Effects of zinc oxide nanoparticles on physiological and anatomical indices of Hordeum sativum L. growth

Zinc based nanoparticles (NPs) have a great importance among the metal-based NPs and widely produced after SiO₂ and TiO₂. The rapid growth of ZnO NPs production and its abundant uses in many industries, and increasing release into an environment from both intentional and unintentional sources, create risks to human health. The recent finding of ZnO NPs application indicates positive and negative effects on plant growth. However, studies exploring the effect of ZnO NPs on the internal organelles of plants and their correlation with the function of photosynthesis are a few. The spring barley (Hordeum sativum L.) is one of the most important staples food crop and is identified as an efficient potentially toxic elements accumulator with phytoremediation potential. Thereby, the present work aimed to investigate the toxic effects of ZnO NPs on physiological and anatomical indices of H. sativum growth grown in a hydroponic condition as it allows to simplify model, where the parameters of plant growth can be easily controlled. Thus, the commercial-grade ZnO NPs (particle size 30-50 nm) was used with the scheme; control (0 ppm), low (300 ppm ZnO NPs), and high (2000 ppm ZnO NPs) the dose of NPs. The low and high levels of ZnO NPs were selected with consideration for the existing levels of Zn in the polluted soils. The results showed ZnO NPs affected the photosynthetic efficiency of H. sativum plants by affecting chlorophyll fluorescence emission, deformation in stomatal and trichomes morphology, damaged cellular organizations, i.e., irregularities in chloroplasts, disruption in grana and thylakoid organizations. Cytomorphometric quantification revealed that ZnO NPs decreased chloroplasts size 4 times in 2000 ppm and 1.5 times in 300 ppm ZnO NPs treated plants. The lower number of chloroplasts per cell were observed in ZnO NPs treated H. sativum leaf cells. The destructive methods of Zn elemental analysis showed 10.2 folds and 3.8 folds higher accumulation in 2000 ppm and 300 ppm ZnO NPs treated leaves than the control, respectively. Additionally, the presence of Zn content in H. sativum leaf tissue was also confirmed by the X-ray fluorescence spectroscopy elemental analysis. The high contents of Zn were visible in several spots in ZnO NPs treated leaf tissues. The accumulation of Zn content dissolute from ZnO NPs damaged the structural organization of the photosynthetic apparatus and reduced the photosynthetic activities. The modifications in anatomical indices were significantly correlated with physiological observations. The work presented here will help to increase the scientific understanding of the toxicity of ZnO NPs in plants and will widen the scope to tailor the nanomaterials to improve the effectiveness, smart and targeted delivery to avoid damages of most significant tissues of plants.  

This study was funded by the RFBR, project nos. 19-34-60041 and 18-29-25071, and the Ministry of Science and Higher Education of Russia, project no. 0852-2020-0029.