C.A. Parker

survival of field-grown rhizobia over the dry summer period in Western Australia

Abstract The survival over summer of field-grown root nodule bacteria was studied in the field and the laboratory during the course of an investigation into a nodulation problem of annual clovers. Dry field soils containing Rhizobium trifolii and R. lupini were subjected to a range of temperatures in the laboratory, the bacteria surviving 6 h exposure to temperatures as high as 80°C. Soil temperatures during summer were recorded at different depths. Populations of rhizobia were estimated at these depths from the end of the growing season (October) to early autumn (April) in plots which had carried dense swards of subterranean clover and Serradella. High populations of R. lupini were maintained in the Serradella plots throughout the summer. Populations of R. trifolii in the subterranean clover plots were initially much lower, and declined with both time and depth. The problem known as ‘second-year clover mortality’ is primarily due to low numbers of clover rhizobia in the soil at the end of the growing season. This situation is aggravated over the long hot dry summer, when there is a further decline in numbers.

The identification of vesicular-arbuscular mycorrhizal fungi using immunofluorescence

Abstract An antiserum was produced to a species of Gigaspora using cell wall antigen from hyphae collected from germinating spores. Using this antiserum in the fluorescent antibody (FA) technique, vesicular-arbuscular mycorrhizal (VAM) fungi could easily be distinguished from non-endogonaceous fungi. Differentiation among different VAM fungi, though not clear cut, was possible, and specificity was improved by absorption of the serum with hyphae of Acaulospora laevis . Variations in staining reactions between different fungi suggest that wall structure interferes with the expression of the FA test.

Interactions between earthworms and their soil environment

Laboratory experiments were used to study the effect of food quantity and quality on the biomass of earthworms, and the influence of earthworms on plant growth and infiltration of water into soil. Earthworms with the most food gained weight faster than those with little or no supplementary food. The latter also failed to become reproductively mature. Earthworms lost weight on a nitrogenpoor diet, but this was not rectified by supplementing such food with inorganic nitrogen added to the soil 2 weeks before the worms. Ryegrass grown in soil in which earthworms (Allolobophora trapezoides) had been kept grew more slowly than in soil which had no previous worm activity, perhaps indicating that earthworms had converted relatively-available organic N into less available forms. Microscolex dubius gave the fastest infiltration rates of water into soil when clover mulch was present. With Eisenia foetida there was little effect of worm density on infiltration rates; the highest density significantly increased infiltration but only when clover hay had been mixed in the soil. The surface casting behaviour of the two earthworm species varied with the placing of the food offered.

Growth of Gaeumannomyces graminis var. tritici in soil: Effects of temperature and water potential

Abstract The effects of temperature and water on the growth of the take-all fungus, Gaeumannomyces graminis var. tritici ( Ggt ), were examined in two factorial experiments. The first examined the effects of temperature and water potential on the growth of two isolates of Ggt on agar media, using osmotically-adjusted water potentials. The second experiment was concerned with the growth of the Ggt isolates in one sterile and two natural soils at two water regimes in the absence of a living host. Three temperatures (10, 18 and 26°C) were used in these experiments. A third experiment determined growth through soil. Growth was greatest at high temperatures and low water potential in axenic culture, but in unsterile soil growth at different temperatures and water potentials was strongly influenced by competition from the soil biota. The best temperature for growth in unsterile soil was 18°C. Growth at 26°C in unsterile soil was greatly reduced, this being attributed to more intense microbial competition. In sterile soil Ggt grew equally well at 18 and 26°C. At 10°C, both isolates of Ggt grew better in unsterile soil than in sterile soil. Under suitable conditions Ggt grew out readily from infected straw into unsterile soil (up to 5 cm in 10 days) in the absence of a host plant, forming melanized, hyaline and branched hyphae. These hyphae were infectious after dry storage for 5 months in the laboratory. Ggt thus appears to be a more successful soil inhabitant than is widely believed. Our experiments could explain many of the host-based concepts related to field expression of disease. The technique presented here could be of value for testing the suppressiveness or conduciveness of soils by measuring fungal growth in soil.

