Wheat leaves emit nitrous oxide during nitrate assimilation

Nitrous oxide (N 2 O) is a key atmospheric greenhouse gas that contributes to global climatic change through radiative warming and depletion of stratospheric ozone. In this report, N 2 O flux was monitored simultaneously with photosynthetic CO 2 and O 2 exchanges from intact canopies of 12 wheat seedlings. The rates of N 2 O-N emitted ranged from <2 pmol⋅m −2 ⋅s −1 when NH [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{{₄⁺}}{equation*}{document} was the N source, to 25.6 ± 1.7 pmol⋅m −2 ⋅s −1 (mean ± SE, n = 13) when the N source was shifted to NO [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{{₃⁻}}{equation*}{document} . Such fluxes are among the smallest reported for any trace gas emitted by a higher plant. Leaf N 2 O emissions were correlated with leaf nitrate assimilation activity, as measured by using the assimilation quotient, the ratio of CO 2 assimilated to O 2 evolved. 15 N isotopic signatures on N 2 O emitted from leaves supported direct N 2 O production by plant NO [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{{₃⁻}}{equation*}{document} assimilation and not N 2 O produced by microorganisms on root surfaces and emitted in the transpiration stream. In vitro production of N 2 O by both intact chloroplasts and nitrite reductase, but not by nitrate reductase, indicated that N 2 O produced by leaves occurred during photoassimilation of NO [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{{₂⁻}}{equation*}{document} in the chloroplast. Given the large quantities of NO [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{{₃⁻}}{equation*}{document} assimilated by plants in the terrestrial biosphere, these observations suggest that formation of N 2 O during NO [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{{₂⁻}}{equation*}{document} photoassimilation could be an important global biogenic N 2 O source.