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This group includes elements that are used as nutrients by only a limited number of plant species.  When high concentrations of an element, not commonly found in most plants, is seen in a particular species, it raises the possibility that the element is essential for that species.  The plants may use these for non-nutritional purposes.  It is thought that species where these elements are present may use them for protection against invading bacteria. For example, Barium is found in brazil nuts; Aluminum in tea; Chromium, Nickel and Cobalt in gramineae; Silicon in rice and cucumbers.

SELENIUM  (Se) 78.96 Atomic Weight

(Essential Trace Element?)

Selenium usually occurs combined, and is distributed throughout the world, but only in small quantities.  It is not found combined in conjunction with free Sulfur.  It has photovoltaic properties and is a good conductor, making it used primarily in the electrical industry in photocells, solar cells, and rectifiers.  It is considered an essential element, but in very limited quantities.  In excess, it is carcinogenic, teratogenic, and toxic, depending on the compound.

Selenium is needed by Astragalus spp. (a legume) and can replace sulfur in certain Sulfur amino acids giving selenocystathionine, elenomethionine, selenocysteine and ethylselenocysteine; these compounds are potentially poisonous.  Some of these have also been isolated from wheat grown in a seleniferous soil.  Selenomethionine competes with methionine in the synthesis of protein.