Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 133

methyl coenzyme M reductase, carbon monoxide dehydrogenase, acetyl coenzyme-A
synthase, hydrogenases, and RNase-A (Ermler et al. 1998 ; Brown 2006 ; Küpper and
Kroneck 2007 ; Ragsdale 2009 ) . A list of various Ni-dependent enzymes is presented
in Table 1 . Any deiciency of nickel in plants is likely to disrupt speciic metabolic
phenomena, thereby producing visual symptoms of Ni deiciency (Brown et al. 1990 ;
Bai et al. 2006a ; Wood et al. 2006 ). However, the amount of Ni normally required by
plants is generally very low, and is adequately provided by most soils; hence, dei-
ciency symptoms usually do not appear in plants under natural conditions.
The most important effect of Ni deiciency, when it appears, is on the activity of
urease, which catalyzes the conversion of urea to ammonium, and is thus involved
in N-assimilation in plants (Spears et al. 1977 ; Bast 1988 ; Gerendás and Sattelmacher
1997a ; Colpas and Hausinger 2000 ; Benoit et al. 2007 ). Plants grown in Ni-deicient
nutrient solutions may develop visual symptoms of Ni-deiciency that are generally
associated with hampered N-metabolism (Gerendás et al. 1999 ; Brown 2006 ; Reddy
2006 ; Wood et al. 2006 ). For example, Ni-deicient soybean ( Glycine max L.) plants
were shown to have depressed urease activity in their leaves that resulted in accu-
mulation of urea at toxic levels in lealet tips (Eskew et al. 1983 ). Similarly, Walker
et al. ( 1985 ), working with cowpeas [ Vigna unguiculata (L.) Walp], suggested that
Ni (and urease) participates in N metabolism of legumes during the reproductive
phase of growth. Checkai et al. ( 1986 ) reported that Ni-deicient tomato plants
( Lycopersicon esculentum L.) developed chlorosis in young leaves, and ultimately,
meristematic necrosis. In some aquatic organisms, nickel is involved in the mainte-
nance of homeostasis, and thus plays an important role in water balance, which is
required for optimal growth (Muyssen et al. 2004 ). Lower levels of nickel are also
known to be essential for initiating the reproductive phase and for seed development
(Brown 2006 ; Tabatabaei 2009 ). Hence, some Ni-deicient plants may fail to pro-
duce viable seeds and are unable to complete their life cycle. From the foregoing, it
can clearly be seen that Ni is essential for the growth and development of most
plants, including some crops.

6 Causes and Symptoms of Nickel Deiciency in Plants

Despite the fact that nickel is essential for normal plant functioning, symptoms of
natural deiciency are uncommon because most soils contain adequate amounts of
the element (Lindberg and Greger 2002 ). Moreover, deiciency symptoms produced
by Ni are very similar to those produced by deiciencies of other essential plant
nutrients (Wood et al. 2004, 2006 ). For example, Brown ( 2006 ) showed that the
symptoms of nickel deiciency are very similar to those of zinc and copper, the dei-
ciency of which may arise from complex soil and environmental interactions that
inluence plant uptake. In addition to soil physicochemical properties (such as pH),
nickel deiciency may be induced by excessive concentrations of zinc, iron, manga-
nese, copper, magnesium, or calcium in the soil solution (Wood et al. 2004 ).
Observations also suggest that long-term and excessive use of fertilizers containing