The mechanisms of iron plaque formation on the surface of rice roots induced by phosphorus starvation

The possible mechanisms of iron plaque formation induced by phosphorus (P) starvation for rice seedlings were studied in solution culture or agar culture. Rice roots appeared reddish after 24 h in nutrient solution without P, which might indicate that iron plaque was induced on the surface of rice roots; however, they remained white in solution with P. The rice roots growing in medium with P and without P were explored with Scanning Electron Microscope in accord with EDAX (SEM/EDAX) to confirm Fe existence in reddish materials on the root surface of rice. The results showed that there was iron in the reddish materials accumulated on the surface of rice root with P starvation. Iron plaque formation induced by P starvation was an interesting phenomenon observed during the experiments. A series of experiments had been done to find the possible mechanisms of iron plaque induced on the roots without P application. The value of pH and Eh in rhizosphere might be two main factors to induce iron plaque formation on the rice root surface. Furthermore, the changes of pH on the surface of rice root in agar with P and without P were investigated using pH indicators. The pH was lower in P0 treatment than that in P treatment. However, the oxidation of α-NA of intact rice root of P starvation for three genotypes was significantly stronger than that of P treatment. It suggested that P starvation enhanced the oxidizing ability of rice root. The results also showed that Fe concentrations in apoplast of root without P application were significantly higher than those with P treatment, and Fe (Ⅲ) concentrations were much higher than Fe(Ⅱ) concentrations on the surface of rice root. Therefore, the formation of iron plaque on rice root induced by P starvation might be the results of bio-chemical reactions occurred on the rice root surface between rice root and iron in nutrient solution.