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 [CO ₂ ] through changes in nitrogen allocation characteristics in leaves.

Abstract

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 N₂-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 μmol mol^?1) and at elevated [CO₂] (500 μmol 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.