Interaction of Cobalt with Metals and Other Chemicals in Mineral Metabolism

The interaction of cobalt with other metals depends to a major extent on the concentration of the metals used. The cytotoxic and phytotoxic responses of a single metal or combinations are considered in terms of common periodic relations and physicochemical properties, including electronic structure, ion parameters (charge-size relations), and coordination. But, the relationships among toxicity, positions, and properties of these elements are very specific and complex (65). The mineral elements in plants as ions or as constituents or organic molecules are of importance in plant metabolism. Iron, copper, and zinc are prosthetic groups in certain plant enzymes. Magnesium, manganese, and cobalt may act as inhibitors or as activators. Cobalt may compete with ions in the biochemical reactions of several plants (66,67).

Iron

Many trace elements in high doses induce iron deficiency in plants (68). Combinations of increased cobalt and zinc in bush beans have led to iron deficiency (69). Excess metals accumulated in shoots, and especially in roots, reduce ion absorption and distribution in these organs, followed by the induction of chlorosis, decrease in catalase activity, and increase in nonreducing sugar concentration in barley (70,71). Supplying chelated iron ethylenediamine di(o-hydroxyphenyl) acetic acid [Fe-(EDDHA),] could not overcome these toxic effects in Phaseolus spp. L. (72). Simultaneous addition of cobalt and zinc to iron-stressed sugar beet (Beta vulgaris L.) resulted in preferential transport of cobalt into leaves followed by ready transport of both metals into the leaf symplasts within 48 h (73). A binuclear binding site for iron, zinc, and cobalt has been observed (74).

Zinc

Competitive absorption and mutual activation between zinc and cobalt during transport of one or the other element toward the part above the ground were recorded in pea (Pisum sativum L.) and wheat seedlings (75). Enrichment of fodder beet (Beta vulgaris L.) seeds before sowing with one of these cations lowers the content of the other in certain organs and tissues. It is apparently not the result of a simple antagonism of the given cations in the process of redistribution in certain organs and tissue, but is explained by a similar effect of cobalt and zinc as seen when the aldolase and carbonic anhydrase activities and intensity of the assimilators’ transport are determined (76). Cobalt tends to interact with zinc, especially in high doses, to affect nutrient accumulation (77). The antagonism is sometimes related to induced nutrient deficiency (69). In bush beans, however, cobalt suppressed to some extent the ability of high concentration of zinc to depress accumulation of potassium, calcium, and magnesium. The protective effect was stated to be the result of zinc depressing the leaf concentration of cobalt rather than the other nutrients (69). Substitution of Zn2+ by CO2+ reduces specificity of Zn2+ metalloenzyme acylamino-acid-amido hydrolase in Aspergillus oryzae Cohn (78).

Cadmium

Combinations of elements may be toxic in plants when the individual ones are not (72). Trace elements usually give protective effects at low concentrations because some trace elements antagonize the uptake of others at relatively low levels. For example, trace elements in various combinations (Cu–Ni–Zn, Ni–Co–Zn–Cd, Cu–Ni–Co–Cd, Cu–Co–Zn–Cd, Cu–Ni–Zn–Cd, and Cu–Ni–Co–Zn–Cd) on growth of bush beans protected against the toxicity of cadmium. It was suggested that part of the protection could be due to cobalt suppressing the uptake of cadmium by roots. Other trace elements in turn suppressed the uptake of cobalt by roots (69). These five trace elements illustrated differential partitioning between roots and shoots (40). The binding of toxic concentration of cobalt in the cell wall of the filamentous fungus (Cunninghamella blackesleeana Lender) was totally inhibited and suppressed by trace elements (79).

Copper

The biphasic mechanism involved in the uptake of copper by barley roots after 2 h was increased with 16 µM CO2+, but after 24 h, a monophasic pattern developed with lower values of copper absorption, indicating an influence of CO2+ on the uptake site (80).

Manganese

Cobalt and zinc increased the accumulation of manganese in the shoots of bush beans grown for 3 weeks in a stimulated calcareous soil containing Yolo loam and 2% CaCO3 (40).

Chromium and Tin

The inhibitory effects of chromium and tin on growth, uptake of NO3- and NH4+, nitrate reductase, and glutamine synthetase activity of the cyanobacterium (Anabaena doliolum Bharadwaja) was enhanced when nickel, cobalt, and zinc were used in combination with test metals in the growth medium in the following degree: Ni>Co>Zn (81).

Magnesium

The activating effect of cobalt on Mg2+-dependent activity of glutamine synthetase by the blue–green alga Spirulina platensis Geitler may be considered as an important effect. Its effect in maintaining the activity of the enzyme in vivo is independent of ATP (82).

Sulfur (Sulphur)

The mold Cunninghamella blackesleeana Lendner, grown in the presence of toxic concentration of cobalt, showed elevated content of sulfur in the mycelia. Its cell wall contained higher concentrations of phosphate and chitosan, citrulline, and cystothionine as the main cell wall proteins (79).

Nickel

In moss (Timmiella anomala Limpricht), nickel overcomes the inhibitory effect of cobalt on protonemal growth whereas cobalt reduces the same effect of nickel on bud number (83).

Cyanide

Cyanide in soil was toxic to bush beans and also resulted in the increased uptake of the toxic elements such as copper, cobalt, nickel, aluminum, titanium, and, to a slight extent, iron. The phytotoxicity from cyanide or the metals led to increased transfer of sodium to the leaves and roots (40).