The mechanism responsible for phosphorus inhibition of vesicular-arbuscular mycorrhiza formation in sudangrass (Sorghum vulgare Pers.) was investigated in a phosphorus-deficient sandy soil (0.5 micrograms phosphorus per gram soil) amended with increasing levels of phosphorus as superphosphate (0, 28, 56, 228 micrograms per gram soil). The root phosphorus content of 4-week-old plants was correlated with the amount of phosphorus added to the soil. Root exudation of amino acids and reducing sugars was greater for plants grown in phosphorus-deficient soil than for those grown in the phosphorus-treated soils. The increase in exudation corresponded with changes in membrane permeability of phosphorus-deflcient roots, as measured by K' (86Rb) efflux, rather than with changes in root content of reducing sugars and amino acids. The roots of phosphorus-deficient plants inoculated at 4 weeks with Glomusfascicuaww were 88% infected after 9 weeks as compared to less than 25% infection in phosphorus-sufficient roots; these differences were correlated with root exudation at the time of inoculation. For plants grown in phosphorusdeficient soil, infection by vesicular-arbuscular mycorrhizae increased root phosphorus which resulted in a decrease in root membrane permeability and exudation compared to nonmycorrhizal plants. It is proposed that, under low phosphorus nutrition, increased root membrane permeability leads to net loss of metaboiltes at sufficient levels to sustain the germination and growth of the mycorrhizal fungus during pre-and postinfection. Subsequently, mycorrhizal infection leads to improvement of root phosphorus nutrition and a reduction in membrane-mediated loss of root metabolites.During the past 20 years, there has been a growing appreciation of the importance of VAM' in the improvement of plant growth through increased uptake of phosphorus (P) and other mineral nutrients, especially in soils of low fertility (5, 9). As evidence has mounted for the role of VAM in the enhancement of P uptake from P-deficient soils, it has also been recognized that high soil P levels severely limit VAM infection (9, 10).Early work failed to distinguish whether P was inhibiting the activity of the mycorrhizal fungus in the soil or during the hostfungus interaction (10,16 of sudangrass inoculated with Glomusfasciculatus (Thaxt.) Gerd. and Trappe only became heavily infected if the host was not receiving adequate P from the other half of the root system. It is now clear that the P content of the host plant is the critical factor controlling the mycorrhizal symbiosis.Ratnayake et al. (12) proposed that the mechanism of P control of VAM formation was associated with a membrane-mediated decrease in root exudation. They were able to correlate low P content of sudangrass and citrus roots with a decrease in phospholipid levels and a large increase in permeability of root membranes, which results in a greater net leakage of amino acids and sugars from the root. They suggested that under P-sufficient conditions, metabolites requi...

SUMMARYThe mechanism responsible for inhibition of the establishment of mycorrhizal associations in Sorghum vulgare Pers. (herbaceous monocot) and Citrus aurantium L. (woody dicot) under high levels of soil phosphorus (P) was studied. Plants were grown on low fertility loamy sand (4.5 ppm P), receiving superphosphate [Ca (H2PO4)2H2O] at 0, 6, 28, 56, 228 and 556 ppm P along with all the other necessary nutrients. The percentage P content of root tissue was correlated with the amount of P added to the soil. Root exudation, measured in terms of the net leakage of soluble amino acids and reducing sugars from the roots within a 17-h period, was significantly higher under low P levels (0, 6 and 28 ppm P) than under high P levels (56, 228 and 556 ppm P). The amount of exudation was correlated with a Pinduced decrease in phospholipid levels and associated changes in permeability properties of root membranes, rather than with changes in the root content of sugars and amino nitrogen. The hypothesis is proposed that phosphorus inhibition of mycorrhizal symbiosis is associated with a membrane-mediated decrease in root exudation.

SUMMARYUsing a 'split root' technique, it was found that phosphorus fertilization of half of the root system of sudangrass could significantly reduce the number of chlamydospores of the mycorrhizal fungus Glomus fasciculatus in the unfertilized half of the root system. In a second experiment, vials of soil containing different concentrations of phosphorus were inserted into pots of sudangrass which were fertilized with different amounts of phosphorus and inoculated with G. fasciculatus. The numbers of chlamydospores, vesicles and arbuscles and the amount of hyphae produced by G. fasciculatus on roots within the vials were not influenced by the soil P in the vials but were inversely correlated with the concentration of P in roots outside the vial. All evidence indicates that it is the concentration of P within the plant and not the soil P which leads to a reduction in colonization, infection, and spore production of G. fasciculatus.

SUMMARYSix citrus cultivars were grown with and without the mycorrhizal fungus Glomus fasciculatus under three fertilizer regimes all without phosphorus. The average mycorrhizal dependency (Gerdemann 1975) of Rough lemon and Brazilian sour orange were greater than the mycorrhizal dependecies of Alemow, Troyer citrange, Bessie sweet orange and Trifoliate orange. The mycorrhizal dependency of each cultivar, except Bessie sweet and Trifoliate orange, was substantially altered by at least one of the fertilizer regimes, and therefore the order of mycorrhizal dependency was different at all three fertilizer regimes. On the average, citrus rootstocks exhibited the greatest mycorrhizal dependency with the least fertilization. The average percentage phosphorus in non-mycorrhizal leaf tissues was inversely correlated with the mycorrhizal dependency of citrus cultivars at the medium fertilizer regime. An inverse correlation was observed between the dry weights of non-mycorrhizal roots of the citrus cultivars and the mycorrhizal dependency of the citrus cultivars.

