Several workers, following Heller (1953), have included aluminium and nickel in their micronutrient formulations. However, the general benefit of adding the former metal does not seem to have been adequately demonstrated.
It was believed that in most plants Ni2+ is not absolutely required for normal growth and development (Mishra and Kar, 1975). However, more recently, it has been found by careful experimentation that nickel is essential (Gerendas et al, 1999). The ion is a component of urease enzymes (Dixon et al, 1975; Polacco, 1977a), which convert urea to ammonia. It has been shown to be an essential micronutrient for some legumes and to actvate urease in potato microshoot cultures (Witte et al., 2002). In tissue cultures the presence of 0.1 mM Ni2+ strongly stimulates the growth of soybean cells in a medium containing only urea as a nitrogen source. Slow growth occurs on urea without the deliberate addition of nickel, possibly supported by trace amounts of the element remaining in the cells (Polacco, 1977b). Cells and tissues are not normally grown with urea as a nitrogen source, and as urease is the only enzyme, which has been shown to have a nickel component, it could be argued that nickel is not essential. However, without it soybean plants grown hydroponically, accumulate toxic concentrations of urea (2.5%) in necrotic lesions on their leaf tips, whether supplied with inorganic nitrogen, or with nitrogen compounds obtained from bacterial symbiotic nitrogen fixation. These symptoms can be alleviated in plants growing in hydroponic culture by adding 1 mg/l Ni to the nutrient solution. Absence of nickel in a hydroponic solution also results in reduced early growth and delayed nodulation (Eskew et al., 1983).
Despite these findings nickel has not been added deliberately to tissue culture media. However, it should be noted that agar contains relatively high levels of nickel (Table 3.2) and the possibility of urea toxicity may have been avoided because, in tissue cultures, urea diffuses into the medium (Teasdale, 1987). Quoirin and Lepoivre (1977) showed that at the concentrations recommended by Heller, Al3+ and Ni2+ were without effect on the growth of Prunus meristems and were inhibitory at higher levels. If it is thought that Ni should be added to a culture medium, 0.1 mM is probably sufficient.
Aluminium has been said to be necessary for the growth of some ferns (Taubck, 1942), but is not generally added to tissue culture media for fern propagation.
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