Beneficial Effects of Sodium

Growth Stimulation

Halophytes. The responses of halophytes and glycophytes to salinity have been reviewed many times (4,7,22–28). One feature of the response of halophytes, and, particularly the succulent halophytes predominantly from the family Chenopodiaceae, is that maximum biomass is achieved at moderate-to-high salinity (29–33). In other species, growth can be stimulated at low salinity, compared with the absence of salt (34), but this effect may depend on the overall nutritional status of the plant and the purity of the sodium chloride.

A part of the biomass of halophytes is the inorganic ions that they accumulate, especially in the shoots (23,26,27,30). It has been argued that, for a better assessment of plant productivity, only the organic portion of the biomass should be considered—that is, the ash-free dry weight (35–37). This consideration certainly reduces the apparent stimulation of ‘growth’ by sodium in the salt-accumulating, succulent euhalophytes, but a positive effect on ash-free dry weight is still apparent.

Interaction with Other Nutrients

The role of potassium in generating turgor can be fulfilled by sodium and to some extent, by calcium and magnesium, particularly at low concentrations of potassium (38-41). The estimated extent to which potassium can be replaced by sodium in the edible portions of crops varies from 1% in wheat (Triticum aestivum L.) and rice (Oryza sativa L.) to 90% in red beet (Beta vulgaris L.) (42). The interactions among cations in terms of uptake and accumulation rates are complex. The ability of low concentrations (^-500 �M) of sodium to stimulate potassium uptake when potassium concentrations are low does not appear to be of importance outside the laboratory (43). The extensive literature on the physiology and genetics of potassium-sodium interactions, especially related to membrane transport, is beyond the scope of this section and has been reviewed comprehensively by other researchers (44-50). Some evidence suggests that shoot sodium concentrations (altered by spraying sodium onto leaves) affects the transport of potassium to the shoots, or at least leaf potassium concentrations (51).

Interactions between sodium and other nutrients have been observed (52-54). Excessive sodium inhibits the uptake of potassium (43,55), calcium (56-67), and magnesium (53). A deficiency of calcium, or a high calcium ratio, results in enhanced sodium uptake. For most species, this calcium requirement is satisfied at a few moles per cubic meter of calcium in solution and is rarely detected in soils. It can become a problem in hydroponics if the calcium concentration in the nutrient solution is low, and no extra calcium is added. Maintaining low calcium ratios (as a general rule, not ⁺¹0:1 for dicots and 20:1 for monocots) will prevent this problem. Similar considerations apply to silicon (68-75).

Nitrogen nutrition modifies the effects of sodium on Chenopodiaceae such as goosefoot (Suaeda salsa L.) (76). Plants of this family accumulate large amounts of nitrogen in the form of nitrate and glycinebetaine (30,77-80). The interactions among salinity, nitrogen, and sulfur nutrition have been investigated in relation to the accumulation of different organic solutes in the halophytic grasses of the genus Spartina (81-83). Generally, adequate nitrogen nutrition is necessary to minimize the inhibition of growth caused by excess salt, but with some differences between the ammonium- and nitrate-fed plants (84-94).

Salinity may interfere with nitrogen metabolism in a number of ways, starting with the uptake of nitrate and ammonium (87,95). Under nonsaline conditions, nitrate is an important vacuolar solute in many plants, including members of the Chenopodiaceae and Gramineae. Under saline conditions, much of the vacuolar nitrate may be replaced by chloride, possibly releasing some nitrate-nitrogen for plant growth and metabolism. On the other hand, salinity can result in the synthesis of large amounts of nitrogen-containing compatible solutes such as glycinebetaine (and in a few cases, proline) and lead to the accumulation of amides and polyamines. Changes may occur at the site of nitrate reduction from the leaves to the roots, and hence changes in nitrate transport to the shoots. Since the latter is linked to potassium recirculation (96,97) and long-range signaling mechanisms controlling growth and resource allocation (98), the implications of such changes are wide ranging. The activity of nitrate reductase may also be affected by salinity. Although toxic ions can affect all aspects of nitrogen metabolism, little evidence suggests that nitrogen supply directly limits the growth of plants under conditions of moderate salinities (99).

In comparison with the other nutrients, the interactions between salinity and phosphorus have received relatively little attention (100) and depend to a large extent on the substrate (52,53). When investigating interactions between salinity and nutrients, one has to be aware of the effects of the substrate, the environment, the genotype-nutrient balances, the nutrient and salt concentrations, the time of exposure to salinity, and the phenology of the plant. These interactions are complex and cannot be comprehended adequately from one or two experiments.