ZnO nanoparticles impact on the photosynthetic activity of Vicia faba: Effect of particle size and concentration

Abstract

The present study evaluated the impact of ZnONPs on the photosynthetic activity of Vicia faba plants. Time evolution of chlorophyll a fluorescence parameters (Fv/Fm, NPQ, Fq′/Fm′, Fq′/Fv′, and Fv′/Fm′), leaf temperature, and stomatal conductance (gs) were monitored during 144 h for leaves subjected to ZnONPs (25 and 70 nm diameter) at 50, 100, 200, and 300 mg L−1 as well as for control leaves. The ZnONPs morphology and size were characterized by transmission electron microscopy. Their capacity to generate reactive oxygen species (ROS) and release Zn ions were assessed by fluorescence and dialysis experiments, respectively. The results revealed that ZnONPs internalized in the leaves, impacting the photosynthetic performance of Vicia faba, reducing the quantum efficiency of photosystem II (PSII), steady-state PSII quantum yield, coefficient of photochemical quenching in steady-state, and stomatal conductance. The particle size played a key role as the photosynthetic functioning of the plant was more affected by ZnONPs of 25 nm diameter than those of 70 nm. A similar rate constant ROS production (kROS) of about 4.6 10−1 M−1 s−1 was found for both ZnONPs. In contrast, a rate constant of Zn2+ dissolution (KdZn+2) of 1.064 10−3 and 0.743 10−3 h−1 was determined for ZnONPs of 25 and 70 nm diameter, respectively. Thus, our findings indicate that Zn2+ ions played the major role on the size-dependent phytotoxicity because ZnONPs of 25 and 70 nm presented similar ability of ROS generation by themselves while 25 nm ZnONPs released 30% more Zn2+ than 70-nm.