Effect Of Phosphorus Iron Ratio Of Soil And Urea Peroxide On Formation Of Iron Plaque On Rice Root Surface And Cadmium Uptake Of Rice Grains

Under waterlogging condition, rice roots could secrete gasousoxygen on root surface, thus oxidize free Fe2+to iron oxides, and form iron plaque on root surface. Further, iron plaque can block Cd uptake to rice shoots. However, it remains unclear how to regulate the quantity of iron plaque on rice root surface and block Cd transfer to rice grains under soil cultivation. In this study, two rice cultivars(ZCSM and TY998) were selected to investigate impacts of different Phosphorus-Iron(P-Fe) ratio and application of urea peroxides on the formation of iron plaque and Cd uptake. The treatments were P0-Fe0(0:0 in molar ratio basis), P1-Fe0(1:0), P2-Fe0(2:0), P0-Fe1(0:1), P0-Fe2(0:2), UP-0(0 kg/hm2),UP-20(20 kg/hm2), UP-40(40 kg/hm2), UP-80(80 kg/hm2). The results were as follows.Rice seedlings subjected to 3-d drying and 14-d wetting later. Different P-Fe ratio significantly influenced the concentrations of amorphous and crystalline Fe oxides in the rhizosphere of rice seedlings. After 10-d wetting, the level of crystalline Fe oxides reached the minimum in P1-Fe0 and P2-Fe0 treatments. In comparison to P0-Fe0 treatment, crystalline Fe oxides was increased by 4.2% and 14.7%. Wetting treatment increased the concentration of amorphous Fe oxides. P supply inhibited the increase of amorphous Fe oxides. Treatment with urea peroxides did not influence the concentrations of amorphous and crystalline Fe oxides.Results from root activity measured by α-naphthylamine oxidation indicated that P-Fe ratio did not influence root activity of rice seedlings, while urea peroxide increased root activity obviously. The root activities of ZCSM and TY998 were 236.1-243.9 μg/(g·h) and 243.2-264.6 μg/(g·h) respectively. Treatment with P1-Fe0 or P2-Fe0 weakenedcolor of iron plaque, while P0-Fe2 deepenedcolor of iron plaque. In comparison to those without urea peroxide, treatment with urea peroxide also made the color of iron plaque deeper. The IV results suggested that urea peroxide increased the formation of reddish iron plaque.Results from iron plaque on root surface of rice seedlings at booting stage measured by DCB method indicated that P2-Fe0 treatment decreased DCB-Fe by 20.3% and 7.8% in ZCSM and TY998, respectively, while the corresponding values under P0-Fe2 treatment were 15.7% and 13.4%. These results suggested that high P-Fe ratio did not contribute to the formation of reddish iron plaque, but low P-Fe ratio induced the formation of iron plaque on root surface. Exposed to UP-40 and UP-80 treatment, the contentof iron plaque on root surface of ZCSM and TY998 were elevated by 15.8%~25.5%and 16.2%~34.8% respectively.Nutrient-element analysis of rice shoots indicated that in comparison to P0-Fe0, treatment with P1-Fe0 and P2-Fe0 decreased Fe uptake by 24.6%-33.4%. P0-Fe2 treatment decreased shoot P uptake of ZCSM by 45.3%. Treatment with P1-Fe0 and P2-Fe0 reduced shoot Cd uptake of TY998 by43.9%~67.2%,P0-Fe142.8% and 63.1%reduced shoot Cd uptake of TY998 by. In comparison to UP-0, UP-20,UP-40 and UP-80 treatment enhanced N, P and K uptake in shoots of ZCSM by 41.7%~64.9%,38.3%~54.3%and 83.9%~95.4%. Exposed to UP-80 treatment, Cd concentrations in shoots of both ZCSM and TY998 were decreased by 22.4% and 30.0%, respectively.In comparison to P0-Fe0 treatment, P1-Fe0 and P2-Fe0 treatment decreased Cd concentration in the grain of ZCSM and TY998 by 43.3%and 15.9%~30.4%. P0-Fe1 treatment decreased Cd concentration in the grain of ZCSM and TY998 by 38.9%~60.3% and28.4%. Compared with UP-0 treatment, UP-80 treatment decreased grain Cd concentration of ZCSM and TY998 by 37.8-41.7%. Results from field experiments indicated that P0-Fe2 treatment decreased the grain production of ZCSM and TY998 by 21.0% and 13.0% respectively. The results suggested that high Fe addition led tonegative effects on grain production. In comparison to without urea peroxide, two treatments with urea peroxide enhanced the biomass and grain production by 36.4%~83.3% and 36.6%~51.5%.Taken together, the above results indicated that the soil with low P-Fe ratio could enhance the quantity of reddish iron plaque on root surface of rice seedlings, and decrease the grain Cd uptake significantly in ZCSM, but not influence the grain Cd uptake in TY998. Application of urea peroxide increased the biomass and grain production of two rice cultivars. Treatments with UP-40 and UP-80 reduced the grain Cd uptake in TY998 significantly, while had no impacts on the grain Cd uptake in ZCSM.