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Rice (Oryza sativa L.) is one of the most favoured world cereals and is staple food for approximately half of the worlds population (Dowling et al., 1998; Itani et al., 2002). According to an estimate more than 148 million hectares area is under rice cultivation in 114 countries of the world (FAO, 2006). Two fold increase in the rice consumers is expected by 2025. On a global basis, rice provides 21 and 15% per capita of dietary energy and protein, respectively (Maclean et al., 2002).
Approximately, 90% of the world's rice is grown in less developed countries, especially in Asia, including Pakistan (Mae, 1997). Khush (2001) indicated that, to ensure world food security, the rice production in Asia must increase from 545 million tons to 700 million tons by year 2025 (Cooper, 1999; Munson, 1985; Marschner, 1995; Fageria and Baligar, 1997).
Rice ranks as second largest staple food grain crop and a major earning source of foreign exchange of Pakistan. High quality of rice is produced for export and fulfilling of domestic needs in Pakistan, which account's 1.0% of Pakistan's GDP and 4.9 % of the value added in agriculture. During 2011-12, in Pakistan it was cultivated on 2571 thousand hectares with the production of 6160 thousand tons (MOF, 2012). However present national yield is far lower than its demand. Beside other environmental factors (biotic and abiotic) heavy metals are the major constraints that reduce the rice production. Recently the environment is polluted by various heavy metals which creat a danger for all living beings. These metals reduce/retard farming efficiency and destruct the health of plants and animals. Metals in terrestrial ecosystems are important for their influence on development and growth of plants (Lepp, 1981, Alloway, 1995, Hall & Williams, 2003). However, soil ecosystems are contaminated with heavy metals by human-induced activities (Naidu et al., 1996, Younas & Shahzad, 1998). A toxic concentration of heavy metals is not known in agricultural soils; however, land disposal of wastes as soil amendments for crop production is responsible for temporal accumulation of heavy metals in soil (Nriagu & Pacyna, 1988, Younas and Shahzad, 1998). Once present in the soil, the heavy metals are persistent (Alloway, 1995).
Heavy metals are phytotoxic either at all concentrations or above certain threshold levels. Toxic metals are biologically magnified through the food chain. They infect the environment by affecting soil properties its fertility, biomass and crop yields and ultimately human health. It is a big issue of accumulation of heavy metals in soils as a result of industrial effluents and atmospheric emissions like paper mill, fertilizers, glasses and Mining wastes. The presence of heavy metals in toxic concentrations can result in the formation of superoxide radicals (O2-), hydrogen peroxide (H2O2), hydroxyl radicals (OH-), etc., can cause severe oxidative damage to biomolecules like lipids, proteins and nucleic acids (Van Assche and Clíjsters, 1990).
Heavy metals also interfere with chlorophyll synthesis either through direct inhibition of an enzymatic step or by inducing deficiency of an essential nutrient (van Assche and Clíjsters, 1990). Heavy metal stress leads to proline accumulation (Alía and Saradhi, 1991). Proline increases the stress tolerance of plants through such mechanisms as osmoregulation, protection of enzymes against denaturation, and stabilization of protein synthesis (Kuznetsov and Shevyakova, 1997). Accumulation of free proline in response to heavy metal exposure seems widespread among plants (Costa and Morel, 1994). Pakistan is an agrarian country with high population growth. Soils are intensively cropped to meet the increasing demand for food production. However, soils of Pakistan are inherently low in fertility to support economic crop production (Rashid, 1993, Jamal et al., 2002). Due to high cost and scarcity of chemical fertilizers, the land disposal of agricultural, municipal and industrial wastes is widely practiced as a major and economic source of nutrients and organic matter for growing cereal crops by poor farmers in Pakistan (Rashid, 1993, Younas & Shahzad, 1998, Jamal et al., 2002). The most reported heavy metals in waste amended agricultural soils are Cu, Pb and Zn (Nriagu & Pacyna, 1988, Younas & Shahzad, 1998, Jamal et al., 2002).
Zinc is one of the micronutrients essential for plant growth but is toxic to plants at higher concentrations and can retard plant growth and disrupt various essential physiological processes (Cakmak and Marschner, 1993; Salt et al., 1995). An excess of Zn is indicated by a decrease in growth and development, metabolic activity and an induction of oxidative damage in various plant species (Panda et al., 2003).
Plants respond to heavy metals by adapting different strategies. While adaptation is considered as comparatively fast inheritable, biochemical, physiological and / or morphological changes for improving plant's resistance to the impact of a stress factor and enabling it to survive in modified/stressed environment (Lichtenthaler, 1996; Dat et al., 1999). The process of adaptation and increase resistance need additional energy and metabolites to restore homeostasis. As the general amount of energy and nutrients available to plants are limited, the growth and biomass formation is reduced to maintain adaptation. Therefore, it was assumed that adaptation to one stressor result in reduced tolerance, to the other stresses requiring another resistance pathway(Larcher, 1995; Godbold, 1998). However, adaptation of a plant to one stress factor can result in its increase tolerance to other stress requiring the same physiological and / or morphological modi¬cations, this phenomenon is called cross-adaptation or cross-tolerance (Godbold, 1998; Dat et al., 1999; Alexieva et al., 2003; Streb et al., 2008).
A number of investigations have shown that cross adaptation is not only possible in case of subsequent impact of heavy metals , but a much more general event. Plant's exposure to any moderate stressor induces increased resistance to different stress factors (Sabehat et al., 1998, Streb et al., 2008). For instance, salt stress increase cold tolerance (Ryu et al., 1995); and heat stress increase tolerance against heavy metal toxicity (Bonha-Smith et al., 1987). Cross adaptation is attributed to the fact that at a cellular level the effects of different stressors are the same (Noctor, Foyer,1998; Dat et al., 2000; Taiz, Zeiger, 2002). Plant's tolerance to different stressers seems to involve a process with integrated network of multiple response (Chrispeels and Sadava, 2003; Fosket, 1994).
However, the adverse effects of heavy metals on cereal crops in response to widespread land disposal of unregulated wastes as agricultural soil amendments have received a very limited attention in Pakistan (Mahmood, 1995, Mahmood & Islam, 2005). Keeping this in view in current study an attempt has been made to investigate the growth and physiological responses of a stress adapted rice cells line towards elevated level of Zn with the following objective.