Plant substances metabolized

The nutritional requirements of insect species exhibiting different feeding habits like scavengers, parasites, predators and phytophagous insects, are similar in a qualitative sense (O. Each insect species needs, however, a particular quantitative composition of nutrients in its diet to complete development ( ). The presence of toxic substances in plants, secondary plant substances as they were formerly called by phytochemists, forms a barrier which phytophagous insects have overcome by specialization. Thus, an insect can tolerate or detoxify the secondary plant substances present in its host plants, while the majority of these substances being present in other plants still acts as toxins (J ). In this way phytophagous insects are adapted to the metabolic qualities of their host plants, i.e. a particular chemical composition of nutrients and secondary plant substances. 

The term essential oil does not reflect its role in the plant s functioning and metabolism. Plant volatile oils funcbon as secondary metabolites. (A metabolite is a compound produced by the plant s metabolism.) Primary metabolites are the compounds needed for the plant to live and include the food substances produced in photosynthesis. The secondary metabolites vary widely in chemical structure and serve a variety of purposes within the plant. They include protective, survival and reproductive roles. However, they are responsible for giving a plant its aroma and flavour and have significant physiological and psychological effects on animals and people. 

The mineralocorticoid aldosterone is also produced by the adrenal cortex and promotes retention of H20 and Na+ and loss of K+ by the kidney. Cortisol is also an agonist of the aldosterone receptor but the level of cortisol is kept low by type 2 11 (Thydroxysteroid dehydrogenase, which converts cortisol to the inactive cortisone (11-dehydrocortisol). Accordingly inhibition of this enzyme by 18(i>-glycyrrhetinic acid (from liquorice) elevates cortisol with consequent effects of H20 and Na+ retention, oedema and hypertension. Further potential sites of interference by plant substances with steroid hormone metabolism include enzymes involved in steroid hormone synthesis such as the cytochrome P450-linked 11 -hydroxylase that catalyses the last step of corticosterone synthesis. 

Urea herbicides reaching the natural environment are gradually decomposed over a shorter or longer period, the steps and the rate of decomposition depending on the stability of the molecule and on the medium. The active substance reaching the soil surface or into water is chemically decomposed by the action of the ultraviolet radiation of the sun or of the acid or alkaline components of the soil. Compounds absorbed by the plants are metabolically degraded, and compounds in the soil are similarly metabolised by the microflora and microfauna of the soil. 

Saturated soil conditions in wetlands affect plant growth and productivity in several ways. The abundance of water seriously interferes with plant root metabolism, creating root oxygen deficiency. In addition, microbial processes in wetland soils can produce reduced substances potentially toxic to wetland plants. Saturated soil conditions in wetlands affect the reactivity of many inorganic redox-mediated processes, thus influencing adaptations of wetland plants. 

The fact that THC produces an effect on the human brain is an accidental outcome of a complex chemistry that serves other purposes for the plant Scientists are still researching the value of cannabinoids to the plant but it appears certain that these substances are important to the plants sun/h/al. Because of the complex structure of the cannabinoid compounds and the resin glands that produce them, scientists do not believe that THC and other cannabinoids are merely by-products or waste products of the plant s metabolism. Instead, cannabinoids appear to be used by the plant for a number of purposes, most having to do with protecting the plant from predators and helping it reproduce successfully. 

Luckner, M. Expression and Control of Secondary Metabolism. In Encyclopedia of Plant Physiology, New Series, Vol. 8, Secondary Plant Products (E. A. Bell, B. V. Charlwood. eds.), pp. 23-63, Springer, Berlin-Heidelberg-New York 1980 Mothes, K. Secondary plant substances as materials for chemical high quality breeding in higher plants. In Biochemical Interactions between Plants and Insects (J. W. Wallace, R. L. Mansell, eds.), pp. 385-405. Plenum, New York 1976 Zahner, H. What are secondary metabolites Folia Microbiol. 24, 435-443 (1979)... 

