similar to that of the averaged whole cell [88]. Conceptually, at least 90% of the plant’s K (when grown
under K-sufficient conditions) can be replaced by Na (if Na is equivalent in function to K) without af-
fecting the specific functions of K in the cytoplasm or the turgor of the cell.
Growth responses of halophytes to Na reflect the high salt requirement for osmotic adjustment
[6,48]. For the halophyte Salicornia herbacea[91], both Na and K are effective in promoting hypocotyl
elongation. Potassium has only 87% of the effect of Na in S. herbacea, although both ions are effective
in glycophytes but at much lower concentrations (about 10 mM as opposed to 100 to 200 mM in halo-
phytes). In halophytes, Na accumulation and its contribution to s reach a maximum [48]. There is evi-
dence that this reflects the ability of the tonoplast in the leaf cells to restrict Na efflux from vacuoles
[20,92,93]. In Ricinus communis, K initially contributes to vacuole and cell expansion but can be replaced
by Na following maturation of the root tissues [90].
Many halophytes osmotically compensate for high external osmotic potential by accumulating Na
salts, often NaCl from the environment [74,94]. The sap s of 2 to 5 MPa required to maintain tur-
gor in halophytes under seawater salinity (s2.3 MPa) can be accounted for by the 400 to 700 mM Na
and Cl concentrations in the sap [20,95]. Many members of Chenopodiaceae and halophytes show marked
selectivity for Na over K at low concentrations of both ions [96].
Biophysical functions of K in cells are nonspecific [89], and Na may at times be more suitable than
K [6]. With a limited K supply, Na, Mg, and Ca can replace K in the vacuole as an alternative inorganic
osmoticum [6]. Our studies with red beet have shown that Na can replace K for vacuolar function; nearly
95% of the plant’s K was replaced by Na (Table 3). For crops such as spinach and lettuce, the ability to
substitute Na for K is low (G.V. Subbarao et al., unpublished results). Thus, it appears that Na can fill the
“biophysical” function of K provided the plants have the ability to take up this element, translocate it to
the shoot, and compartmentalize it in their vacuoles.
In contrast, the location of major metabolic processes such as protein synthesis, photosynthesis, and
glycolysis within the cytoplasmic compartment places restrictions on the type and concentrations of so-
lute in this compartment [56,74]. Solutes that accumulate in cytoplasm must not disrupt metabolism and
must be maintained at concentrations that permit the various processes to proceed at favorable rates.
There is evidence that mechanisms exist for regulating cytoplasmic concentrations of a range of ions in-
cluding K, Na, H, and Pi[56,77,89,97–99]. For all eukaryotic organisms, the composition of cytoplasm
appears to be highly conservative during evolution. Despite wide variations in the concentrations of K,
Na, and Cl in the vacuolar compartment, the cytoplasm is characterized by 100 to 200 mM K, with little
potential for Na replacing K in cytoplasm [56,74,90].
SODIUM—A FUNCTIONAL NUTRIENT IN PLANTS 369
TABLE 2 Leaf Sap K Levels, Osmotic Potential (s), and Contribution of K to the Leaf Sap s for Red
Beet, Spinach, and Lettuce Grown Under 5.0 mM K Using Nutrient Film Technique
Leaf sap K Leaf sap s % contribution of K
Plant species (mM) (MPa) to the lamina sap
Red beet (Beta vulgaris) 435 0.97 96
Spinach (Spinacea oleracea) 242 0.91 57
Lettuce (Lactuca sativa) 244 0.98 53
Source: Subbarao et al., unpublished data.
TABLE 3 Effect of Replacing K with Na in Hydroponic Solution on Leaf Na Levels of Red Beet
% Na substituted for K
in nutrient solution K in sap Na K in sap % Na to total Na K
(from a total of 5 mM) (mM) (mM) in sap
0 414 415 0.2
75 83 222 63
95 25 237 89
98 13 294 96
Source: Subbarao and Wheeler, unpublished data.