A direct chemical method for the rapid, sensitive and cost effective detection of phosphite in plant material

Abstract

Phosphite application mitigates diseases caused by oomycete plant pathogens. Tissue concentrations of phosphite above 1 mM are generally required for disease protection. Determining the concentration of phosphite in plant material requires extensive extraction and derivatisation procedures prior to separation by gas–liquid chromatography (GLC). This paper describes a direct chemical method to estimate the concentration of phosphite using a silver nitrate reagent. Glass fiber filter papers were saturated with a 1 M aqueous solution of silver nitrate (adjusted to pH 2.5 with nitric acid) and dried for 2 h at 60 °C. 20 μL of polyvinylpolypyrrolidone treated aqueous plant extract was adsorbed onto the filter paper and incubated in the dark at room temperature (25 °C) for 1 h. The presence of phosphite in the extract reduces the silver ions to elemental silver resulting in a grey-black precipitate that is clearly visible. The method is rapid, sensitive and inexpensive, and can detect phosphite at concentrations of 1 mM in 20 μl of aqueous extract from 100 mg of fresh plant material. Samples analysed by this method gave similar results to analysis by GLC, indicating the method can be used in the field or the laboratory to determine uptake and distribution phosphite in the plant, the retention of phosphite over time and the timing of phosphite reapplication.