Copper is an essential micronutrient, even though plants normally contain only a few parts per million of the element. Two kinds of copper ions exist; they are the monovalent cuprous [Cu(I)] ion, and the divalent cupric [Cu(II)] ion: the former is easily oxidized to the latter; the latter is easily reduced. The element becomes attached to enzymes, many of which bind to, and react with oxygen. They include the cytochrome oxidase enzyme system, responsible for oxidative respiration, and superoxide dismutase (an enzyme which contains both copper and zinc atoms). Detrimental superoxide radicals, which are formed from molecular oxygen during electron transfer reactions, are reacted by superoxide dis-mutase and thereby converted to water. Copper atoms occur in plastocyanin, a pigment participating in electron transfer.
Several copper-dependent enzymes are involved in the oxidation and hydroxylation of phenolic compounds, such as ABA and dopamine (Lerch, 1981). The hydroxylation of monophenols by copper-containing enzymes leads to the construction of important polymeric constituents of plants, such as lignin. These same enzymes can lead to the blackening of freshly isolated explants. Copper is a constituent of ascorbic acid oxidase and the characteristic growth regulatory effects of ethylene are thought to depend on its metabolism by an enzyme, which contains copper atoms.
High concentrations of copper can be toxic. Most culture media include ca. 0.1-1.0 ^M Cu2+. Ions are usually added through copper sulphate, although occasionally cupric chloride or cupric nitrate have been employed. In hydroponic culture of Trifolium pratense, uptake of copper into the plant depended on the amount of nitrate in solution. Uptake was considerably reduced when NO3 was depleted (Jarvis, 1984). The concentration of Cu in tissue culture media is very small relative to the level in plants (Table 3.1). It is therefore not surprising that various authors report strong increases of growth when Cu is added at 1- 5 ^M (Dahleen, 1995; Nirwan and Kothari, 2003; Kintzios et al., 2001; Nas and Read, 2004; Bouman and Tiekstra, 2005)
Plants utilise hexavalent molybdenum and absorb the element as the molybdate ion (MoO42-). This is normally added to culture media as sodium molybdate at concentrations up to 1 mM. Considerably higher levels have occasionally been introduced [e.g. in the media of Abou-Mandour (1977) and Asahira and Kano (1977)] apparently without adverse effect, although Teasdale (1987) found pine suspension cultures were injured by 50 mM. Molybdenum is a component of several plant enzymes, two being nitrate reductase and nitrogenase, in which it is a cofactor together with iron: it is therefore essential for nitrogen utilisation. Tissues and organs presented with NO3 in a molybdenum-deficient medium can show symptoms of nitrate toxicity because the ion is not reduced to ammonia.
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