2 Journal of Food Legumes 263 4, 2013
the chloroplast development in young leaves and the inhibition of photosynthesis Boddi et al. 1995.
The sources of heavy metals pollution can be both natural and anthropogenic. Natural sources include the mother
rocks and minerals of the metals while the anthropogenic sources are agriculture, black and colored metallurgy and
transport Budnikov 1998.There are many records that the places adjacent to the industrial places have been affected
mainly by the toxic heavy metals Rao 1979. Human activity has contributed metal concentrations in soil range from less
than 1 mgkg to as high as 100,000 mgkg Blaylock and Huang 2000, which have lead to the disruption of natural, aquatic
and terrestrial ecosystems. Above certain concentrations and over a narrow range these heavy metals can turn into toxic
products Babich and Stotzky 1978. It is reported that the continuous exposure to the heavy metals have severely
affected the health of human beings, plants, animals and the microbial populations that have disrupted the ecological
habitats Sterritt and Lester 1980, Brynhildsen and Rosswall 1997.
2. Toxic effects of various heavy metals on plants
Heavy metals cause acute toxic effects on plants grown on the contaminated soils. Their toxicity results in chlorosis,
weak plant growth, yield depression accompanied by reduced nutrient uptake, disorders in plant metabolism and, in
leguminous plants, a reduced ability to fix molecular nitrogen Chaudri et al. 2000, Dan et al. 2008. Agricultural yield is
decreased due to their prevalent use and cause dangerous health effects as they enter into the food chains Schickler
and Caspi 1999. Heavy metals are transmitted through the food chains and strongly affect the human health Brun
et al.
2001, Gincchio et al. 2002. There are metals like Pb, Hg, Cd, Ar and Cr, which have no biological function but are toxic
to life even at very low concentration Salt et al. 1995. Many metals are nutrients at permissible levels but act as toxins
when the concentrations found in the soil exceed to those required as the nutrients. Arsenic is non-essential element
and generally toxic to plants. Symptoms include poor seed germination and marked reductions in root growth. Plants may
show reduced growth, nutrient deficiencies and chlorosis resulting from reduced chlorophyll biosynthesis Mascher
et al.
2002.Cadmium is also a non-essential element, the accumulation of which may cause several physiological,
biochemical and structural changes in plants Feng et al. 2010. It also alters mineral nutrients uptake, cause inhibition in
stomatal opening by interacting with the water balance of plant Hossain et al. 2010, disturbs the Calvin cycle enzymes,
photosynthesis, carbohydrate metabolism Shi et al. 2010 and ultimately reduces the crop productivity di Toppi and
Gabreilli 1999.There are reports that excess of chromium causes inhibition of plant growth, chlorosis in young leaves,
nutrient imbalance, wilting of tops, and root injury Scoccianti et al.
2006. It inhibits the chlorophyll biosynthesis in terrestrial plants Vajpayee et al. 2000. High levels of Cr affected total
dry matter production and yield of plants Shanker et al. 2005. Like other heavy metals, chromium toxicity produces chlorosis
and necrosis in plants Cervantes et al. 2001.
Copper is considered as an important micronutrient for
plants Thomas et al.1998 and plays important role in CO
2
assimilation and ATP synthesis. It is known to be an essential component of various proteins like plastocyanin of
photosynthetic system and cytochrome oxidase of respiratory electron transport chain Demirevska-kepova et al. 2006 as
well as other various proteins. In acute Cu toxicity, leaves may become wilted before eventually becoming necrotic Yau
et al
. 1991. Plants can accumulate some amount of cobalt from the soils but the uptake and distribution of cobalt in
plants is species-dependent and is controlled by different mechanisms Kukier et al. 2004, Li et al. 2004 and Bakkaus
et al. 2005. There are some reports that Co adversely affects
the shoot growth and biomass in some plants Li et al. 2009. Lead is a major heavy metal that has gained importance as a
potent environmental pollutant Sharma and Dubey 2005. Lead affects a number of plants by decreasing the productivity of
plants growing in lead- polluted soils Johnson and Eaton 1980. Its phytotoxicity leads to inhibition of number of enzyme
activities, disturbances in the mineral nutrition, water imbalance, and changes in the hormonal status and alteration
in the membrane permeability. At high concentrations, lead may even lead to cell death Seregin and Ivanov 2001.
Mercury is also a very important toxic metal. Exposure to the mercurials in crop plants are by direct administration as
antifungal agents, through the seed treatment or foliar spray, or accidentally Patra and Sharma 2000. It affects both the
light and dark reactions of photosynthesis by the substitution of magnesium of chlorophyll thereby resulting in the
breakdown of photosynthesis. It binds to water channel proteins, thus inducing leaf stomata to close and physical
obstruction of water flow in plants Zhang and Tyerman 1999. High levels of mercury interferes with the mitochondrial
activity and induces oxidative stress by triggering the generation of ROS thus leading to the disruption of
biomembrane lipids and cellular metabolism in plants Israr et al.
2006, Cargnelutti et al. 2006. Nickel also causes phytotoxicity by altering
physiological reactions and diverse toxicity symptoms such as chlorosis and necrosis in different plant species Zornoza
et al. 1999, Pandey and Sharma 2002, Rahman et al. 2005,
including rice Samantaray et al. 1997. Yadav 2009 have reported that plants grown in high Ni
2+
containing soil showed imbalance of nutrients and resulted in disorder of cell
membrane functions.Zn concentrations in the range of 150 to 300 mgkg has been categorized as toxic that inhibits various
plant metabolic processes, results in reduced growth and cause senescence. Shoot and root growth also decrease by
Kaur Nayyar : Heavy metal toxicity to food legumes: effects, antioxidative defense and tolerance mechanisms 3
the application of zinc Fontes and Cox 1998. Prolonged exposure to zinc causes chlorosis in the younger leaves,
which can extend to older leaves Ebbs and Kochian 1997.
3. Effects on Legumes
