ME (malic enzyme) type (such as Zea maysandSorghum bicolor), the H/pyruvate cotransport system
operates instead of the Na/pyruvate cotransport system in mesophyll chloroplasts isolated from P. mil-
iaceum[22]. This stresses the necessity of differentiating between metabolic types of C 4 species when
studying the role of Na.
B. Replacing Potassium Functions
Next to N, K is the mineral nutrient required in largest amounts by plants for metabolic functions and
growth [69]. Plant nutritionists have been intrigued for decades by this high requirement for K. Potassium
plays a vital role in a wide range of biochemical and biophysical processes in plants. It is a highly mobile
charge carrier, it neutralizes the effects of anions, and it plays an important role in enzyme activation and
membrane transport. The process that is generally regarded as being the most sensitive to K is the main-
tenance of turgor pressure and, as a consequence, cell expansion. Biochemically, most K-requiring en-
zymes need only 10–50 mM K for maximum activity, and other monovalent cations can sometimes sub-
stitute for K [70,71]. It is reported that protein synthesis in vitro in plant systems is maximal at 100 mM
K or higher [56,72,73]. Potassium exists as a monovalent cation in biological systems and does not par-
ticipate in covalent bonding. Moreover, it forms only weak coordination complexes because of its small
ionic charge, low electronegativity, and its completed 3pelectron shell [69]. However, K is the major cy-
toplasmic cation in cells and plays a number of interrelated and integrated roles [56,74]. These
include:
Cofactor in enzyme activation, especially protein synthesis (translation)
Stabilization of the active conformation of enzymes and possibly membranes
Cytoplasmic volume regulation
Energy conservation across membranes
Cytoplasmic pH regulationIn general, K functions in plants can be summarized as both biophysical (non–K-specific role as an
osmoticum in the vacuole) and biochemical (specific and nonspecific roles in the cytoplasm). According
to prevalent concepts, the need of monovalent cations can be completely filled by K, but some functions
can also be exercised by Na and other monovalent cations, thus reducing the total amount of K required
by the plant.
- Internal Osmoticum
The large central vacuole of the plant cell (which occupies nearly 90% of the cell’s volume) provides a
large buffer volume, primarily of inorganic ions for satisfying the osmotic requirements of terrestrial
plants without maintaining a large volume of cytosol filled with energy-expensive organic solutes [75].
The peripheral cytosol layer facilitates the distribution of chloroplasts close to the cell surface, thus max-
imizing the penetration of light to the photosynthetic apparatus [75]. Potassium makes a major contribu-
tion to the solute potential (s) of the cell [76,77]. Investigations of 200 plant species by Iljin [78] have
shown that the contribution of K to the total s varied from 66 to 90%. The accompanying anions are pri-
marily Pi, Cl, NO 3 , and SO 4 [69,75]. Although K salts are the most common inorganic osmotica in the
vacuole, there is no known absolute requirement for K in this compartment as vacuoles contain high con-
centrations of many other solutes such as Na salts [79], sugars [75,80], and amino acids [81]. If K salts
are the only vacuolar solutes, the s of the vacuole containing 200 mM K (and associated anions) would
be about 0.9 MPa [69]. Reported sap s varied from 0.7 to 1.2 MPa for plants not subjected to salt
or water stress [77,82], suggesting that turgor maintenance in the vacuole is generated primarily by K salts
under K-sufficient conditions. Examples of the contribution of the K to the total leaf sap s for red beet,
spinach, and lettuce under K-sufficient conditions are given in Table 2.
The vacuole is considered to be a storage organelle, and the nutrients accumulated within it are
largely removed from active metabolism. Nevertheless, they are important to generate and maintain cell
turgor [75,83,84]. It has been shown that the vacuolar K levels can be highly variable (10–200 mM), de-
termined mostly by the external K concentrations in the root zone. In contrast, cytoplasmic levels are rel-
atively stable, near 200 mM for most plant species [56,76,77,84–90]. In well-fertilized crops, the vacuo-
lar K concentration can be high, on the order of 200 mM. Because the vacuole occupies nearly 80–90%
of a mature plant cell volume, it has most of the cell K in K-sufficient plants, providing a K concentration
368 SUBBARAO ET AL.