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Trace Elements in Abiotic and Biotic Environments
(Rühling and Tyler 2004). Its contents of moss (Hylocomium splendens) from the
mountains in Poland (0.7–1.4 mg/kg) are similar to those of the same moss species
from Alaska (0.8–4.7 mg/kg). This may also suggest similar Ni atmospheric deposition in both regions (Migaszewski et al. 2009).
29.5 PLANTS
There is no evidence of an essential role of Ni in plant metabolism, although there
are several suggestions that it might be needed for plants. Its essentiality for some
bacteria has been proven, mainly bacteria involved in processes of nitrification and
mineralization of OM. Thus, Ni is considered as essential for legumes, in which it
plays a role in urease metabolism. It is involved in N transport from roots to tops and
in H 2 metabolism.
Studies on phytoavailability and behavior of Ni in plants are related mainly to its
toxicity and implications with respect to human and animal health. Its toxicity and biological effects are highly related to its species. The cationic form, Ni 2+ , is more readily
absorbed and more toxic than its complexed species. However, all species of Ni are
easily available to plants and controlled by both soil and plant factors. Soil pH has the
most pronounced impact; increased pH from 4.5 to 6.5 decreases Ni in oats by a factor
of about 8. Plants uptake Ni added to soil easier than from its lithogenic sources. High
cation-exchange capacity values of soils may limit its availability to plants.
The phytoavailability of Ni depends on its origin and soil properties, as well as
on plants’ abilities to absorb Ni, (e.g., accumulators and hyperaccumulators). About
200 plant species have been identified as Ni hyperaccumulators, which may contain
more than 0.1% of this metal (Mesjarz-Przybyłowicz et al. 1994). The processes of
Ni uptake are still little known; however, its availability is positively correlated with
its concentrations in soil solutions. There are observations indicating an extracellular,
metabolically independent Ni adsorption and a possible displacement of Mg + from
cell membranes. All Ni 2+ retained by bacteria was accumulated externally to cells
(Kabata-Pendias 2011). The transport and storage of Ni in plants is metabolically controlled. It is mobile in plants and is likely to be accumulated in leaves and seeds. The
Ni speciation in the plant extracts indicates that regardless of chemical forms added to
soils, it is found only in neutral and negative complexes (Khellat and Zerdauori 2010).
In plants under Ni stress, the absorption of nutrients, some metabolic processes,
and root development are strongly retarded. Photosynthesis and transpiration
processes, as well as N 2 fixation by legume plants, are also inhibited. The most
common symptom of Ni phytotoxicity is chlorosis, which may be associated with
Fe-induced chlorosis. Under natural conditions, Ni toxicities are associated with
serpentine or other Ni-rich soils. Elevated Ni concentration in nutrient solution
decreases the activity of superoxide dismutase and catalase in wheat shoots, while
increasing the activity of peroxidases and glutathione S-transferase (Gajewska
and Skłodowska 2008). Addition of Ni, at the concentration of 200 mg/kg, to two
soils, heavy loamy sand and light silty loam, decreases oat yields by 65% and 40%,
respectively (Wyszkowska et al. 2007).
The phytotoxic Ni concentrations in plants widely range from about 40 to over
300 mg/kg, depending mainly on plant properties. Generally, the range of excessive