see Chapter 44 by Subbarao and Johansen). Physiological mechanisms that regulate Na levels of these na-
trophobes are addressed elsewhere (see Chapter 44 by Subbarao and Johansen).
The capability to translocate significant amounts of Na to the shoot even among natrophiles vary
widely among plant species, with halophytes representing the extreme on the high end. The general as-
sumption about Na tolerance in plants is that they compartmentalize the absorbed Na in vacuoles and use
it as an inorganic osmoticum in place of or along with K. It is widely believed that the cytoplasm does not
tolerate high levels of Na as it interferes with normal metabolic functioning [53]. This seems to be true
for both natrophiles and natrophobes, with the only difference between these groups being the ability of
natrophiles to compartmentalize the absorbed Na effectively. Natrophobes that have limited or no ability
to compartmentalize Na spend substantial amounts of energy in preventing Na from entering the plant
(see Chapter 44 by Subbarao and Johansen). These Na-excluding mechanisms are a drain on the plant’s
carbon and energy resources and thus natrophobes can suffer substantial growth reductions or death when
grown in high-Na environments [8] (see chapter by Subbarao and Johansen).
Plant cell cytosol typically contains about 100 mM K and rarely tolerates Na levels above 20 mM
[54–56]. Enzymes isolated from salt-sensitive Phaseolusand salt-tolerant AtriplexandSalicorniaare
equally sensitive to NaCl when bioassayed [53]. This was established for four different enzymes, which
included the rather salt-sensitive aspartate transaminase as well as salt-tolerant glucose-6-phosphate de-
hydrogenase [53]. Furthermore, growth of PhaseolusandAtriplexin saline cultures failed to alter the spe-
cific activity or NaCl sensitivity of the enzymes [53]. This indicates that even natrophilic species, such as
AtriplexandSalicornia, cannot tolerate high Na levels in their cytoplasm. They maintain this relatively
constant cytoplasmic level of Na by compartmentalizing high levels of Na in their vacuoles [53]. This is
an important survival feature of halophytic plants under saline conditions [57] (see Chapter 44 by Sub-
barao and Johansen).
Most crop plants belong to the category of glycophytes. Some crop plants such as sugar beet, red beet,
Swiss chard, celery, and turnip have a substantial ability to translocate Na to the shoot. For red beet, a
considerable buildup of Na in the tops can occur whenever Na is present in the nutrient solution [3]. Our
studies indicate that Na is absorbed from the nutrient medium and translocated to the shoot relatively
freely in red beet but not in either spinach or lettuce (G.V. Subbarao and R.M. Wheeler, unpublished).
The extent to which Na is taken up by plants is influenced by other nutrients, particularly K and N
[58]; however, this does vary with species [59,60]. The rate of transpiration can influence uptake and
movement of some ions in plants [61]. For example, Pitman [62,63] showed that higher rates of transpi-
ration increased the ratio of K to Na reaching the leaves of barley and white mustard (Sinapis albaL.). In
tomato, the ability of roots to exclude Na from the rest of the plant decreased rapidly as the level of K in
the nutrient solutions fell [64].
V. SODIUM FUNCTION IN METABOLISM
A. C 4 Metabolism
In the Calvin cycle of C 4 plants, CO 2 is concentrated in the bundle sheath cells. An extensive flow of
metabolites between mesophyll and bundle sheath cells is required to operate this CO 2 concentration
mechanism [4]. Sodium deficiency has been reported to impair this conversion of pyruvate to phospho-
enolpyravate (PEP), which takes place in the mesophyll chloroplasts [4]. In certain C 4 species, e.g., Ama-
ranthus tricolor, the C 3 metabolites alanine and pyruvate accumulate, whereas C 4 metabolites PEP,
malate, and aspartate decrease under Na deficiency [21]. Sodium deficiency leads to a reduction in pho-
tosystem II (PSII) activity and ultrastructure changes in mesophyll chloroplasts but not in bundle sheath
chloroplasts of A. tricolorandKochia childsii[65,66]. Resupplying Na to these species restored PSII ac-
tivity [22] in thechloroplast. In C 4 species, nitrate assimilation also appears to be confined to the meso-
phyll cells. For A. tricolor, nitrate reductase activity is substantially decreased in leaves of Na-deficient
plants but is restored after resupplying Na [67]. Sodium reportedly enhances nitrate uptake by roots and
nitrate assimilation in leaves of A. tricolor[68].
Using isolated chloroplasts of Panicum miliaceum(a C 4 species), it was shown that Na enhanced
pyruvate uptake, indicating that Na/pyruvate was cotransported through the envelope into the chloro-
plast by light-stimulated Na efflux pumps [4]. In contrast, no Na effect on pyruvate uptake rates was noted
in mesophyll chloroplasts of Zea mays. Additional evidence suggests that in C 4 species of the NADP-
SODIUM—A FUNCTIONAL NUTRIENT IN PLANTS 367