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

Abstract

Phoebe bournei 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 μmol·mol−1 by the end of this century; however, little is known about what causes the photosynthetic decline of P. bournei seedlings under eCO2 or whether this photosynthetic decline could be controlled by fertilization measures. To explore this problem, one-year-old P. bournei seedlings were grown in an open-top air chamber under either an ambient CO2 (aCO2) concentration (350 ± 70 μmol·mol−1) or an eCO2 concentration (700 ± 10 μmol·mol−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 P. bournei seedlings treated with a moderate nitrate application was 27.0% lower than that of seedlings grown under an aCO2 concentration (p < 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 (p > 0.05), and photosynthetic declineoccurred more slowly, accompanied by a significant increase in rubisco content, RCA content, and stomatal density. The Pn of P. bournei 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—there was no photosynthetic decline—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 P. bournei seedlings under eCO2 by changing the leaf structure and photosynthetic physiological characteristics.