Potassium is the major cation (positive ion) within plants reaching in the cytoplasm and chloroplasts concentrations of 100 - 200 mM. The biphasic uptake kinetics suggest two uptake systems: a high-affinity and a low-affinity one. K+ is not metabolised. It contributes significantly to the osmotic potential of cells. K+ counterbalances the negative charge of inorganic and organic anions. It functions in cell extension through the regulation of turgor, it has a major role in stomatal movements and functions in long-distance nutrient flow. Potassium ions are transported quickly across cell membranes and two of their major roles are regulating the pH and osmotic environment within cells. Potassium, calcium, sodium and chloride ions conserve their electrical charges within the plant, unlike the cation NH4+ and the anions NO3-, SO42-, and H2PO4-, which are rapidly incorporated into organic molecules. In intact plants, potassium ions are thought to cycle. They move, associated with cations (particularly NO3), upwards from the roots in the xylem. As nitrate is reduced to ammonia and assimilated, carboxylic acid ions (RCO3-, malate) are produced. These become associated with the released K+ ions and are transported in the phloem to the roots, where they are decarboxylated, releasing K+ for further anion transport (Ben-Zioni et al., 1971). Carboxylate transported to the roots gives rise to OH ion, which is excreted into the soil (or medium) to counterbalance NO3 uptake (Touraine et al., 1988). Potassium ions will clearly have a similar role in cultured tissues, but obvious transport mechanisms will usually be absent.

Many proteins show a high specificity for potassium which, acting as a cofactor, alters their configuration so that they become active enzymes. Potassium ions also neutralise organic anions produced in the cytoplasm, and so stabilise the pH and osmotic potential of the cell. In whole plants, deficiency of potassium results in loss of cell turgor, flaccid tissues and an increased susceptibility to drought, salinity, frost damage and fungal attack. A high potassium to calcium ratio is said to be characteristic of the juvenile stage in woody plants (Boulay, 1987). Potassium deficiency in plant culture media is said to lead to hyperhydricity (Pasqualetto et al., 1988), and a decrease in the rate of absorption of phosphate (Chin and Miller, 1982). However quite wide variations in the potassium content of MS medium had little effect on the growth or proliferation of cultured peach shoots (Loreti et al., 1988).

Lavee and Hoffman (1971) reported that the optimum rate of callus growth of two apple clones was achieved in a medium containing 3.5 mM K+: when the concentration was much higher than this, or when it was less than 1.4 mM, the callus grew less vigorously. However, the growth rate of wild carrot suspensions was said by Brown et al., (1976) to be at, or near, the maximum when K+ concentration was 1 mM: for embryogenesis 10-50 mM K+ was required. Uptake of potassium into plants is reduced in the absence of calcium (Devlin, 1975).

Within a large sample of different macronutrient compositions, it is found that authors have tended to relate the concentration of potassium to the level of nitrate. This is correlated with a coefficient of 0.78, P<0.001 (George et al, 1988). The average concentration of potassium in these media was 13.6 mM and the most common value (median), 10.5 mM. Murashige and Skoog (1962) medium contains 20.04

Sodium ions (Na+) are taken up into plants, but in most cases they are not required for growth and development and many plants actively secrete them from their roots to maintain a low internal concentration. The element can function as an osmotic stabilizer in halophytic plants; these have become adapted so that, in saline soils with low water potential, they can accumulate abnormally high concentrations of Na+ ion in vacuoles, and thereby maintain sufficient turgor for growth.

Sodium does appear to have a beneficial nutritional effect on some plants and is therefore considered as a functional element (Subbarao et al., 2003). Small amounts of sodium chloride (e.g. 230 mg/l) can stimulate the growth of plants in the families Chenopodiaceae and Compositae even when there is no limitation on the availability of K+ (Brownell, 1979). In other plants such as wheat, oats, cotton and cauliflower (Sharma and Singh, 1990), sodium can partially replace potassium, but is not essential.

Sodium only appears to be essential to those salttolerant plants, which have a C4 (crassulacean acid) metabolism. Examples are Bryophyllum tubiflorum (Crassulaceae) and Mesembryanthemum crystallimum (Aizoaceae). In these plants the element is necessary for CO2 fixation in photosynthesis.

