Plants factors affecting toxicity

The book reviews our knowledge of the chemistry and mineralogy of Mo, the extraction of available Mo from various soils, the various analytical methods of determining Mo content in soils and plants, the biochemical role of Mo in crop production, the technology and application of Mo fertilizers to crops, the responses to Mo of various temperate and tropical crops. Mo deficiency and toxicity in various plant species, the interaction of Mo with other plant nutrients, and the distribution of Mo within the plant. Factors affecting the availability of soil Mo to plants and Mo status in the semiarid and subhumid tropics are also discussed. 

Figure 9 3 Major plant and soil factors affecting plant-available potentially toxic metals ...

In contrast to the aforementioned toxicity tests, in situ toxicity tests involve exposing organisms to contaminants on-site. This provides for more environmental realism, but there is also less control over confounding variables that may affect toxicity (spatial or temporal variation in temperature, sunlight, nutrients, pH, etc.), or other factors that may disturb or disrupt the test (animals, winds, floods, vandalism, etc.). For these tests, animals may be placed in mesh cages or corralled by impermeable barriers, such as wood, metal, or plastic sheets, at various locations throughout the contaminated zone. Plants may be planted in plots of contaminated soils. Toxicity endpoints may include survival, sublethal effects, or accumulation of contaminants in body tissues. For these tests, organisms are also placed in less contaminated sites for comparison. 

The realization that selenium (Se) may be an essential micronutrient for human diets has arisen only recently, in the second half of the twentieth century. Selenium deficiency, attributable to low soil selenium levels in farm animals, especially sheep that are afflicted by selenium-responsive white muscle disease, has been recognized for at least half a century. However, the more recent identification of Keshan and Kashin-Beck diseases as endemic selenium-responsive conditions, occurring in a central 4000-1— km-wide belt of central China and in areas of Russia, demonstrated conclusively that not only is selenium an essential element for man but also deficiencies occur naturally and require public health measures to alleviate them. Selenium incorporation into plants is affected by the acidity of the soil and by the concentrations of iron and aluminum present so that selenium content of human diets is modulated by these components of the environment. The very recent discovery that these diseases probably arise through the interaction of selenium deficiency with enhanced viral virulence has added a further layer of complexity, but it does not alter the fact that selenium is an essential dietary component that cannot be substituted by any other element. Another complicating factor is that moderately increased soil selenium concentrations result in the opposite condition of seleno-sis, or selenium overload, with equally debilitating consequences. Of all elements, selenium has a very narrow safe intake range, and unlike some other potentially toxic elements, it is absorbed efficiently by the intestine over a wide range of concentrations and across a variety of different molecular forms. 

The bulk of the allelopathy literature has dealt with direct toxic effects on other plants. However, as developed in this review, it is obvious that allelochemicals may have a major impact on plant root-microbial interactions. Such interactions could lead to growth inhibition in the microorganisms (or in roots) and affect other factors of the root-microbe association resulting in effects... 

A number of external and internal factors are influential in determining the susceptibility of a specific plant to an airborne toxicant. Some generalizations can be stated relative to factors that affect susceptibility to oxidant air pollutants ... 

Plants are known to produce toxicants and antinutrilional factors, such as protease inhibitors, hemolytic agents, and alkaloids, which often protect the plant against pests and disease. Many of these toxicants are present in today s crops ai levels that do not cause acute toxicity or do not affect humans or animals when the food is properly prepared. New plant varieties should not contain levels of such toxicants that are above the range that exists in today s crops. 

The physical factors (Figure 3) affecting the fate and persistence of pesticides in our environment are primarily climatic parameters such as sunlight, temperature, humidity, rain, wind, and the toxicants vapor pressure (13,42). Other important physical factors involve the plants or animals growth which will dilute and occasionally store pesticides in restricted sites, such as in oil or fat depots. 

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