Incorporation of 137Cs by tropical tree species and its correlation with plant physiology

Environmental phenomena associated with potential releases of radionuclides to terrestrial ecosystems have attracted the interest of several researches around the world. However, such interest has not been focused exclusively in its effects on human health. Several radionuclides have also been used as tracers of physicochemical or biological phenomena. These studies depend on both the development of auxiliary analysis techniques, which help to improve the results of traditional methods, and the production of additional data, so that a wide variety of natural processes can be understood. In this sense, the same radiometric results can be analyzed or interpreted from different viewpoints, allowing the interdisciplinary understanding of the overall structure of terrestrial ecosystems. 137Cs, for example, is one of the main fission products of nuclear reactors and atmospheric nuclear testing that is persistent in the biosphere because of its half-life of 30.2 years and biological mobility. This radionuclide remains in the top soil due to fixation and adsorption to clay minerals and organic matter, persisting in the root zone of plant for a long period of time. As its chemical behavior is similar to other monovalent inorganic cations (such as NH+4 , Na+ and K+), Cs+ can be readily absorbed by roots and translocated to the aboveground parts of plant. In this work, concentrations of 137Cs+, K+ and Na+ in fruits of lemon (Citrus limon B.) and coconut (Cocos nucifera L.) were measured by both gamma spectrometry and neutron activation analysis, with the aim to understand the behavior of monovalent inorganic cations in tropical plants as well as the plant ability to store these elements. In contrast to K+ and Na+, Cs+ is not an essential element to plants. However, our results have shown that 137Cs+ incorporation to vegetal tissues is positively correlated to K+ distribution inside the tropical plant species, suggesting that these two elements might be assimilated in a similar way, and that they pass through the biological cycle together. Correlation between 137Cs+ and Na+ is strongly dependent on plant species. A simple mathematical model is also proposed to describe the temporal evolution in the incorporation of monovalent inorganic cations by the fruits of such tropical species. This model exhibits close agreement with the experimental results during the fruit development. (author)