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Nickel (Ni) is a micronutrient required at very low concentrations by the plants (Brown
et al., 1987), but high concentrations are reported to be inhibitory (Gonnelli et al., 2001;
and Wang et al., 2001).
Mercury is a nonessential element, extremely toxic to plants and has a long half-life and
thus extremely persistent in the environment. Exposure of plants, even to its minute
concentrations, may lead to the alteration of many cellular processes and structure.
Heavy metals produce physiological stress leading to formation of free radicals at high
concentration. Stresses form Reactive Oxygen Species (ROS). Therefore, a mechanism to
interrupt the autocatalytic process is required. Under normal situation, concentration of
oxygen radicals remains low because of the activity of protective enzymes, including
Superoxide Dismutase (SOD), catalase and ascorbate peroxidase (Asada and Takahashi,
1987). SOD is a metalloenzyme that catalyzes dismutation of superoxide anion into oxygen
and hydrogen peroxide. Such enzymes provide a defense system for the survival of aerobic
organisms (Beyer et al., 1991). Plants resort to heavy metals by altering the levels of
antioxidants and antioxidative enzymes (Noctor and Foyer, 1998; and Pandey et al., 2005).Several hormones are incriminate in tempering the plant responses to oxidative stress,
including ethylene (Vahala et al., 2003), abscisic acid (Kovtun et al., 2000), salicylic acid
(SA) (Metwally et al., 2003) and brassinosteroids (BRs) (Cao et al., 2005).
Brassinosteroids are a class of phytohormones distributed all over the plant kingdom. These
compounds, which include the highly bioactive brassinolide (BL) and its analogous, have been
regarded as new plant growth regulators essential for plant development and for improving
their quality and yield when applied. They protect the plants against a number of stresses
like chilling (Dhaubhadel et al., 1999), salt (Ozdemir et al., 2004), heat (Dhaubhadel
et al., 2002) and heavy metals (Janeczko et al., 2005). However, it is not clear whether
BRs are involved in the modulation of plant responses to oxidative stresses. The influence
of BRs on the response of the antioxidative enzymes of plants under stress conditions was
studied recently (Cao et al., 2005; and Hayat et al., 2007a and 2007b). The available data
show that the changes in the activity of antioxidative enzymes by BRs differed with plant
species and stress conditions (Ozdemir et al., 2004; Almeida et al., 2005; and Hayat
et al., 2007a and 2007b).
Brassica juncea is an important oil seed crop chiefly grown in semi-urban areas which
are generally irrigated with sewage water containing high levels of heavy metals. Hence, the
present investigation is an attempt to study the effects of Hg and Ni toxicity pertinent to
seedling growth, antioxidant defense mechanism and their possible mitigation by
homobrassinolide.
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