On the basis of their tolerance or sensitivity, plants are commonly distinguished as halophytes or glycophytes. Glycophytes ("sweet" plants) tolerate only low concentrations of salt, while halophytes (halas = salt, salt plants) tolerate relatively high concentrations of salt (Flowers and Yeo 1986; Flowers and Yeo 1988). It was estimated by Flowers et al. (1986) that there were at least 800 species of halophytic angiosperms in more than 250 genera. This illustrates the point that there are many species of plants that possess the necessary features to enable them to grow and survive in a saline environment (Austin 1989).
Some halophytes possess glands and bladders, which actively excrete excess salts. Examples of these are Spartina, Armeria, Limonium and Glaux and Mesembryanthemum (Long and Mason 1983; Agarie et al. 2007). Each gland may excrete up to 0.5 ^l of salt solution in an hour. Obligate halophytes, for example Halogeton glomeratus, only grow in saline soil, and Salicornia europaea grows well in the presence of NaCl (Wainwright 1984; Aghaleh et al. 2009). For example, a salt bush (Atriplex halimus) indigenous to Australia, has developed a mechanism to control the Na+and Cl- ion concentration of its tissues. The epidermal bladders on the surface of the aerial parts of the plant are specialized cells that accumulate salt. As the leaf ages the salt concentration in the cell increases and eventually the cell bursts or falls off the leaf, releasing the salt outside the leaf (Troughton and Donaldson 1972).
In non-halophytes, resistance to salinity is commonly correlated with the ability to restricted entry of ions into the shoot. Their growth will be retarded when the salt content of the soil exceeds a rather low value. Glycophytes lack specialized anatomical features as well as tolerance to ions accumulated in the tissues. Typical of glycophytic dicotyledons is the uptake of ions from the external medium but the upward movement of these ions through the shoots is restricted by mechanisms of varying effectiveness (Greenway and Munns 1980; Dajic 2006).
In most halophytes osmotic adjustment results from the increase in concentrations of Na+ and Cl- in the tissue. In glycophytes, tolerance to salinity is related to the exclusion of these ions from tissues. This became clearer by comparing ionic concentrations in the tissues of salt-tolerant and non-salt tolerant cultivars of the same species. Many salt tolerant non-halophytes tend to restrict Na+ uptake and take up more K+ than do the less tolerant ones (Greenway and Munns 1980). For example, salt tolerant clones of Agrostis stolonifera contained lower Na+ in the shoots than a salt-sensitive inland clone (Ahmad et al. 1981). This showed that restricted Na+ uptake and maintenance of high Na/K ratios were features of salt tolerance in A. stolonifera, a result later confirmed by Hodson et al. (1981).
However, Na+ "exclusion and accumulation" have often been implicated, as mechanisms of salt-tolerance in non-halophytes, but this conclusion cannot be generalized. The wild maritime tomato species Lycopersicon chesmanii was a salt accumulator but the commercial species L. esculentum exhibited salt exclusion (Rush and Epstein 1976; Santa-Cruz et al. 1999: Rajasekaran et al. 2000).
The high concentrations of the ions in the tissues of halophytes suggest that their metabolic process may be tolerant to salt stress when compared to glyco-phytic metabolism. However, comparison shows the enzymes of halophytes and glycophytes have a similar degree of sensitivity to salt (Gibson et al. 1984). The sensitivity of enzymes from halophytes to salt, despite the presence of high ionic concentrations, suggests that plant cells have the capability to compartmentalize the toxic ions away from sensitive metabolic sites (Flowers et al. 1977). Most importantly, halophytes have developed 'controls' in Na+ influx strategy in roots to lower Na+ accumulation compared to glycophytes (Wang et al. 2006). Halophytes also have a capacity for osmotic adjustment in that these plants accumulate osmolytes such as glycine betaine and proline that maintain the osmotic balance disrupted by the presence of ions in the vacuole (Wang et al. 2004). Halophytes can maintain high metabolic activity even at inhibitory concentrations of intracellular Na+ and possess enhanced antioxidant mechanism (Fang et al. 2005). On the other hand, Jithesh et al. (2006) concluded that the antioxidant enzymes protected halophytes from deleterious ROS production during salt stress. It is clear that salinity induces oxidative stress in plants. Therefore, increases in malondialdelyde and lipid peroxidation are generally used as indicators for ROS production during or after salt stress conditions. Works with halophytes suggested that maintenance of malondialdehyde levels after salt stress and the induction of antioxidant enzymes confirmed the role of antioxidants in salt tolerance trait in halophytes (Parida et al. 2004; Fang et al. 2005). In these circumtances, induction of antioxidant enzymes was shown to protect halophytes against ROS, thus preventing lipid peroxidation during salt stress. This suggests that the antioxidant enzymes are essential components of an adaptive defense mechanism against salt stress in halophytes (Jithesh et al. 2006).
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