As we know P and Arsenic have the same channel pathway in xylum/phloem cells in plants. Will increasing P dose affect phytpextraction of arsenic or will it only have impact on the plant's health (whether it improves/deteriorates).
Excess phosphate in the soil solution can suppress uptake of arsenate by the plant. On the other hand, depending on the soil chemistry application of phosphate can increase plant availability of soil arsenate through competition for sorption sites on mineral surfaces. In the case of sunflower where arsenic uptake and accumulation is relatively low, unless you are adding very high rates of phosphate I doubt you will see a practical difference. You will likely achieve greater arsenic removal by supplying adequate phosphorus to support vigorous root growth and high biomass yields than trying to limit phosphate to encourage arsenate uptake. We have observed effects of phosphate suppression of arsenic accumulation in P. vittata, but arsenic concentrations in the plant were substantially greater (>2000 mg/kg) than what you would obtain in sunflower.