J. M. Lynch

Substrate flow in the rhizosphere

The major source of substrates for microbial activity in the ectorhizosphere and on the rhizoplane are rhizodeposition products. They are composed of exudates, lysates, mucilage, secretions and dead cell material, as well as gases including respiratory CO2. Depending on plant species, age and environmental conditions, these can account for up to 40% (or more) of the dry matter produced by plants. The microbial populations colonizing the endorhizosphere, including mycorrhizae, pathogens and symbiotic N2-fixers have greater access to the total pool of carbon including that recently derived from photosynthesis. Utilization of rhizodeposition products induces at least a transient increase in soil biomass but a sustained increase depends on the state of the native soil biomass, the flow of other metabolites from the soil to the rhizosphere and the water relations of the soil. In addition, the phenomena of oligotrophy, cryptic growth, plasmolysis, dormancy and arrested metabolism can all influence the longevity of rhizosphere organisms. With this background, microbial growth in the rhizosphere will be discussed.

Cultivation and the soil biomass

Abstract The fumigation-respirometric determination of soil biomass was modified to assess the effects of cultivation on the biomass in the upper 5 cm of intact soil samples. We found the method suitable for comparative studies, but not for providing an absolute measure. The soil biomass increased during the growth of a wheat crop and then decreased to an approximately constant amount. The biomass was significantly greater where the soil had been direct-drilled than where it had been ploughed, probably because plant roots were more abundant after direct drilling. The size of the oil biomass in relation to substrate input is discussed.

The use of colony development for the characterization of bacterial communities in soil and on roots

A simple agar plating method for the description of microbial communities is described. This method is based on the quantification of the numbers of bacterial colonies in 6–7 age-based classes as they appear on agar media over a period of 6–10 days. The method can be used to quantify microbial communities in different habitats (roots and soil) and can be related to the ecophysiology of the microbial communities present. Significant differences in distribution patterns were found in time and depth on the roots. In general, as roots matured, the microbial communities changed from one dominated by r-strategists to one that was more distributed towards K-strategists. The soil had the greatest percentage of organisms that could be characterized as K-strategists. The method was also used to compare microbial communities on wheat roots and in soil in both the field and in microcosms in the glasshouse. In general, the method enabled differentiation between r- and K-strategists in environmental samples, something that could not be done using an ecophysiological index (a modification of the Shannon diversity index) or total bacterial numbers alone.

Microbial growth in the rhizosphere

Abstract Barley plants were grown for up to 16 days in solution culture either under axenic conditions or in the presence of a mixed population of microorganisms. The quantities of soluble carbohydrate released by the roots grown in the absence of microorganisms and the numbers of bacteria which developed in the inoculated solutions were determined. Except for the first 4 days after germination, a greater biomass was produced than could be accounted for by the utilization of the carbohydrates released by the roots grown in the absence of microorganisms; this supports the view that the microorganisms stimulate the loss of soluble organic materials. These results are considered in relation to microbial activity in the soil and in particular to the significance of N2 fixation by free-living rhizosphere bacteria in the nitrogen economy of plants.

Aureobasidium pullulans in applied microbiology: A status report

Aureobasidium pullulans is of widespread ecological occurrence. It occurs on the leaves of a wide range of crops where it can be an indicator organism of environmental pollution, being able to withstand pollutants. A wide range of enzymes and products which are useful in industry have been characterized, and it is also a proven source of single-cell protein. On the debit side it is involved in biodeterioration, for example of paint surfaces. A pullulans is an unexplored candidate for the application of both classical genetics and genetic engineering.

