ABSTRACT
Sodium nutrition of plants has remained a fascinating and elusive topic despite several decades of intensive 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 C4 plants may be an exception). This is despite the fact that in some plants, internal Na tissue levels can become extremely high, nearly reaching K in tissue concentrations [3]. Sodium and K are chemically and structurally 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 channels [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].
