D. Distribution of Glycine Betaine in the Plant
Although GB synthesis is largely confined to chloroplasts, GB is readily translocated to other tissues
[21,203]. In sugar beet, there is a linear relationship between GB levels in leaves and roots [21]. In red
beet, leaf GB levels are linearly related to petiole and root GB, indicating that GB is readily translocated
from leaves to other parts of the plant (Figure 8a and b). Thus, the distribution of GB in red beet appears
to be fairly uniform in various plant parts (leaves, storage root, and fibrous roots), although slightly
higher concentrations are found in leaf lamina (G. V. Subbarao and R. M. Wheeler, unpublished results).
In other chenopods, GB accumulates primarily in mature leaves under salt stress, but under nonstress
conditions, GB is found mostly in young expanding leaves [34,127,203]. In barley under water stress,
GB accumulates mostly in mature leaves but is then translocated to the expanding leaves upon rewater-
ing [104,203].
E. Stability of Glycine Betaine in Plants
Glycine betaine constitutes essentially an inert end product that is not metabolized by the plant
[4,204,205]. Glycine betaine is not degraded upon relieving the stress in wheat [206,207], barley
[29,128,203], sugar beet [21], alfalfa [208], tobacco [118], or spinach [191]. But GB levels may decline
once stress is removed due to growth in the absence of subsequent synthesis [204,207]. Because of this
stability, GB content may serve as a cumulative index of the internal water status [204], with potential ap-
plications in both plant breeding and crop management.
F. Physiological Costs Associated with Glycine Betaine
AccumulationEnergetically, synthesis of GB is expensive for plants as it contains N and its biosynthesis requires
NADPH [14]. In barley leaves, GB can represent nearly 2% of the Kjeldahl nitrogen of the tissue
[204,209]. In GB accumulators (such as halophytes), where concentrations can reach 500 to 1000 mM,
betaine represents 20 to 30% of the total N of the plant [2,15,16].
Conventional genetic approaches provide a glimpse of possible benefits and costs of GB accumula-
tion. Homozygous GB-accumulating (Bet1/Bet1) maize plants exhibited less growth inhibition than near-
isogenic GB-deficient (bet1/bet1) plants under salinity stress. However, high GB accumulation in maize
is associated with a 5% reduction in grain yield under well-irrigated field conditions. The molar concen-
GLYCINE BETAINE IN STRESS RESISTANCE 895
Figure 8 (a) Relationship between lamina and petiole glycine betaine levels of red beet. (b) Relationship be-
tween lamina and root glycine betaine of red beet.