Commercial use of vesicular–arbuscular mycorrhizae (VAM) may be an alternative to rising agricultural energy and fertilizer costs. Vesicular–arbuscular mycorrhizae may be able to increase crop yields while reducing fertilizer and energy inputs. Since mycorrhizal fungi are naturally present in most soils, their unique fertilizer abilities are already being utilized by most crop plants. Commercial uses of VA mycorrhizal fungi are therefore currently restricted to situations where the natural populations of VAM fungi have been destroyed or damaged such as in fumigated or chemically treated areas, greenhouses, and disturbed areas such as coal spoils, strip mines, waste areas, or road beds. Commercial production of VAM inoculum is presently being attempted at several locations in the U.S. Vesicular–arbuscular mycorrhizal inoculum is produced by growing VAM fungi on the roots of suitable host plants under aseptic greenhouse conditions The inoculum consists of the host-plant growth medium and host roots associated with VAM hyphae and spores which have been ground and dried. Most large-scale uses of VAM involve the establishment of the mycorrhizae on seedlings which will be transplanted to the field. Large-scale methods for direct inoculation with VAM have not yet been devised, but in small trials, layering, banding, broadcasting, and pelleting seed with VAM inoculum have proved effective. Methods for determining what soils are most likely to benefit from applications of VAM fungi are available. The potential for employing VAM fungi on a wide scale in agriculture is dependent on the development of crop growth-promoting strains of VAM which are superior to native soil populations of VAM fungi.

SUMMARYThe hypothesis was tested that the amount of external hyphae of a vesicular-arbuscular mycorrhizal (VAM) fungus extending from roots out into soil is not always proportional to the extent of colonization of the root cortex. Growth enhancement and amount of external hyphae were compared for eight isolates of five Glomus spp. that differed in their geographic origin and capacity to enhance growth of Troyer citrange, but were similar in their capacity to extensively colonize Troyer citrange roots. In general, isolates from California increased growth in a P-deficient (9-8 mg kg"^) California soil more than did non-native isolates from Florida soils. The difference between the capacity of California and Florida isolates to enhance growth was not a function of the degree to which they colonized the roots since all had colonized over 95 % of the root length by the time of harvest. Differences in growth enhancement did appear, however, to be a function of the amount of external hyphae that had developed as estimated by the weight of soil they had bound into aggregates. This study suggests that isolates of VA mycorrhizal fungi may differ in their capacity to develop an external hyphal system independent of their capacity to colonize the root cortex, and that we cannot assume that high levels of colonization will necessarily nnean the fungus has also developed the mycelium in the soil necessary to transport nutrients responsible for plant growth enhancement.

SUMMARYThe interaction between light intensity and soil temperature with phosphorus inhibition of vesicular-arbuscular mycorrhiza (VAM) formation in sudangrass (Sorghum vulgare, Pers.) was investigated in P-deficient (0-5 mg P kg~^) sandy soil amended with 0 (OP), 15 (15P) and 30 (30P) mg P kg"' soil as superphosphate. For seedlings inoculated with Glomus fasciculatus and grown at 100, 75, and 50"0 of glasshouse light intensity, the reduction in VAM formation by added soil P (15 P and 30P) was greatest at the lower light levels, whereas in the OP treatment, VAM fornaation was unaffected by a decrease in light intensity. Increases in root P concentration resulted from both soil P amendment and, in the 15P treatment, from a decrease in light intensity, and were associated with decreases in root membrane permeability and corresponding decreases in root exudation of amino acids and sugars. The reduction in sugar exudation in the 15P treatment at lower light intensity was paralleled by a decrease in root extract levels of sugars, but decreases in amino acid exudation could not be explained by changes in amino acid levels in root extracts. These results suggest that P inhibition of VAM formation is more severe at lower light intensities because of a decrease in membrane-mediated exudation, although a reduction in sugar levels in root cells may also contribute to tbe observed decrease in exudation.When inoculated seedlings were grown at 25, 30, and 35°C soil temperatures, the reduction in VAM formation in the 1 5P and 30P treatments was overcome by increase in soil temperature. The increase in VAM formation by increased temperature in the 15P plants was associated with significant increases in root membrane permeability and exudation without a corresponding change in root P concentration. In the OP treatment, an increase in soil temperature increased VAM formation with little effect on root exudation, which suggested an effect of temperature on Glomus fasciculatus. Higher soil temperature may increase VAM formation through either a direct effect of temperature on tbe fungus or an indirect effect via an increase in leakage of root metabolites necessary for fungal activity, or both.

No abstract