Trewavas AJ (1983 a) Plant growth substances - metabolic flywheels for plant development. Cell Biol Internat Rep 7 569-575... 

Quinic acid, a carboxylated tetrahydroxycyclohexane, is a secondary plant substance formed from dehydroquinic acid, an intermediate in the shikimic acid pathway, the metabolic pathway leading to aromatic compounds. It, therefore, is ubiquitous in living plant cells. It has been isolated from cinchona bark and detected in the cambial sap of conifers (114). 

The terpenoids, the phenols and the alkaloids constitute the three most important groups of the secondary plant substances. This is a very unfortunate designation since it is all too easy to equate secondary with of secondary importance or, even, unimportant. In actual fact it is not known for many of the substances so classified in what way they could be of use to the plants that produce them. In many cases they do indeed seem to be waste products of metabolism which are withdrawn from metabolism by being transferred to vacuoles or stored in the bark without any demonstrable benefit to the plant other than an easing of the... 

This completes our discussion of the phenols, the second largest group of secondary plant substances after the terpenoids. Listing the biosynthetic pathways in order of increasing importance for higher plants, phenols are formed by the acetate-mevalonate pathway, the acetate-malo-nate pathway, and, particularly, the shikimic acid pathway (Fig. 112). The cinnamic acids occupy a central position in phenol metabolism, participating as they do in the biosynthesis of all other important phenols. 

Porphyrins are a small but important group of secondary plant substances. We dealt with their structure when we discussed the chlorophylls. The biosynthesis of the porphyrins is interlinked with the citric acid cycle and amino acid metabolism. (Fig. 132). The precursor succinyl CoA is derived from the citric acid cycle and the precursor glycine from amino acid metabolism. The two are combined to give an unstable intermediate which loses CO2 to form 8-aminolevulinic acid. Two molecules of 8-aminolevulinic acid are combined to yield porphobilinogen, a pyrrole system, which is the building block of the porphyrins. Four such porphobilinogen molecules become linked in a series of steps to form the porphyrin system. The first porphyrin to appear in the course of biosynthesis is uroporphyrinogen III. Of the subsequent intermediates protoporphyrin IX is worthy of mention. This is because introduction of Mg " on the one hand leads to Mg-protoporphyrin IX and then further to the chlorophylls, whereas introduction of Fe++ on the other yields Fe-... 

The literature of alkaloids can conveniently be divided into five sections, dealing with (1) the occurrence and distribution of these substances in plants (2) biogenesis, or the methods by which alkaloids are produced in the course of plant metabolism (3) analysis, ranging from the commercial and industrial estimation of particular alkaloids to the separation, purification and description of the individual components of the natural mixture of alkaloids, which normally occurs in plants (4) determination of structure and (5) pharmacological action. 

Hartung, W., Gimmler, H. Heilmann, B. (1982). The compartmentation of abscisic acid, of ABA - biosynthesis, ABA - metabolism and ABA conjugation. In Plant Growth Substances 1982, ed. P.F. Wareing, pp. 325-334. London Academic Press. 

An environmental protocol has been developed to assess the significance of newly discovered hazardous substances that might enter soil, water, and the food chain. Using established laboratory procedures and C-labeled 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), gas chromatography, and mass spectrometry, we determined mobility of TCDD by soil TLC in five soils, rate and amount of plant uptake in oats and soybeans, photodecomposition rate and nature of the products, persistence in two soils at 1,10, and 100 ppm, and metabolism rate in soils. We found that TCDD is immobile in soils, not readily taken up by plants, subject to photodecomposition, persistent in soils, and slowly degraded in soils to polar metabolites. Subsequent studies revealed that the environmental contamination by TCDD is extremely small and not detectable in biological samples. 

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. 

This chapter focuses on the effects of humic substances present at the rhizo-sphere on plant growth and nutrient uptake. The main structural features of humic substances, their nutritional function, and the capacity to interact with plant metabolism are also presented. 

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