Cobalt is not regarded as an essential element. Nevertheless, it was found to have been included in approximately half of a large sample of published plant culture media (George, et al., 1987). Murashige and Skoog (1962) included Co in their medium because it had been shown to be required by lower plants (Holm-Hansen et al., 1954) and that it might have a role in regulating morphogenesis in higher plants (Miller, 1954; Salisbury, 1959). However, no stimulatory effect on the growth of tobacco callus was observed by adding cobalt chloride to the medium at several concentrations from 0.1 ^M and above, and at 80.0 and 160 ^M the compound was toxic. Similarly Schenk and Hildebrandt (1972) obtained no clear evidence for a Co requirement in tests on a wide variety of plants, but retained the element in their medium because they occasionally observed an apparent stimulation to the callus growth of some monocotyledons. Pinus suspension cultures do not require cobalt (Teasdale, 1987). The concentration most commonly added to a medium is ca. 0.1 ^M, although ten times this amount has sometimes been used. Cobalt is the metal component of Vitamin B12 which is concerned with nucleic acid synthesis (Fries, 1962), but evidence that the element has any marked stimulatory effect on growth or morphogenesis in plant tissue cultures is hard to find. Cobalt may replace nickel in urease and thereby render it inactive, e.g., in potato (Witte et al., 2002).

Advantage from adding cobalt to plant culture media might be derived from the fact that the element can have a protective action against metal chelate toxicity and it is able to inhibit oxidative reactions catalysed by ions of copper and iron (Albert, 1958). The Co2+ ion can inhibit ethylene synthesis.

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