Excess lead alters growth, metabolism and translocation of certain nutrients in radish
Introduction
In the past few decades increasing urbanization, cultivation and industrialization including mining activities have caused enormous chemical pollution. Among the various chemicals, lead is one of the most dangerous of the metals to human beings and other life (Rubio and Hardisson, 1999). Lead is omnipresent in various environmental matrices including air, water, soil and plants. Automotive sources are the major contributors of Pb emission to the atmosphere in the developing countries like India. Although in European and American countries the ambient Pb concentration has decreased considerably in last ten years. The toxicity of Pb is often hidden in plants as many accumulator plants can absorb heavy amount of Pb though not showing any visible symptoms of Pb toxicity (Judel and Stelte, 1977).
Lead has been shown to accumulate in plants from several sources including soil but the reports on accumulation of the Pb within plants are variable (Al Salman and Abdul Aziz, 2002). There are large differences in Pb deposition that has been reported in different plant species (Parasyuta et al., 2000). On leaf surfaces Pb deposition are known to cause adverse physiological effects either by blocking the stomata or by disrupting metabolic pathways after entering into the leaf (Liu et al., 1999). The ill effects of Pb include interference with other nutrients uptake and translocation (Fodor et al., 1998), growth retardation (Uveges et al., 2002), changed activity of several enzymes (Geebelen et al., 2002), disturbed respiration (Romanowska et al., 2002) and photosynthesis (Banaszak et al., 2001, Sarma et al., 2006) within the plant system. An important consequence of heavy metal stress in plants is the excessive generation of reactive oxygen species (ROS) such as superoxide anions (), hydrogen peroxide (H2O2) and the hydroxyl radicals (OH) particularly in chloroplast and mitochondria (Dat et al., 1998, Mittler, 2002). These active oxygen species can rapidly attack all types of biomolecules such as nucleic acid, proteins, lipids and aminoacids, leading to irreparable metabolic dysfunction and cell death.
The present investigation therefore, attempts to evaluate detrimental effects of excess Pb on growth, yield, chloroplastic pigments, photosynthesis, translocation of certain essential nutrients (P, S and Fe) and antioxidative enzyme activities by growing the plants in refined sand with excess Pb.
Section snippets
Materials and methods
Radish (Raphanus sativus) cv. Jaunpuri was grown in refined sand at an ambient temperature (15–32 °C) under glasshouse conditions (Agarwala and Sharma, 1976), based on the modification of the method of Hewitt (1966) for Indian conditions in polyethylene containers (10 in.) with complete nutrient solution. Each pot has provided with a central drainage hole covered with an inverted watch glass with glass wool under the rim to allow free drainage of nutrient solution. The composition of complete
Visible symptoms
Radish grown at excess supply of Pb (0.05 mM) show visual symptoms of Pb toxicity late. After 20 days of metal supply (d 50) the growth of plant was depressed become more pronounced after 35 days of metal supply (d 65). Young leaves show mild interveinal chlorosis near apex. Later lamina curled inward and folded. At this stage leaves of these plants appeared rough and tapered. The number and size of the leaves were reduced. The length, girth and volume of radish roots reduced drastically at both
Discussion
The visible symptoms of excess Pb were apparent very late. At excess Pb (0.5 mM) the young leaves of the plants showed chlorosis and the symptoms were somewhat similar to that observed in barley leaves (Wozny et al., 1995). The decrease in growth of radish was gradual and depressed with increase in Pb concentration at 0.1 and 0.5 mM. At maturity, root development was very poor. The reduction in roots development under excess Pb conditions has been reported earlier for several other plants (Tomar
Conclusion
The detrimental effect of excess Pb in radish was evident in the form of retarded growth and reduced biomass. This was supported by increased activity of peroxidase, acid phosphatase and ribonuclease and decreased activity of catalase, in addition to the imbalance in the availability of essential nutrients including iron. In recent times, exposure to lead occurs mainly through inhalation of air and ingestion of Pb in food, water, soil or dust. It accumulates in blood, bones and soft tissues and
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