V. SALT STRESS AND ROLE OF PROLINE
Water stress produces numerous metabolic irregularities in plants [44]. An increased proline concentra-
tion in water-stressed plants is due either to the inhibition of protein oxidation or to the breakdown of pro-
tein from its precursors [50]. In the Indian desert, 65 plant species were examined for proline content [51];
of these, 54 showed the presence of proline. These studies further revealed that some of the well-adapted
desert plants do not accumulate proline at all [51]. Proline accumulation in plants is not governed by the
environment but rather by the plants’ internal factors [52].
The accumulation of proline in plants is correlated with the extent of the water stress in the plant.
Mechanisms of regulation of proline accumulation during normal plant development are quite different
from those operating during the abiotic stress response [53]. Treichel [54] determined that the activity
level of delta-1-pyrroline-5-carboxylate reductase increases with progressive adaptation to NaCl stress.
The increase in proline concentration is associated with the activity of delta-1-pyrroline-5-carboxylate
reductase. It has been suggested that proline at a high concentration acts as a source of solute for in-
tracellular osmotic adjustment [20] and a storage compound for both nitrogen and carbon for utiliza-
tion in growth after stress [50]. The accumulation of proline upon dehydration related to water deficit
or increasing osmotic pressure is the most frequent and extensive response of saline plants [55]. In halo-
phytes, a positive correlation was found between the proline content and the NaCllevels in the
cell sap [54].
Venkatesalu et al. [56] grew Sesuvium portulacastrum, a salt marsh halophyte, at different salinity
levels. The total amino acids decreased with increased salinity. Proline and glycinebetaine levels in-
creased as the salinity level increased. Proline concentration in Cressa creticaincreased with an in-
crease in salinity levels [57]. The proline concentration increases greatly in the growing regions of
maize (Zea mays) primary roots at low water potentials, largely as a result of an increased net rate of
proline deposition [58]. Naidoo and Naidoo [59] reported that concentrations of proline in roots and
shoots increased significantly with salinity increase in Sporobolus virginicus. The proline content in-
creased under saline conditions over control in Vigna radiataandCicer arietinumas reported by
Muthukumaraswamy and Panneerselvam [60] and Muthukumaraswamy et al. [61], respectively. Joshi
and Khairatkar [62] observed that asparagine, aspartic acid, glutamic acid, phenylalanine, proline, glu-
tamine, glycine, serine, and threonine constituted major fractions of amino acids in 40-day-old
seedlings of Juncusspp. The concentrations of asparagine, aspartic acid, glutamic acid, and proline in-
creased while the others decreased in response to salinity stress. However, the total amount of amino
acids increased under saline conditions.
Garcia et al. [63] reported that in rice (Oryza sativa) proline either has no effect or in some cases pro-
motes the effect of NaCl on growth inhibition, chlorophyll loss, and induction of a highly sensitive marker
for plant stress, the osmotically regulated Sa/T gene. However, a high concentration (10 mM) of proline
prevents NaCl-induced chlorophyll loss in blades, preserves its integrity, and enhances growth. Proline
does not play an important role in salt tolerance in rice.
InCarthamus tinctorius, an increased proline content under NaCl saline conditions did not help to
maintain growth because productivity at flowering was less than the control value at all levels of NaCl
salinity [64]. Thus, it appears that an increased proline content in safflower under NaCl salinity helps in
survival and not in maintaining growth. However, increased proline at Electrical Conductivity (ECe) 5.0
mS cm^1 of NaCl helps in increasing productivity at maturity of this variety [64].
The data on proline accumulation in the leaves of halophytes in the Indian desert revealed that plants
that grow in saline areas exhibited higher proline during winter followed by summer and least in rainy
seasons (Table 1) [65]. All plant species at site I (Pachpadra salt basin) accumulated more proline as com-
pared with sites II (Didwana salt lake) and III (Jodhpur nonsaline), which may be due to the high salinity
of this habitat. Because site I is more saline than the latter two, it can be concluded that salt stress caused
more proline. Perhaps free proline contents play an essential role in survival of these plants.
Sangwan et al. [66] reported that accumulation of free proline in the calli derived from seedlings of
Cicer arietinumunder the influence of chloride salinity was more than the sulfate salinity and was in-
creased with increasing concentration of salts. This indicates that the proline production depends upon the
ions and the degree of stress and the plant species on which the stress is imposed, as reported by Yang et
al. [67]. Free proline accumulation under chloride salinity may also be attributed to the fact that oxidation
of proline was inhibited more by chloride ions than sulfate ions as reported by Stewart et al. [68]. One of
BIOLOGY AND PHYSIOLOGY OF SALINE PLANTS 567