Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 127

Hasinur et al. 2005 ). Therefore, any Ni deiciency negatively affects plant growth and
metabolism in many ways. Such deiciency may affect (a) plant growth, (b) plant
senescence, (c) N metabolism, and (d) Fe uptake. Ni-deicient plants may develop
chlorosis in the youngest leaves, and ultimately Ni deiciency may produce
meristematic necrosis (Brown et al. 1987b, 1990 ; Yossef et al. 1998 ; Bai et al. 2006a ;
Wood et al. 2006 ). In addition, Ni is a constituent of several metalloenzymes, e.g.,
urease (Eskew et al. 1984 ; Brown et al. 1987a ; Sakamoto and Bryant 2001 ).
Therefore, a deiciency of Ni leads to reduced urease activity and disturbs N assimila-
tion (Ermler et al. 1998 ; Küpper and Kroneck 2007 ). In addition to the foregoing
functions, Ni is also proposed to participate in several important metabolic reactions
(e.g., hydrogen metabolism, methane biogenesis, and acetogenesis in bacteria) (Maier
et al. 1993 ; Collard et al. 1994 ; Ragsdale 1998 ; Mulrooney and Hausinger 2003 ).
Finally, nickel is known to have a role in phytoalexin synthesis and in plant resistance
to various plant stresses (Graham et al. 1985 ; Barker 2006 ; Wood and Reilly 2007 ).
Although small Ni concentrations are essential to normal plant growth, high con-
centrations may exert deleterious effects on plant growth and produce symptoms of
toxicity. The symptoms associated with nickel toxicity to plants include the follow-
ing: reduced shoot and root growth, poor development of the branching system,
deformation of various plant parts and abnormal lower shape, decreased biomass
production, leaf spotting, mitotic root tip disturbances, inhibition of germination,
and chlorosis that can result in foliar necrosis (Ewais 1997 ; Rao and Sresty 2000 ;
Pandey and Sharma 2002 ; Nakazawa et al. 2004 ; Rahman et al. 2005 ; Gajewska
et al. 2006 ). All of these toxic effects ultimately reduce the yield of agricultural
crops (Balaguer et al. 1998 ; Ahmad et al. 2007 ). Other symptoms of Ni plant toxic-
ity include Fe deiciency-induced chlorosis and foliar necrosis (Brown et al. 1987a ;
Wood et al. 2006 ). Excess nickel affects nutrient absorption by roots, plant develop-
ment and metabolism, and inhibits photosynthesis and transpiration (Nedhi et al.
1990 ; Kochian 1991 ; Pandey and Sharma 2002 ; Hasinur et al. 2005 ; Rahman et al.
2005 ). Nickel also has the ability to replace Co and certain other heavy metals
located at active sites in metalloenzymes and can, thereby, disrupt their functioning
(Ermler et al. 1998 ; Küpper and Kroneck 2007 ).
In view of the foregoing, it is amply clear that Ni enters the environment from
several sources, and although it is beneicial for optimum plant growth at low con-
centrations, it may produce hazardous effects on growth and key metabolic pro-
cesses of most plants at high concentrations. It is our purpose in the present review
to address both the beneicial and toxic effects that Ni displays in plants. Moreover,
we also address the key processes used by plants, whether morphoanatomical or
metabolic, that enables them to tolerate excessive Ni levels.

2 Sources of Nickel Emission to the Environment

Nickel is released into the environment from a variety of natural and anthropogenic
sources. Among industrial sources, a considerable amount of environmental Ni
derives from the combustion of coal, oil, and other fossil fuels. Other industrial