Short- and Long-Term Effects of Ca(OH)2/ZnO Heteronanostructure on Photosystem II Function and ROS Generation in Tomato

Among different formations, inorganic/inorganic assemblies can be considered "two in one" systems offering collective and/or new physical-chemical properties and substantial activity. Herein, a post-synthetic approach involving the assembly through Van der Waals forces and/or hydrogen bonding of the preformed ZnO@OAm NPs and Ca(OH)₂@OAm NPs of non-uniform sizes (9 nm and 27 nm, respectively), albeit coated with the same surfactant (oleylamine-OAm), is reported. The resulting semiconductor hetero-nanostructure (named CaZnO) has been physicochemically characterized. The X-ray diffraction (XRD) peaks correspond to both ZnO and Ca(OH)₂, confirming the successful formation of a dual-phase system. Field emission scanning electron microscopy coupled with energy-dispersive spectroscopy (FESEM-EDS) of CaZnO indicated the formation of Ca(OH)₂ NPs decorated with irregular-shaped ZnO NPs. The synthesized hetero-nanostructure was evaluated by assessing any negative effects on the photosynthetic function of tomato plants as well as for the generation of reactive oxygen species (ROS). The impact of the CaZnO hetero-nanostructure on photosystem II (PSII) photochemistry was evaluated under both the growth light intensity (GLI) and a high light intensity (HLI) at a short (90 min) and long (96 h) duration exposure. An enhancement of photosystem II (PSII) function of tomato plants by 15 mg L^-1 CaZnO hetero-nanostructure right after 90 min was evidenced, indicating its potential to be used as a photosynthetic biostimulant, improving photosynthetic efficiency and crop yield, but pending further testing across various plant species and cultivation conditions.