Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 126

10 Yield and Yield Components ............................................................................................. 148
11 Anatomical Changes .......................................................................................................... 149
12 Conclusions........................................................................................................................ 150
13 Summary ............................................................................................................................ 151
References.................................................................................................................................. 152

1 Introduction

Nickel is one of 23 metal pollutants of great concern to the environment and to
human health (Sunderman 1992 ; Jarup 2003 ; Duda-Chodak and Baszczyk 2008 ).
Nickel is the 24th most abundant element (twice as Cu) and comprises approxi-
mately 0.008% of the content of the earth's crust; hence, it is a natural component of
soil (parent material) and water (Alloway 1995 ; Hostýnek and Maibach 2002 ; Hedi
et al. 2007 ). Most of the earth's nickel, however, is inaccessible, as it is locked in the
iron-nickel molten core, which constitutes approximately 10% nickel. The second
largest Ni deposits of the earth rest in the sea. It is estimated that the sea contains
approximately eight billion tons of Ni, either dissolved in seawater or deposited in
the seabed (Birch 1964 ; Stixrude et al. 1997 ). Soils may contain nickel levels as low
as 0.2 mg kg −1 or as high as 450 mg kg −1 . The average nickel content in soil is
approximately 20 mg kg −1 ; however, the content level may vary greatly depending
upon the mode of origin of the soil's parent material (Assembly of Life Sciences
1975 ; Aubert and Pinta 1978 ; Wilson and Kordybach 2000 ). Because organic matter
strongly absorbs some metals, particularly nickel, fossil fuels such as coal and oil
may contain considerable amounts of nickel (Sigel et al. 2005 ). Moreover, Ni natu-
rally occurs in a few plants (legumes) where it functions as an essential component
of some enzymes (e.g., ureases) that are involved in nitrogen assimilation (Eskew
et al. 1984 ; Brown et al. 1987a ; Sakamoto and Bryant 2001 ). Tea plants may contain
high Ni levels (at concentrations up to 5.3 mg kg −1 in dried leaves), whereas Ni levels
in other plants vary: instant tea (15.5 mg kg −1 ), cacao powder (9.8 mg kg −1 ), cashews
(5.1 mg kg −1 ), soy protein (4.3 mg kg −1 ), walnuts (3.6 mg kg −1 ), ilberts and peanuts
(1.6 mg kg −1 ), almonds (1.3 mg kg −1 ), wheat germ (1 mg kg −1 ), pistachios
(0.8 mg kg −1 ), and rice (0.4 mg kg −1 ) (Nielsen 1993 ). The nickel content in some
vegetables varies from 0.26 mg kg −1 (beans) to 0.08 mg kg −1 (tomatoes). Some fruits,
such as peaches (0.16 mg kg −1 ) and apples (0.03 mg kg −1 ), may also contain moder-
ate amounts of nickel (Nielsen 1993 ).
The National Pollutant Inventory (NPI) of the Government of Australia ranked
400 hazardous materials in order of their relative potential hazards and probable
exposure levels. Nickel and its compounds were ranked as number 54 (National
Pollutant Inventory 1999 ) on this list. In another ranking by the same organization,
environmental contaminants were ranked on a scale of 0-3, based on (1) the extent
of their toxic or poisonous nature, (2) their ability to remain active as an environ-
mental pollutant, and (3) their ability to accumulate in living organisms. Nickel and
its compounds were rated at 1.2 as a health hazard and 1.0 as an environmental
hazard (National Pollutant Inventory 1999 ).
Nickel has long been known as an important plant micronutrient, and for having a
multitude of biological functions (Eskew et al. 1983 ; Kochian 1991 ; Welch 1995 ;