Abstract: After use of PPT,it is apt to accumulate,but its effect on cotton root growth was not reported at present.Therefore, this paper studied effects of different PPT concentrations on root growth and development under hydroponic cultur in greenhouse. The results showed that the total root surface area,total root length and total root volume significantly decreased when PPT concentration was 1 mg?L-1, to maximum at 5 mg?L-1 concentration of PPT. The activity of glutamine synthetase was reduced significantly as the concentration of PPT rised. It maybe ammonium’s accumulation inhibited root growth , thus decreased the activity of root and glutamine synthetase .