Effects of lead chloride on chloroplast reactions

Lead toxicity in plants

Contamination of soils by heavy metals is of widespread occurrence as a result of human, agricultural and industrial activities. Among heavy metals, lead is a potential pollutant that readily accumulates in soils and sediments. Although lead is not an essential element for plants, it gets easily absorbed and accumulated in different plant parts. Uptake of Pb in plants is regulated by pH, particle size and cation exchange capacity of the soils as well as by root exudation and other physico-chemical parameters. Excess Pb causes a number of toxicity symptoms in plants e.g. stunted growth, chlorosis and blackening of root system. Pb inhibits photosynthesis, upsets mineral nutrition and water balance, changes hormonal status and affects membrane structure and permeability. This review addresses various morphological, physiological and biochemical effects of Pb toxicity and also strategies adopted by plants for Pb-detoxification and developing tolerance to Pb. Mechanisms of Pb-detoxification include sequestration of Pb in the vacuole, phytochelatin synthesis and binding to glutathione and aminoacids etc. Pb tolerance is associated with the capacity of plants to restrict Pb to the cell walls, synthesis of osmolytes and activation of antioxidant defense system. Remediation of soils contaminated with Pb using phytoremediation and rhizofiltration technologies appear to have great potential for cleaning of Pb-contaminated soils.

Response of higher plants to lead contaminated environment

Lead concentration is increasing rapidly in the environment due to increased use of its sources by human society. Alarming concentrations of the metal have been reported in dust of densely populated urban areas and, water and land of various areas near the industrial waste disposals. Plants absorb lead and accumulation of the metal have been reported in roots, stems, leaves, root nodules and seeds etc. which increases with the increase in the exogenous lead level. Lead affects plant growth and productivity and the magnitude of the effects depend upon the plant species. Photosynthesis has been found to be one of the most sensitive plant processes and the effect of the metal is multifacial. Nitrate reduction is inhibited drastically in roots by the metal but in the leaves a differential effect is observed in various cultivars. Lead also inhibits nodulation, N-fixation and ammonium assimilation in the root nodules. It appears that the toxic effect of the metal is primarily at physiological level and provision of certain inorganic salts can antagonize the toxic effects to some extent. Further responses of plants to the metal depend on various endogenous, environmental and nutritional factors. Some plants are able to tolerate excess of Pb+2 by involving processes like exclusion, compartmentalization or synthesizing metal detoxifying peptides-the phytochelatins.

Monitoring of auto exhaust pollution by roadside plants

The changing levels of SO2 and Pb in the air and vegetation, along ten road transections of Lucknow city (having varying traffic densities) have been investigated, with a view to authenticate a possible correlation between SO2 and Pb concentration in air and sulphate and lead accumulation in the foliage of avenue trees. The study showed that the road transection at Alambagh (traffic density 4835 for 2 h) revealed the highest level of pollutants (SO2, 202 µg m(-3); SPM, 1080 µg m(-3); and lead, 2.96 µg m(-3), 2 h average) in air, as well as in the foliage of plants, whereas the road stretches with less traffic density correspondingly showed lower levels of pollutants. Pb and sulphate in leaves were found to be positively correlated with Pb and SO2 pollution in the air. Results suggest that Dalbergia sissoo and Calotropis procera are the ideal plant species to monitor as indications of Pb and SO2, respectively, in the air.

Fluorescence emission spectra of chloroplasts and subchloroplast preparations at low temperature

A study was made of the chlorophyll fluorescence spectra between 100 and 4.2 K of chloroplasts of various species of higher plants (wild strains and chlorophyll b mutants) and of subchloroplast particles enriched in Photosystem I or II. The chloroplast spectra showed the well known emission bands at about 685, 695 and 715--740 nm; the System I and II particles showed bands at about 675, 695 and 720 nm and near 685 nm, respectively. The effect of temperature lowering was similar for chloroplasts and subchloroplast particles; for the long wave bands an increase in intensity occurred mainly between 100 and 50 K, whereas the bands near 685 nm showed a considerable increase in the region of 50--4.2 K. In addition to this we observed an emission band near 680 nm in chloroplasts, the amplitude of which was less dependent on temperature. The band was missing in barley mutant no. 2, which lacks the light-harvesting chlorophyll a/b-protein complex. At 4.7 K the spectra of the variable fluorescence (Fv) consisted mainly of the emission bands near 685 and 695 nm, and showed only little far-red emission and no contribution of the band at 680 nm. From these and other data it is concluded that the emission at 680 nm is due to the light-harvesting complex, and that the bands at 685 and 695 nm are emitted by the System II pigment-protein complex. At 4.2 K, energy transfer from System II to the light-harvesting complex is blocked, but not from the light-harvesting to the System I and System II complexes. The fluorescence yield of the chlorophyll species emitting at 685 nm appears to be directly modulated by the trapping state of the reaction center.

Fluorescence changes in isolated broken chloroplasts and the involvement of the electrical double layer

We studied the effects of a variety of cations on chlorophyll fluorescence yield of broken chloroplasts prepared under carefully controlled ionic conditions. In the absence of light-induced electron transport and associated proton pumping, two types of cation-induced chlorophyll fluorescence changes could be distinguished in broken chloroplasts. These are termed "reversible" and "irreversible" fluorescence yield changes. Reversible fluorescence yield changes are characterized by antagonistic effects of monovalent and divalent cations and are prevented by the presence of 5 mM Mg2+ in the suspending media. Reversible-type fluorescence yield changes show little or no dependence on the structure, lipid solubility, or coordination number of the cation, but depend strictly on the net positive charge carried by the ion. It is proposed that these fluorescence changes are brought about through the interaction of monovalent or divalent cations with an electrical double layer at the interface of the outer surface of the thylakoid membrane and the surrounding aqueous solution. The results are interpreted in terms of the Gouy-Chapman theory of the diffuse double layer, indicating that the thylakoid outer surface bears an excess fixed negative charge density of about 2.5 muC/cm2, or approximately 1 negative charge per 640 A2 of membrane surface. Chlorophyll fluorescence quenching in isolated broken chloroplasts suspended in media containing 5 mM MgCl2 is also observed on addition of certain polyvalent cations to the medium. This type of cation-induced fluorescence change appears to be largely irreversible and may occur through specific binding of the cation to the thylakoid as a result of the high electrostatic attraction exerted by the negatively charged membrane surface.