Rhizosphere organisms

It is a general observation that herbicide degradation occurs more readily ia cultivated than fallow sod, suggesting that rhizosphere organisms are effective herbicide degraders. Whether this can be effectively exploited ia a phytoremediatioa strategy remains to be seea. 

I. Kraffczyk, G. Trolldenier, and H. Beringer, Soluble root exudates of maize influence of potassium supply and rhizosphere organisms. Soil Biol. Biochem. 76 315 (1984). 

A. D. Rovira, L. F. Elliott, and R. J. Cook, The impact of cropping systems on rhizosphere organisms affecting plant health. The Rhizosphere (J. M. Lynch, ed.), Wiley, Chichester, 1990, p, 389. 

Cultures are being found that can degrade both polychlorinated biphenyls and petroleum hydrocarbons. There is also interest in the role of rhizosphere organisms in polychlorinated biphenyl degradation, particularly since some plants exude phenolic compounds into the rhizosphere that can stimulate the aerobic degradation of the less chlorinated biphenyls. 

While there may be phylogenetic reasons for particular distributions of the polyhydroxylated alkaloids in plants, caution should nevertheless be exercised in using the presence of these compounds as taxonomic markers. One reason is that these alkaloids can be released into the soil by producer plants and micro-organisms from where some, such as DMDP and castanospermine can be readily taken up and accumulated in plant tissues of completely unrelated neighbouring species. It may also be the case that micro-organisms (Rhizobium, other rhizosphere organisms, or endophytes) closely associated with specific plants may also produce polyhydroxylated alkaloids which could then be mistakenly considered of plant origin. 

The microorganisms surrounding the roots are active in the breakdown of any crop residues or other energy sources, and in the overall humification process. Since the rhizosphere organisms are known to be more active than are those located elsewhere in the soil mass (Katznelson, 1960), it follows that this humification process in the rhizosphere is accelerated at least to the extent that the numbers are increased. Starkey (1931a) observed such a correlation. 

Another kind of root-microorganism association that involves many kinds of plants is that in which certain fungi live in or on the roots. These fungi, mentioned in Chapter 3, are commonly called mycorrhizal fungi, and are not true rhizosphere organisms in the sense that this term is usually used. The fungi are not merely loosely associated with the plant roots as are the true rhizosphere microflora, but are so closely and permanently attached to the roots that they function as essentially a part of the root system (Schmidt, 1947 Harley, 1948, 1959 Wilde, 1954 Garrett, 1960 Hacskaylo, 1967). 

Effects of organic matter on rhizosphere organisms and root development of Sorgham plants in two different soils. Plant and Soil, 7 , 3-18. 

Rhizosphere organisms produce organic acids such as 2-keto-gluconic acid which was suggested to have a chelating action on calcium, thereby improving... 

Steeghs et at. have described the use of PTR-MS, in conjunction with GC-MS, to explore VOCs in the rhizosphere " and their generation resulting from biotic stresses [230]. This area of research is important because plant roots are believed to release between 5% and 20% of all photosynthetically fixed carbon and therefore these roots create and maintain a carbon-rich environment for rhizosphere organisms, such as plant pathogens and symbiotic microbes from which VOCs can also be emitted. The research team focused on root-secreted VOCs from Arabidopsis. Simple volatile metabolites were mainly observed, such as ethanol, acetaldehyde, acetic acid, ethyl acetate, 2-butanone, 2,3,-butanedione and acetone, together with the monoterpene 1,8-cineole. Some VOCs were produced regardless of how the plants were treated (e.g. with compatible or incompatible bacteria), whereas other VOCs were induced specifically as a resull of differenf compatible and incompatible interactions between microbes and insects and Arabidopsis roots. 

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