Many early reports of the role of nickel in agricultural productivity have been questioned since they did not adequately exclude the possibility that nickel was acting directly as a fungicidal element (27). Regardless of the many questionable reports, a compelling body of literature exists in which appropriate concentrations of nickel were applied or where the plant response is consistent with current knowledge of nickel functions including effects on nitrogen fixation, seed germination, and disease suppression (26,27,31,34,38,39).
The first demonstration of an agricultural Ni deficiency did not occur until 2004 (Wood et al., 2004), when it was observed in pecan (Carya illinoinensis). Nickel deficiency in pecan is associated with a physiological disorder 'mouse-ear' which occurs sporadically, but with increasing frequency, throughout the southeastern United States (portions of South Atlantic region) where it represents a substantial economic impact. In agreement with the results of Brown et al. (1), Ni deficiency in pecan results in a disruption of nitrogen metabolism and altered amino acid profiles (72).
The value of addition of nickel to Murashige and Skoog plant tissue-culture medium was shown by Witte et al. (41). These authors suggested that the lack of nickel and urease activity may represent a stress factor in tissue culture and recommended that the addition of 100 nM Ni be adopted as a standard practice. The benefits of adding nickel to solution cultures was also demonstrated by Khan et al. (42), who determined that a mixture of 0.05 mg Ni L-1 and 20% nitrogen as urea resulted in optimal growth of spinach (Spinacia oleracea L.) under hydroponic conditions.
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