Deposition and absorption capacity of Populus deltoides × P. nigra to different size zinc oxide aerosol

To investigate the deposition and absorption of particulate matters on leaves, this study firstly uses the zinc oxide (ZnO) nanoparticles to simulate sedimentation, attachment and retention of fine particulate matter (PM_2.5) on Populus deltoides × P. nigra leaves. The contents of ZnO on the leaf surface and in the leaf tissue were quantitatively determined by water-washing and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Furthermore, ZnO particle number on the leaf surface and its microstructure were analyzed by an scan electron microscope (SEM)and the response characteristics of photosynthetic physiology was also investigated by the Li-COR 6400 portable photosynthesis system. The results showed that ZnO particles could be adhered by leaves which were treated with NPs (particles with a mean diameter of 30 nm), BPs (particles with a mean diameter of 100 nm) or MPs (particles with a mean diameter of 1 μm). The deposition capacity of Populus deltoides × P. nigra to different particles was mainly analyzed from two aspects of total quality and total particle number. The mass concentration of MPs on leaf surface was the highest after treated for 16 days, reaching 653.03 mg/g, which was significantly higher than NPs. Analyzed by Image J software, the number of particles on the foliar surface was the highest under NPs treatment, followed by BPs and MPs, indicating that the smaller the size of the particles were, the larger the number of particles distributed on the surface of the leaves, while the weight of the smaller particles was relatively lower. Moreover, P. deltoides × P. nigra could uptake particle less than 1 μm. Compared with weight of deposition, the uptake of NPs by leaves was higher, reaching 1.17 mg/g after 16 day treatment, which was 2.59 and 2.89 times of BPs and MPs, respectively. This indicated that the smaller size of particle was easily absorbed by plants. Phytotoxicity was tested for plants exposed to different size of ZnO aerosol. Leaf surface microstructure had obvious change, stomata was often filled with ZnO particles or was also clogged and injured cells, and slightly disturbed striations were also visible in the ZnO-treated leaves. Moreover, acute phytotoxicity was also observed in photosynthetic. After 16 days treatment of NPs, BPs and MPs, net photosynthetic rate (P_n) was reduced by 22%, 44% and 19%, respectively, which was caused by stomatal and non-stomatal factors.