Effects of nitrogen levels on photosynthetic and fluorescence characteristics in seedlings of endangered plant Phellodendron amurense.

To investigate the photosynthetic responses of Phellodendron amurense to nitrogen levels, we detected the photosynthetic parameters and chlorophyll fluorescence of liquid-cultured seedlings under four nitrogen (NH4NO3) levels, which are 1 mmol/L (N1), 4 mmol/L (N4), 8 mmol/L (N8), and 16 mmol/L (N16). The results showed that the contents of chlorophyll a, chlorophyll b, carotenoid, and total chlorophyll increased initially, and then decreased with the increasing nitrogen supply. They reached the maximum under N8 (4 mmol/L NH4NO3), followed by that under N16 (8 mmol/L NH4NO3), N4 (2 mmol/L NH4NO3) and N1 (0.5 mmol/L NH4NO3). The chlorophyll a/b reached the maximum under N4. The light response curves of photosynthesis were consistent under N4, N8 and N16, while the net photosynthetic rate (Pn) increased rapidly, then slowed down significantly, and finally presented gentle changes when photosynthetically active radiation (PAR) was enhanced gradually. The maximum net photosynthetic rate (Pnmax) under N8 condition was the highest, and Pnmax under N16 and N4 were lower. However, the light response curve under N1 was slightly different with others. After reaching the peak, Pn gradually decreased with the increase of PAR, and Pnmax under N1 was the lowest. Under N8, the light saturation point (LSP) and the stomatal conductance (Gs) were the highest, the apparent quantum yield (AQY) and the intercellular CO2 concentration (Ci) were relatively higher, while the light compensation point (LCP) and the dark respiration rate (Rd) were relatively lower. With the increase of nitrogen concentration, the maximum fluorescence (Fm), variable fluorescence (Fv), photochemical quenching coefficient (qP), original light energy conversion efficiency of optical systemⅡ (Fv/Fm), PSⅡquantum efficiency (ΦPSⅡ), and the electron transport rate (ETR) all increased first and then decreased, and reached the maximum under N8 and the minimum under N1. The initial fluorescence (Fo) and the non-photochemical quenching coefficient (NPQ) under N4 were the highest ones. All these results indicate that increase of nitrogen supply can significantly enhance the photosynthetic capability of P. amurense seedlings, but excessive nitrogen is not conducive to seedling growth, and N8 is the optimal condition for seedling growth.