the shoot. Certain morphological features, such as increased width or early development of casparian
strips [100] or formation of a double endodermis [101,102], have been reported to develop under high
evaporative demands, thus minimizing the passive influx and bypass flow of Na and Cl ions into the
xylem. We are not aware of such anatomical changes reported in any crop species under saline conditions.
It may be worthwhile to examine genotypes that show high salinity tolerance for such kinds of adaptive
features. Because water use is tightly linked to ion uptake and selectivity, the morphological and physio-
logical traits that increase water use efficiency (WUE) in a given genotype could have a role in deter-
mining salinity tolerance [93]. In rice, genotypes that showed higher WUE also had a higher level of salin-
ity tolerance [93].
III. ORGANIC SOLUTE ACCUMULATION
A wide variety of organic solutes have been reported to accumulate in plant tissues during water and salt
stress and are hypothesized to have functions including cytoplasmic osmotic adjustment, protecting cy-
toplasm and chloroplasts from sodium damage, and stabilizing proteins and membrane structure
[103–105] (this volume, Chapter 45 on glycine betaine for further discussion). The chemical nature of the
compatible solutes varies from one taxonomic group to another, but most are derivatives of polyols or ni-
trogen dipoles [39] (Table 1). Osmotic adjustment by the plant promotes turgor maintenance and is thus
associated with adaptation to both high soil salinity and low soil moisture [5,106] (see Chapter 45). Com-
patible solutes are an important factor in the osmotic balance of the cytoplasm under salt stress [33],
where sodium salts are sequestered to play a complementary osmotic role in the vacuole [4,33,35] (see
Chapter 17). However, this is considered to be a halophytic mode of osmoregulation [2,107], which is en-
ergetically more efficient than overall osmoregulation by organic solutes [6,108], a common feature of
glycophytes [103].
These organic solutes may comprise common metabolites such as sugars, amino acids such as pro-
line [109–112], and organic acids such as prolinebetaine [113] and other aliphatic quaternary ammonium
compounds [114] (see Chapter 45). There is evidence that solute accumulation is a regulated process and
not merely the result of a discrepancy between the sensitivity of the growth process and photosynthesis
to stress [115]. Nevertheless, metabolites such as glucose and sucrose accumulate in tissues whose growth
has been inhibited by stress [103].
The type of stress would determine which compounds act as osmotic solutes [116]. In grain sorghum,
betaine accumulates only under moderate levels of salt stress, not under water stress [116]. However, in
crops such as wheat, barley, and rye, betaine accumulates under water stress as well as salinity stress
[116]. Nevertheless, salinity is reported to be the more effective stimulator of betaine accumulation [117]
(see Chapter 45). In barley, more glycinebetaine is accumulated under gradual stress, but proline is the
predominant solute under sudden stress [118].
A. Role in Osmoregulation
High concentrations of organic solutes in the cytoplasm could contribute to the osmotic balance when
electrolytes are lower in the cytoplasm than in the vacuole [109,119]. These compatible solutes could also
act as a nitrogen source [120] or protect membranes against salt inactivation [121,122]. These proposed
activities may complement each other within the integrated metabolic and ontogenic pattern of a particu-
lar species [123].
Under saline conditions, the large quantities of Na, K, and Cl and other ions that are translocated
to the shoot and contribute to the osmotic adjustment are believed to accumulate mainly in the vacuole
after reaching threshold levels in the cytoplasm [8]. This concentration of inorganic ions could be con-
sidered as a threshold level at which accumulation of organic solutes such as proline, betaine, or other
compounds begins in the cytoplasm, thus maintaining the intracellular osmotic balance between cyto-
plasm and vacuole [116]. For instance, in wheat, proline began accumulating when NaK exceeded a
threshold value of 200 mol/g fresh weight [124]. Also, in grain sorghum, a moderate level of salt stress
(0.4 MPa or more) is required to induce a significant betaine concentration [125,126]. Further studies
are needed to determine the extent to which this threshold level varies among genotypes of a given
species.
864 SUBBARAO AND JOHANSEN