Elevated CO2 decreases the photorespiratory NH3 production but does not decrease the NH3 compensation point in rice leaves
2014
Miyazawa, S. (National Institute of Agrobiological Sciences, Tsukuba (Japan). Functional Plant Research Unit) | Hayashi, K. | Nakamura, H. | Hasegawa, T. | Miyao, M.
The exchange of gaseous NH3 between the atmosphere and plants plays a pivotal role in controlling the global NH3 cycle. Photorespiration generates NH3 through oxygenation instead of carboxylation by the CO2-fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The future increase in the atmospheric CO2 concentration, [CO2], is expected to reduce plant NH3 production by suppressing RuBisCO oxygenation (Vsub(o)). We measured the net leaf NH3 uptake rate (Fsub(NH3)) across NH3 concentrations in the air (nsub(a)) ranging from 0.2 to 1.6 nmol /mol at three [CO2] values (190, 360 and 750 micro mol /mol) using rice plants. We analyzed leaf NH3 gas exchange using a custom-made whole-leaf chamber system, and determined the NH3 compensation point (gamma), a measure of potential NH3 emission, as the x-intercept of the linear relationship of Fsub(NH3) as a function of nsub(a). Our gamma values were lower than those reported for other plant species. Gamma did not decrease under elevated [CO2], although leaf NH4sup(+) content decreased with decreasing Vsub(o) at higher [CO2]. This was also the case for gamma estimated from the pH and NH4sup(+) concentration of the leaf apoplast solution (gamma'). Gamma' of rice plants, grown at elevated [CO2] for months in a free-air CO2 enrichment facility, was also not decreased by elevated [CO2]. These results suggest that suppression of RuBisCO oxygenation by elevated [CO2] does not decrease potential leaf NH3 emission in rice plants.
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