homeostasis. Part of this newly formed K-malate may be retranslocated to the roots for subsequent reuti-
lization of K as a counterion for NO 3 transport. The high mobility of K in the phloem and its continuous
circulation within the plant are indications of a function for K in the long-distance transport processes of
higher plants [9,118]. This function of K as a counterion in long-distance transport may not be specific
and other cations should be able to replace K in this function provided they are phloem mobile. For sev-
eral crops, e.g., maize, Na is not phloem mobile [9]. But in crops such as sugar beet, Na is reported to be
phloem mobile and thus could be as effective as K for the long-distance transport functions [9]. Our stud-
ies with red beet indicate that NO 3 levels in shoot were not affected by large drops in the plant’s tissue K
concentration when Na was available as an alternative ion (G.V. Subbarao et al., unpublished data). This
indicates that in red beet, Na may be able to replace the K in this function. Replacing the plant K with Na,
particularly at higher degrees of substitution (90%), has resulted in higher levels of NO 3 in leaves of red
beets, spinach, and lettuce (G.V. Subbarao et al., unpublished). Nitrate reductase was reported to require
K for its activation [119,120].
- Enzyme Activation
Potassium has a direct metabolic role within the cytoplasm [4,121]. Several enzymes are activated by K
ions, and that activation was generally maximal at a concentration of about 100 mM, the same as the nor-
mal K range in cytoplasm [70]. Potassium ions have an important role in protein and starch synthesis [77]
as well as in respiratory and photosynthetic metabolism [76]. The precise mechanisms of activation are
not known, but it is important to note that Na is frequently (but not always) less effective as an activating
cation for these enzymes [77]. Regardless of the fluctuations of K levels in the vacuole compartment, it
seems that plant cells maintain cytoplasmic K levels in the range 100 to 200 mM [69]. Cytoplasmic K lev-
els are thought to be affected only under severe K deficiency afer the vacuolar K pools have been ex-
hausted. Thus, the cytoplasm is very conservative in its K requirements. Because most of the metabolic
processes and enzyme action are located in the cytoplasm, it is thought that most of the cytoplasmic en-
zymes require K for their functions.
Protein synthesis [77,122,123] and oxidative phosphorylation [124] are equally inhibited by high Na
in vitro whether the organelles are isolated from glycophytes or halophytes [53]. Starch synthetase has a
requirement of about 50 mM K for normal functioning. Other monovalent cations such as rubidium, ce-
sium, and ammonium are about 80% as effective as K, while Na is only about 20% as effective at main-
taining starch synthetase activity [125]. Several reports indicate that K is needed for the normal func-
tioning of starch synthetase in sweet potato, taro, white potato, wheat, bush beans, field corn, soybeans,
peas, and rice [126–129]. Sodium seems unable to replace this K function even in sugar beet. Potassium
deficiency results in the accumulation of solute carbohydrates and reducing sugars due to the inhibition
of starch synthesis [70]. Glucose transport across the plasmalemma of sugar beet storage cell protoplasts
is faster in the presence of KCl than in the presence of NaCl [130]. Sodium, however, is more effective
in catalyzing the transport of sucrose across the tonoplast into the vacuole and in stimulating sucrose ac-
cumulation in the storage tissue [130]. This effect of Na on sucrose storage seems to be related to stimu-
lation of adenosinetriphosphatase (ATPase) activity at the tonoplast of beet storage cells [4,130].
Vacuolar ATPase is substrate specific and Mg dependent and is distinguished from nonspecific phos-
phatase [75]. Beet root ATPase is stimulated about 100% by both Na and K ions. The highest ATPase ac-
tivity is obtained in sugar beet and Avicenniaroots with combinations of K and Na but not with either K
or Na alone [131]. This agrees with observations that growth of sugar beet is highest when both Na and
K are present in the growing medium. Green and Taylor [132] proposed that ATPase activation requires
two binding sites, one for K and one for Na, and that maximal activation is obtained when both sites are
occupied. During in vitro studies of the effect of high concentrations of KCl and NaCl on the K,Na-
ATPases, genotypic differences were found suggesting differences in cellular localization of Kand Na
[133,134].
Using isolated mitochondria from Brassica rapa, it was shown that the esterification of phosphate
and PO 4 /O 2 ratio are increased more by Na than K [135]. But it is unclear whether this increased rate of
phosphate metabolism has some connection with the formation of complexes with ATP. It is also unclear
whether an increased rate is desirable in terms of overall plant growth. Sodium is able to form stable com-
plexes with polyphosphates [136]. The effect of Na on the enzymatic activity of pyruvic kinase is small
in comparison with that of K [9]. Also, Na is not effective in activating acetic thiokinase from spinach
leaves [137].
The existence of isozymes has not been considered when evaluating whether Na could replace K dif-
SODIUM—A FUNCTIONAL NUTRIENT IN PLANTS 371