The Influence of the Rhizosphere on Crop Productivity

The rhizosphere region is a variable zone containing a proliferation of microorganisms inside and outside the plant root. Many compounds are both taken up and passed out. Under normal growth conditions the rhizosphere exists because of the continuous loss of many forms of plant metabolites, which are rapidly utilized by microorganisms. Consequently, these rhizosphere microorganisms are in a position to affect both subsequent loss of material from the roots and nutrient uptake by the roots. In natural ecosystems an equilibrium develops between the plant and microorganisms that is affected only by the normal growth of plant and seasonal changes in the environment. However, in agriculture, man continually changes the normal equilibrium by manifold means. (e.g., plant monoculture, herbicide, fungicide and pesticide treatments, fertilizer application, and cultivation), all of which modify subsequent plant growth and the associated rhizosphere biota. Because of the importance of agriculture, the majority of work on the rhizosphere and its effects on plant growth has involved research on crop plants and, although this has provided great insight into rhizosphere—plant interactions in these relatively few species, some care should be taken in extrapolating such results to all natural ecosystems. With this proviso, we attempt to show, first, the effect the plant has on development and maintenance of the rhizosphere and, second, the influence the rhizosphere has on plant physiology and consequently crop productivity, highlighting areas of research likely to be rewarding both scientifically and commercially in the future. We do not attempt a complete review of the literature, since there have been reviews on many aspects of rhizosphere biology in recent years (Barber, 1978; Hale, et al. 1978; Newman, 1978; Balandreau and Knowles, 1978; Hale and Moore, 1979; Bowen, 1979, 1980, 1982; Woldendorp, 1981; Foster and Bowen, 1982; Lynch, 1982, 1983; Subba Rao, 1982a; Suslow, 1982), but rather choose specific examples to illustrate our major points.

Potential of Trichoderma spp. as consistent plant growth stimulators

In a series of repeated trials, six Trichoderma spp. strains, applied as a dried powder from a liquid fermentation in molasses/yeast medium, proved to be consistent at promoting the growth of lettuce (Latuca sativa L.) seedlings grown in a peat-sand potting compost in the glasshouse. Strains WT, 92, 20, and 75 at 0.75% or 1% w:w concentrations increased shoot dry weight by up to 26%, although WT did inhibit germination. For example, after 4 days only 13% of seeds sown in WT 1% w:w treated compost had germinated, whereas in other treatments germination was consistently greater than 32%. WT increased shoot fresh and dry weights by 14.3 g and 0.6 g per pot, respectively, without affecting the root dry weights, to give concomitant increases in shoot: root ratios of fresh and dry weight. The potential use of these Trichoderma spp. strains for plant growth promotion is discussed.

Formation of Ethylene by a Soil Fungus

SUMMARY: Methionine is a substrate for ethylene formation in Mucor hiemalis, but glucose is also required for maximal ethylene production. The formation of ethylene from these substrates in a defined mineral salts medium was studied both with sealed shaken flasks and with a chemostat. Oxygen promoted growth and ethylene production per unit weight of organism. That anaerobic conditions appear to be necessary to observe ethylene accumulation in the soil is probably because soil anaerobiosis mobilizes the substrates required for ethylene biosynthesis.

Variations in the size of the soil biomass.

Abstract In the surface 5 cm of a clay soil in 2 successive years the microbial biomass, as measured by a fumigation-respiration technique, was constant. The biomasses in two clay soils were approximately ten times greater than that in a silt loam. When straw from a preceding crop was chopped and left on the soil surface the biomass after 8 months was greater by a factor of two than that in soil where the straw had been burnt. Where soil had been kept in grass for 9 yr, the biomass was greater by a factor of three than that in soil of the same kind that had been in arable cultivation for the previous 4 yr. As the fumigation-respiration technique measured only the microbial and microfaunal contribution to the biomass, the total biomass can only be assessed by measuring the root contribution separately; when this was done the total biomass in the grassland was found to be greater by a factor of about six.

Colonization and decomposition of straw by fungi

A range of fungi isolated from decomposing straw were compared for their ability to grow on straw and its components. Extension rates were measured on agar containing water-soluble extract and on straw internodes. Activity was measured by clearing of cellulose and weight loss from straw. Penetration of fungi into the lumen of sterile wheat straw was also determined. In all the tests Sordaria alcina and Trichoderma harzianum showed outstandingly high growth and activity. In respiratory measurements on inoculated straw, Chaetomium globosum showed the greatest activity.