Increase of nitrogen to promote growth of poplar seedlings and enhance photosynthesis under NaCl stress

The solution culture method was used to study the effect of increasing nitrogen on the growth and photosynthesis of poplar seedlings under 100 mmol L−1 NaCl stress. I Increase in nitrogen reduced stomatal limitation of leaves under NaCl stress, improved utilization of CO2 by mesophyll cells, enhanced photosynthetic carbon assimilation capacity, significantly alleviated saline damage of NaCl, and promoted the accumulation of aboveground and root biomass. I Increased nitrogen enhanced photochemical efficiency (ФPSII) and electron transport rates, relieved the reduction of maximum photochemical efficiency (Fv/Fm) under NaCl, and reduced the degree of photoinhibition caused by NaCl stress. Increased nitrogen applications reduced the proportion of energy dissipating in the form of ineffective heat energy and hence a greater proportion of light energy absorbed by leaves was allocated to photochemical reactions. Under treatment with increased nitrogen, the synergistic effect of heat dissipation and the xanthophyll cycle in the leaves effectively protected photosynthetic PSII and enhanced light energy utilization of leaves under NaCl stress. The increased nitrogen promoted photosynthetic electron supply and transport ability under NaCl stress evident in enhanced functioning of the oxygen-evolving complex on the electron donor side of PS II. It increased the ability of the receptor pool to accept electrons on the PSII electron acceptor side and improved the stability of thylakoid membranes under NaCl stress. Therefore, increasing nitrogen applications under NaCl stress can promote poplar growth by improving the efficiency of light energy utilization.