Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 129

Mandrand-Berthelot 2000 ). However, insoluble Ni compounds enter plant cells
mainly via endocytosis (Costa et al. 1994 ).
The factors that inluence Ni bioavailability and uptake by plants include the fol-
lowing: (1) soil concentrations of nickel (Cataldo et al. 1978 ), (2) acidity of the soil
or the soil solution (McIlveen and Negusanti 1994 ; Antoniadis et al. 2008 ), (3) pres-
ence of other competitive metals (Kochian 1991 ), and (4) organic composition of
the soil (Burke et al. 2000 ; Jean et al. 2008 ). Among these, the most important factor
that determines Ni solubility is soil pH, and this, in turn, determines nickel's avail-
ability for plant uptake (Smith 1994 ; Weng et al. 2004 ; Antoniadis et al. 2008 ).
Therefore, anthropogenic processes that change soil pH alter Ni solubility in soils.
For example, at low pH in acidic soils Ni is in a more soluble form and thus becomes
more mobile (Zhang et al. 2006 ). Therefore, symptoms of metal toxicity to plants
can easily be observed in acid soils, sometimes even if no other metal is naturally
present or is added to the soil system by human activities (Rautaray et al. 2003 ).
Soil colloidal materials have the ability to absorb nickel. Nickel is present in an
exchangeable form when bound to organic matter and can, therefore, be easily
exchanged with the crystal lattice of minerals in the soil solid phase (Misra and
Pande 1974 ; Karaca 2004 ; Sukkariyah et al. 2005 ). Although Ni solubility in soils
increases with soil acidity, high Ni mobility may also occur under neutral or alka-
line conditions (Willaert and Verloo 1988 ; Alloway 1995 ). Generally, mobile forms
of nickel are readily available for uptake and most of the nickel ion absorbed by
plants accumulates primarily in roots. Still, Ni, in some cases, is known to translo-
cate to aerial plant parts in some species. Hence, uptake and accumulation of Ni is
species-, pH-, amount-, and available-form-dependent.

4 Transport and Distribution in Plants

Nickel and certain other metals are primarily transported from roots to shoots
(Peralta-Videa et al. 2002 ) and to leaves (Krupa et al. 1993 ) via the xylem, through
the transpiration stream (Neumann and Chamel 1986 ). Nickel is highly mobile in
plants and can be easily retranslocated from old to young leaves (Zhao et al. 1999 ;
Gray and Mclaren 2006 ). As it is an essential element, Ni may also be translocated
via the phloem to neonatal plant parts, such as buds, fruits, and seeds (McIlveen and
Negusanti 1994 ; Welch 1995 ; Fismes et al. 2005 ; Page et al. 2006 ). Such transport
and retranslocation is strongly regulated by metal-ligand complexes (i.e., nicotiana-
mine, histidine, and organic acids) (Vacchina et al. 2003 ; Kim et al. 2005 ; Pianelli
et al. 2005 ; Haydon and Cobbett 2007 ) and by proteins that speciically bind and
transport Ni (Hausinger 1997 ; Colpas and Hausinger 2000 ).
Approximately, half of the Ni taken up by plants is retained in the plant root
system (Cataldo et al. 1978 ). This may result from its sequestration at cation
exchange sites of vessel walls, xylem parenchyma cells and/or immobilization in
the root (Seregin and Kozhevnikova 2006 ). Furthermore, root vascular cylinders
contain a high percentage of Ni (over 80%), while less than 20% is present in the