In the present study, impacts of nitrate/ammonium ratios, aluminum and phosphate stress on the reduction properties of five-leaf maize root exudates were investigated using hydroponic culture method. Appropriate nitrate/ammonium ratio (3:1) prompted maize roots to secrete more organic reducing substances which led to more release of Mn2+ from MnO2. Under stress of low phosphorous and high aluminum, maize roots release more reducing substances, and more Mn2+ was released into the solution after the root exudates were reacted with MnO2. The amount of reducing substances secreted by maize roots gradually increased with the culture time with the initial Al concentration of 20 μmol L-1. The similar trend was observed for the dissolved Mn2+ after the reaction of root exudates with MnO2. Therefore, root exudates can alleviate Al toxicity and activate phosphate in acid soils under low P and high Al, but they may increase the toxicity of Mn2+ to plants.
Key words: Active reducing substances, MnO2, reduction reaction, release of Mn2+, maize.