Lynette Abbott

Factors influencing the occurrence of vesicular-arbuscular mycorrhizas

Abstract Vesicular-arbuscular (VA) mycorrhizas are widely distributed but there is limited knowledge of the occurrence of individual species in relation to soil, climate and vegetation. Recent developments in methods for identifying and quantifying VAM fungi, both in soil and within roots, should enable the clearer definition of the factors that influence their distribution. Careful attention must be paid to sampling procedures to avoid bias and subjectivity. Soils commonly contain more than one VAM fungus. The development of VA mycorrhizas varies with soil type and depth, season and vegetation. The dynamics of root colonization by individual species within a population have not been adequately explored. Minor or gradual disturbances in agricultural and natural ecosystems may, or may not, lead to marked changes in mycorrhiza formation. Populations of VAM fungi appear to be capable of adjusting to gradual changes in the environment without abrupt changes in the extent of colonization. In contrast, more extreme, or rapid environmental changes such as those associated with mining or erosion, may markedly decrease mycorrhiza formation. The restoration of populations of mycorrhizal fungi will depend on the availability of accessible sources of propagules and on the suitability of the disturbed soils for plant and fungal growth.

Vesicular-arbuscular mycorrhizas and soil salinity

This review discusses the growth and activity of vesicular-arbuscular (VA) mycorrhizal fungi in saline conditions. The review includes examination of the effects of high concentrations of salts on the occurrence of VA mycorrhizal fungi in field soils, and on spore germination, growth of hyphae, establishment of the symbiosis and production of spores in controlled conditions. Information on the growth and reproduction of VA mycorrhizal fungi under saline conditions is scarce and is often circumstantial. There is clear evidence that germination of spores and subsequent hyphal growth of some VA mycorrhizal fungi are reduced by increasing concentration of salts. However, in plant growth experiments, experimental designs and methodologies have generally not allowed the direct effects of salinity on fungal growth to be separated from plant-mediated effects. There is a need for controlled studies to investigate the responses of VA mycorrhizal fungi to soil salinity. Research is required which distinguishes between effects on different phases of the fungus lifecycle and which includes in its design the ability to separate direct effects from plant-mediated influences on fungal growth and reproduction.

Comparative Ecology of Galapagos Ground Finches (Geospiza Gould): Evaluation of the Importance of Floristic Diversity and Interspecific Competition

Distribution, abundance, diet, and beak morphology of the six Geospiza finch species were studied at eight sites on seven Galapagos islands. The resulting information was used to test the theories of Lack and Bowman that interspecific competition (Lack) and floristic and food differences among islands (Bowman) determine the ecological and morphological characteristics of the finches. Both factors were found to be important in different aspects of finch ecology. Most of the 21 populations studied spent more time foraging for seeds and fruits than for insects. Most populations had generalized diets and nine out of 18 pairs of sympatric populations had diets overlapping by 50% or more. Diet breadth was positively correlated with variety of available foods (classified by an index comprising measures of size and hardness), but was not correlated with abundance or biomass of sympatric populations of finches (potential competitors). Competitive release, in the form of large population biomass on islands with few congeners, was not found. Nor was population biomass related to variety of available food. Overlap in diet between two species was greatest when the species were most similar in abundance and body size. Interspecific competition is indicated by the distribution and morphology of the finches. The absence of four combinations of species cannot be attributed to change: (1) G. conirostris and G. fortis, (2) G. conirostris and G. scandens, (3) G. difficilis and G. fuliginosa, and (4) G. scandens and G. difficilis. These absences cannot be attributed to the absence of suitable foods. and sites with similar floras and suites of food types do not necessarily have similar ground finch faunas. The ecological and morphological similarity of members of each of the four pairs suggests an inability to coexist for competitive reasons. In contrast, where only two species occur together on an island they tend to differ markedly in size (beak and body) and diet. Species with large beaks can eat larger and harder seeds and fruits than species with small beaks, and as a consequence they tend to have more diverse diets. Species with large beaks also eat medium—sized seeds and fruits quicker than species with small beaks. Species with small beaks may be able to deal with small soft seeds more efficiently than species with large beaks, but the data are not clear on this point. In addition, small species with small beaks have a metabolic advantage over large species with large beaks. Variety of available foods influences the large species more than the small species: the two largest species, G. magnirostris and G. conirostris, occur only on sites where plants producing large and hard seeds and fruits are plentiful. Food variety apparently does not influence the degree of diet overlap. The number of plant species on an island statistically determines the number of land—bird species in general, and Geospiza species in particular, which occur on that island. The influence is probably mediated through food supply, since 53% of the variation in Geospiza species diversity among the eight sites was accounted for in a multiple linear regression analysis by diversity of available seeds and fruits. We conclude that food supply and interspecific competition have jointly determined the ecological course of the radiation of Darwins finches and the resulting pattern of species diversity. Interisland variation in vegetation favored the initial steps of differentiation. Competitive interactions among species influenced later stages by determining which ecological types could coexist on an island with a given array of foods.

