FePO4 nanoparticles as a source of nutrients for plants: synthesis and evaluation of their effects on hydroponically grown cucumber and maize seedlings

The nutrient use efficiency (NUE) of crops is typically low, in particular referring to the uptake of nutrients applied through fertilizers. A strategy to improve the NUE could be the development of new and more efficient fertilizers. A promising field in order to achieve this goal could be the use of nanotechnology. Nanomaterials are widely used in medical and pharmaceutical fields, but their application in agriculture and in particular in plant nutrition is at its infancy. A continuous method of FePO4 nanoparticles (FePO4 NPs) synthesis based on the extremely fine and rapid mixing of a FeCl3 solution with a K2HPO4 solution in a mixing chamber was tested for its effectiveness with a laboratory-made system. The proof-of-concept could produce FePO4 particles smaller than 100 nm, reaching the threshold of 50% of particles smaller of 100 nm, a value that is recommended by the European Union for the definition of nanomaterial. A pilot plant for the continuous FePO4 NPs synthesis was set up, using two dosing pumps for solutions pumping, and an HPLC mixing tee as mixing chamber. The system could produce 15 L•h-1 of raw FePO4 NPs suspension. Purification through dyalisis was optimized, together with a stabilization method of FePO4 NPs, called citrate capping, based on the adding of tribasic potassium citrate and thorough vortexing, in order to reduce aggregation and sedimentation of particles on long time periods. FePO4 NPs were then tested for their effectiveness as source of P and Fe on two hydroponically grown crop species, cucumber (Cucumis sativus) and maize (Zea mays). The experiments were designed in order to evaluate the effect of FePO4 NPs as source of both nutrients, or source of sole P and Fe. For this reason, as negative controls were used plants grown without P (-P), without Fe (-Fe), or without both nutrients (-P-Fe). In addition, in order to analyze if the size of FePO4 particles could cause different effects on plants, we included in the experiment a treatment with non-nanometric FePO4 (bulk FePO4). The results showed that nano-sized FePO4 improved the availability of P and Fe, if compared to the non-nano counterpart, as demonstrated by SPAD indexes of leaves and the determination of nutrients concentrations in tissues. Transmission Electron Microscopy (TEM) observations on cucumber roots treated with FePO4 NPs revealed that these particles did not enter into the plant, suggesting as mechanism of delivery of nutrients the dissolution in the apoplast. Gene expression analysis of homologs of AtPHR1, a key regulator of the response to P starvation in Arabidopsis, revealed in cucumber an upregulation of Csa3M608690 in plants grown with FePO4 NPs. The transcriptional behavior of Csa1M024210, homologs of AtBTS, suggested that plants grown with both forms of FePO4 are, with respect of Fe, in good nutritional conditions thus confirming physiological parameters. For maize, the negative modulation of ZmFER-Like gene in response to all treatments suggested a minor role of this gene in the regulation of Fe homeostasis in this plant species, while the upregulation of ZmIRO2 in plants grown with both forms of FePO4 confirmed the sub-optimal nutritional state of the plants.