Manganese distribution and speciation help to explain the effects of silicate and phosphate on manganese toxicity in four crop species.

Soil acidity and waterlogging increase manganese (Mn) in leaf tissues to potentially toxic concentrations, an effect reportedly alleviated by increased silicon (Si) and phosphorus (P) supply. Effects of Si and P on Mn toxicity were studied in four plant species using synchrotron-based micro X-ray fluorescence (μ-XRF) and nanoscale secondary ion mass spectrometry (NanoSIMS) to determine Mn distribution in leaf tissues and using synchrotron-based X-ray absorption spectroscopy (XAS) to measure Mn speciation in leaves, stems and roots. A concentration of 30 μM Mn in solution was toxic to cowpea and soybean, with 400 μM Mn toxic to sunflower but not white lupin. Unexpectedly, μ-XRF analysis revealed that 1.4 mM Si in solution decreased Mn toxicity symptoms through increased Mn localization in leaf tissues. NanoSIMS showed Mn and Si co-localized in the apoplast of soybean epidermal cells and basal cells of sunflower trichomes. Concomitantly, added Si decreased oxidation of Mn(II) to Mn(III) and Mn(IV). An increase from 5 to 50 μM P in solution changed some Mn toxicity symptoms but had little effect on Mn distribution or speciation. We conclude that Si increases localized apoplastic sorption of Mn in cowpea, soybean and sunflower leaves thereby decreasing free Mn2+ accumulation in the apoplast or cytoplasm.