Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 136

50% germination (Espen et al. 1997 ; Leon et al. 2005 ). Maheshwari and Dubey
( 2008 ) reported a signiicant reduction in the germination of rice seeds, when high
concentrations of Ni were present. These authors showed that exposure of seedlings
to Ni for 120 h reduced germination by 12-20%, reduced plumule and radicle
lengths by 20-53%, and reduced fresh weight by 8-34%. In addition, Ni stress
increased the concentration of soluble proteins by 58-101%, and augmented
amounts of total free amino acids present in the endosperm and embryo axes by
39-107%. The authors suggested that the high concentration of Ni in the growth
medium of rice seedlings imposed stress that resulted in decreased hydrolysis and
delayed mobilization of the protein reserves present in endosperm. Hence, high
levels of Ni can cause imbalances in the concentration of proteins and amino acids
in growing embryo apices. Such effects of high Ni levels may ultimately result in
decreased seed germination of most plants.

8.2 Seedling Growth

Although lower Ni levels improve early seed growth, high levels may be extremely
toxic and may inhibit seedling growth (Veer 1989 ; Nedhi et al. 1990 ; Leon et al.
2005 ). The reduction in plumule and radicle lengths and reductions in fresh and dry
weights were reported for several plant species exposed to high Ni concentrations
(e.g., sunlower (Singh et al. 2004 , Ahmad et al. 2009 ), radish (Espen et al. 1997 ),
Vigna radiata (Jagetiya and Aery 1998 ), and tomato (Kowalczyk et al. 2003 )). Such
high Ni level-induced effects resulted from the inhibition of key enzymes involved
in digestion of food reserves (protease and a- and b-amylases), protein synthesis,
carbohydrate metabolism, and mobilization of food reserves (Bishnoi et al. 1993a ;
Lin and Kao 2006 ; Maheshwari and Dubey 2007 ). In addition, high Ni levels are
known to strongly interfere with mineral nutrient uptake, thereby altering nutrient
concentrations in germinating seeds (Gabbrielli et al. 1990 ; Rubio et al. 1994 ; Molas
1997 ; Ahmad et al. 2007 ). Because micro- and macronutrients are often involved in
a variety of metabolic processes, and some are required as cofactors and enzyme
activators, any deiciency of them may suppress certain metabolic phenomena
involved in regulating seed germination.

8.3 Nutrient Uptake and Transport

The uptake and translocation of plant nutrients during germination and early seed-
ling growth is highly sensitive to the presence of high Ni levels in the growth
medium (Cataldo et al. 1978 ; Gabbrielli et al. 1990 ). This sensitivity may result
from either the direct effect of high Ni on seed imbibition or alterations in the nutri-
ent uptake pattern in germinating seeds (Seregin and Kozhevnikova 2005 ). However,
symptoms of Ni toxicity, caused by Ni stress, may take some time to appear when