Plant surface microbiology

Plant surface microbiology , Plant surface microbiology , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Phosphorus and the formation of vesicular-arbuscular mycorrhizas

Abstract Infection of ryegrass roots by vesicular-arbuscular mycorrhizal (v.a.m.) endophytes occurring in an unfertilized virgin soil of low P status was more sensitive to increasing P supply than that by endophytes occurring in an adjacent fertilized agricultural soil. P application to soil depressed mycorrhiza formation in subterranean clover by increasing plant P status and not by direct effects of soil P on v.a.m. endophytes. Localized placements of superphosphate by banding or topdressing did not affect the development of mycorrhizas of roots in the fertilized zones differently from those of roots not in fertilized zones. The amount of infection was not correlated with P concentrations within the plant at harvest. However, the extent of infection could be correlated with P concentrations of roots at early stages of penetration by the fungi. Additionally, the effects of P supply on frequency of penetration by hyphae and subsequent mycorrhiza development closely paralleled effects of P supply on soluble carbohydrate concentrations in roots.

Seasonal Variation in Feeding Habits of Darwin's Ground Finches

We investigated the effects of seasonal variation in food abundance on food partitioning among Darwins ground finches of the genus Geospiza. Finch populations, the availability of seed of fruits, and finch feeding habits were measured during 2 visits to each of 4 study sites on 3 Galapagos islands. Our 1st visit was made near the end of the 1973 rains at a time of seasonal food abundance and the 2nd, 6—8 mo later, when food was scarce. If interspecific competition prevails, finch populations should decline from wet to dry season in response to declining food availability. At 2 sites where food supplies declined sharply, so did an index of finch numbers; at the other 2 sites, both food supplies and finch numbers remained high. Total finch biomass at 5 dry—season study sites was correlated with food abundance but was not at 8 wet—season study sites. If interspecific competition is stronger, diets of finch species should diverge as food becomes scarce, but if intraspecific competition is stronger, diets should expand and overlap among species increase. At all 4 sites, finch species diverged in their diets and took a narrower range of foods in the dry season. All species shifted from a common wet—season diet of soft, easy—to—handle seeds and fruits to different diets reflecting the morphological specializations of each species. The results are consistent with interspecific competition for food occuring at all sides, but other explanations cannot be ruled out. Even where food remained relatively abundant, species diets changed and overlapped less. Intense competition was not, therefore, a necessary condition for seasonal diet changes; these may have been simple proximate responses to food availability. Existing foraging theory does not predict these results or others, because the assumption that available foods do not change qualitatively between seasons is violated. Where food greatly decreased, inter— and intraspecific competition may have caused the death or dispersal of most of the finches in the wet—season populations and diet divergence minimized interspecific competition are probably complemented by year—to—year differences caused by the unpredictable Galapagos rainfall.

