Absorption of aqueous inorganic and organic silicon compounds by wheat and their effect on growth and powdery mildew control

Soluble silicon, absorbed in the form of silicic acid by plant roots, confers efficient protection against many diseases. However, owing to its low solubility, there is a practical limit to increasing silicic acid availability to plants in the field. For this reason, methyl silanols – mono-, di- and trimethyl silicic acid – the water-soluble degradation products of silicone polymer, were evaluated for their ability to protect against powdery mildew in wheat grown under peat-based and hydroponic conditions. X-ray microanalysis indicated that only the monomethyl derivative was absorbed and deposited by wheat plants. Inductively coupled plasma–optical emission spectrometry measurements of total plant Si content confirmed this observation and indicated that monomethyl silicic acid remained mostly in its soluble monomeric form in planta. Mono- and trimethyl silicic acid offered no protection against powdery mildew and, indeed, yielded higher levels of infection than the control. Unexpectedly, dimethyl silicic acid induced severe symptoms of phytotoxicity early into the experiments at nutrient solution concentrations as low as 0.1 mM. For their part, both mono- and trimethyl silicic acid also caused symptoms of phytotoxicity by the end of the experiments. Our findings would suggest that silanols cannot be considered as substitutes for silicic acid in plants. Moreover, application of silicone-contaminated waste or sewage sludge to agricultural land needs to be evaluated with considerable care.