The colonization of host-root and soil by Rhizobia—I. Species and strain differences in the field

Abstract Growing-season populations of rhizobia associated with annual host-plant roots and nearby soil were examined in a field soil showing a nodulation problem in the second year after establishment. Rhizobium lupini reached higher populations at a faster rate than R. trifolii . A sharp drop in the population of R. trifolii associated with subterranean clover roots early in the growing season was followed by a recovery to high numbers. No such phenomenon occurred with R. lupini . The numbers of rhizobia under patches of non-nodulated plants in second-year stands were very low, usually R. trifolii in the first year were reflected in the second-year nodulation.

Death of rhizobia on inoculated seed

Abstract The death of R. trifolii WU1 and R. meliloti WU96 was studied with and without protection during the first hour after inoculation on mung bean ( Vigna radiata ) seeds. For unprotected rhizobia, death occurred in four distinct phases: a first phase of relatively slow death rate while the seeds remained moist; a second phase of very rapid death shortly after loss of visible moisture on seed; a third phase where numbers tended to stabilize, and a fourth phase in which a significant death rate continued. The use of protectant greatly reduced the overall death rate, especially the rapid death following loss of visible moisture. Some possible mechanisms of cell death and protection are discussed.

Inhibition of rhizobia by toxic soil-water extracts

Abstract Soil-water extracts from soils in which clovers nodulate poorly proved inhibitory to Rhizobium trifolii , in seeded agar plates. The same extracts did not inhibit R. lupini . The toxic extracts were found mainly in the growing season, but not after heavy rain. Neither the soil nor the soil-water extracts retained their toxicity on storing. Filtered broths from pure cultures of 59 soil micro-organisms, isolated from soil and clover roots in problem stands, were tested for their effect on both R. lupini and R. trifolii : nine inhibited both species, 19 inhibited R. trifolii only, and 31 had no inhibitory effect on either species. None inhibited R. lupini without also inhibiting R. trifolii . The possible role of microbial antagonism in second year mortality is discussed.

Self-inhibition of rhizobial strains and the influence of cultural conditions on microbial interactions

Abstract Interactions between Rhizobium and soil rhizosphere microorganisms on agar were examined using the giant-colony and the streak methods. The interactions between microorganisms often varied when the test method was changed. Soil and rhizosphere microorganisms produced greater stimulation or inhibition of Rhizobium when the Rhizobium was suspended within the agar. Giantcolonies of Rhizobium were limited in their effects upon soil microorganisms. When microorganisms were streaked at right angles to each other, interaction occurred only between those streaked 3 days apart and not between those streaked on the same day. Inhibition effects were very pronounced and stimulation was less evident with the streak method. Toxin production was implicated with selfinhibition of rhizobia and soil microorganisms. Stimulatory, inhibitory and lytic activity of soil microorganisms were confirmed with sterile broth extracts of 2 and 5 day old cultures. pH changes in the media as a result of microbial activity were not considered responsible for the interactions observed.

Establishment and persistence of Rhizobium trifolii in western australian soils

Abstract Large scale experiments with inoculated and drill sown Trifolium subterraneum, T. hirtum , and T. cherleri showed that recent isolates of Rhizobium trifolii from healthy plants in problem pastures were superior to the strains used in commercially available inoculants. The new rhizobia are also shown to persist in the soil longer than the commercial strains. Evidence was obtained of different levels of performance by R. trifolii strains on different soils. Following the inclusion of one of the superior isolates in commercial peat inoculants, a number of farmer-sown pastures were examined for strain persistence. The new isolates showed much improved persistence over the older inoculant strains.

Effect of infection of seminal and nodal roots by the take-all fungus on tiller numbers and shoot weight of wheat

Abstract A simple technique is described for the separation of seminal and nodal roots of wheat grown in pots. The effect of infection of either of the root-systems of wheat by the take-all fungus on tiller numbers and shoot weight was investigated using a variation of the technique. Both root systems were necrosed by the introduced inoculum of the pathogen to which they were exposed when they were about 7 days old. Although infection of either of the root systems resulted in significant reduction of tiller numbers, only infection of nodal roots caused significant reduction of shoot weight.