Among nonchenopods, tomato has been reported to respond positively to additional Na in the nutri-
ent medium [155]. For example, there was a 12% increase in the dry weight of tomato after the addition
of 1 mM NaCl in the nutrient medium [155]. Sodium alleviated symptoms of K deficiency and decreased
the critical foliar K concentration at the symptoms appeared [64]. Also, there is some evidence that potato
responds positively to Na. On the basis of a series of field trials on sandy soils, Na application improved
potato yields up to 6% for plots where adequate K was given and nearly 10% in plots where K fertiliza-
tion was not given [24]. Barley, oats, and carrots showed positive responses to supplemental Na at low K
levels [13,25,26], and alfalfa, flax, and celery showed a moderate response to Na at adequate K levels
[25,26].
A. Sodium Improves the Quality
For a few crops, Na has been reported to improve the quality of the product. For celery, Na improved re-
sistance to blight (Septoria petroselini appli), crispness, and thus market value [156]. Also, taste tests
demonstrated that addition of Na tended to reduce a strong celery flavor (improved flavor) [13,24].
Sodium can also improve the taste of carrots by increasing their sweetness [24].
VII. SODIUM AND POTASSIUM INTERACTIONS
Crops vary widely in their ability to substitute Na for K in their growth requirements. For instance, crops
that have halophytic ancestors or crops that evolved near seashores typically have a high potential to sub-
stitute Na for K. The following list of plants and their origins illustrates this phenomenon:
Beets—evolved in sandy soils near the sea in the Canary Isles, Persia, Babylon, and western India
(cited in Harmer and Benne [13])
Cabbage—found in rocky coastlines (cited in Harmer and Benne [13]) on the Isle of Lolland in Den-
mark, the island of Heligoland, Germany, the south of England and Ireland, the Channel Isles,
islands off the coast of Charente, France, and on the north coast of the Mediterranean near Nice,
Genoa, and Lucca [96]
Horseradish—known in Holland as sea-radish, grows wild in the salty soils in the east of Russia
Turnip—common in the sand on the seacoast in Sweden, Holland, and England
Celery—in damp places from Sweden to Algeria, Egypt, Abyssinia, and in Asia (cited in Harmer and
Benne [13])Harmer and Benne [13] and Harmer et al. [157] presented good surveys of the potential of Na sub-
stitution for K in plants. In addition to variation between species, benefits of Na uptake can vary be-
tween genotypes of the same species such as sugar beet [158,159], celery [160], red beet [3], and
tomato [161].
A. Sodium Replacement of Potassium
On the basis of their growth response to or tolerance for Na, crop plants have been classified into four
response groups (Table 5) or three levels of tolerance [162]. Plants that discriminate less against Na are
likely to have a higher ability to utilize Na for their monovalent cation requirements. Accordingly,
group 1 plants, which do not respond favorably to Na even under K deficiency, have little potential for
utilizing Na (Table 5). For group 2 plants, there is slight potential (about 10%) for replacing some K
with Na in these functions and thus tissue K with Na (Table 5). The largest potential for replacing K
with Na in these functions lies in group 3 and group 4 plants (Table 5). From their known levels of tol-
erance to external Na and the reported translocation of Na to the shoot and edible portion of the plant,
we have estimated the amount of tissue K in edible plant parts that is potentially replaced by Na with
minimum effects on growth (Table 6).
B. Influence of Sodium on Critical Potassium Levels
Sodium has a major role in determining critical K levels (the tissue K level at which 95% of the maxi-
mum yield can be achieved) [164]. For crops that have a capacity to substitute Na for K in metabolic func-
SODIUM—A FUNCTIONAL NUTRIENT IN PLANTS 373