Elevated CO₂ concentration enhance carbon and nitrogen metabolism and biomass accumulation of Ormosiahosiei

Elevated CO₂ concentrations may inhibit photosynthesis due to nitrogen deficiency, but legumes may be able to overcome this limitation and continue to grow. Our study confirms this conjecture well. First, we placed the two-year-old potted saplings of Ormosia hosiei (O. hosiei) (a leguminous tree species) in the open-top chamber (OTC) with three CO₂ concentrations of 400 (CK), 600 (E1), and 800 μmol·mol^-1 (E2) to simulate the elevated CO₂ concentration environment. After 146 days, the light saturation point (LSP), light compensation point (LCP), apparent quantum efficiency (AQE), and dark respiration rate (Rd) of O. hosiei were increased under increasing CO₂ concentration and obtain the maximum ribulose diphosphate (RuBP) carboxylation rate (V_{c max}) and RuBP regenerated photosynthetic electron transfer rate (J_max) were also significantly increased under E2 treatment (P < 0.05). This results in a significant increase of the maximum assimilation rate (A_max) under elevated CO₂ concentrations. Sucrose phosphate synthase (SPS) activity in sucrose metabolism increased in the leaves, more soluble sugars, starches, and sucrose was produced, but sucrose content only in leaves increased at E2, and more carbon flows to the roots. The activity of the NH₄⁺ assimilating enzymes glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) in the leaves of O. hosiei increases under elevated CO₂ concentrations to promote nitrogen synthesis that reduces the content of ammonium nitrogen and increases the content of nitrate nitrogen. In addition, under E1 conditions, sucrose synthase (SS), direction of synthesis activity was highest and sucrose invertase (INV) activity was lowest, this means that the balance of C and N metabolism is maintained. While under E2 conditions SS activity decreased and INV activity increased, this increased C/N and nitrogen use efficiency. So, the elevated CO₂ concentration promotes the accumulation of O. hosiei biomass, especially in the aboveground part, but did not have a significant effect on the accumulation of root biomass. This means that O. hosiei is able to cope under the elevated CO₂ concentration without showing photosynthetic adaptation during the experimental period.