High Nitrate or Ammonium Applications Alleviated Photosynthetic Decline of <i>Phoebe bournei</i> Seedlings under Elevated Carbon Dioxide

<i>Phoebe bournei</i> is a precioustimber species and is listed as a national secondary protection plant in China. However, seedlings show obvious photosynthetic declinewhen grown long-term under an elevated CO₂ concentration (eCO₂). The global CO₂ concentration is predicted to reach 700 &mu;mol&middot;mol^&minus;1 by the end of this century; however, little is known about what causes the photosynthetic decline of <i>P. bournei</i> seedlings under eCO₂ or whether this photosynthetic decline could be controlled by fertilization measures. To explore this problem, one-year-old <i>P. bournei</i> seedlings were grown in an open-top air chamber under either an ambient CO₂ (aCO₂) concentration (350 &plusmn; 70 &mu;mol&middot;mol^&minus;1) or an eCO₂ concentration (700 &plusmn; 10 &mu;mol&middot;mol^&minus;1) from June 12th to September 8th and cultivated in soil treated with either moderate (0.8 g per seedling) or high applications (1.2 g per seedling) of nitrate or ammonium. Under eCO₂, the net photosynthetic rate (Pn) of <i>P. bournei</i> seedlings treated with a moderate nitrate application was 27.0% lower than that of seedlings grown under an aCO₂ concentration (<i>p</i> < 0.05), and photosynthetic declineappeared to be accompanied by a reduction of the electron transport rate (ETR), actual photochemical efficiency, chlorophyll content, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), rubisco activase (RCA) content, leaf thickness, and stomatal density. The Pn of seedlings treated with a high application of nitrate under eCO₂ was 5.0% lower than that of seedlings grown under aCO₂ (<i>p</i> > 0.05), and photosynthetic declineoccurred more slowly, accompanied by a significant increase in rubisco content, RCA content, and stomatal density. The Pn of <i>P. bournei</i> seedlings treated with either a moderate or a high application of ammonium and grown under eCO₂ was not significantly differentto that of seedlings grown under aCO₂&mdash;there was no photosynthetic decline&mdash;and the ETR, chlorophyll content, rubisco content, RCA content, and leaf thickness values were all increased. Increasing the application of nitrate or the supply of ammonium could slow down or prevent the photosynthetic declineof <i>P. bournei</i> seedlings under eCO₂ by changing the leaf structure and photosynthetic physiological characteristics.