Pastures often experience a pulse of phosphorus (P) when fertilized. We examined the role of arbuscular mycorrhizal fungi (AMF) in the uptake of P from a pulse. Five legumes (Kennedia prostrata, Cullen australasicum, Bituminaria bituminosa, Medicago sativa and Trifolium subterraneum) were grown in a moderate P, sterilized field soil, either with (+AMF) or without (−AMF) addition of unsterilized field soil. After 9-10 weeks, half the pots received 15 mg P kg −1 of soil. One week later, we measured: shoot and root dry weights; percentage of root length colonized by AMF; plant P, nitrogen and manganese (Mn) concentrations; and rhizosphere carboxylates, pH and plant-available P. The P pulse raised root P concentration by a similar amount in uncolonized and colonized plants, but shoot P concentration increased by 143% in uncolonized plants and 53% in colonized plants. Inoculation with AMF decreased the amount of rhizosphere carboxylates by 52%, raised rhizosphere pH by ∼0.2-0.7 pH units and lowered shoot Mn concentration by 38%. We conclude that AMF are not simply a means for plants to enhance P uptake when P is limiting, but also act to maintain shoot P within narrow boundaries and can affect nutrient uptake through their influence on rhizosphere chemistry.

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Aims: Arbuscule-producing fine root endophytes (FRE) (previously incorrectly Glomus tenue) were recently placed within subphylum Mucoromycotina; the first report of arbuscules outside subphylum Glomeromycotina. Here, we aimed to estimate nutrient concentrations in plant and fungal structures of FRE and to test the utility of cryoscanning electron microscopy (cryoSEM) for studying these fungi. Methods: We used replicated cryoSEM and X-ray microanalysis of heavily colonized roots of Trifolium subterraneum. Results: Intercellular hyphae and hyphae in developed arbuscules were consistently very thin; 1.35 ± 0.03 µm and 0.99 ± 0.03 µm in diameter, respectively (mean ± SE). Several intercellular hyphae were often adjacent to each other forming "hyphal ropes." Developed arbuscules showed higher phosphorus concentrations than senesced arbuscules and non-colonized structures. Senesced arbuscules showed greatly elevated concentrations of calcium and magnesium. Conclusion: While uniformly thin hyphae and hyphal ropes are distinct features of FRE, the morphology of fully developed arbuscules, elevated phosphorus in fungal structures, and accumulation of calcium with loss of structural integrity in senesced arbuscules are similar to glomeromycotinian fungi. Thus, we provide evidence that FRE may respond to similar host-plant signals or that the host plant may employ a similar mechanism of association with FRE and AMF.

Crops with improved uptake of fertilizer phosphorus (P) would reduce P losses and confer environmental benefits. We examined how P‐sufficient 6‐week‐old soil‐grown Trifolium subterraneum plants, and 2‐week‐old seedlings in solution culture, accumulated P in roots after inorganic P (Pi) addition. In contrast to our expectation that vacuoles would accumulate excess P, after 7 days, X‐ray microanalysis showed that vacuolar [P] remained low (<12 mmol kg−1). However, in the plants after P addition, some cortex cells contained globular structures extraordinarily rich in P (often >3,000 mmol kg−1), potassium, magnesium, and sodium. Similar structures were evident in seedlings, both before and after P addition, with their [P] increasing threefold after P addition. Nuclear magnetic resonance (NMR) spectroscopy showed seedling roots accumulated Pi following P addition, and transmission electron microscopy (TEM) revealed large plastids. For seedlings, we demonstrated that roots differentially expressed genes after P addition using RNAseq mapped to the T. subterraneum reference genome assembly and transcriptome profiles. Among the most up‐regulated genes after 4 hr was TSub_g9430.t1, which is similar to plastid envelope Pi transporters (PHT4;1, PHT4;4): expression of vacuolar Pi‐transporter homologs did not change. We suggest that subcellular P accumulation in globular structures, which may include plastids, aids cytosolic Pi homeostasis under high‐P availability.

Tedera is a forage legume that can provide out-of-season green feed in Mediterranean climates. To date, growers have had no comprehensive soil nutrition guidelines to optimise tedera production. We undertook field and glasshouse studies to understand tedera’s macronutrient requirements. Three field experiments were sown with tedera cv. Lanza® at Cunderdin, Dandaragan and Three Springs in Western Australia. These experiments evaluated seven levels of phosphorus (P) (0–30 kg ha−1) and potassium (K) (0–80 kg ha−1) and two combined treatments with P and K. Glasshouse pot experiments were conducted using tedera cultivars Lanza® and Palma and lucerne cultivar SARDI Grazer. Ten concentrations of added P (0–256 mg kg−1), ten of K (0–256 mg kg−1) and ten of sulphur (S) (0–16 mg kg−1) were tested. There was no significant response to P or K in field soils at Cunderdin or Three Springs. There was no response to K at Dandaragan, but P produced a positive response in the July and October growing season cuts. In the glasshouse, tedera cultivars reached peak productivity at lower soil Colwell P (7.6 to 12 mg kg−1) than lucerne (22 mg kg−1). Lanza® had a moderate biomass response, and Palma did not show a significant response to Colwell K (0.8 to 142 mg kg−1) or soil S (1.3 to 12.5 mg kg−1). Nodulation was greatly reduced at the extremes in P and K treatments. For the first time, these field and glasshouse results have allowed us to establish guidelines for optimal soil nutrition for tedera that growers can use to benchmark the soil or shoot nutrient status of their tedera pastures and assess the economic benefit of correcting deficiencies.