Integrated Biophysical Assessment of the Effect of Zinc Oxide Nanoparticles on the Photosynthetic Apparatus of Spinach (Spinacia oleracea L.)

Zinc oxide nanoparticles have been reported as the most common nanostructured materials in agricultural areas and, recently, researchers found that the concentration of zinc oxide nanoparticles in the soils and air is growing, causing concerns in society regarding their possible effects on plant photosynthesis. This study used an integrated biophysical approach that incorporated three different techniques to assess the short-term effect of zinc oxide nanoparticles on the photobiochemical apparatus of spinach (Spinacia oleracea L.): (i) delayed luminescence measurements of isolated thylakoid membranes to determine Photosystem II charge recombination, (ii) Clark-type oxygen electrode measurements for the determination of photosynthetic oxygen evolution from isolated chloroplasts, and (iii) UV-Vis spectrophotometric analysis of acetoneextracted chlorophyll pigments exposed to red, blue, and white LED light for photostability assessment. Zinc oxide nanoparticles treated samples displayed a gradual decrease in the levels of delayed luminescence over 60 min of dark incubation, while control samples remained quite unchanged, implying disturbed charge recombination in P680+/QA - pair of PSII (reaction center). It was observed that photosynthetic oxygen production gradually declined in both the control and the treated chloroplast samples, yet the initially high rates were retained in the latter. UV-Vis spectrophotometric analysis of the extracted chlorophyll pigments showed a faster decrease in the main chlorophyll absorption peaks at 437 nm (Soret) and 668 nm (Qy) in ZnO-treated samples, an effect that was most evident under white light.