Abstract:

Objective: Excessive nitrogen addition can restrain the leaf photosynthesis and growth of woody plants. However, the underlying ecophysiological mechanism is still not very clear, which is important for precision fertilization to seedlings and plantation. Poplar (Populus spp.) is an important afforestation species and biological model tree. This study aims to investigate the photosynthetic and chlorophyll fluorescence characteristics, nonstructural carbohydrate (NSC) and nitrogen distribution of hybrid poplar seedlings under nitrogen addition for 2 consecutive years, in order to reveal the ecophysiological mechanism of trees in responding and adapting to nitrogen addition. Method: In this study, the potted cuttings of P. × euramericana 'Zhongjin7' were used as experimental materials, NH₄NO₃ was used as nitrogen source and 4 gradients (CK: 0 g·plant^-1a^-1, N3: 3 g·plant^-1a^-1, N6: 6 g·plant^-1a^-1, N9: 9 g·plant^-1a^-1) were set. After nitrogen addition for 2 years, changes of leaf photosynthetic characteristics, organ biomass accumulation, non-structural carbohydrate storage and distribution were investigated. Result: After nitrogen addition for consecutive 2 years, the nitrogen content and chlorophyll content in leaves and roots significantly increased, stomata became smaller but higher density, transient water use efficiency remarkably increased, but photosynthetic nitrogen use efficiency continuously decreased. Under moderate and low dose nitrogen addition, the net photosynthesis (P_n) of leaves on top twig was suppressed by sink and high carbohydrate feedback inhibition. But the P_n of lateral branch leaves was higher than that of the control. Under high nitrogen addition, P_n and starch content were significantly decreased, but with actual fluorescent quantum yield and electron transport rate increased significantly, suggesting that photorespiration and other processes supplied large amount energy for nitrate reduction and assimilation. Nitrogen addition continued to promote lateral branch growth and weaken the apical dominance. Under N9 conditions, the starch content in main stem decreased with a lower biomass, which maybe due to the source inhibition of P_n. The root drymass and biomass distribution percentage of N9 were no significantly different with those of N3 and N6. The results of organ growth, and carbon and nitrogen distribution implied that the decline of aux in synthesis and transport might play an important role in whole plant carbon and nitrogen relationships under excessive nitrogen addition. Conclusion: Nitrogen addition with moderate and low doses can significantly enhance carbon assimilation of leaves on the lateral branch and increase plant growth of whole poplar seedlings, with a sink limit on leaf net photosynthesis. But leaf photosynthesis may be inhibited by nitrogen assimilation under high dose nitrogen addition, with a declined apical dominance. And whole plant biomass accumulation could be restricted by source.

Key words: Populus×euramericana'Zhongjin7', nitrogen addition, photosynthetic traits, nonstructural carbohydrate, carbon and nitrogen distribution,