Plants and Microorganisms

The intent of this chapter is to give a critical review of all research related to the effects of photochemical oxidants, including ozone and the perozyac lnitrates, on plants and microorganisms. 

Injury to vegetation was one of the earliest indicators of photochemical air pollution. Injury was first observed in the Los Angeles area in 1944. Since then, there has been a slow but steady increase in research efforts to understand the effects of these pollutants on vegetation. In the late 1960 s and early 1970 s, there has been a flood of published information. 

The chemical composition of the photochemical-oxidant complex is discussed in Qiapter 2. The major phytotoxic components are ozone, nitrogen dioxide, and the peroxyaiylidtrates. The latter homologous 

Before ozone - and PAN were identified as specific phytotoxic components of the photochemical complex, researchers used a number of artificial chemical reaction systems to simulate the ambient photochemical-oxidant situation. These efforts involved a number of irradiated and nonirradiated reaction systems unsaturated hydrocarbon-ozone mixtures, unsaturated hydrocarbon-NOx mixtures, and dilute auto exhaust). Most research before 1960 involved one or more of these reaction systems. This research has been well reviewed - 451.459.488.505 extenslvely covered here. Although the 

A number of reviews of varied quality cover general or special effects of photochemical oxidants on vegetation (Table 11-1). Thomas fairly comprehensively covered the available information on the effects of photochemical oxidants on plants. Middleton gave the first comprehensive coverage of the phytotoxic effects of photochemical oxidants in 1961. A number of excellent reviews have appeared since 1%1. Rich presented an early review of ozone effects. Dugger and associates presented the physiologic and biochemical effects of oxidants on plants. Heck covered factors that influenced the expression of oxidant dam- 

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Plants and microorganisms produce unique and diverse chemical stmctures, some of which act as immunomodulators (18—28). Of specimens used in traditional medicine, approximately 450 plant species have shown antiviral activity out of 4000 plants screened (19). Several tannins (20) exhibit strong inhibition of tumor promotion experimentally. Pretreatment of mice with small amounts of tannins for several days strongly rejected transplanted tumors. This activity has been claimed to be effected through enhancement of host-mediated antitumor activity. 

Persistence of pesticides in the environment is controlled by retention, degradation, and transport processes and their interaction. Retention refers to the abihty of the soil to bind a pesticide, preventing its movement either within or outside of the soil matrix. Retention primarily refers to the sorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. In contrast to degradation that decreases the absolute amount of the pesticide in the environment, sorption processes do not affect the total amount of pesticide present in the soil but can decrease the amount available for transformation or transport. 

Another important relationship between the salts of the soil and corrosion has to do with biological activity. Since the growth of plants and microorganisms depends upon the proper inorganic mineral nutrients, the action of these forms of life varies with the mineral content of the soil. While many of the possible indirect effects, such as the role of various nitrogenous... 

Humans are able to synthesize only 11 of the 20 amino acids in proteins, called nonessential amino acids. The other 9, called essential amino acids, are biosynthesized only in plants and microorganisms and must be obtained in our diet. The division between essential and nonessential amino acids is not clearcut, however tyrosine, for instance, is sometimes considered nonessential because humans can produce it from phenylalanine, but phenylalanine itself is essential and must be obtained in the diet. Arginine can be synthesized by humans, but much of the arginine we need also comes from our diet. 

Pterins make no contributions to the colors of plants and microorganisms. One important pterin is the folate produced by plants and microorganisms. Folate and its derivatives are present in plants in various concentrations in mitochondria, cytosols, vacuoles, and plastids. The total amount of fohc acid depends on the plant species, on the developmental stage, and on external factors. Good sources of folates are beans, lentils, spinach, and wheat germ. ... 

Several candidate wildlife indicators are suggested and discussed in this chapter. In addition, we recognize that valuable sources of data on residue-effect relationships are available to assist in the selection of habitat-specific indicators (Jarvinen and Ankley 1999 USCOE and USEPA 2005). Although this chapter emphasizes animals, similar considerations and literature exist for plants and microorganisms as bioindicators and biomarkers (National Research Council 1989 USEPA 1997 Gawel et al. 2001 Citterio et al. 2002 Yuska et al. 2003). 

The Rhizosphere as a Site of Biochemical Interactions Among Soil Components, Plants, and Microorganisms... 

Signal molecules exchanged between plants and microorganisms have been identified that favor beneficial plant colonization. Some compounds present in... 

It has been found that many environmental factors influence the amount and composition of root exudates and hence the activity of rhizosphere microbial populations. Microbial composition and species richness at the soil-plant interface are related either directly or indirectly to root exudates and thus vary according to the same environmental factors that influence exudation. In es.sence, the rhizosphere can be regarded as the interaction between soil, plants and microorganisms. Figure 2 shows some of the factors associated with these interactions, which will be discussed during the course of the chapter. Here we mention briefly the influence of some plant and microbial factors on root exudation and rhizosphere microbial populations, while soil factors are discussed later. 

In addition to the interactions between plants and microorganisms, a third factor, the soil, also plays a role in determining root exudation and the activity and diversity of rhizosphere microbial populations. In this section, physical and structural aspects of the soil are discussed in relation to their effects on root exudation and microbial populations. Consideration is also given to the role of agricultural management practices on rhizosphere processes. In addition, the role of other biotic factors, such as microfaunal predation, is discussed in relation to nutrient cycling in the rhizosphere. 

The production and release of organic molecules by the root systems of plants have been extensively studied under a wide range of soil conditions (nutrient and water availability, pre.sence of pollutants, etc.,. see Chaps. 2 and 3). Furthermore it has been clearly demonstrated that soil microorganisms are able to produce molecules that can affect the physiology and architecture of roots (3) evidence has been also provided that molecular signals between plants and microorganisms are exchanged (see Chap. 7). 

Under aerated conditions at neutral to alkaline pH, inorganic iron is extremely insoluble (8), such that plants and microorganisms rely absolutely on iron uptake from organic matter complexes or iron that has been solubilized by siderophores and organic compounds contained in root exudate. Low-molecular-weight root exudates that dissolve iron include organic acids that are secreted by plant roots as a specific response to iron deficiency (9) or that are released constitutively at... 

Depending on the ability of specific transport systems to utilize the predominant metal chelates present in the soil solution, competition may occur between plants and microorganisms and between different types of microorganisms for available iron. This has been particularly well studied for Pseudomonas sp., which produce highly unique iron chelators that are utilized in a strain specific manner but which also retain the ability to use more generic siderophores pro-... 

V. Rdmheld, Existence of two different strategies for the acquisition of iron in higher plants. Iron Transport in Animals. Plants, and Microorganisms (G. Winkel-mann, D. Van der Helm, and J. B. Neilands, eds.), VCH Chemie, Weinheim, Germany, 1987, pp. 353-374. 

The first chapter defines the spatial and functional features of the rhizosphere, which make this environment the primary site of interaction between soil, plant, and microorganisms. Among the multitude of organic compounds present in the rhizosphere tho.se released by plant roots are the most important from a qualitative and quantitative point of view furthermore, the relationships with soil components of any released compound need to be considered (Chapter 2). The release of these compounds strongly depends on the physiological status of the plants and is related to the ability of plant roots to modify the rhizosphere in order to cope with unfavorable stress-reducing conditions. These aspects are dis-... 

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