SODIUM AS AN ESSENTIAL ELEMENT
Some uncertainty exists about the status of sodium as a nutrient, partly arising from the semantics of ‘essentiality’. The original criteria of Arnon and Stout  were that an essential element should be necessary for completion of the life cycle, should not be replaceable by other elements, and should be involved directly in plant metabolism. Sodium fails to meet all the three criteria for most plants and is generally regarded as a beneficial nutrient (see below). Only a few plants have any difficulty completing their life cycles in the absence of sodium, and these include some euhalophytes and some C4  species. The osmotic functions of cations in the vacuoles of plants growing at low salinity can be performed to some extent by any of the common cations. In particular, the monovalent alkali metals can perform similar functions in generating solute osmotic pressures and turgor .
The term ‘functional nutrient’ has been suggested for sodium, and, perhaps also for silicon and selenium . It might equally be applied to some of the rare earth elements that promote plant growth in certain circumstances . As Tyler has pointed out for the latter group, research on essentiality, even of sodium, has examined only a small proportion of the total number of species in the Plant Kingdom. Even so, it is clear that for most species, sodium is not essential in any sense.

BENEFICIAL EFFECTS
1
GROWTH STIMULATION
Halophytes. The responses of halophytes and glycophytes to salinity have been reviewed many times . 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 . In other species, growth can be stimulated at low salinity, compared with the absence of salt , 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 . 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 . This
 

Handbook of Plant Nutrition
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.

2 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 . 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.) . The interactions among cations in terms of uptake and accumulation rates are complex. The ability of low concentrations ( 500 μM) of sodium to stimulate potassium uptake when potassium concentrations are low does not appear to be of importance outside the laboratory . The extensive literature on the physiology and genetics of potassium-sodium interactions, especially related to membrane transport, is beyond the scope of this chapter and has been reviewed comprehensively by other researchers . 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 .
 Interactions between sodium and other nutrients have been observed  . Excessive sodium inhibits the uptake of potassium , calcium , and magnesium . A deficiency of calcium, or a high sodium/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 sodium/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 .
 Nitrogen nutrition modifies the effects of sodium on Chenopodiaceae such as goosefoot (Suaeda salsa L.) . Plants of this family accumulate large amounts of nitrogen in the form of nitrate and glycinebetaine . 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 . 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 .
 Salinity may interfere with nitrogen metabolism in a number of ways, starting with the uptake of nitrate and ammonium . 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 and long-range signaling mechanisms controlling growth and resource allocation , 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 .
 In comparison with the other nutrients, the interactions between salinity and phosphorus have received relatively little attention  and depend to a large extent on the substrate . 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.
EFFECTS ON C4  SPECIES
Sodium was reported to be necessary for the growth of some halophyte species ; notably, bladder saltbush (Atriplex vesicaria Heward, Chenopodiaceae). Sodium specifically stimulates the growth of Joseph’s coat (Amaranthus tricolor L., Amaranthaceae) , possibly by an effect on nitrate uptake and assimilation . Sodium appears to be essential for the C4 grasses such as proso millet (Panicum miliaceum L.), kleingrass (P. coloratum L.) and saltgrass (Distichlis spicata Greene)  and has been found to stimulate the growth of grasses such as marsh grass (Sporobolus virginicus Kunth) and alkali sacaton (S. airoides Torr.) in some studies . Subsequent work showed that this requirement was linked with the C4 pathway of photosynthesis  and specifically with pyruvate-Na   co-transport into mesophyll chloroplasts , a step that is necessary for the regeneration of phosphoenolpyruvate and the fixation of CO2. Not all C4 plants require sodium for photosynthesis or grow better when it is present . The C4 species of the NADP  -malic enzyme (ME) type have a different co-transport system for pyruvate that uses protons rather than sodium ions.
 In sorghum species (Sorghum L.), there is a specific effect of higher concentrations of sodium (and low concentrations of lithium) on the kinase that regulates the activity of phosphoenolpyruvate (PEP) carboxylase, the primary carbon-fixing enzyme in C4  and crassulacean acid metabolism (CAM) plants . The kinase also seems to be linked to the responses of PEP carboxylase to nitrate in C3  and C4  Alternanthera Forssk. species  . There was a report that sodium was required for CAM in Chandlier plant (Kalanchoe tubiflora Hamet) , but little further work has been published on this aspect, and no relationship occurs between CAM and halophytism . On the other hand, salinity and other stresses are known to induce CAM photosynthesis in  the  facultative  CAM  species,  ice  plant (Mesembryanthemum  crystallinum  L., Aizoaceae) .

2 TOXICITY OF SODIUM
Application of sodium to recently transplanted seedlings or cuttings runs the risk of uncontrolled bypass flow of water and sodium to the shoots through damaged roots. Hence sodium is often applied in the laboratory, greenhouse, or growth-chamber experiments after the plants have become established in the growing medium. For such situations, Munns  has described a series of events that occurs in most plants. At its simplest, these effects start with the initial osmotic stress caused by making the external (medium) water potential more negative. Subsequently, external inorganic ions are taken up and organic solutes synthesized for osmotic adjustment of the plant cells. Failure to
 

Handbook of Plant Nutrition
properly control the influx of inorganic salts results in the direct toxicity of high intracellular (particularly cytoplasmic) concentrations of ions or to osmotic imbalances within tissues such as the accumulation of salts in the apoplast of species like rice . Although this description has been challenged in detail regarding the implications for stress-resistance breeding  and the point at which specific ion effects become evident , it is still the best model of physiological responses to applied salinity. The same concepts, with modifications of timescale and phenology, can be useful in the crop field and in natural environments, although in both cases the severity of salinity (and other stresses) is subject to fluctuations that the laboratory experiment is designed to avoid.