine betaine is hypothesized to stabilize photosynthetic reactions, the structure of extrinsic proteins
of photosystem II (PSII) complex, and ATP synthesis under Na stress conditions [186]. Chloroplasts of
sugar beet can accumulate high levels of Na but still retain structural and functional integrity [142]. In
contrast, chloroplasts of bean leaves showed pronounced swelling when 75% of their K was replaced
by Na and consequently were unable to function [142]. This is consistent with reports of sensitivity of
leaf photosynthesis to salinity stress in beans [187]. Glycine betaine can prevent chlorophyll loss and
proteolysis and may improve resistance to drought or salinity [188,189]. In sugar beet leaves, most of
the GB is localized in the chloroplasts [2,43,190,191]. In spinach [43] and Suaeda[148], GB concen-
trations in chloroplast reached 300 mM under salt stress. Glycine betaine may also protect the oxygen-
evolving PSII complex against the inhibitory effects of NaCl [148,175,192,193] and freezing stress
[194].
In red beet, leaf photosynthetic rates are highly tolerant to internal Na. For example, leaf photosyn-
thetic rates showing no trend across internal Na levels ranging from 40 to 100 g kg^1 dwt (G. V. Sub-
barao and R. M. Wheeler, unpublished results). Stress may cause lesions in the reaction centers of PSII
[195,196]. Unlike the case of beets, when tissue K levels were replaced with Na in spinach, GB levels de-
creased linearly as tissue Na increased (Figure 7). Chlorophyll levels and photosynthetic rates are sensi-
tive to leaf Na in spinach (G. V. Subbarao and R. M. Wheeler, unpublished results).
In wheat, pretreatment with GB alleviated NaCl-induced stomatal and nonstomatal inhibition of
photosynthesis completely [197]. In vitro studies have shown that GB can protect thylakoid membranes
from freezing stress [194]. Glycine betaine–deficient maize lines are more sensitive to high-tempera-
ture stresses than GB-producing lines. Membrane stability, resistance to photoinhibition, and steady-
state yield of electron transport over PSII are adversely affected by high temperatures in GB-deficient
lines [198]. Williams et al. [199] reported that GB can increase the thermal stability of PSII. Several
studies have indicated that GB protects the PSII against high Na in vitro [175]. Also, transgenic Ara-
bidopsis thalianalines that overexpress GB maintain normal PSII function under high salt levels [200]
or cold stresses [201]. Betaine-treated spring and winter wheat seedlings exhibited a greater capacity to
prevent the closure of PSII reaction centers than control plants when subjected to cold stress [153]. It
is hypothesized that GB protects PSII from denaturation and inactivation (from Na) by forming layers
of preferentially oriented dipoles on protein surfaces that shield them both sterically and electrostati-
cally from chaotropic solutes (such as Na) [192]. Incharoensakdi et al. [172] reported that GB may pro-
tect ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity from high salts by acting at the
protein-water interface, decreasing the effects of excess salts on the enzymes and other macromolecules
[182,202].
894 SUBBARAO ET AL.
Figure 7 Relationship between leaf sodium and glycine betaine accumulation in spinach.