II. SODIUM AND ESSENTIALITY
According to Arnon and Stout [1], the following conditions should be met for an element to be consid-
ered as an essential nutrient:
The organism cannot complete its life cycle without it.
Its action must be specific and cannot be replaced by any other element.
Its effect on the organism must be direct.This set of criteria was expanded by Epstein [2] to include
The element is part of an essential compound, or metabolite.These criteria are based exclusively on the ecological considerations of survival and reproduction,
where high yield and biomass production may or may not be an important aspect or even associated with
nutrient essentiality. It is possible that some nutrients such as Na and Si may promote maximal biomass
production without meeting the preceding requirement for essentiality. In addition, not all metabolic func-
tions require a unique nutrient to function. Many essential metabolic processes can function equally well
with a number of different chemically and physically similar elements. It appears that it is possible for
similar elements such as Na and K to replace each other fully in certain nonspecific metabolic functions.
Thus, even though an element may function completely in an essential function (may be even more ef-
fectual than any other element), it would not be considered an essential nutrient unless it has a unique
function that it alone can meet. It could be argued, at least from agronomic considerations, that additional
levels of essentiality should be differentiated to denote nutrients that may be required for maximal yield
or are able to replace other nutrients in certain essential metabolic functions, reducing the critical level of
an essential nutrient.
Based on the expanded criteria of Arnon and Stout [1], Na has been shown to meet the criteria for
essentiality only for certain C 4 plant species, such as Atriplex vesicaria, A. tricolor, Kochia childsii,and
Panicum miliaceum[4,13–19]. In the absence of Na, these C 4 plant species grew poorly, showed visual
deficiency symptoms such as chlorosis and necrosis, or failed to form flowers. Supplying 100 M Na en-
hanced growth and alleviated visual symptoms. Even for extreme halophytes, in which Na is beneficial if
not essential, it is required only at the micronutrient level [20]. Even in C 4 families, Amaranthaceae,
Chenopodiaceae, and Cyperaceae, in which Na has been shown to be essential, the amount of Na required
is at a level typical of a micronutrient [21]. For C 4 species such as maize, sorghum, and sugarcane, Na has
not been shown to be essential [22].
Despite the fact that Na is not essential for many species, application of Na to the growth medium
has been shown to stimulate the growth of asparagus, barley, broccoli, brussels sprouts, caraway, car-
rot, chicory, cotton, flax, millet, oat, pea, rutabaga, tomato, vetch, wheat, cabbage, celeriac, horseradish,
kale, kohlrabi, mustard, radish, rape, celery, marigold, sugarbeet, red beet, Swiss chard, and turnip
[13,23–25]. Visual leaf symptoms of low Na on sugarbeet, marigold, and red beet appear as a dull dark
green color, rapid wilting in drought, and a tendency for leaves to grow out horizontally from the
crown. In some cases marginal intervenal scorch may develop, similar to that of K deficiency [24]. The
presence of Na tends to reduce K content in the leaves of sugarbeet and red beet. Several researchers
believe that Na promotes growth and vigor of sugarbeet, which results in increased yield [26,27]. How-
ever, the specific function of Na in the metabolic processes of these crops is still unknown. There is not
sufficient evidence to indicate that Na is essential for these crops. But this is open to further investiga-
tions because it appears that if Na is required, it is required at micronutrient levels, and that the older
analytical methods (before 1955 and the use of flame photometry or atomic absorption) were not suf-
ficiently sensitive to evaluate adequately very low concentrations of Na. It is unknown therefore
whether the deficient solutions employed in these early physiological experiments were completely free
of Na. In addition, there is such a large potential for Na contamination that every step must be checked
for Na. Thus, the question of essentiality is still open and needs to be readdressed with modern analyt-
ical techniques.
SODIUM—A FUNCTIONAL NUTRIENT IN PLANTS 365