ABSTRACT
Heavy metal contamination is an environmental problem and is currently evaluated in terms of its health risks for human beings (Dal Corso et al. 2010). Cadmium (Cd) is a toxic heavy metal found in soil, sediments, air, and water. Volcanic eruptions, forest burn-off, and wind transportation of particles from the soil are some of the natural sources of cadmium. However, in the last few years, Cd contamination has increased as a consequence of industrial activities such as mining, metal smelting, electroplating, use and puri—cation of Cd, fossil fuel burn-off, use of phosphatised fertilizers (Cupit et al. 2002), and manufacture of batteries, pigments and plastics (Popova et al. 2009). Cadmium’s high toxicity and solubility with water has been widely recognized. It can form chemical compounds in combination with other elements such as chlorides, oxides, and sul—des, which bond
24.1 Introduction .......................................................................................................................... 429 24.1.1 De—ning Biomarkers, Tolerance, and Sensitivity ..................................................... 430
24.1.1.1 Biomarkers ................................................................................................. 430 24.1.1.2 Tolerance .................................................................................................... 430 24.1.1.3 Sensitivity ................................................................................................... 431
24.2 Aspects Associated with Cadmium Tolerance in Plants ...................................................... 431 24.2.1 Physiological and Biochemical Characteristics of Biomarkers Signi—cant for
Plant Cadmium Tolerance ........................................................................................ 431 24.2.1.1 Metal Homeostasis in Plants ...................................................................... 431 24.2.1.2 Cadmium Absorption through Roots ......................................................... 431 24.2.1.3 Cadmium Mobility from the Root to the Shoot ......................................... 431 24.2.1.4 Defense and Detoxi—cation Mechanisms through Chelation and
Sequestration .............................................................................................. 432 24.3 The Concept of Bioindicator: Different Reported Interpretations ....................................... 433 24.4 Conclusion and Perspectives ................................................................................................. 436 References ...................................................................................................................................... 437
with soil particles, and remain in the soil for many years. The relevance of Cd contamination and its consequences increases in the face of its ecological, nutritional, and environmental implications. Stress caused by heavy metals is one of the major problems affecting plant agricultural productivity. On the other hand, natural ¬ora shows some relative differences in its tolerance capacity to heavy metals. Some plants can grow in a soil enriched with toxic levels of heavy metals, while others cannot (Yadav 2010). Several effects of cadmium exposure in plants, such as chlorosis, growth reduction, brown coloration of the root apex (Burzynski and Klobus 2004; Wojcik et al. 2005), chlorophyll content and photosynthetic rate reduction (Haag-Kewer et al. 1999; Burzynski and Klobus 2004; Mobin and Khan 2007; Tukaj et al. 2007; Monteiro et al. 2009) have been reported. Decrease in ribulose 1,5-bis-P regeneration, photosystem II quantic ef—ciency (Pankovic et al. 2000), and Rubisco activity (DiCagno et al. 2001) can be found among the photosynthetic parameters affected by cadmium exposure, as well as transpiration reduction (Haag-Kewer et al. 1999), and polymorphism induction in DNA (Aina et al. 2004; Liu et al. 2005; Aina et al. 2007). A relevant aspect of cadmium’s toxicity is its chemical similarity to essential elements such as Zn, Fe y Ca, producing irregularities in their homeostasis or causing their displacement from proteins (Verbruggen et al. 2009).
