PHYSIOLOGICAL AND BIOCHEMICAL STUDIES OF HEAVY METAL TOXICITY IN AQUATIC PLANTS

Transcription

1 PHYSIOLOGICAL AND BIOCHEMICAL STUDIES OF HEAVY METAL TOXICITY IN AQUATIC PLANTS CHAPTER 1 INTRODUCTION

2 Increase in environmental pollution caused by heavy metals, due to industrial and agricultural activities, is becoming a significant problem in the world. In natural environment, trace amounts of heavy metals are essential, but become toxic when their concentration exceeds certain levels. Plants are the best indicators of such hidden killers and they also develop the capacity to tolerate metabolically such toxic pollutants including heavy metals. Aquatic ecosystem is one of the most vulnerable places for such pollution because of different effluents that may drain off finally to some water bodies subjecting the flora of that system to encounter toxic substances. The aquatic plants absorb metals from air, water and soil. The uptake depends upon the soil ph, drainage status, plant species, chemical form and location. Aquatic ecosystem act as the receptacle for various contaminants from natural and anthropogenic sources such as effluents from mines, smelters, excessive use of fertilizers and pesticides, and partially from aerial deposition. Although most environments are not heavily contaminated, the low levels observed nonetheless pose a high risk of heavy metal accumulation in the food chain. Therefore, approaches to develop plants with reduced heavy metal uptake are important. In natural condition, plant may be exposed during their ontogeny to a variety of stresses of which heavy metal is known to be one of the great environmental stress affecting tremendously the mortality of plants and low yield growing in both soil and aquatic environments. Heavy metals toxicity may be best defined as activity of a toxic ion (Levitt, 1980). Compared to extended and systematic studies of heavy metal toxicity on various plants processes, very little has been done for Arsenic (As), Copper (Cu) and Zinc (Zn). Aquatic plants are one of the important components of the natural ecosystems (Culley et al., 1981). Despite this, it has been the focus of intense

3 genetic, biochemical, physiological and molecular study for over 40 years because of several traits that make it very desirable for laboratory study. Heavy metal stress imposes reduced growth, impaired metabolism and affects various others plant processes. Though it is well documented in certain literatures that heavy metal stress affects various plant processes but very little is known about the biochemical and molecular alterations to the detrimental effects of heavy metals responses of aquatic plants. Heavy metal toxicity is an important abiotic stress, which induces oxidative damage in both aquatic and terrestrial plants and affects antioxidant systems, altering different physiological and biochemical processes (Upadhyay and Panda, 2005; Panda and Upadhyay, 2008) and causes significant losses in productivity. Imposition of heavy metal stress may increase generation of reactive oxygen species (ROS), such as superoxide radicals (O2"), hydrogen peroxide (H2O2), hydroxyl radical (OH), alkoxyl radical (RO), singlet oxygen ('O2) within the cell, particularly within the chloroplast of water stressed leaves. This leads to lipid peroxidation, protein degradation, enzyme inactivation and affect nucleic acid and almost every component of cell leading to cell death (Hendry 1993, Panda 2002). To effectively eliminate the ROS, plant cells have well developed an integrated enzymic and non-enzymic antioxidant defense system (Asada, 1994). The non-enzymic antioxidant system includes reduced molecules like, glutathione; ascorbate, a-tocopherols etc. play important function in aquatic plants. The enzymic antioxidant includes superoxide dismutase (SOD, EC ), Catalase (CAT, EC ), ascorbate peroxidase (APX, EC ). Ascorbate in cell is regulated by ascorbic acid oxidase (AAO), where as glutathione an intermediary redox metabolite in the ascorbate-glutathione cycle of scavenging H2O2 is maintained

4 in the reduced state by glutathione reductase (GR, EC ). In addition, nonspecific peroxidase horseradish peroxidase (POX, EC ) and polyphenol peroxidase (PPO) also play important role in antioxidative defense, particularly in aquatic plants. The present investigation was undertaken for understanding the mechanism of heavy metal toxicity induced oxidative damage on imposition and its tolerance mechanisms, and the possible role of essential elements in enhancing tolerance process in the two selected model plants, Pistia stratioles L. and Spirodela polyrhiza L. The ability of plants to recover and resume rapid growth following metal imposition and its tolerance mechanisms is important for their vegetations. Various other nutrients are also essential throughout all the stages of plant development. A common response to heavy metal stress for some aquatic plants is the structural and morphological change of roots and leaves. Various hypotheses were proposed to account for the change of root and leaf aspects in stressed plant. Our study also revealed that heavy metal stress induced oxidative damage of aquatic macrophytes was correlated with dysfunction of antioxidant defense system (Upadhyay and Panda 2005; Upadhyay and Panda, 2009). Heavy metals are also a significant limiting factor for agricultural productivity and generally inhibit plant growth through reduced water absorption and nutrient uptake. In order to better understand the mechanisms of nutrient uptake in stress condition, an experiment was conducted with these two plants under heavy metal stressed and controlled conditions. In this study, we also investigate that the response of these two model plants to heavy metal stress in order to elucidate whether the nutrient status play any role to metal tolerance in freshly grown aquatic plants. In addition, differences in ultrastructural changes, various

5 physiological and biochemical changes including LOX and MDA were compared between the two selected model plants. In this study, an efforts has been made to understand the antioxidant regulations of heavy metal toxicity (metals like Cu, As,...etc) and some interaction studies on both the aquatic plants and their mechanism of tolerances to selected heavy metals. The present investigation aims at abridging the gaps on the possible biochemical mechanisms of heavy metal stress in aquatic plants that will give clues to future molecular studies. The major objectives set in the proposed investigation are as follows:- Measurement of various growth parameters as affected by heavy metals (like Cu, Zn, As... etc) stress under different exposure in Pislia stratiotes L. and Spirodella polyrhiza L. Determination of biochemical mechanisms of heavy metal toxicity and tolerance using model aquatic plants like P. Stratiotes L. and S. Polyrhiza L Determination of Lipoxygenase activity in Pistia and Spirodella plants under heavy metal stress. Atomic Absorption Spectroscopy analysis for metal uptake studies in both the aquatic plants. Electron microscopic studies in both aquatic plants under heavy metal stress.