Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 135

the branching system (Reeves et al. 1996 ), deformation of various plant parts (Wright
and Welbourn 2002 ), abnormal lower shape (Mishra and Kar 1974 ; McIlveen and
Negusanti 1994 ), decreased biomass production (Rahman et al. 2005 ), leaf spotting
(Gajewska et al. 2006 ), mitotic root tip disturbances (McIlveen and Negusanti 1994 ),
inhibition of germination (Nedhi et al. 1990 ), Fe deiciency leading to chlorosis
(Ewais 1997 ; Kirkby and Römheld 2004 ), and foliar necrosis (Kukkola et al. 2000 ).
Excess Ni levels also affect nutrient absorption via roots (Kochian 1991 ; Hasinur
et al. 2005 ), impair plant metabolism (Pandey and Sharma 2002 ), and inhibit photo-
synthesis and transpiration (Sheoran et al. 1990 ). Ultimately, impairment of these
processes leads to reduced agricultural crop yields, when excessive Ni levels are
present in soils.

8 Effect of Nickel on Seed Germination

Seed germination is one of the most important stages in crop development that
preordains establishment of healthy crop stands, better growth and yield at the later
growth stages. Germination is strongly inluenced by several factors, such as pres-
ence or absence of light, germination time, water status, and mineral composition
of soil (Mott 1974 ; Chachalis and Reddy 2000 ; Zlesak 2007 ). In addition, environ-
mental stresses such as salinity, drought, and presence of excessive amount of met-
als in soil also inluence the germination ability of plant species (Peralta-Videa
et al. 2001 ; Houlam and Fares 2001 ; Li et al. 2002 ; Munzuroglu and Geckil 2002 ;
Zia and Khan 2004 ).
Although excess Ni, when present, may produce effects at any crop development
stage, plant sensitivity to heavy metal or other environmental stress is high at the
germination stage (Leon et al. 2005 ). Increasing concentrations of Ni have been
shown to inhibit seed germination and seedling growth in many plant species (Espen
et al. 1997 ; Leon et al. 2005 ). Ni-induced growth inhibition has been ascribed to
downregulation of protein synthesis and to effects on the activities of certain key
enzymes responsible for mobilizing food reserves from the endosperm of cotyle-
dons (Foy et al. 1978 ; Bishnoi et al. 1993a ). In addition, Ni is known to be a com-
petitor for several essential micro- and macroelements and may reduce the uptake of
elements into germinating seeds, thereby producing poor germination and seedling
establishment (Cataldo et al. 1978 ; Körner et al. 1987 ; Kochian 1991 ). The inhibi-
tory effects of high Ni concentrations on seed germination are elaborated on below.

8.1 Germination Percentage and Rate

Nickel, acting as a micronutrient, can improve both the rate and percentage of
seed germination. However, high levels of Ni in the growth medium has led to a
marked inhibition of germination and has delayed the time necessary to achieve