Effects of simulated atmospheric nitrogen deposition on foliar chemistry and physiology of hybrid poplar seedlings.

During recent decades, the southern and eastern regions of Asia have experienced high levels of atmospheric N deposition. Excess N deposition is predicted to influence tree growth and species composition in the regions, but visual or physiological assessments alone are not sufficient to determine the real effects of atmospheric N deposition. In this study, we simulated atmospheric wet deposition of inorganic N by spraying a NO3- solution (20 mmol⋅L-1) or a mixture of NO3- (20 mmol⋅L-1) plus NO2- (100 or 300 μmol⋅L-1) on leaves of hybrid poplar (Populus alba × Populus berolinensis) seedlings and examined morphoanatomical traits and physiological processes. Leaves of seedlings sprayed with single or mixed N solutions developed marginal necrosis, curling, and small cracks on the adaxial surface. The silicon (Si)-rich crystals were larger (about 100% increase in crystal diameter compared to untreated seedlings) on the adaxial leaf surface, with a significant positive correlation between the atomic percentage of N and Si on the crystal areas of the surface. Leaves were sensitive to NO2- compared with NO3- even at a low concentration; water content, dry mass, and photochemical variables significantly declined and dark respiration increased only in leaves treated with mixed N form. Mixed N foliar applications significantly increased leaf concentrations of the free amino acids Glu, Gln, and Asn and organic acids oxaloacetic acid and citric acid. Besides, mixed N treatment stimulated leaf transamination, as indicated by significant increases in Ala and Asp concentrations and activities of glutamic oxalacetic transaminase and glutamic pyruvic transaminase. However, mixed N applications led to declines in leaf concentrations of putrescine (by 65%, p = 0.01) and spermine (by 53%, p = 0.01). A higher proportion of NO2- (300 μmol⋅L-1) in mixed N solution was inhibitory to key N-metabolic enzymes and N translocation via the phloem. Our results showed that wet deposition of airborne N pollutants modified surface properties and induced additional detrimental effects related to N-compound foliar absorption. Furthermore, our findings indicate that detoxification of reactive N is apparently related to N assimilation and export from the treated leaves via the phloem.