Salt Tolerance In Plants

Soil salinity is a major constraint that adversely affects crop yields in many parts of the world. About one billion hectares of land is adversely affected by salinity worldwide, due to flooding and ocean surge, soil salinity increases by 10% annually (FAO, 1988). High sodium ion (Na+) concentration which is the major cause of salinity is toxic to the plant cell metabolic activities as it imparts both ionic and osmotic stresses that adversely affects plant growth, disrupt the ionic equilibrium and elicits salt toxicity in the cells. Seawater contains about 40 grams per litre (g/l) of dissolved salts, mostly of sodium chloride sodium chloride). Beans and rice can tolerate about 1–3 g/l, and are considered glycophytes (as are most crop plants). At the other extreme, Salicornia bigelovii(dwarf glasswort) grows well at 70 g/l of dissolved solids, and is a promising halophyte for use as a crop. [3] Plants such as barley (Hordeum vulgare) and the date palm (Phoenix dactylifera) can tolerate about 5 g/l, and can be considered as marginal halophytes

Halophytes are the plants that grows and survive in environments where salt concentration is 200 mM NaCl or more (Flowers and Colmer, 2008). Adaptation to saline environments by halophytes may take the form of salt tolerance or salt avoidance. Plants that developed mechanism for reduced uptake of salt even though they occurred in a saline environment are described as facultative halophytes, while those that take up the salt but shows minimal effects are 'true', or obligate halophytes. Different plant species employ different mechanisms to mitigate the effect of hyper-concentration of Na+. Some halophytes accumulates proline for osmotic adjustment in response to salinity and abiotic stress (Kerep et al., 2002). Meanwhile, some halophytes adapts to high sodium concentration in the by regulating a wide array of genes or by evolving unique functional and structural mechanisms such as salt gland, thick suberin layer, salt-responsive gene etc. to withstand salt stress. For instance, to avoid high salt concentration intake, a plant species may complete its reproductive life cycle during periods (such as wet season) when soil salt concentration is low to avoid excessive soil sail uptake. In some cases, halophytes may maintain its internal salt concentration by excreting excess salts through leaves, or by accumulation of the salts in leaves that later die and drop off. Recently, salt responsive genes have been isolated and cloned from halophytes and introduced into a variety of plants to cope with high salt tolerance in such transgenic plants (Yadav et al., 2012).

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Aeluropus lagopoides, a member of the family Poaceae. The plant is a stoloniferous perennial C4 halophytic grass that grows in the coastal areas where it is incessantly exposed to high salinity from sea waters. The roots are adventitious, the leaves are small with epicuticularly wax which confers some adaptive characteristics on the plant. A. lagopoides could be utilize as a fodder for cattle and the grass has been investigated for molecular and physiological adaptation to environmental conditions (Mohsenzadeh et al., 2006). A. littoralis and A. lagopoides are reported to survive high salinity condition with low salt accumulation in dry grounded tissue (Bodla et al., 1995). The physiological and biochemical effect of water stress in two species of halophytic plants (A. lagopoides and A. littoralis) has been investigated (Vaziri et al., 2011).

Salt tolerance mechanism in A. lagopoides as revealed by Express Sequence Tag (EST) showed that the plant has ability to excrete excess salt (Mehta et al., 2005). Barhoumi’s (2006) study on A. littoralis for salt excretion reported that excreted salts were in favour of sodium salts while salts of potassium, calcium and magnesium were retained in plants. The amount of biochemical substances such as proline, soluble sugars, potassium levels, total amino acid, protein and chlorophyll contents have been used to monitor stress in higher plants (Ashraf and Harris 2004; Mohsenzadeh et al., 2006). It was noted that plants could accumulated relatively low amount of inorganic ions to achieve a water potential gradient between soil and plant (Gulzar et al., 2003). Several differentially expressed genes were identified using molecular tools to understand the involvement of genes in signalling, regulation and expression that controls osmotic adjustment processes in abiotic stress condition in plants (Mehta et al., 2005).

Although, studies have been conducted on the mechanisms involved in salt tolerance in halophytic plants, there is a need for further investigation on phenotypic and biochemical response of plants to salinity. The present work therefore considers the phenotypic and biochemical characterization of the response of halophytic plant Aeluropus lagopoides to sodium chloride (NaCl) of varying concentrations with the view to improve our current understanding of salt tolerance as it relates to morphology and biochemical constituents of the plant. The information generated may serve as a resource for further identification of novel traits related to salt tolerance in plants.