Root dynamics of phosphorus and iron interaction in rice under variable phosphorus availabilit

Rice paddy soils are characterized by a peculiar redox environment: the reductive dissolution of Fe minerals during field flooding leads to an increase in soil porewater Fe(II) concentrations. Rice (Oryza sativa) plants adapt to the anerobic conditions with the development of root aerenchyma, in turn affecting the redox chemistry of the rhizosphere as a consequence of radial oxygen loss (ROL). The sharp gradients in O2 and Fe(II) concentrations from roots to reduced bulk soil result in the oxidation of the Fe(II) in the rhizosphere and formation of iron plaque on rice root surface. Changes in root morphology and physiology as a function of nutrient availability are expected to influence these gradients in the rhizosphere, affect the formation of Fe-plaque, and feedback on nutrient availably for the plant. This is particularly the case for phosphorus (P) as rice plants are known to respond to P deficiency by changing root architecture, and increasing the rate of H+ release and ROL in the rhizosphere [1]. Moreover, considering the high affinity of Fe minerals towards P, Fe-plaque can interact with P regulating nutrient availability and uptake by the plants [2] [3] [4]. Nonetheless, little is yet known on how P availability influences Fe-plaque formation and its role in serving as a source or sink of plant available P. We hypothesised that under P deficient conditions (i) an increase in root development and ROL contribute to enhancing Fe plaque formation on the root surface with respect to P sufficient conditions; and (ii) the pool of Fe-plaque-associated P can nonetheless serve as a source of P as the plants respond by activating strategies that promote Fe-plaque dissolution. Rice plants (cv. Selenio) were cultivated for 60 days under hydroponic conditions. Nutrient solutions containing 13 M and 130 M P were tested to represent P-deficient and P-sufficient conditions respectively. To appreciate the role of Fe-plaque in regulating P availability it is necessary to test plant responses at P concentrations as close as possible to soil porewater concentrations. Plant response to the different P availabilities were evaluated by determining the rate of H+ release in the nutrient solution during the growing period, as well as measuring root morphology, root biomass and root aerenchyma development after 30 days. Fe plaque formation was induced at 30 days after seeding (DAS) and at 45 DAS, by exposing half of the P-deficient and P-sufficient plants to a 800 M Fe(II) solution buffered at pH 6.0 for 48 hours. During the induction Fe(II) concentration in solution was monitored hourly and compared to a control without plant. Rice roots were extracted with 0.1 M acid ammonium oxalate both before and after each Fe-plaque induction and at the end of the cultivation period (30 DAS, 45 DAS and 60 DAS). No significative differences in root biomass were underlined at 30 DAS, but a higher root surface area and volume was observed in P-deprived plants. The different root traits could be explained by the higher aerenchyma development (i.e. porosity) under P-deficient conditions. During Fe-plaque induction, P-deficient plants showed a faster rate of Fe(II) oxidation and an enhanced Fe-plaque formation with respect to P-sufficient plants confirming our first hypothesis. Despite the greater amount of Fe-plaque formed on the roots of P-deficient plants after induction, no significant differences were observed between the two treatments after 60 days of development. This together with the higher release of H+ observed by P-deficient plants, suggest that under nutrient deficient conditions plants are able to acquire P by enhancing the dissolution of Fe plaque, confirming our second hypothesis. In conclusion, our results show that P availability can strongly drive Fe-P interactions at root level and that P associated with Fe-plaque represents a dynamic pool of plant available P under deficient conditions, regulated by higher rates of Fe-plaque formation and dissolution with respect to P-sufficient conditions.