Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 139

8.7 Changes in Proteins and Amino Acids

A considerable increase in free amino acid levels occurs in germinating seeds
exposed to heavy metals. When such exposure occurs in root growing media, a
variety of biochemical and molecular responses are initiated, and these help the
plant to tolerate hazardous metal ions. Exposure to Ni caused enhanced accumula-
tion of free amino acids, particularly alanine, proline and asparagine, which is
regarded as an indicator of metabolic disruption, in young seedlings of soybean
(El-Shintinawy and El-Ansary 2000 ). These authors reported that Ni stress (200 mM)
induced the accumulation of free amino acids in the roots of soybean seedlings.
They also reported that cysteine was the most accumulated amino acid (17% of total
free amino acids present), when exposed to Ni stress. Maheshwari and Dubey ( 2008 )
reported that protease activity decreased and protein concentration increased, when
plants were exposed to high Ni regimes. The level of free amino acids in rice seed-
lings was increased under a high Ni regime, and the authors suggested that free
amino acids may be produced in seedlings under Ni stress at the cost of mainte-
nance of developmental processes (Ali and Saradhi 1991 ). In summary, protein deg-
radation under high Ni regimes may enhance the accumulation of crop plant amino
acids, particularly at initial growth stages.

9 Effects of Nickel at the Vegetative Stage

9.1 Growth Attributes

High concentrations of Ni reduce vegetative growth parameters, including plant
height and fresh and dry biomass production in several agricultural crops (MacNicol
and Beckett 1985 ; Seregin et al. 2003 ). As mentioned, the general signs of Ni toxic-
ity in plants include reduced shoot and root growth (Rahman et al. 2005 ; Sabir et al.
2011 ), poor branching (Reeves et al. 1996 ), deformed plant parts (Wright and
Welbourn 2002 ), abnormal lower shape (Mishra and Kar 1974 ; McIlveen and
Negusanti 1994 ), decreased biomass production (Pandey and Sharma 2002 ; 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-induced chlorosis (Ewais 1997 ; Kirkby and Römheld 2004 ), and foliar
necrosis (Kukkola et al. 2000 ).
Since roots are directly exposed to excess Ni levels, they may display reduced
growth, and proliferation and deformation in shape long before symptoms of toxicity
appear in aerial plant parts (Wong and Bradshaw 1982 ; Yang et al. 1996 ; Kopittke
et al. 2007 ). For example, a high concentration of Ni is known to inhibit the produc-
tion of new root hairs and deform existing ones in many plants (e.g., Betula papy-
rifera (paper birch) and Lonicera tatarica (honeysuckle); Patterson and Olson 1983 ;
Rauta et al. 1995 ; Atta-Aly 1999 ). Similarly, the process of initiation and proliferation