Nutrient Acquisition Strategies Research Articles

Mycorrhizas and Other Specialized Nutrient-Acquisition Strategies: Their Occurrence in Woodland Plants From Kakadu and Their Role in Rehabilitation of Waste Rock Dumps at a Local Uranium Mine

The presence of mycorrhizas, proteoid roots and leguminous nodules was determined in a range of woodland species (from a variety of habitats in soils formed on different parent materials) in the Kakadu area in the monsoonal tropics of northern Australia. In addition, the chemical fertility and the occurrence of mycorrhizal fungi and rhizobia in rudimentary soils ('minesoils') forming in situ on waste rock dumps at a mine site in the region were compared with stockpiled topsoils from the mine area and undisturbed topsoils collected from the surrounding native woodland. A major aim of these investigations was to assess the feasibility of rehabilitating the waste rock dumps without spreading topsoils. More than 90% of the woodland flora examined had one or more specialised nutrient-gathering mechanism. Mycorrhizas were found on 82% of the species, with some 16% of species having both ecto and VA mycorrhizas, often on the same individual plant. Many of these observations are the first records of mycorrhizal infection in the particular genera and species involved. Soil baiting and dilution experiments showed that rhizobia and mycorrhizal fungi were ubiquitous components of the soil biota in all undisturbed woodland soils. However, they were absent or poorly represented in the stockpiled topsoils and in some of the rudimentary soils formed in waste rock at the mine site. The diversity of spore types and/or numbers of infective propagules of VAM fungi was lower in stockpiled topsoils and in minesoils than in the undisturbed woodland soils. Nutrient omission experiments identified that acute deficiency of P, and to a lesser extent N, was a limitation to growth of seedlings on all soils. Zn deficiency was detected in the only soil for which this was examined. A glasshouse experiment, using a young minesoil and application of basal nutrients, demonstrated that inoculation of Acacia holosericea seedlings with rhizobium could completely alleviate the effects of N deficiency. Under conditions of both N and P deficiency, dual inoculation of A. holosericea with rhizobium and spores of the VAM fungus, Glomus, only partly overcame the limitations of P deficiency on seedling growth. Induction of deficiencies of P and Zn in a second minesoil (through application of basal nutrients), demonstrated that inoculation of seedlings of Eucalyptus pellita with spores of the ectomycorrhizal fungus, Scleroderma, partly alleviated the effects of both deficiencies. Rehabilitation strategies implemented at the mine site using either soils forming in situ on the waste rock dumps, or by spreading stockpiled topsoils, will need to ensure deficiencies of P and other nutrients are alleviated and that viable populations of mycorrhizal fungi and rhizobia are introduced and maintained during early phases of vegetation establishment.

Differential responses of C3 and C4 grasses to mycorrhizal symbiosis, phosphorus fertilization, and soil microorganisms

The responses of five C4, warm-season and five C3, cool-season tallgrass prairie grasses to phosphorus (P) fertilization, mycorrhizae, and soil microorganisms were compared in greenhouse studies. The warm-season grasses responded positively to mycorrhizae or to P fertilization, but mycorrhizal plants did not respond to P. The soil microflora reduced mycorrhizal plant dry weight and root colonization. In contrast, cool-season grasses did not respond to mycorrhizae or P fertilization. Soil microorganisms did not suppress cool-season plant growth, but root colonization was reduced in nonsterile soil. For the warm-season grasses there was an inverse relationship between mycorrhizal root colonization and P fertilization and a positive relationship between root colonization and plant dry weight. For the cool-season grasses there was also an inverse relationship between root colonization and P fertilization, but the relationship between root colonization and plant dry weight was negative. In both the warm-season and cool-season grasses, low levels of mycorrhizal root colonization persisted even when P fertilization was sufficient to eliminate mycorrhizal effects on plant growth. Thus, warm- and cool-season grasses display profoundly different strategies for nutrient acquisition. Key words: cool-season grasses, warm-season grasses, vesicular–arbuscular mycorrhizae.

Short term 22Na+ and 42K+ uptake in intact, mid‐vegetative plants

Spergularia marina (L.) Griseb. is. a rapidly growing, annual, coastal halophyte. Because of its small size, it is suitable for isotope studies of ion transport well beyond the seedling stage. The purpose of this report is to establish the similarities and differences between 22Na+ and 42K+ uptake in S. marina and in more commonly used mesophytic crop species. Vegetative plants were used 18 days after transfer to solution culture. Plants were grown either on Na+‐free medium or on 0.2 × sea water. 22Na+ uptake was linear with time for several hours. The rate was relatively insensitive to external concentration between 1 and 180 mol Na+ m−3, particularly in Na+‐free plants. Transport to the shoot accounted for 40 to 70% of the total uptake, dependent on salinity but largely independent of time. 42K+ uptake decreased with increasing salinity in Na+‐free plants and increased in 0.2 × sea water plants. Both uptake and transport to the shoot were non‐linear with time, upward concavity suggesting recovery from a manipulative and/or osmotic injury. Steady state root contents were compared with predicted contents based on cortical cell electrical potentials using the Nernst equation. Reasonable agreement was found in all cases except Na+ content of 0.2 × sea water plants, in which active efflux was indicated. Uptake studies conducted in the presence of chemical modifiers (dicyclohexylcarbodiimide, dinitrophenol and fusicoccin) showed responses of 42K+ uptake as expected from studies on agronomic species, and implied the presence of a similar active uptake here despite the appearance of equilibrium. Active Na+ uptake was suggested at low Na+ levels. We conclude that S. marina is a promising experimental system combining the rapid nutrient acquisition strategy of agionomically important annuals with a high degree of salt tolerance.