Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 134

light metals (e.g., Zn, Mg, and Cu) is one primary cause of Ni deiciency (He et al.
2005 ; Brown 2006 ). This may be because Cu and Zn inhibit Ni uptake competi-
tively (i.e., these three soluble metal ions are absorbed by the same transport sys-
tem) (Cataldo et al. 1988 ; Körner et al. 1987 ; Kochian 1991 ). In addition, the soluble
compounds of Ni and Mg-ions are absorbed by this same transport system, as both
ions have similar charge-size ratio. High application rates of Mg-containing fertil-
izers may produce Ni-deiciency (Oller et al. 1997 ). Therefore, it is often dificult to
identify whether observed symptoms result from a deiciency of Ni or are caused by
a deiciency of certain other metals (Brown 2006 ; Wood et al. 2006 ).
The earliest symptoms of Ni deiciency in plants, or plant growth effects in
response to the addition of Ni to growth media, under controlled experimental con-
ditions, were reported by Brown et al. ( 1987a, b ). These authors indicated that Ni
deiciency has a wide range of effects on plant growth and metabolism, including
some prominent ones: (a) reduced vegetative growth (lengths and fresh and dry
weights of root and shoot), (b) enhanced plant senescence, (c) changes in N metabo-
lism, and (d) normal Fe uptake. Preliminary investigations also indicate that Ni may
have a role in phytoalexin synthesis and in reducing plant disease resistance (Graham
et al. 1985 ).
At lower levels, Ni improves the seed germination of many species. Therefore,
under Ni deicit conditions, seeds may experience delayed seed germination (Brown
et al. 1987b ; Ahmad et al. 2009 ). Nickel is essential for the proper plant absorption
of Fe (Kopittke et al. 2008 ). Therefore, Ni-deicient plants may develop chlorosis or
necrotic spots when the deiciency is extreme (Checkai et al. 1986 ; Voss 1993 ;
Bennett 1993 ). When Ni is completely absent, plants show increased lower and
fruit loss and reduced crop yield (Brown et al. 1987b ; Balaguer et al. 1998 ).
Moreover, Ni-deicient plants display weak and broken branches and poor plant
architecture (Wood et al. 2006 ; Brown 2006 ).
Other symptoms of Ni deiciency also exist, but they vary by plant species and
the extent of the deiciency. Among these symptoms are the following: development
of premature leaf chlorosis that may extend to necrosis of entire leaves and/or lealet
tips; blunting of leaf and/or lealet tips; dwaring of foliage; curled leaf and/or lealet
margins; failure of lamina to develop properly; reduced internode distance; distor-
tion of bud shape; shoot brittleness; death of overwintering shoots resembling cold
injury; resetting and loss of apical dominance; a diminished root system with dead
ibrous roots; dwarf trees; and ultimately, death of whole trees or tree parts (Wood
et al. 2006 ).

7 Toxicity of Nickel in Plants

Although Ni is essential to several metabolic phenomena, it is extremely toxic to
plants when present at excessive levels in the soil or in nutrient solutions to which
plants are exposed. The general signs associated with Ni toxicity in plants include the
following: reduced shoot and root growth (Rahman et al. 2005 ), poor development of