Metal ecotypes

Lead is considered to be a non-essential metal to plants, and only a small proportion of the lead in soils is biovailable to plants (Alloway, 1990). Visible symptoms of toxicity, though unspecific to Pb, are smaller leaves and a stunted growth. Leaves may become chlorotic and reddish with necrosis and the roots may turn black. Several plant species, ecotypes and bacterial strains have been known to develop Pb tolerance. The phytotoxicity of Pb is low as it has very limited availability and uptake from soil and soil solutions. However, plant roots are usually able to take up and accumulate large quantities of Pb2+ in soil and culture solutions but translocation to aerial shoots is generally limited due to binding at root surfaces and cell walls (Lagerwerff, 1971 Jones et al., 1973 Lane and Martin, 1977). 

Two processes are involved in speciation. Firstly, the two populations must evolve sufficient ecological differences that they do not suffer from competitive exclusion. In the case of metal-tolerant ecotypes, this condition has been fulfilled. Secondly, they must evolve sufficient barriers to gene exchange that the two gene pools are effectively isolated, and will evolve independently. There are a number of ways in which this could occur, and the first stages of this have been observed in a number of studies of metal-tolerant plants. 

Heavy metal tolerance remains one of the clearest examples of microevolution, and one of the best systems to study the relationship between adaptation and ecology. It provides a model for the evolution of ecotypes and edaphic endemics. At the physiological and biochemical level, it is a model system for the study of the mechanisms of resistance to stress and pollution. Many questions remain unanswered, however, among the principal ones of which are ... 

There have been many attempts to reduce the migration of toxic metals from soils, and at the same time improve the appearance of derelict sites by the introduction of metal-tolerant ecotypes (Bradshaw et al., 1965 Gemmel, 1977 Bradshaw and Chadwick, 1980). Many such sites are poor in micronutrients and must first be treated with soil amendments such as lime and fertilisers. The maintenance of plant cover at such sites minimises soil erosion and reduces contamination from wind-blown dust. It has... 

The environmental characteristics that seemed most logical to explain the ecotypes and their differences were based upon trace metals and their coordination chemistry There are at least two consequences of this hypothesis insect avoidance and trace-metal uptaUce (for use in the "polishing stages of sewage treatment) ... 

Unfortunately, by March, 1976 it was evident that the weevil avoided the obvious waterhyacinth plants, i e, the 10 percent of the population that were superhyacinth plants The reason was not plant adaptability (13) The basis seemed to be differences in the trace metal content of the ecotypes most notably the superhyacinth plants had a greater concentration of iron in leaves and stem and roots Though this might be expected on an absolute basis, the plants being notably larger (Table I), it was also true on a relative basis, i e, when the concentration as expressed as mg metal per kg dry plant ... 

Cadmium has been found to be a micronutrient for an ecotype of Thalassiosira weissflogii, a marine alga [1] and many other heavy metals such as copper, nickel and zinc are well-known for a long time already as essential trace elements for plants. While general aspects of the entry of Cd into the environment are dealt with in detail in Chapter 2 of this book [2], we will summarize here in Sections 1.1 and 1.2 a few plant-specific aspects before discussing mechanisms of Cd toxicity in plants. 

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