VIII. SODIUM, OTHER PERSPECTIVES
A. Sodium Tissue Levels
Regardless of Na supply, most crops translocate very little Na to reproductive structures such as seeds,
fruits, or storage roots, which are the edible portions of many staple crops [167]. This is the case for ce-
reals such as wheat and rice, fruit crops such as tomatoes, tubers such as potatoes, and roots such as sweet
potatoes. The main reason for this low Na concentration in the reproductive or storage organs is that they
are fed mostly through phloem, where there is heavy discrimination against Na translocation.
In contrast, the edible portions of leafy salad crops are vegetative structures that maintain a heavy
xylem flow, thus allowing Na to accumulate in their tissues. If plant nutrient supplies are modified to fa-
vor Na uptake (e.g., by reducing K supply in the presence of Na), then it is possible for some leafy crops
to accumulate Na in edible structures at reasonably high levels without adversely affecting productivity
or quality. This approach could be adapted for greenhouse crops such as lettuce or spinach, where a sig-
nificant portion of the K salts could be replaced with relatively inexpensive Na salts. Also, for field crops
such as beets, celery, and radish, there is a large potential to replace K with Na. Sodium levels of the ed-
ible portions could also be increased by substituting some Na fertilizers for K fertilizers [3,10,168].
B. Genetic Variation in Tissue Sodium
As noted previously, there are genotypic differences within species for Na uptake and partitioning to ed-
ible structures, namely in sugar beet, red beet, and celery. Some wild relatives of the cultivated tomato
translocate substantially higher levels of Na to fruits (M. Shannon, personal communication). Some of the
tomato lines derived from crosses of these wild relatives accumulate nearly 10 times higher Na levels in
the fruits than the cultivated tomatos growing under identical conditions (Figure 5).
There are considerable differences among forage species in the contents of Na in their shoots, even
when the supplies to their roots are similar [50,60]. Such differences could be due to selectivity during
absorption [50] or to differences in transport to shoots [49]. Identification and testing of germplasm
adapted to low-K conditions could be useful in understanding the physiological roles of K as well as in
providing breeding materials for forage production on soils low in K and where K fertilizers are costly or
unavailable [169]. The Na content of forage and pasture crops is also important in animal nutrition [4].
376 SUBBARAO ET AL.Figure 5 Sodium levels of tomato fruits derived from the cross between L. esculutum X L. cheesmani. (From
M. Shannon, unpublished data from USDA Salinity Laboratory, Riverside.)