Nearly 60 years ago, Prat (1934) initiated the research of heavy metal resistance in plants when he was analysing the growth performance of two populations of Silene dioica (Melandrium sylvestre), one from a copper mine and one from a non-mine soil. He was able to demonstrate a heritable copper resistance in the mine population, relative to the non-mine population, which he explained as a result of evolution by natural selection. Nearly 20 years later Bradshaw (1952) and Baumeister (1954) started further research on ecological and physiological differentiation between plants from metal-enriched and noncontaminated habitats. The species chosen for study were predominantly Agrostis capillaris in the Bradshaw group (Jowett 1959; Gregory 1965; McNeilly 1965; Antonovics 1966) and Silene vulgaris in the Baumeister group (Broker 1962; Ernst 1964; Gries 1965; Riither 1966). In the late 1950s Duvigneaud (1958), while studying the vegetation on metalliferous soils in Central Africa, added to the above approaches a phytogeographic one and introduced the study of speciation processes in metallophytes. In the 1950s, the study of evolutionary and physiological aspects of metal resistance was hampered by the absence of convenient techniques for measuring metal concentrations in small plant samples. The techniques available for metal analysis were either timeconsuming, such as phase separation (Ernst 1964), or costly and only applicable for laboratory-raised plant material, i.e. radiolabelling (Turner & Gregory 1967; Peterson 1969). Only after applying atomic absorption spectrophotometry on wet-ashed plant material (Reilly 1967) did time and cost-effective metal analyses become possible.

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Metal tolerance in plants

Toxicity and Tolerance in the Responses of Plants to Metals

The effect of lead on seed imbibition and germination in different plant species

Deterioration of membranes caused by lipoxygenase (LOX) activity under 10 µM PbCl2 or 10 µM HgCl2 was partially alleviated by the exogenous application of 100 µM salicylic acid (SA). In two cultivars of rice (Oryza sativa L. cvs. Ratna and IR 36), the presence of SA ameliorated the increased leakage of electrolytes, injury index, and the content of malondialdehyde caused by these heavy metals. Lead decreased H2O2 content whereas Hg increased it in both cultivars. Application of SA increased H2O2 in presence of Pb, while decreased it in presence of Hg. Both Pb and Hg decreased superoxide dismutase activity, while increased peroxidase activity. The activity of catalase was decreased by Hg but increased by Pb and SA reversed their effects. Thus, SA ameliorated the damaging effects of Pb and Hg on membranes.

Effects of Salicylic Acid on Heavy Metal-Induced Membrane Deterioration Mediated by Lipoxygenase in Rice

The present work describes the changes that take place in phytohormone contents in germinating chickpea (Cicer arietinum cv. Aziziye-94) seeds in response to heavy metal stress. For this aim, endogenous abscisic acid (ABA), gibberellic acid (GA3), zeatin (Z) and zeatin riboside (ZR) contents were followed for 24, 48 and 72 h in chickpea seeds germinating at the concentrations of 0.1, 1.0 and 5.0 mM Pb or 0.1, 1.0 and 10 mM Zn. The results showed that Pb and Zn significantly delayed and impeded the germination of chickpea seeds. The negative effect of Pb on germination was higher than that of Zn. Further, Pb increased ABA and Z contents while decreased GA3 content in the germinating seeds. The high concentrations of Zn (1.0 and 10 mM) decreased contents of Z, ZR and GA3 while 0.1 mM Zn increased the content of the same hormones. The ABA content was enhanced by Zn in all concentrations used.

P. Maitra

Papers

Growth and metabolism of germinating rice (Oryza sativa L.) seeds as influenced by toxic concentrations of lead