Plant, bioavailability

The plant-bioavailable fraction of PTMs can be defined as the fraction of a metal total content in the soil that can be absorbed by plants via roots uptake (Kabata-Pendias and Pendias, 2001). Usually, this fraction is only a small proportion of the total element content of soils and shows much higher spatio-temporal variability than the total concentration. A plant uptakes mobile ion from the soil solution and the soil element fraction which is in solution is that which is considered immediately available. Nevertheless, the soil solid phases, inorganic as well as organic, take part in the supply and buffering of elements and allow their retention under wet conditions, which would otherwise leach all soluble elements from the soil. Therefore, the solid-bound elements take part to the available pool and this is why element concentrations in the soil solution are one to three orders of magnitude lower than those in plants (Bargagli, 1998). 

Single chemical methods to assess phytoavailable metals in soil 

Neutral salts of strong acids are likely to release soluble and non-specifically adsorbed metals. These solutions have the advantage that the pH of the soil suspension is determined by the soil mainly, and not by the extractant. Displacing 

Soil sampling technique Soil Soil sol. Soil Soil sol. Soil Soil sol. Soil Soil sol. Soil Soil sol. Ref. 

Extractant/presumed PTMs pools Elements References 

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Metal bioavailability is the fraction of the total metal occurring in the soil matrix, which can be taken up by an organism and can react with its metabolic system (Campbell, 1995). Metals can be plant-bioavailable, if they come in contact with plants (physical accessibility) and have a form which can be uptaken by plant roots (chemical accessibility). Soil metals become accessible for humans by ingestion, inhalation and dermal contact. Available forms of PTMs are not necessarily associated with one particular chemical species or a specific soil component. Main soil PTMs pools of different mobility, target organisms and routes of transfer are sketched in Fig. 9.2. The most labile fraction, corresponding to the soluble metal pool, occurs as either free ions or soluble complexed ions and is considered the... 

Most foods of animal origin contain nicotinamide in the coenzyme form (high bioavialability). Liver and meat are particularly rich in highly bioavailable niacin. Most of the niacin in plants, however, occurs as nicotinic acid in overall lower concentrations and with a lower bioavailability. The major portion of niacin in cereals is found in the outer layer and its bioavailability is as low as 30% because it is bound to protein (niacytin). If the diet contains a surplus of L-tryptophan (Ttp), e.g., more than is necessary for protein synthesis, the liver can synthesize NAD from Trp. Niacin requirements are therefore declared as niacin equivalents (1 NE = 1 mg niacin = 60 mg Trp). 

Milk, milk products, and foods of animal origin contain high amounts of (free) riboflavin with good bioavailability. In foods of plant origin, the majority of riboflavin is protein-bound and therefore less bioavail-able. Cereal germs and bran are plant sources rich in riboflavin [1]. 

Plants contain to some extent less bioavailable forms of vitamin B6, e.g., glycosylates, or biologically inactive metabolites, e.g., e-pyridoxin-lysin-complexes. In addition, the release of vitamin B6 from foods rich in fiber is assumed to be delayed. The bioavailability of vitamin B6 from animal-derived foods is therefore overall higher than from plant-derived foods. Good dietary sources of vitamin B6 include chicken, fish, pork, beans, and pulses [1]. 

Another contaminant of concern for the Delta is Se. Selenium occurs in high concentrations in the soils of the western San Joaquin Valley associated with salts that have accumulated in this region [10]. Selenium is recycled through agricultural retom flows to the river and transported to the Delta and San Francisco Bay. The Se is transformed into a more bioavailable form by microbial communities and aquatic plants. The Se is passed through the foodweb with particular concern for bottomfeeding migratory waterfowl and predatory fishes. The hydrodynamics of water from the San Joaquin River is an important consideration in the intensity and distribution of Se contamination within the Delta. 

Klienfeld and Tabershaw 1954 Prout et al. 1985 Stephens 1945 Stevens et al. 1992 Templin et al. 1993 Withey et al. 1983), or dermal (Bogen et al. 1992 Jakobson et al. 1982 McCormick and Abdul-Rahman 1991 Sato and Nakajima 1978 Steward and Dodd 1964 Tsuruta 1978) exposure. All these routes of exposure may be of concern to humans because of the potential for trichloroethylene to contaminate the air, drinking water, food, and soil. More information on the absorption of trichloroethylene following ingestion of contaminated soil and plants grown in contaminated soil near hazardous waste sites are needed to determine bioavailability of the compound in these media. 

