Abstract
The systemic fungicide phosphite is increasingly being used to treat oomycete plant diseases in horticultural crops and native ecosystems around the world. The effects of phosphite application on a plant are variable. Resistance depends on the species, the time of application and the method of application. Very little is known about how phosphite is transported and stored within plants, or how these processes are affected by the physiological status of the plant. This is mainly due to the unavailability of a simple, efficient method for measuring the concentration of phosphite. We have developed an enzymatic assay that enables phosphite to be quantified as an alternative to high-performance liquid chromatography or gas chromatography. Phosphite is oxidised to phosphate by a modified phosphite dehydrogenase. NADH formed in the reaction reduces p-iodonitrotetrazolium violet (INT) via an intermediate electron carrier to produce a stable red formazan that can be measured spectrophotometrically at 490 nm. This method is being further developed to quantify phosphite in plant-tissue samples. Dried plant material is powdered and resuspended in water. Phosphite is separated from the plant extract solution by dialysis and determined with the phosphite dehydrogenase assay. The method is rapid and inexpensive and could potentially be developed to measure the phosphite concentration within plant samples in the field.