Abstract: 【Objective】 Phosphorus (P) deficiency could significantly induce the formation of reddish brown iron plaque on the surface of rice roots. However, it remains unclear how P induces the formation of reddish brown iron plaque on root surface.【Method】 In this study, 24 rice varieties of different origins were used to measure the iron plaque on their root surface. Rice seedlings were firstly cultivated for 21 days hydroponically, and then submitted to P-deficient solution and 0.1 mmol•L-1 Fe2+ solution, respectively. The parameters including root length, root surface area, root and shoot dry weight and content of iron plaque were determined after treatments. Based on the above parameters, a rice variety (Tianyou 998) was screened with higher amount of iron plaque. In the following experiments, effects of different nutrient deficiencies (N deficiency, P deficiency, K deficiency, and their interactive treatments, etc.), root-split treatments (root system was divided into two parts, both two parts were treated with P supply treatments, one part was treated with P-deficient treatment, and the other was with P supply treatment, or both two parts were treated with P-deficient treatment) and different P/Fe treatments (1﹕1, 1﹕3, 1﹕5, 1﹕8) on the formation of iron plaque were examined with Tianyou 998 as materials. After treatments, contents of iron plaque, inorganic P content on root surface, root and shoot P or Fe uptake were determined to investigate the effect of P on iron plaque and their nutritional effects.【Result】Different rice varieties had various content of iron plaque. The content of iron plaque on root surface among 24 varieties ranged from 3.67 to 9.78 mg•g-1. The amount of iron plaque on root surface was correlated with root length, root surface area and rice biomass. Results from nutrient-deficient experiments indicated that iron plaque by DCB method consisted of reddish brown and non reddish brown iron plaque. Only P deficiency could induce the formation of reddish brown iron plaque on root surface, while N or K deficiency could not. Root-split experiment indicated that the formation of reddish brown iron plaque was affected by exogenous P, rather than internal inorganic P. Results from different P/Fe treatments indicated that P/Fe in solution influenced the amount of reddish brown iron plaque significantly. DCB-Fe concentration on root surface was related to Fe level in solution. The maximal value of iron plaque was 36.50 mg•g-1. Reddish brown iron plaque increased with decreasing P/Fe. When P/Fe was less than 1:3, the reddish brown iron plaque could be induced on root surface of rice seedlings obviously. The formation of reddish brown iron plaque contributed to P and Fe uptake in both roots and shoots of rice seedlings.【Conclusion】The formation of reddish brown iron plaque was induced by exogenous P deficiency, rather than endogenous P signal. The formation of reddish brown iron plaque was associated with external P/Fe levels around root system, while DCB-Fe was related to Fe concentration in solution. Reddish brown iron plaque formed on root surface of rice seedlings can be regarded as a nutrient pool, which would contribute to the uptake of P and Fe.

Key words: phosphorus , rice seedlings , reddish brown iron plaque , P/Fe