Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 144

Rauser and Dumbroff ( 1981 ) examined water relations in bean seedlings exposed
to excess Ni and Co. These authors reported no signiicant change in the relative
water content (RWC) of bean leaves during early periods of metal stress. However,
RWC decreased signiicantly during later periods of stress. These authors also
reported that metal-treated plants had a signiicant increase in leaf water potential of
unifoliate leaves on the irst day the metal was applied. However, on the second day,
the water potentials in leaves treated with Co and Zn returned to the levels observed
in the controls, whereas leaves in the Ni treatment showed a steep decline in water
potential values.
When Ni stress is present, plant water relations may undergo changes, possibly
from the accumulation of compatible solutes/osmotica. For example, proline accu-
mulates in plants under Ni stress (Ali and Saradhi 1991 ; Mehta and Gaur 1999 ;
El-Enany and Issa 2001 ; Lin and Kao 2007 ). In addition, other osmotica, such as
soluble sugars, and other free amino acids, also accumulate in Ni-stressed plants
(Baccouch et al. 1998a ). These solutes can signiicantly reduce the water and solute
potentials in Ni-stressed plants. Sharma and Dietz ( 2006 ) believe that the proline
effects on osmoregulation under Ni stress are obvious, but the effects of other
osmotica (e.g., sugars, amino acids, and proteins), are less clear.

9.6 Free Amino Acids

In metal stressed environments, free amino acids may accumulate in cultivated
crops. The accumulation results in a decrease in the total protein concentration
(Ali et al. 2009 ). Various amino acids that include alanine (Bhatia et al. 2005 ),
arginine (Ali et al. 2009 ), asparagine (Smirnoff and Stewart 1987 ), glycine (Bhatia
et al. 2005 ; Ali et al. 2009 ), cysteine (Freeman et al. 2004 ; Ali et al. 2009 ), glu-
tamic acid (Homer et al. 1997 ), histidine (Krämer et al. 1996 ; Ali et al. 2009 ),
lysine (Ali et al. 2009 ), methionine (Ali et al. 2009 ), proline (Gajewska et al. 2006 ;
Maheshwari and Dubey 2007 ; Gajewska and Skłodowska 2005 ; 2008 ), and serine
(Ali et al. 2009 ), are known to accumulate at high concentrations, when metal (i.e.,
Ni, Zn, Cr, Cd, Pb) concentrations are high. Some amino acids, such as proline,
function as protectants, while others, such as asparagine, function as chelating
agents that form metal-ligand complexes with Cd, Pb, Ni, Zn, etc. (Homer et al.
1997 ). High accumulations of free amino acids may, therefore, protect stressed
plants from the toxic effects of speciic metals through a detoxiication mechanism
(Clemens 2001 ; Hall 2002 ).
The accumulation of an amino acid in response to heavy metal exposure is a
species-speciic trait that varies greatly among species, particularly in hyperaccu-
mulator or metal-tolerant plants. White et al. ( 1981 ) reported that the major portion
of Cu and Ni was bound to asparagine and histidine in xylem luids. Smith and
Martell ( 1989 ) suggested that, among all the organic acids and proteinacious amino
acids, histidine has the highest association constant for complexation with Ni.
Harmens et al. ( 1993 ) reported that cysteine titers increased in tolerant and sensitive
ecotypes of Silene vulgaris by factors of 4.5 and 3.8, respectively, in response to