Organs, essential plant

The term bioavailability has different meanings in different contexts and disciplines. Numerous definitions of bioavailability exist. Research on the relationship between bioavailability and chemical speciation (forms) originated in the field of soil fertility in the search for a good predictor for the bioavailability of essential plant nutrients (Traina and Laperche 1999). It is well accepted that dissolved nutrients are more labile and bioavailable to plants than solid-phase forms (including sorbed species). The same has been considered to be true for organic contaminants and their availability for microbial degradation. 

Although arsenic is not an essential plant nutrient, small yield increases have sometimes been observed at low soil arsenic levels, especially for tolerant crops such as potatoes, com, rye, and wheat (Woolson 1975). Arsenic phytotoxicity of soils is reduced with increasing lime, organic matter, iron, zinc, and phosphates (NRCC 1978). In most soil systems, the chemistry of As becomes the chemistry of arsenate the estimated half-time of arsenic in soils is about 6.5 years, although losses of 60% in 3 years and 67% in 7 years have been reported (Woolson 1975). Additional research is warranted on the role of arsenic in crop production, and in nutrition, with special reference to essentiality for aquatic and terrestrial wildlife. 

Boron is also an essential trace mineral in plants. A trace mineral is an element needed in minute amounts for the good health of an organism. Boron is critical to production of certain essential plant proteins and to help plants extract water from the soil. Low levels of boron show up as yellowing, blackening, twisting, or crumpling of leaves. 

The similarities between mechanisms involved in production and accumulation of toxic defence compounds in plants, and metabolism and transport of the same compounds as xenobiotics in animals, mean that essential building blocks of such complex and divergent organisms as plants and animals have a very ancient common origin, and that, through evolution, they were sometimes recruited to functions that oppose to each otter in nature. 

Tryptophan is an essential amino acid for most organisms. In plants and bacteria, this compound is derived from chorismic acid. Many groups of secondary compounds are formed from tryptophan among these are several simple amine derivatives and a number of alkaloids. 

Vitamins (Latin vita + amine) substances present in the animal diet in only small amounts, and indispensable for growth and maintenance of the organism. A dietary requirement is implicit in the definition of a V. Most of the substances that are V. in animals are essential for the metabolism of all living organisms, but plants and microorganisms can synthesize them (some fat-soluble V., however, may have metabolic roles unique to animals). The dietary re quirement in the animal results from the evolutionary loss of this biosynthetic ability. Animals differ in their ability to synthesize certain V., and they therefore display difierent dietary requirements for V. For example, ascorbic acid (V.C) is a V. only for primates and a few other animals (e.g. guinea pig) most animals can synthesize it, and for them it is therefore not a V. Some V. can be synthesized from provitamins in the diet. In addition some of the V. requirement of humans and higher animals is supplied by the intestinal flora, e.g. most of the V.K required by humans is supplied in this way. 

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