R. S. Pacovsky

Micronutrient uptake and distribution in mycorrhizal or phosphorus-fertilized soybeans

SummarySoybean plants were grown in a soil very low in available P. Seedlings were inoculated with two vesicular-arbuscular mycorrhizal (VAM) fungi or were left non-inoculated and fertilized with P. Assimilation and allocation of micronutrients (Fe, Mn, Zn, and Cu) were determined during host development, and the uptake of trace elements in VAM plants was compared to P-fertilized, non-VAM plants of similar weight, growth stage, and P status. Copper and zinc concentrations were always higher in VAM plants, while iron and manganese concentrations were lower than in the equivalent P-fertilized soybeans. Differences in the micronutrient content of fully-mature soybean pods reflected differences in the leaves and roots. Thus, for trace elements, seed quality can be altered by VAM colonization in a fashion not duplicated by P fertilizer.

Measurement of the extraradical mycelium of a vesicular-arbuscular mycorrhizal fungus in soil by chitin determination

SummaryDevelopment of a vesicular-arbuscular mycorrhizal (VAM) fungus in association with soybean was determined in a greenhouse soil mix by chitin assay. Samples were sieved to eliminate hexosamine-containing contaminants. This preparation reduced the interference caused by extraneous soil substances and permitted quantitative measurement of extraradical VAM fungal mycelium in the soil mix by colorimetric assay. Recovery of added chitin, used as an internal standard, was greater in the soil mix than in an inert medium indicating that some hexosamine was stabalized from chemical degradation by other soil components.

Influence of soil on the interactions between endomycorrhizae and Azospirillum in sorghum

Abstract Sorghum ( S. bicolor L. Moench cv. Bok 8) plants were grown in soil or sand-perlite low in plant-available N and P. Plants were inoculated with a vesicular-arbuscular mycorrhizal (VAM) fungus, or a strain of Azospirillum brasilense or both endophytes together. Plants received a nutrient solution which did not contain N or P. Increases in plant dry weight, shoot-to-root ratios, and the N content of dually-infected plants could be accounted for by summing the VAM and Azospirillum effects. For sorghum inoculated with both endophytes, the presence of A. brasilense in the rhizosphere increased VAM colonization and biomass, while the N input due to Azospirillum decreased, possibly due to competition for carbohydrates. Comparisons between sorghum grown with or without VAM-fungal infection in four growth media showed that edaphic factors other than P availability determined the host response to VAM infection. The P-fixing capacity of the soil, rather than the amount of available (NaHCO 3 -extractable) P, influenced the balance between mutualistic and parasitic VAM-fungal growth.

Nutrient and growth interactions in soybeans colonized with Glomus fasciculatum and Rhizobium japonicum

SummaryGlycine max (L. Merr. cv. Amsoy 71) plants were grown in a greenhouse in a sand/perlite medium low in plant-available N and P. Plants were either inoculated with a vesicular-arbuscular mycorrhizal (VAM) fungus alone, a strain ofRhizobium japonicum alone, both endophytes together or were left non-inoculated to serve as a control. All combinations received a N-and P-free nutrient solution. Nodulated plants contained 4 to 5 times the phytomass of non-inoculated controls, and plants colonized with both the VAM fungus and Rhizobium were 18% greater in dry weight than nodulated, non-VAM plants due to a positive VAM times Rhizobium interaction. Nitrogen fixation, calculated from C2H4 and H2 data, was significantly higher in the tripartite symbiosis, with 80% of the increase attributable to increased nodule mass and 20% due to increases in specific nodule activity. Colonization by the VAM fungus and the development of vesicles increased significantly following nodulation. The synergistic interactions between the microsymbionts suggests that the response of the host to dual colonization is complex and depends on a balance between the three members of the symbiosis.

Mycotrophic growth and mutualistic development of host plant and fungal endophyte in an endomycorrhizal symbiosis

SummarySoybean plants colonized by the vesicular-arbuscular mycorrhizal (VAM) fungusGlomus fasciculatum were grown in pot cultures utilizing a composite greenhouse rooting medium. Development of fungal mycelia inside and outside the host root and total fungal biomass were determined from assays of fungal chitin. Growth and phosphorus uptake by VAM plants and uncolonized controls were compared. Mycotrophic growth in VAM plants occurred during the final six weeks of the 19-week growth period, when the concentration of available soil P fell below 10 μg P/g soil. Growth enhancement was most pronounced in the reproductive organs. The data suggest a relationship between the initiation of the reproductive phase in the host and the cessation of growth in the endophyte. Source-sink relationships and P availability appear to be factors influencing interactions between the symbionts.

Nutrition of sorghum plants fertilized with nitrogen or inoculated withAzospirillum brasilense

SummarySorghum plants were inoculated withAzospirillum brasilense or received an N-amended nutrient solution. Azospirillum inoculation increased plant dry weight and nitrogen assimilation by 25%. Most plant growth responses to Azospirillum were comparable to application of 2.0 mM N. Increased scavenging of nutrients, altered root permeability or nitrogen fixation are possible explanations for these effects.

