Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 151

13 Summary

With the world's ever increasing human population, the issues related to environ-
mental degradation of toxicant chemicals are becoming more serious. Humans have
accelerated the emission to the environment of many organic and inorganic pollut-
ants such as pesticides, salts, petroleum products, acids, heavy metals, etc. Among
different environmental heavy-metal pollutants, Ni has gained considerable atten-
tion in recent years, because of its rapidly increasing concentrations in soil, air, and
water in different parts of the world.
The main mechanisms by which Ni is taken up by plants are passive diffusion
and active transport. Soluble Ni compounds are preferably absorbed by plants pas-
sively, through a cation transport system; chelated Ni compounds are taken up
through secondary, active-transport-mediated means, using transport proteins such
as permeases. Insoluble Ni compounds primarily enter plant root cells through
endocytosis. Once absorbed by roots, Ni is easily transported to shoots via the xylem
through the transpiration stream and can accumulate in neonatal parts such as buds,
fruits, and seeds. The Ni transport and retranslocation processes are strongly regu-
lated by metal-ligand complexes (such as nicotianamine, histidine, and organic
acids) and by some proteins that speciically bind and transport Ni.
Nickel, in low concentrations, fulills a variety of essential roles in plants, bacteria,
and fungi. Therefore, Ni deiciency produces an array of effects on growth and metab-
olism of plants, including reduced growth, and induction of senescence, leaf and mer-
istem chlorosis, alterations in N metabolism, and reduced Fe uptake. In addition, Ni
is a constituent of several metallo-enzymes such as urease, superoxide dismutase,
NiFe hydrogenases, methyl coenzyme M reductase, carbon monoxide dehydrogenase,
acetyl coenzyme-A synthase, hydrogenases, and RNase-A. Therefore, Ni deiciencies
in plants reduce urease activity, disturb N assimilation, and reduce scavenging of
superoxide free radical. In bacteria, Ni participates in several important metabolic
reactions such as hydrogen metabolism, methane biogenesis, and acetogenesis.
Although Ni is metabolically important in plants, it is toxic to most plant species
when present at excessive amounts in soil and in nutrient solution. High Ni concen-
trations in growth media severely retards seed germinability of many crops. This
effect of Ni is a direct one on the activities of amylases, proteases, and ribonu-
cleases, thereby affecting the digestion and mobilization of food reserves in germi-
nating seeds. At vegetative stages, high Ni concentrations retard shoot and root
growth, affect branching development, deform various plant parts, produce abnor-
mal lower shape, decrease biomass production, induce leaf spotting, disturb mitotic
root tips, and produce Fe deiciency that leads to chlorosis and foliar necrosis.
Additionally, excess Ni also affects nutrient absorption by roots, impairs plant
metabolism, inhibits photosynthesis and transpiration, and causes ultrastructural
modiications. Ultimately, all of these altered processes produce reduced yields of
agricultural crops when such crops encounter excessive Ni exposures.

Acknowledgment This review article has been extracted from “Review of Literature” section of
the PhD thesis of Mr. Muhammad Sajid Aqeel Ahmad (99-ag-1464).