tration of GB in sugar beet can reach 5 to 10% that of sucrose and conceivably reduce sugar yields indi-
rectly as the synthesis of GB requires about the same energy input as that of sucrose [164,210]. Photo-
synthate diverted to GB represents an appreciable cost in energy and carbon that is available neither for
storage as sucrose nor for plant processes that contribute indirectly to economic yield.
V. ROLE IN ADAPTATION TO DROUGHT AND SALINITY
Mechanisms that either permit resistance to cellular dehydration or minimize water loss contribute
to productivity in water-limited environments [211,212]. Osmotic adjustment, a well-defined adapta-
tion to water deficit, is associated with maintenance of protoplast volume, cell turgor [211], and avoid-
ance of lethal relative water content [213–215]. In some plant species, osmotic adjustment is par-
tially achieved by accumulation of GB [4,130]. Increase in cellular osmolarity, particularly in the
cytoplasm, results from the accumulation of nontoxic, osmotically active solutes and is accompanied
by either water influx or reduced efflux from cells to provide the turgor necessary for cell expansion and
growth [9].
Salt resistance may be correlated with the accumulation of GB and choline in a number of species
[3,216]. Sugar beet, which has higher resistance to salinity than spinach, also accumulates higher levels
of betaine [21,121,217]. There is some evidence that GB accumulation contributes to salt resistance in
Lophopyrum elongatum[218]. In sorghum, drought-resistant genotypes accumulated nearly three times
more GB than sensitive genotypes under well-watered and drought-stressed conditions [219]. Neverthe-
less, the adaptive value of betaine accumulation in salt-stressed and water-stressed plants is still specula-
tive and needs further investigation [107,164,168].
A. Exogenous Glycine Betaine Application for Improving Drought
and Salt ResistanceAlthough traces of GB are detected in legumes, e.g., bean (Phaseolus vulgarisL), pea, soybean, lupins,
and alfalfa [52], and other crops such as tomato, potato, rice, maize, and rape (Brassica napusL.), no
significant amounts of GB accumulate in these crops [4,220]. It is hypothesized that foliar application
of GB in nonaccumulating species can ameliorate the negative effects of drought and salinity on pro-
ductivity [221,222]. Foliar-applied GB rapidly penetrates leaf tissue and is quickly translocated to other
plant organs [223]. Also, surfactants (such as ‘kinetic’, ‘lus-50’, and ‘sito’) can enhance the penetra-
tion and uptake of GB in leaf tissue [223]. Foliar application of GB to potato and tomato is reported to
reduce crop failures in arid climates [221,223–228]. Fruit yield of tomato increased up to 40% by the
foliar application of GB when grown in saline soils or exposed to high temperatures in California
[223,226].
Exogenous GB application is reported to improve growth and yield of tobacco under drought condi-
tions [221,222]. Field experiments with wheat and barley have indicated that foliar application of GB at
18 kg ha^1 increased yield [227,229]. Rice growth was improved under salinity by foliar application of
GB [230]. Foliar application of GB to sorghum, barley, wheat, and soybean has improved growth under
field drought conditions [221–223,226,229]. Exogenous GB application has also resulted in improvement
of growth under drought and saline conditions for lupins (cited in Agboma et al. [222]), alfalfa
[62,208,231], cotton [232], and maize [221]. Thus, application of GB, which is a by-product of sugar beet
[226], environmentally safe, nontoxic, and water soluble, should be explored further for its potential use
as a management strategy in mitigating the negative impact of water stress and salinity on crop produc-
tion [226].
However, for some crops that do not naturally accumulate it, GB application had no effect or re-
duced the yield and hence may not be compatible in crops that do not naturally accumulate it [233].
Foliar application of GB on turnip, rapeseed, and spring cereals did not improve yield in drought stud-
ies in Finland [227]. In rape (Brassica napusL. var. oleiferacv. Samourai), the viability of GB-treated
leaf disks was substantially reduced when subjected to PEG stress. This was attributed to toxic effects
of GB on Rubisco and protein synthesis [234]. In contrast, GB application enhanced the viability of leaf
disks under PEG stress in spinach. This suggests that GB is not a compatible organic osmoticum for all
plants [234].
896 SUBBARAO ET AL.