Plant tissue decomposition

Unidentified Inhibitors. Many reports have been published relative to inhibitory responses which were obtained with extracts of plant tissue or from products associated with decomposition processes. For the most part, the inhibitory responses have been noted, but the inhibitors have not been identified chemically. Garb 48) tabulated approximately 25 references in which inhibitors were reported but were not characterized. These will not be relisted here. Uncharacterized inhibitors have also been reportd by Le Tourneau et al. 91), Patrick and Koch 112), Lapusan 85), Guenzi and Mc-Calla 63), Lawrence and Kilcher 86), Grodzinskii et al. 60), Brown 24), Patrick et al. 114), and Hoveland 73). 

Chemicals with allelopathic potential are present in virtually all plant tissues, including leaves, stems, roots, rhizomes, flowers, fruits, and seeds. Whether these compounds are released from the plant to the environment in quantities sufficient to elicit a response, remains the critical question in field studies of allelopathy. Allelochemics may be released from plant tissues in a variety of ways, including volatilization, root exudation, leaching, and decomposition of the plant residues. 

Up to about 70 C, plant tissues are thermally stable, as they must be in nature to avoid damage from prolonged direct exposure to the sun. Pyrolysis, the chemical decomposition by heat, starts in dry lignocellulosics around 100 C, in moist ones below 80 C. It accelerates as temperature rises, peaking in many organic materials between 275 and 300 C, at which point cellulose disintegrates. 

Soils amended with arsenic-contaminated plant tissues were not measurably affected in C02 evolution and nitrification, suggesting that the effects of adding arsenic to soils does not influence the decomposition rate of plant tissues by soil microorganisms (Wang et al. 1984). The half-life of cacodylic acid is about 20 days in untreated soils and 31 days in arsenic-amended soils (Hood 1985). Estimates of the half-time of inorganic arsenicals in soils are much longer, ranging from 6.5 years for arsenic trioxide to 16 years for lead arsenate (NRCC 1978). 

Wang, D.S., R.W. Weaver, and J.R. Melton. 1984. Microbial decomposition of plant tissue contaminated with arsenic and mercury. Environ. Pollut. 34A 275-282. 

Histamine also occurs naturally in plant tissues. It arises from the decomposition of histidine, but its function has not been elucidated. Histamine levels in some plants are surprisingly high - 1,340 pg/g in the blossoms of the spinach plant ( ). It is the exposure of man and animal to this botanical histamine with a possible physiological action that makes histamine of agricultural importance. The inhalation of cotton dust, for instance, has been related to byssinosis, a respiratory disease involving a lung dysfunction. 

The high water solubility of arbutln provides efficient leach-ablllty of the compound from plant tissues and subsequent transport and absorption by other plants. The ultimate chemical form (hydroqulnone vs. arbutln) available for absorption would be dependent upon the extent of decomposition (le. hydrolysis) affecting arbutln transport. 

Opsahl, S., and R. Benner. 1995. Early diagenesis of vascular plant tissues Lignin and cutin decomposition and biogeochemical implications. Geochimica et Cosmochimica Acta 59 4889-4904. 

FIGURE 6 Potential interactive pathways and processes of humic substances emanating from decomposition products of higher plant tissues with extracellular and surface-bound enzymes and photolytic reactions, particularly with UV irradiance. Humic acid-enzyme complexes can be stable for long periods (weeks and months) and subsequently reactivated upon exposure to weak UV light. Further photolysis can cleave simple compounds from the macromolecules for subsequent utilization by microbes. 

Cairney, J. W. G., and Burke, R. M. (1998). Extracellular enzyme activities of the ericoid mycorrhizal endophyte Hymenoscyphus ericae (Read) Korf Kernan Their likely roles in decomposition of dead plant tissue in soil. Plant Soil 205(2), 181-192. 

Figure 23.2 Conceptual model showing the possible fates of seagrass-hound nitrogen. Nitrogen temporarily immobilized in plant tissue can become available through exudation from live tissue, remineralization through decomposition, and grazing and subsequent excretion. Some material may be removed from the system through burial or export as dissolved or particulate organic matter.

Formation of intense flavours during the ripening period of fruit is traditionally well known. In many cases the characteristic flavours are only produced during the auto-lytic decomposition or during mechanical destruction of plant tissue (apple juice flavour from mash, woodruff in withered clove). The cause of this post-mortem flavour liberation is always a hydrolytic process mediated by plant-borne enzymes. 

As plant tissues senesce and die, three processes may ensue almost simultaneously. First, enzymes within the dead but sterile and physically intact cells cause proteolysis and other autolytic degradations. The released amino acids, sugars, tannins, phenols, and quinones may be oxidized by chemical or enzymatic catalysis to produce humus-like pigments, or proto-humus as discussed by Stevenson in Chapter 2. This was well illustrated by Cohen (Given and Dickinson, 1975) who observed cellular material of partially polymerized eaco-anthocyanins in residues of Rhizophora mangle deposited in a mangrove swamp in Florida. The autolytic reactions may be prominent in situations where microbial decomposition is slow due to acidity, anaerobiosis, or lack of basic nutrients. 

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