Photosynthetic characteristics and nitrogen allocation in the black locust (Robinia pseudoacacia L.) grown in a FACE system

Key message The black locust is adapted to elevated [CO2] through changes in nitrogen allocation characteristics in leaves. The black locust (Robinia pseudoacacia L.) is an invasive woody legume within Japan. This prolific species has a high photosynthetic rate and growth rate, and undergoes symbiosis with N-2-fixing micro-organisms. To determine the effect of elevated CO2 concentration [CO2] on its photosynthetic characteristics, we studied the chlorophyll (Chl) and leaf nitrogen (N) content, and the leaf structure and N allocation patterns in the leaves and acetylene reduction activity after four growing seasons, in R. pseudoacacia. Our specimens were grown at ambient [CO2] (370 mu mol mol(-1)) and at elevated [CO2] (500 mu mol mol(-1)), using a free air CO2 enrichment (FACE) system. Net photosynthetic rate at growth [CO2] (A(growth)) and acetylene reduction activity were significantly higher, but maximum carboxylation rate of RuBisCo (V-cmax), maximum rate of electron transport driving RUBP regeneration (J(max)), net photosynthetic rate under enhanced CO2 concentration and light saturation (A(max)), the N concentration in leaf, and in leaf mass per unit area (LMA) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) content were significantly lower grown at elevated [CO2] than at ambient [CO2]. We also found that RuBisCo/N were less at elevated [CO2], whereas Chl/N increased significantly. Allocation characteristics from N in leaves to photosynthetic proteins, N-L (Light-harvesting complex: LHC, photosystem I and II: PSI and PSII) and other proteins also changed. When R. pseudoacacia was grown at elevated [CO2], the N allocation to RuBisCo (N-R) decreased to a greater extent but N-L and N remaining increased relative to specimens grown at ambient [CO2]. We suggest that N remobilization from RuBisCo is more efficient than from proteins of electron transport (N-E), and from N-L. These physiological responses of the black locust are significant as being an adaptation strategy to global environmental changes.