Fig. 3.4 Top: Depletion of P in the medium compared to the growth of Dahlia cultures. Bottom (Right) P content in 1-week old Dahlia shoots taken from the culture after 1 week when P had not yet been exhausted. (Left) 6-week old shoots in which the upper part had been formed after all P had been taken up from the medium. The high content in the newly formed upper part of the shoots indicates massive retranslocation of P after uptake from older to newer tissue (G. de Klerk, unpub. data).

Fig. 3.4 Top: Depletion of P in the medium compared to the growth of Dahlia cultures. Bottom (Right) P content in 1-week old Dahlia shoots taken from the culture after 1 week when P had not yet been exhausted. (Left) 6-week old shoots in which the upper part had been formed after all P had been taken up from the medium. The high content in the newly formed upper part of the shoots indicates massive retranslocation of P after uptake from older to newer tissue (G. de Klerk, unpub. data).

Most macronutrient formulations do not contain any sodium at all, and the average concentration in 615 different preparations was 1.9 mM (George et al., 1988). Even if the element is not deliberately added as a macronutrient, small amounts are incorporated in most media from the salts added to provide micronutrients. Plant macronutrient preparations containing high concentrations of both sodium and chloride ions are not well formulated.


Magnesium is an essential component of the chlorophyll molecule and is also required non-specifically for the activity of many enzymes, especially those involved in the transfer of phosphate. ATP synthesis has an absolute requirement for magnesium and it is a bridging element in the aggregation of ribosome subunits. Magnesium is the central atom in the porphyrin structure of the chlorophyll molecule. Within plants, the magnesium ion is mobile and diffuses freely and thus, like potassium, serves as a cation balancing and neutralising anions and organic acids. Macklon and Sim (1976) estimated there to be 2.1 mM Mg2+ in the cytoplasm of Allium cepa roots while McClendon (1976) put the general cytoplasmic requirement of plants as high as 16 mM. Plant culture media invariably contain relatively low concentrations of magnesium (average 6.8 mM, median 5.3 mM). Very often MgSO4 is used as the unique source of both magnesium and sulphate ions.

Walker and Sato (1981) found there to be a large reduction in the number of somatic embryos formed from Medicago sativa callus when Mg2+ was omitted from the medium. In sympathy with this finding, Kintzios et al., (2004) observed in tissue culture of melon that the highest level of magnesium occurred in direct somatic embryogenic cultures and the lowest level in callus cultures.


The sulphur utilised by plants is mainly absorbed as SO42-, which is the usual source of the element in plant culture media. Uptake is coupled to nitrogen assimilation (Reuveny et al., 1980), and is said to be independent of pH. It results in the excretion of OH-ions by the plant, making the medium more alkaline. However, according to Mengel and Kirkby (1982), plants are relatively insensitive to high sulphate levels and only when the concentration is in the region of 50 mM, is growth adversely affected. Although sulphur is mainly absorbed by plants in the oxidized form, that which is incorporated into chemical compounds is mainly as reduced -SH, -S- or -S-S- groups. The sulphur-containing amino acids cysteine and methionine become incorporated into proteins. Sulphur is used by plants in lipid synthesis and in regulating the structure of proteins through the formation of S-S bridges. The element also acts as a ligand joining ions of iron, zinc and copper to metalloproteins and enzymes. The reactive sites of some enzymes are -SH groups. Sulphur is therefore an essential element and deficiency results in a lack of protein synthesis. Sulphur-deficient plants are rigid, brittle and thin-stemmed. Important sulphur compounds are glutathione, which acts in detoxification of oxygen radicals, and the proteins thioredoxin and ferrodoxin that are involved in redox chemistry.

Growth and protein synthesis in tobacco cell suspensions were reduced on a medium containing only 0.6 mM SO42- instead of 1.73 mM (Klapheck et al., 1982) and when the supply of S in the medium was used up, large amounts of soluble nitrogen accumulated in the cells. Most media contain from 25 meq/l SO42- (1 - 2.5 mM).

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