Direct and residual effect of biochar application on mycorrhizal root colonisation, growth and nutrition of wheat

The influence of biochar (biomass-derived black carbon) on crop growth and nutrient uptake varies based on the rate of biochar applied with fertilisers. We investigated the effect of deep-banded oil mallee biochar at different rates (0, 1.5, 3.0, and 6 t/ha) with 2 types of fertiliser (non-inoculated MultiMAPS® at 30 or 55 kg/ha; inoculated Western Mineral Fertiliser at 100 kg/ha) on wheat growth at a farmer’s field in a low rainfall area of Western Australia. Wheat yield increased significantly when biochar was applied with inoculated fertiliser and 30 kg/ha non-inoculated fertiliser. Mycorrhizal colonisation in wheat roots increased significantly with biochar application with inoculated mineral fertiliser. Mycorrhizal hyphae may have improved water supply to reduce drought stress in these treatments by extending crop exploration of water from the wide inter-rows. Grain yield increases were due to better grain survival and grain fill with reduced drought stress. Early stage phosphorus uptake was not improved by mycorrhizal colonisation—phosphorus supply from the soil and applied fertiliser was already adequate. The residual effect of biochar and mineral fertilisers was assessed using a mycorrhizal bioassay for soil collected from the field trial 2 years after application of biochar. Biochar and both fertilisers increased mycorrhizal colonisation in clover bioassay plants. Deep-banded biochar provided suitable conditions for mycorrhizal fungi to colonise plant roots.

Increasing the length of hyphae in a sandy soil increases the amount of water-stable aggregates

Abstract The contribution from increased lengths of saprophytic and mycorrhizal hyphae to the aggregation of a sandy soil was assessed in a glasshouse experiment. Increased length of mycorrhizal hyphae was encouraged by inoculation of Lolium rigidum with Scutellospora calospora and the effects of the roots on aggregation were restricted by containment within 38 μm mesh bags. The hyphae of saprophytic fungi were increased by the addition of glucose or straw substrates at 2.3 mg g−1. After incubation of the soils for five weeks, changes in > 2 mm water-stable aggregates were assessed in the root-free soil before and after drysieving through 8 mm mesh, to test the resistance of the aggregates to soil abrasion. Hyphal lengths and hot-water extractable carbohydrate C contents were measured in the > 2 mm water-stable fraction and non-aggregate ( The treatments increased hyphal lengths from 3.2 m g−1 to 4–6.2 m g−1, but the hyphal lengths were not different between the treated soils (P 2 mm aggregates from the soils amended with straw and glucose were 2–5.7 m g−1 greater (P 2 mm aggregate fraction of the soil amended with glucose were significantly correlated with aggregation (P 250 μm sand which favoured cross-linking of the sand grains by short lengths of hyphae.

The rate of development of mycorrhizas affects the onset of sporulation and production of external hyphae by two species of Acaulospora.

Subterranean clover was inoculated with Acaulospora laevis , [WUM 11(4)] or an undescribed Acaulospora sp. [WUM 18(1)] at three inoculum quantities and the rate of development of mycorrhizas and the production of spores and external hyphae were monitored. A critical length of mycorrhizal root was found to be associated with the onset of sporulation for both fungi. This was less for Acaulospora sp. than it was for A. laevis and was first reached for plants inoculated with the highest quantity of inoculum for each fungus. Root length colonized and number of spores produced were well correlated at early sampling times once sporulation had begun, but were not well correlated at the end of the experiment. Acaulospora sp. produced six times as many spores as did A. laevis by the final harvest, but this only represented half the volume of spores produced by the latter. After sporulation had commenced the percentage of root length colonized by A. laevis declined while the percentage of root length colonized by Acaulospora sp. remained high, perhaps because of differences in the length of external hyphae produced by each fungus. The length of external hyphae per gram of soil increased with Acaulospora sp. but not for A. laevis , throughout the experiment. However for each fungus there was not a close relationship between the length of external hyphae and the number of spores produced.

Phosphorus, soluble carbohydrates and endomycorrhizal infection

Abstract The effect of phosphorus supply on concentrations of soluble carbohydrate within roots and on the development of mycorrhizas on subterranean clover was examined in two glasshouse experiments. Where increasing phosphorus supply decreased the percentage of root length converted to mycorrhizas, it also decreased the concentrations of soluble carbohydrates within roots. Shading, defoliation and low root temperatures also decreased both the percentage of root length infected and concentrations of soluble carbohydrate within roots. The percentage of root length infected was closely correlated with concentrations of soluble carbohydrates within roots irrespective of the treatment used to vary these concentrations. Small additions of phosphorus to severely deficient plants increased the percentage root length infected possibly by stimulating the growth of the mycorrhizal fungus.