17
Sodium: A Functional Nutrient in Plants
G. V. Subbarao* and Gary W. Stutte
Dynamac Corporation, Kennedy Space Center, Florida
Raymond M. Wheeler
National Aeronautics and Space Administration, Kennedy Space Center, Florida
Wade L. Berry
University of California, Los Angeles, California
363I. INTRODUCTION
Sodium nutrition of plants has remained a fascinating and elusive topic despite several decades of inten-
sive research efforts, particularly during the 1960s and 1970s. Using Arnon and Stout’s [1] definition of
“essential nutrient” as modified by Epstein [2] as the standard to evaluate essentiality, Na has still not
been shown to meet their criteria to be an essential nutrient for all higher plants (certain types of C 4 plants
may be an exception). This is despite the fact that in some plants, internal Na tissue levels can become ex-
tremely high, nearly reaching K in tissue concentrations [3]. Sodium and K are chemically and struc-
turally similar monovalent cations. The hydrated Na ion has a radius of 0.358 nm, the K ion 0.331 nm;
thus, physically, there appears to be no size limitation for them to be taken up through the same ion chan-
nels [4]. The Na concentration in the earth’s crust is similar to that of K (2.8% vs. 2.6%) [5,6]. Sodium
levels are very high in many irrigation waters and in some cases approach 10 times that of K (Table 1).
Many halophytic plants have taken advantage of this close similarity between Na and K and have adapted
to grow in high-salt (NaCl) areas (see review by Glenn et al. [7]) where other less well adapted plants (i.e.,
glycophytes) are limited in growth because of the high salinity stress [8]. Many nonhalophytic plants can
utilize Na under conditions of limited K availability for a number of non–K-specific metabolic functions
[3,9,10]. Glycophytic plants such as beets, celery, turnips, and spinach can utilize Na to such a degree that
it is possible for farmers to substitute the relatively inexpensive Na as a fertilizer for K fertilizer [9].
One of the most noteworthy features of Na metabolism in plants is the remarkable difference among
species in accumulating or excluding Na from their tissues. Despite the the physical and chemical simi-
larity between Na and K, many higher plants have developed a very high degree of selectivity for the up-
take of K even in the presence of large amounts of Na. The same high degree of discrimination is also of-
ten found in the transport of Na from the roots to the shoot; thus, seeds and fruits of most plants are very
low in Na (see Chapter 44 on mechanisms of salinity tolerance by Subbarao and Johansen). This trait con-
serves K, very often a limiting resource for plant growth, but limits the transfer of Na from the soil through
plants on to animals, thus promoting soil salinization and Na deficiency in herbivores.
Sodium is an essential element for animals (including humans) and must be present in relatively
large amounts in the diet. Sodium, the principal electrolyte in animal systems, plays an important role
in maintaining the ionic balance of body tissues; its osmotic characteristics are utilized in the blood
*Current affiliation:Japan International Research Center for Agricultural Sciences, Ibaraki, Japan.