Abstract: To clarify the effects of nitrogen addition on the growth-defense trade-off of seedlings of the endangered species Taxus cuspidata and provide a theoretical basis for its protection, an experiment was conducted on 4-year-old seedlings of T. cuspidata using three N types (ammonium sulfate, ammonium nitrate and potassium nitrate) and four addition levels (30, 60, 90 and 120 kg N·hm^-2·a^-1). The results showed that: (1) Height increment, basal diameter increment, and total biomass of T. cuspidata seedlings increased significantly with increasing N addition level. When the N addition level exceeded 90 kg N·hm^-2·a^-1, the basal diameter increment and total biomass began to decline. Nitrogen addition significantly reduced the root/shoot ratio of seedlings, but without significant differences among different N addition levels. (2) Net photosynthetic rate (P_n) and stomatal conductance increased significantly after N addition, peaking at 90 kg N·hm^-2·a^-1. The change of intercellular CO₂ concentration was opposite to the P_n, indicating that N addition could alleviate non-stomatal limitation of plants. (3) Flavonoid content increased firstly and then decreased after N addition, with a threshold of 30 kg N·hm^-2·a^-1. (4) After N addition, there was a positive correlation between growth and photosynthetic indices, and a negative correlation between flavanols and photosynthetic indices. The changes of net assimilation rate, relative growth rate and flavonoid indices were consistent with the predictions from the growth-differentiation balance hypothesis. (5) Compared with the single N form treatment, the addition of ammonium nitrate promoted aboveground growth, stomatal conductance and anthocyanin synthesis of seedlings. The flavonoid index increased significantly under the potassium nitrate treatment. In general, the response of T. cuspidata seedlings to N addition was related to both the amount and type of N addition. When the N addition was in the range of 30-90 kg N·hm^-2·a^-1, the growth and defense level of seedlings were better. The findings can be used as a reference measure for the ex-situ conservation and reintroduction of T. cuspidata.

Key words: nitrogen addition, growth-defense trade-off, Taxus cuspidata, growth-differentiation balance hypothesis, flavonoid.