Effects of Foliar Nitrogen Treatment and Pruning on Leaf Morphology and Photochemical Efficiency of Populus alba × Populus berolinensis

Cutting seedlings of a hybrid poplar (Populus alba × Populus berolinensis) were pruned firstly as (CK_J), then leaf-sprayed by nitrate (NaNO₃) or a mixed nitrate/nitrite (NaNO₃/NaNO₂) with a concentration of 30 mmol/L for a simulation experiment of nitrogen deposition. The effects of leaf-spraying and pruning on leaf morphology and photochemical efficiency of the seedlings were investigated. Results showed that pruning had no significant impacts on leaf photochemical efficiency (F_v′/F_m′ and ΦPSII), pigment content, relative electric conductivity, and malondialdehyde (MDA) content. Leaf spraying led to blade edge carbonization, rolling symptom at the edge of immature and mature leaves of the pruned seedlings, changes of chemical element contents on leaf surface, and a decline in leaf chlorophyll content. Moreover, leaf spraying led to an increase in leaf anthocyanin content of unpruned seedlings (T_Z-NO3 and T_Z-NO2) on the 6^th and 12^th day. On the 8^th day of leaf spraying, values of F_v′/F_m′ and ΦPSII of the unpruned seedlings were decreased as compared with the control (CK_Z), whereas the values of pruned seedlings were increased; on the 14^th day, leaf photochemical efficiency of the pruned seedlings were increased significantly (P < 0.05). In addition, leaf spraying for 14 days led to a decline of MDA content of the unpruned seedlings as compared with CK_Z. In summary, the morphological and physiological characteristics were influenced not by pruning but significantly by foliar NO₃^- treatment.