Elevated CO₂ Decreases the Photorespiratory NH₃ Production but Does not Decrease the NH₃ Compensation Point in Rice Leaves
Miyazawa, Shin-Ichi | Hayashi, Kentaro | Nakamura, Hirofumi | Hasegawa, Toshihiro | Miyao, Mitsue
The exchange of gaseous NH ₃ between the atmosphere and plants plays a pivotal role in controlling the global NH ₃ cycle. Photorespiration generates NH ₃ through oxygenation instead of carboxylation by the CO ₂-fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The future increase in the atmospheric CO ₂ concentration, [CO ₂], is expected to reduce plant NH ₃ production by suppressing RuBisCO oxygenation (V ₒ). We measured the net leaf NH ₃ uptake rate (F NH₃) across NH ₃ concentrations in the air (n ₐ) ranging from 0.2 to 1.6 nmol mol ⁻¹ at three [CO ₂] values (190, 360 and 750 µmol mol ⁻¹) using rice plants. We analyzed leaf NH ₃ gas exchange using a custom-made whole-leaf chamber system, and determined the NH ₃ compensation point (γ), a measure of potential NH ₃ emission, as the x -intercept of the linear relationship of F NH₃ as a function of n ₐ. Our γ values were lower than those reported for other plant species. γ did not decrease under elevated [CO ₂], although leaf NH ₄⁺ content decreased with decreasing V ₒ at higher [CO ₂]. This was also the case for γ estimated from the pH and NH ₄⁺ concentration of the leaf apoplast solution (γ′). γ′ of rice plants, grown at elevated [CO ₂] for months in a free-air CO ₂ enrichment facility, was also not decreased by elevated [CO ₂]. These results suggest that suppression of RuBisCO oxygenation by elevated [CO ₂] does not decrease potential leaf NH ₃ emission in rice plants.
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