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Annual pasture legumes with a superior ability to acquire soil phosphorus (P) and a low external critical P requirement could reduce the need for P fertiliser. Roots of pasture legumes grown in field soil will commonly be colonised by arbuscular mycorrhizal fungi (AMF). However, recent research suggests a second community of arbuscule-forming root-colonising fungi may be present: fine root endophytes (FRE). AMF are known to enhance P acquisition by plants under certain conditions, whereas very little is known about FRE and their impact on plant P status. We, therefore, grew plants at seven levels of P supply to determine and compare the external critical P requirements of two species of Ornithopus and two cultivars of Trifolium subterraneum when colonised predominantly by FRE (FRE-dominant treatment) or both FRE and AMF (mixed treatment). As expected from previous studies, the Ornithopus species had a lower critical P requirement than the T. subterraneum cultivars. However, for the Ornithopus species only, the FRE-dominant community had a significantly lower external critical P requirement (24–31 mg P kg −1 soil) than the mixed community (50–58 mg P kg −1 soil). Moreover, at low-P supply (≤15 mg applied P kg −1 soil), Ornithopus species in the FRE-dominant treatment had longer specific root length, smaller average root diameter, 45–128% greater total root length, and lower shoot and root P concentration than in the mixed treatment. We concluded that comparisons among experiments of root morphological traits and external critical P requirements can be affected by the presence of different communities of arbuscule-forming root-colonising fungi and that these effects may vary among plant species. [ABSTRACT FROM AUTHOR]

Despite the reported importance of mycorrhizal symbioses for early growth and nutrient acquisition of cotton, little is known about how sodicity affects this relationship. Changes in mycorrhizal colonisation and nutrient uptake of cotton in a range of naturally non-sodic (exchangeable sodium percentages (ESP) < 6) and low-sodic soils (ESP 6–10), from cotton production areas in southern Queensland and northern New South Wales, with different ESP (ranged between 1.4 and 9.8) was investigated in a glasshouse experiment. The experiment was a complete factorial design with 11 recently-collected soils and two mycorrhizae treatments (either inoculated with fresh “live” mycorrhizal inoculum or without inoculum). Linear mixed model analysis showed minimal effects of sodicity, when ESP was less than 10, on mycorrhizal colonisation, associated plant growth and nutrient uptake. Principle component and regression analysis showed that other sources of variation including soil pH and soil P content, rather than sodicity, might drive cotton colonisation in Vertosols with low to moderate ESP. The colonisation percentage was positively linearly correlated with P, Mg, and Zn uptake of cotton plants. Further investigation into mycorrhizal spore density and species diversity under sodic soil conditions is warranted. [ABSTRACT FROM AUTHOR]

Little is known about how the adverse physical and chemical environment in sodic soils affects plant-mycorrhizal relationships. We investigated mycorrhizal colonisation and plant nutrient uptake of cotton plants under highly stressed (Exchangeable Na percentage (ESP) 21) and less stressed (ESP 7) conditions with two rates of applied P. The relative hyphal contribution to P uptake was quantified using dual isotope labelling techniques ( 32 P and 33 P). Root colonisation and P uptake of mycorrhizal cotton plants reduced by 16% and 20%, respectively, in highly sodic soil as compared to plants in low sodic soil, however, the relative proportion of P delivered via hyphal pathways ( 32 P from root-free hyphal compartment) was similar. Under high P conditions, the relative increase in the proportion of 33 P (root + hyphae compartment) taken up by inoculated plants was greater in the low sodic soil relative to the high sodic soil. Mycorrhization improved early seedling vigour, and nutrient uptake. Reduced colonisation and hyphal exploration of the soil, possibly due to the physical and chemical constraints imposed by highly-sodic soil, rather than poorer mycorrhizal function, may be responsible for limited early P uptake of cotton in highly-sodic soil. [ABSTRACT FROM AUTHOR]

Development of more phosphorus (P)-efficient pasture legumes could improve the P balance of pasture systems. Trifolium subterraneum (subterranean clover) is the most widely-grown annual pasture legume in southern Australia and is host to arbuscular mycorrhizal fungi (AMF). To examine the potential to alter colonisation level through breeding, 97 lines from the core collection of subterranean clover (core lines) and 42 cultivars (cultivar lines) were grown in a glasshouse in a low-P sandy acid field soil with indigenous AMF (Experiment 1). To examine the robustness of rankings, two cultivars (Riverina and Woogenellup) were also grown in 11 field soils (Experiment 2). To examine whether the presence of AMF changed other root traits important for P uptake, cultivars Riverina and Woogenellup were grown in two low-P field soils (sand, sandy loam), either pasteurised to remove indigenous AMF or unpasteurised, and with and without addition of inoculum of AMF (Experiment 3). The range of colonisation for core lines and cultivar lines was similar (∼12–68% of root length), however, the core lines had higher mean colonisation (37% vs 33%) and a greater proportion of lines with >40% of root length colonised (41% vs 24% of lines). Woogenellup was more heavily colonised than Riverina in eight soils and had a similar level of colonisation in three soils. In the sand, pasteurisation resulted in shoot DM increasing by up to 100% (possibly due to reduction in root diseases). Inoculation, in pasteurised and unpasteurised soil, resulted in an increase in shoot DM, shoot P concentration and shoot P content of ∼100%, increased average root diameter by up to 13%, decreased specific root length by up 30% and decreased root mass ratio; effects on average root hair length were variable. In the unpasteurised sandy loam, shoot P concentrations were higher than in the sand, and inoculation slightly increased shoot P concentration, but not shoot DM; the effects of inoculation on root traits were mostly small. We conclude that potential may exist to select subterranean clover for higher or lower colonisation by AMF using cultivar lines. Also, indigenous AMF may be less effective at enhancing plant P uptake and growth than inoculants. Finally, key root traits for enhancing plant P-uptake may change in the presence of AMF, but not sufficiently so to warrant deliberate inclusion or exclusion of AMF when screening germplasm for such traits. [ABSTRACT FROM AUTHOR]

Soil sodicity degrades land and more than half of the world’s sodic soils are in Australia. Farmers in Australia produce and export cotton grown in sodic soils. Undesirable physicochemical constraints (e.g. high pH, high bulk density, low porosity and reduced oxygen content) are associated with sodic soils and may adversely affect microbial interactions in the rhizosphere, including mycorrhizal associations. This viewpoint focusses on facts and gaps in our knowledge about mycorrhizal associations in sodic soils, with special attention to cotton systems. We highlight the difference between saline and sodic soil conditions and the impacts they may have on host plant-mycorrhizae symbiosis. This viewpoint identifies the need for more research on the potential impact of sodicity on mycorrhizal species diversity, functionality and benefits to crop growth. Changes in agronomic management strategies to maximize mycorrhizal symbiosis benefits are suggested, especially for those plant species, like cotton, that are highly reliant on mycorrhizal symbiosis for optimal growth and nutrient uptake. [ABSTRACT FROM AUTHOR]