Different concentrations of Ni are found in agricultural soil
released from various environmental sources. The responses
of plants grown in such soil-substratum vary accordingly
depending upon the concentration of metal present and
plant species. To assess the toxic effects of Ni and extent
of plant defensive strategies, increasing Ni doses (50, 100,
and 150 mg kg
-1) were used in sandy-loam soil-substratum
at pH 7.9 to evaluate the performance of
Vigna species
(
V. cylindrica
[L.] Skeels,
V. mungo
[L.] Hepper, and
V. radiata
[L.] R. Wilczek). The experiment was conducted in
a complete randomized design. Nickel stress was induced
by adding various concentrations of Ni chloride in soil
substratum. Malondialdehyde (MDA) and antioxidant
levels were determined in roots and leaves. Escalating
levels of Ni in soil resulted in an affirmative relationship
between MDA with that of antioxidants. A dose-dependent
increase in the activity of Superoxide Dismutases (SOD),
Catalase (CAT), and peroxidases (POD) suggested the
existence of a sequence response of these enzymes to
scavenge oxidative stress in the roots. However, inadequate
production of SOD and CAT appeared to be compensated
by the enhanced activity of POD, which acted as potent
quencher to reactive oxygen species (ROS) in leaves. At
the most elevated Ni dose, SOD, CAT and POD activities
were insufficient to counteract ROS generated that led to
membrane damage manifested by elevated MDA levels.
Nevertheless, SOD and CAT alleviated Ni toxicity in roots
while SOD, CAT and POD acted in a concurrent manner
to protect leaves from oxidative damage in
V. cylindrica.
The study clearly indicated a Ni dependent antioxidant
enzymes defense system in
V. cylindrica.
