Phosphorus fertilization of Phoebe zhennan seedlings under drought reduces nitrogen assimilation

Phosphorus (P) limitation and water stress can affect trees' growth, yet differing levels of nutrition under drought may have negative or positive effects on their growth and metabolism. This study aimed to evaluate the impact of P nutrition upon growth and metabolism in drought-stressed Phoebe zhennan (an endangered forest tree species) seedlings, to provide a basis for strengthening its conservation in the face of future climate change. A complete randomized design was used to investigate the effects of four different levels of P fertilization (no P, low P, moderate P, and high P) under water stress on the growth, photosynthesis, and nitrogen metabolism of P. zhennan. Drought-stressed seedlings under high P (HP) application had enhanced growth, while moderate P (MP) application improved their root biomass and leaf area only, whereas low P (LP) did not significantly affect any growth trait. Moreover, HP fertilization had positive effects on leaf relative water contents (LRWCs), while HP and MP both significantly improved photosynthetic traits vis-a-vis non-fertilized seedlings. The concentration of soluble sugars was significantly higher in non-fertilized than fertilized seedlings as were nitrogenous compounds (NO3- and NH4+) and nitrogen metabolic enzymes (NR, GDH, GOGAT, and GS) activities. Both NO(3)(-)and NH(4)(+)concentrations were significantly decreased under HP application compared with the non-fertilized counterparts. The plant traits improved under HP application counteracted the adverse impact of low nitrate availability and promoted seedling growth. Low-dose fertilization produces depressive effects in P. zhennan, but a high P concentration can overcome those negative effects to improve growth.