Pool concentration of a substance that exceeds the threshold - for example megadose vitamin C - or substances that are excreted unchanged because they cannot be metabolised, such as sugar alcohols, or compounds that are not biologically essential, such as carcinogens, bacterial toxins and some minor plant constituents, are also bioavailable (and thus bioactive) in that they have a metabolic impact, even if this is only the stimulation of detoxification processes, or the use of energy for their excretion. 

WISEMAN H (1999) The bioavailability of non-nutrient plant factors dietary flavonoids and phyto-oestrogens. Proc Nutr Soc. 58 (1) 139-46. 

Specific carotenoid-protein complexes have been reported in plants and invertebrates (cyanobacteria, crustaceans, silkworms, etc.), while data on the existence of carotenoproteins in vertebrates are more limited. As alternatives for their water solubilization, carotenoids could use small cytosolic carrier vesicles." Carotenoids can also be present in very fine physical dispersions (or crystalline aggregates) in aqueous media of oranges, tomatoes, and carrots. Thus these physicochemical characteristics of carotenoids as well as those of other pigments are important issues for the understanding of their bioavailability. 

The release of a compound from the food matrix in which it is incorporated is a determining process for its bioavailability and is largely influenced by the physicochemical characteristics of the compound, the type of food matrix, the subcellular location of the compound in plant tissues, and the food processing. The, food matrix type greatly influences the compound bioaccessibility. 

For carotenoids, the type of matrix varies from relatively simple matrices in which the free carotenoid is dissolved in oil or encapsulated in supplements to more complex matrices in which the carotenoid is within plant foods. It is clear that the efficiency of the process by which the compound becomes more accessible in the gastrointestinal tract is inversely related to the degree of complexity of the food matrix. Carotenoid bioavailability is indeed far greater in oil or from supplements than from foods and usually the pure carotenoid solubilized in oil or in water-soluble beadlets is employed as a reference to calculate the relative bioavailability of the carotenoid from other foods. ... 

Kopsell, D.A. and Kopsell, D.E., Accumulation and bioavailability of dietary carotenoids in vegetable crops. Trends Plant Sci. 11, 499, 2006. 

Algae can be cultivated easily and quickly when compared to plants. They produce very high quantities of carotenoids compared to other sources (3.0 to 5.0% w/w on a dry weight basis). They contain both cis and trans isomers of carotenoids for high bioavailability and bioefflcacy, and also contain oxygenated carotenoids (xantho-phylls), which have greater bioactivity and better anticancer properties. The proteins from Dunaliella biomass can be utilized for bread and other products and whole cells can be utilized for animal, poultry, and fish foods because they are safe. ... 

Phytic acid (inisitol hexakisphosphate) is the main storage form of phosphorus in plants. The phosphorus is not bioavailable to non-ruminants as they lack the enzymes to break it down. Novozyme has developed a commercial enzyme, phytase, that can be added to animal feed to release the phosphorus. No inorganic phosphorus needs to be added. This shift in the source of phosphorous has a large impact on the environmental footprint of pig farming. 

Importance of Partitioning for Bioavailability to Invertebrates, Microbes, and Plants... 

Upon formation of a metal chelate or complex, the next rate-limiting step in delivering iron to the cell is the diffusion of iron complexes through the. soil in response to diffusion gradients. In the vicinity of plant roots, metal chelates and complexes may also move by bulk flow in the transpiration stream as water moves from the soil into the plant. However, depending on their charge characteristics and hydrophobicity, metal chelators and complexes can become adsorbed to clay and organic matter, which may then decrease their mobility and bioavail-... 

Phytoremediation in the root zone. Proteins and enzymes produced by the plant can be exuded by the roots into the rhizosphere. These plant products target contaminants in the surrounding soil, leading to precipitation or immobilization in the root zone. This mechanism within phytostabilization may reduce the fraction of the contaminant in the soil that is bioavailable. 

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