Location and quantification of phosphorus and other elements in fully hydrated, soil‐grown arbuscular mycorrhizas: a cryo‐analytical scanning electron microscopy study

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Abstract
• Concentrations of phosphorus (P), potassium (K), magnesium (Mg) and calcium (Ca) were determined in situ in fully hydrated arbuscular mycorrhizas by cryo-analytical scanning electron microscopy. The field- and glasshouse-grown plants (subterranean and white clovers, field pea and leek) were colonized by indigenous mycorrhizal fungi. • The [P] in intraradical hyphae was generally 60-170 mM, although up to 600 mM was recorded, and formed strong linear relationships with [K], up to 350 mM, and [Mg], up to 175 mM. Little Ca was detected. The turgid branches of young arbuscules contained 30-50 mM P, up to 100 mM K and little Mg. Collapsing arbuscule branches and clumped arbuscules had greatly elevated Ca (30-250 mM), but otherwise differed little from young arbuscule branches in elemental concentration. • The [P] was low or undetectable in 86% of uncolonized cortical cell vacuoles, but was generally elevated in vacuoles surrounding an arbuscule and in the liquid surrounding hyphae in intercellular spaces. • Our results suggest that both young arbuscules and intercellular hyphae are sites for P-transfer, that Mg2+ and K+ are probably balancing cations for P anions in hyphae, and that host cells may limit arbuscule lifespan through deposition of material rich in Ca.