title: Elevated CO2 decreases the Photorespiratory NH3 production but does not decrease the NH3 compensation point in rice leaves.
abstract: 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 (Vo). We measured the net leaf NH3 uptake rate (FNH3) across NH3 concentrations in the air (na) ranging from 0.2 to 1.6 nmol mol(-1) at three [CO2] values (190, 360 and 750 µmol mol(-1)) using rice plants. We analyzed leaf NH3 gas exchange using a custom-made whole-leaf chamber system, and determined the NH3 compensation point (γ), a measure of potential NH3 emission, as the x-intercept of the linear relationship of FNH3 as a function of na. Our γ values were lower than those reported for other plant species. γ did not decrease under elevated [CO2], although leaf NH4 (+) content decreased with decreasing Vo at higher [CO2]. This was also the case for γ estimated from the pH and NH4 (+) concentration of the leaf apoplast solution (γ'). γ' 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.
authors.affiliation: Functional Plant Research Unit, National Institute of Agrobiological Sciences, Kannondai, Tsukuba, 305-8602 Japan Present address: Department of Molecular and Cell Biology, Forestry and Forest Products Research Institute, Matsunosato 1, Tsukuba, 305-8687 Japan.
authors.forename: Shin-Ichi
authors.initials: S
authors.lastname: Miyazawa
authors.affiliation: Carbon and Nutrient Cycles Division, National Institute for Agro-Environmental Sciences, Kannondai, Tsukuba, 305-8604 Japan.
authors.forename: Kentaro
authors.initials: K
authors.lastname: Hayashi
authors.affiliation: Taiyo Keiki Co. Ltd., 1-12-3 Nakajujo, Kita-ku, Tokyo, 114-0032 Japan.
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Nakamura
authors.affiliation: Agro-Meteorology Division, National Institute for Agro-Environmental Sciences, Kannondai, Tsukuba, 305-8604 Japan.
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Hasegawa
authors.affiliation: Functional Plant Research Unit, National Institute of Agrobiological Sciences, Kannondai, Tsukuba, 305-8602 Japan mmiyao@affrc.go.jp.
authors.forename: Mitsue
authors.initials: M
authors.lastname: Miyao
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D002245:Carbon Dioxide / Q000494:pharmacology*; D019069:Cell Respiration / Q000187:drug effects; D008027:Light; D009584:Nitrogen / Q000378:metabolism*; D012275:Oryza / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D010788:Photosynthesis / Q000187:drug effects; D018515:Plant Leaves / Q000378:metabolism / Q000528:radiation effects; D018526:Plant Transpiration / Q000187:drug effects
chemical_list: D002245:Carbon Dioxide; D000641:Ammonia; D009584:Nitrogen
keywords: Ammonia; Elevated CO2; NH3 compensation point; Nitrogen metabolism; Photorespiration