Development of two endomycorrhizal symbioses on soybean and comparison with phosphorus fertilization

SummarySoybean (Glycine max L. Merr. cv. Amsoy 71) plants were grown in a greenhouse in a soil very low in plant-available P, and plants were harvested 5 times over a 21-week growth period. Soybeans were inoculated with one of two species of VAM fungi or received daily one of three nutrient solutions of different P concentrations (0.0, 0.2, or 1.0mMP). Until week 9, the dry weights, leaf areas and developmental stage of soybeans inoculated withG. fasciculatum orG. mosseae were similar to the 1.0 or 0.2mMP-treated plants, respectively. Phosphorus concentrations were significantly lower in VAM plants at weeks 6 and 9 as compared to non-VAM soybeans given 1.0mMP, suggesting P input in VAM plants was immediately used for new growth. Total P input for VAM plants was linear over 21 weeks, and the average rate of P uptake for these plants was 0.19mg P d−1. Estimated specific P uptake rates (SPUR) for the mycorrhizae (VAM roots) were twice that of the control (0.0mMP) roots. The calculated SPURs forG. fasciculatum andG. mosseae hyphae were 95 and 120μg P g−1 VAM d−1 respectively, a 4 to 5 fold increase over non-inoculated roots, indicating more attention must be paid to P assimilation by VAM fungi in P-fixing substrates.

Metabolic differences in Zea-Glomus-Azospirillum symbioses

Abstract Zea mays L. (cv. Early Sunglow) plants were inoculated with either the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus etunicatum , with a strain of Azospirillum brasilense , or with both endophytes together. All plants were grown in a relatively fertile, peat-based potting mix to minimize differences in macronutrient assimilation between treatments. In addition, non-inoculated maize plants were fertilized with amounts of N and P that had been found to compensate for the input of nutrients following colonization by Azospirillum or Glomus . In general, maize plants colonized by Glomus contained more Zn and Cu, but less P, starch, proline, threonine and alanine than corresponding P-fertilized plants. Corn plants colonized by Azospirillum contained less N, soluble sugars, soluble protein, leucine and isoleucine. but more leaf area and glutamate than corresponding N-fertilized plants. Roots of mycorrhizal plants contained a number of lipids [fatty acids 16:1(11 c ), 18:3 (6,9,12), 20:3, 20:4 and 20:5] not found in P-fertilized controls. The physiology of the plants inoculated with G. etunicatum or A. brasilense was altered in a manner independent of host mineral status.

Influence of inoculation withAzospirillum brasilense andGlomus fasciculatum on sorghum nutrition

Sorghum (Sorghum bicolor (L.) Moench cv. Bok 8) plants were inoculated with either the vesicular-arbuscular mycorrhizal (VAM) fungusGlomus fasciculatum, with a strain ofAzospirillum brasilense, or with both endophytes together. Non-inoculated plants were fertilized with quantities of N and P that had been found to compensate for the input of nutrients following azospirillum or glomus colonization. Total plant dry weight in all treatments was statistically indistinguishable at harvest (10 weeks). In general, plants colonized by Glomus contained less P, Mn, starch and sucrose, but more Cu, Zn and proline than P-fertilized plants. Azospirillum-inoculated sorghum contained less N, glucose, threonine and glutamine, but more Fe and glutamate than N-amended plants. Mycorrhizal roots contained five specific fatty acids not found in non-VAM plants. Inoculated plants displayed altered nutrient requirements, membrane composition and metabolite levels, indicating that colonization by these endophytes influenced host physiology, even under conditions where N or P input was negligible.

Growth and nutrient allocation in Phaseolus vulgaris L. colonized with endomycorrhizae or Rhizobium

Two cultivars of Phaseolus vulgaris L., one responsive to colonization with microsymbionts (Mexico 309) and one less-responsive (Rio Tibagi) were grown in Leonard jars containing sand/vermiculite under greenhouse conditions. Bean plants were either left non-inoculated (controls) or were inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus etunicatum or a strain of Rhizobium leguminosarum bv. phaseoli (UMR-1899). Plants from the Mexico 309 cultivar maintained a higher growth rate, supported proportionately more nodules and mycorrhizae, and assimilated relatively more N or P when colonized by Rhizobium or Glomus, respectively, than did plants of the Rio Tibagi cultivar. Estimated specific nodule activity for Mexico 309 beans was more than twice that of Rio Tibagi, whereas the specific phosphorus uptake rate (SPUR) was 35% greater in the non-inoculated roots of Rio Tibagi compared to Mexico 309. Colonization by G. etunicatum more than doubled the SPUR for each cultivar compared to control roots. New acid phosphatase isozymes appeared in VAM-colonized roots of both cultivars compared to controls. Acid and alkaline phosphatase activities were significantly higher in G. etunicatum-colonized Mexico 309 roots, but not in Rio Tibagi mycorrhizae, compared to uninfected roots. Polyphosphate hydrolase activity was elevated in mycorrhizae of both cultivars compared to control roots. These results indicate that the dependence of a host on a specific endophyte increases when there are limitations to the supply of a nutrient that the endophyte can provide. The greater the increase in absorption or utilization capacity following colonization by the microsymbiont, the greater the dependence by the host. More importantly, identification of enzymatic activities that influence these plant-microbe associations opens the possibility that the specific genes that code for these enzymes could be targeted for future manipulation.