Effects of ZnO Nanoparticles on Phaseolus vulgaris Germination and Seedling Development Determined by X-ray Spectroscopy

There is still little information on the potential use of nanomaterials for seed nutrient enhancement through seed priming. The use of nanoparticles (NPs) in agriculture is promising, but in-depth knowledge on their interaction with plants is required. The aim of this study was to evaluate the effects of different concentrations (1–5000 mg L–1) and sizes (20, 40, and 60 nm) of uncoated ZnO NP compared to ionic ZnSO4 (positive control) on common bean (Phaseolus vulgaris) seed germination. The seeds were soaked in ZnO NP aqueous dispersions for 20 min. The ZnO nanoparticles did not affect the germination rate. The 10 mg L–1 40 nm ZnO treatment showed a tendency to increase weight after 5 days (8.26 ± 0.11 g) when compared to the negative control (7.7 ± 0.7 g). However, at 5000 mg L–1 40 nm ZnO NP and ZnSO4 weight was reduced to 7.7 ± 0.8 g and 6.05 ± 0.08 g, respectively. Microprobe X-ray fluorescence showed that most of the Zn absorbed was trapped in the seed coat, while a small fraction entered the cotyledon. X-ray absorption spectroscopy indicated the biotransformation of the ZnO NP. In the hilum and cotyledon, Zn was found associated with organic molecules such as citrate, malate, and histidine-like compounds. Seedling weight reduction depended on the concentration of Zn taken up by the tissue and on the biotransformation of ZnO into organically bound Zn. Considering the properties of the studied NP, in particular the slow Zn release and lower toxicity compared to ZnSO4, the results represent a step forward toward the application of ZnO NP as an agrochemical.