III. CONCEPT OF “FUNCTIONAL NUTRIENT”
To overcome the difficulty associated with this limited definition of “essentiality,” Nicholas [28] sug-
gested the term “functional or metabolism nutrient,” which is defined as “any mineral element that func-
tions in plant metabolism irrespective of whether or not its action is specific.” The term functional nutri-
ent seems appropriate, but the definition needs to be much more specific. It seems that it would be
desirable to define a functional nutrient as one that is essential for maximal biomass production or can
function in an essential metabolic process, reducing the critical level of an essential nutrient, without hav-
ing a unique function itself, as defined by Arnon and Stout [1]. The remaining portion of the chapter deals
with this issue, and we will try to present evidence to support the notion that Na should be considered as
a functional nutrient.
IV. UPTAKE AND TOLERANCE OF SODIUM
A. Uptake Mechanisms
The concept of dual mechanisms has been widely recognized in the absorption of alkali cations by plant
roots [29–32]. Because of the chemical similarity between K and Na, it is generally assumed that K and
Na compete for common absorption sites in the root. Selective ion transport or mechanism 1 K transport
is effective at very low external K concentrations [33], with a maximum rate at an external K concentra-
tion of 1 mM [29]. Sodium even in 20-fold excess fails to compete significantly with K under mechanism
- Mechanism 1 depends on metabolic energy derived from adenosine triphosphate (ATP). At higher con-
centrations of K (up to 50 mM), mechanism 2 becomes important [29,30]. Mechanism 2 does not dis-
criminate K from Na, and thus Na can competitively inhibit the absorption of K [33]. Also, mechanism 2
does not directly require metabolic energy to function and is thought to operate through diffusive forces,
which involve ion channels. Sodium uptake in plants is believed to be primarily through mechanism 2
[34].
Inward-rectifying K channels (Kinchannels) have been reported in different root cells, including cor-
tical, root hair, stelar, and xylem parenchyma cells, that can sense K concentrations [35–41]. These ion
channels transport at rates between 10^6 and 10^8 ions per second per channel protein. Transport is “pas-
sive,” in which diffusion of ions through the channel is a function of both the membrane voltage and the
concentration difference across the membrane; thus, uptake is not directly coupled to the input of other
forms of free energy [42]. Also, selectivity is not absolute and many channels can conduct a range of ions,
although not all at the same efficiency [42]. This property is reflected in the so-called ionic selectivity se-
quence for the channel, which can have physiological significance. Thus, some K channels conduct Na to
a finite extent and can affect the degree to which plants withstand high Na (i.e., salinity) in the root zone
[43]. Another property of ion channels is their ability to reside in “open” or “closed” conformational
states, which either permit or prevent ion permeation. This conformational switching can occur in re-
sponse to ligands or to a change in membrane voltage after which channels activate or deactivate [44].
The control of activation by membrane voltage or by ligands such as Ca may be the key to understanding
the role of these ion channels in cell biology [42].
Although, it is widely believed that mechanism 1 does not have much affinity to transport Na in the
presence of adequate K, for some crops such as beets this mechanism may be transporting Na indepen-
dent of the external concentration [45]. Several Atriplexspecies take up Na in preference to K. In these
species, Na competes with K during uptake, but K does not compete with Na [46–48]. Thus, mechanisms
of Na transport at low concentrations and in the presence of K are open to further investigation.
B. Regulation, Translocation, and Partitioning
Plant species vary widely in their ability to absorb and translocate Na to the shoot [49]. Generally, species
that absorb Na and translocate it freely to the shoot are termed natrophiles[50]. Most plant species do not
readily absorb Na but prefer K instead and are termed natrophobes[49–51]. If natrophobes take up Na,
it is usually retained in the root and with relatively little translocation to the shoot [49]. Natrophobes
translocate Na, but only when subjected to high concentrations (100 mM) in the root zone, which usu-
ally results in a major growth reduction and/or death of the plants as a result of Na toxicity [8,52] (also
366 SUBBARAO ET AL.