PHYTOTOXICITY OF LEAD ON LEGUMINOUS PLANTS PHASEOLUS MUNGO AND LENS CULINARIS

Abstract

Heavy metal is a major source of the edaphic pollution which is extensive incidence as consequences of modernity in industries and agriculture. Lead is a probable contaminant among all heavy metal that eagerly gathers in edaphic factors. In plant Lead is not a vital constituent but it enters and accumulates easily in various parts of the plant. Absorption of Lead in root is controlled by different physico-chemical parameters.Such as cation exchangeability of the soils, pH and particle size, as well as by root exudation. Lead is a predominantly hazardous element, as it can accumulate in individual organisms, but also in entire food chains. Rising in the uses of Lead made stuffs in our environment, lead to significant elevation of its toxicity in the atmosphere. This thesis examined the effect of toxic metal Lead on seedlings of two important crops widely used in Pakistan i.e., Phaseolus mungo L. and Lens culinaris Medik. those were cultivated in natural environment in nutrient medium in which Lead salt was added. Analysis revealed that Lens culinaris Medik. distinctly influence by Lead stress as compared to Phaseolus mungo L. Both leguminous species showed the prominent effect of Lead absorption in roots through the inhibition in seed and seedling germination, alteration in morphology of root and reduction in root elongation. Histomorphological revolution in vascular bundles of root in both pulses showed a rapid response of reduction in the ascent of sap which ultimately influences the other physiological and morphological attributes. Seed and seedling germination index, fresh and dry weight of root and shoot adversely affected by Lead toxicity. The degree of affection of Lead is mostly based upon the nature of the plant species. Lead effect the plant multidimensionality way, photosynthesis is one of the most familiar and sensitive phenomenon. UV/Visible double beam Schimadzo Spectrophotometer was used for examined the absorption spectrum of photosynthetic pigments. The alteration in absorption spectrum in response to elevation of Lead stress related to reduction in synthesis of chlorophyll with values of 1.415 to 0.426 for Lens culinaris Medik. and 1.004 to 0.023 for Phaseolus mungo L. viiAbstract The spectral change in photosynthetic pigments is the signal of the Lead acumulation. The uptake of Lead in root was higher than the other essential nutrients. Decline in Magnesium concentration related with the replacement of this essential ion by Lead. This was linked to the conversion of light green colors leaves to the dark green colors of leaves in both leguminous plants. It was also observed that these leaves remain dark green color after facing the demise. It was the indication of less concentration of real Mg chlorophyll. Total protein contents in the seedlings of Phaseolus mungo L.were found to be increased in shoot as compared to roots whereas it is significantly reduced in root and shoots of Lens culinaris. Amino acids in the roots of both species were found to be increased and decreased in the shoots of Lens culinaris while higher concentration of amino acids was observed in shoots of Phaseolus mungo L. Higher proline content and phenol contents in both species showed adaptation of self defense bio- chemical system to manage the toxicity of Lead while peroxidase and lignin activity were Lead dose depended. The reduced leaves sizes were correlated with an increase in Lead levels, and activities of peroxidase and lignin deposition in it. The intensification of activities of peroxidase and phenol in the Lead treated plants were accompanied by an increase in the biosynthesis of lignin content which perform function of scavenging reactive oxgen species (ROS radical). A strong correlation (r 2 =0.8570) was observed between Lead and lignin deposition in Lens culinaris Medik. whereas it was non- significant relation in Phaseolus mungo L.(r 2 =0.466). Increase in the lignin contents of Lens culinaris Medik. as a chemical adaptation of the cell walls of various leaves tissues for endurance while decreased in the lignin contents in Phaseolus mungo L. at high dose of Lead may be attributed with the decline in the peroxidase activity. Investigations revealed that although plants adopt several bio-physicomorphological and biochemical strategies for their survival but toxicity of Lead was considerably high due to which plant failed to continue to exist. viiiAbstract Bio-physico morphological deviation in anatomy of leaf under the Lead toxicity revealed that the most important immunological strategy. Presence of the trichomes or leave hairs on the margin of the leaves is the self protective phenomenon to neutralize the harmful effect of Lead. It was examined in both Phaseolus mungo L. and Lens culinaris Medik. by using the photo-camera microscope. Enhancement in number of trichome and stomata in the upper surface of the leaf is very important modification for defensive mechanism followed to conduct the normal photosynthesis process or to regulate the light reaction under the exposure of Lead toxicity. Leave morphological modification in Phaseolus mungo L. and Lens culinaris Medik. may be sign of defensive strategy that enables leaves to maintain their physiology in an environment marked by strong variations of Lead toxicity with light, temperature ,wind and relative humidity. Trichom play a role of a bioindicator of atmospheric pollution and biogeochemical signal of Lead contamination in soil. Tricome also act as the protective shield or defensive obstacle against the Lead toxicity. Trichom production increase with an increasing concentration of Lead.This led to reduction in transpiration and stimulation in conservation of water in leaves. Leaves hairs are responsible for protection in response to anxiety circumstance particularly in leave succession.Enhancement in the quantity of stomata is very important attribute in realtion with the gaseous exchange particularly emission of oxygen and absorption of carbondioxide. Elevation in creatinine, CK enzymatic activity, reducing sugar and glucose (p<.001) with the smaller size of stomata were observed in both leguminous plant in response to Lead toxicification. Decrease in mineral ions (Na, K, Ca, Mg, Fe, Mn and Zn) uptake in both species due to Lead accumulation indicates that Lead absorption more frequently as compared to essential mineral ion. The accumulation trend of Lead and Calcium showed a linear relation in between applied dose and accumulation of Lead in Phaseolus mungo L. and Lens culinaris Medik. which inversely related with Calcium content of both members of Fabaceae family. ixAbstract The reduction in Calcium content was due to chemical similarities in between the oxidation states of two ions but smaller ionic radius and high density of Lead helps in accumulation of it in tissues of both species. These interactions may occur at the cellular and molecular level and are the abilities of Lead to displace Calcium during specific physiological process. It is likely that the higher density and smaller ionic radii of heavy metal plays an integral role to block the access of essential mineral ions in plants and alter physiological processes of both species. Results obtained by atomic absorption Spectrophotometry from measurements of different essential macronutrient and micronutrient with Lead content both in roots and shoots showed that Lead accumulation was more pronounced in root than shoot.