A. D. Robson

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.

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.

65Zn uptake in subterranean clover (Trifolium subterraneum L.) by three vesicular-arbuscular mycorrhizal fungi in a root-free sandy soil

Abstract The uptake of Zn by the hyphae of three vesicular-arbuscular (VA) mycorrhizal fungi, Acaulospora laevis Gerdemann & Trappe, Glomus sp. resembling Glomus fasciculatum (Thaxter) Gerdemann & Trappe emend. Walker & Koske and Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders, in association with subterranean clover Trifolium subterraneum L. (cv. Seaton Park) was examined. The plants were grown in PVC tubes for 5 weeks before transfer to an experimental system, where a root compartment was separated from a root-free hyphal compartment by a 37μm nylon mesh. 65Zn was placed in the hyphal compartment at distances of 0, 10, 20 or 40mm from the roots. For the non-mycorrhizal control, 65Zn was placed at 0 or 10 mm from the roots. At 14 and 28 days after transfer (DAT) the youngest opened leaves (YOL) were harvested, and at 21 and 35 DAT whole plants were harvested. Dry weights, P and Zn concentrations, root length and mycorrhizal colonization, length of external hyphae and specific 65Zn activity were recorded. At all harvests 65Zn activity in shoots of plants colonized by A. laevis was greater than for non-mycorrhizal plants and plants colonized by either Glomus sp. or S. calospora. Plants inoculated with A. laevis absorbed 65Zn placed 40 mm from the roots, whereas no 65Zn uptake from this distance occurred with other treatments. The ability of plants colonized by A. laevis to access 65Zn 40mm from the roots was associated with a much greater hyphal length at this distance from the roots than for the two other mycorrhizal fungi. However, immediately adjacent to the roots there was a greater length of external hyphae for Glomus sp. than for A. laevis. Hyphal length was much lower for S. calospora at all distances from the roots than for the other mycorrhizal fungi. The results show great differences in the abilities of the hyphae of VA mycorrhizal fungi to grow in root-free soil and to absorb and transport labelled Zn to the host plants in a soil with a luxury supply of Zn.

Effects of vesicular-arbuscular mycorrhiza on the availability of iron phosphates to plants

SummaryThe effect of inoculation with a mycorrhizal fungus on the growth of subterranean clover and of ryegrass was measured using three sources of phosphorus with different solubilities. These were (in order of decreasing solubility): potassium dihydrogen phosphate, colloidal iron phosphate and crystalline iron phosphate. Mycorrhizal infection increased growth more for subterranean clover than for ryegrass for all sources of phosphorus. For both species the greatest benefit from mycorrhizal inoculation was obtained with the least soluble source of iron phosphate. It is suggested that the mycorrhizas were able to explore the soil more thoroughly and hence were able to locate and use the point sources of phosphorus in the insoluble iron phosphates.

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.

The involvement of mycorrhizas in assessment of genetically dependent efficiency of nutrient uptake and use

This article summarises the way in which mycorrhizal infection of roots affects the mineral nutrition of plants and how the symbiosis may interact with the evaluation of efficiency of nutrient uptake and use by plants. A brief account of the processes of infection and the way they are affected by host genotype and environmental conditions is given and the relationships between this and mineral nutrition (especially phosphate nutrition) are outlined.

Distribution and Transport of Zinc in Plants

Zinc (Zn) distribution and transport in plants is affected by the level of Zn supply and plant species. When plants have low to adequate Zn supply, Zn concentrations are usually higher in growing tissue than in mature tissue; this is true for roots, vegetative shoots and reproductive tissues. In plants tolerant of toxic levels of Zn, accumulation has been observed in the root cortex and in leaves. In these tissues, Zn accumulates in cell walls or is sequestered in vacuoles.

Nodulation of Casuarinaceae in relation to host species and soil properties

A field survey was conducted in Australia to examine nodulation of Casuarinaceae in relation to 22 host species (187 sites) and soil chemical properties (152 sites). Four of the five Casuarina species examined were regularly nodulated (most plants in the 60 out of 88 sites in which nodules were found). Casuarina species occurred more frequently on soils of higher available-phosphorus status than did Allocasuarina species, eight of which did not nodulate in any soils. With the nine Allocasuarina species which nodulated,nodules occurred on only a minority of the individual plants examined. Soil chemical properties other than available-phosphorus level were useful in distinguishing sites at which individual species occurred but had no apparent relationship to nodulation. Under glasshouse conditions, baiting of field soils with seedlings of Casuarinaceae indicated the occurrence of nodulation to be similar to that observed in the field survey. Allocasuarina species formed ectomycorrhizas more commonly than did Casuarina species; both genera formed vesicular-arbuscular mycorrhizas. A second glasshouse experiment supported the hypothesis that low phosphorus supply and the absence of infective Frankia were two of the factors responsible for the absence of nodulation in some field soils. The effects of other soil factors in limiting plant growth and nodulation of Casuarinaceae are considered, and the potential significance of N2 fixation by Casuarinaceae in the field discussed. Nodules were found on Allocasuarina campestris, A. dielsrana and A. lehmanniana, not previously recorded as nodulating.

Specific activity of phosphorus in mycorrhizal and non-mycorrhizal plants in relation to the availability of phosphorus to plants

Abstract Phosphate was allowed to react with a soil to which iron hydroxide had been added. The P was then labelled by a subsequent addition of 32 P. Soil P was extracted by 10 m m CaCl 2 , 0.5 m NaHCO 3 , and acid NH 4 F solutions and the specific activity of P in the extracts was measured. Subterranean clover plants were grown both with and without a mycorrhizal fungus. Phosphorus contents and the specific activities of P in the plant shoots were determined. For mycorrhizal plants, adding iron hydroxide had no effect on the amount of P taken up, but for non-mycorrhizal plants it decreased the uptake. However there was no effect of iron hydroxide or of mycorrhizal infection on the specific activity of P in the plants. Adding iron hydroxide had no effect on the amount of P extracted by acid NH 4 F, but decreased the P extracted by 10m m CaCl 2 and by 0.5 m NaHCO 3 . The specific activity of P in the extracts was not affected by the addition of iron hydroxide and was the same for the three extractants. Further, the specific activity of P in all extractants was similar to that of P in both mycorrhizal and non-mycorrhizal plants. Thus differences in the availability of soil P to mycorrhizal and non-mycorrhizal plants and to the extractants were not reflected by differences in labelling. It therefore follows that lack of difference in specific activity between mycorrhizal and non-mycorrhizal plants does not eliminate the possibility that mycorrhizal plants can obtain P that was unavailable to non-mycorrhizal plants.