Wheat leaves emit nitrous oxide during nitrate assimilation

Nitrous oxide (N2O) is a key atmospheric greenhouse gas that contributes to global climatic: change through radiative warming and depletion of stratos pheric ozone. In this report, N2O flux was monitored simultaneously with ph otosynthetic CO2 and O-2 exchanges from intact canopies of 12 wheat seedlin gs. The rates of N2O-N emitted ranged from <2 pmol(.)m(-2.)s(-1) when NH4was the N source, to 25.5 +/- 1.7 pmol(.)m(-2.)s(-1) (mean +/- SE, n = 13) when the N source was shifted to NO3-. Such fluxes are among the smallest r eported for any trace gas emitted by a higher plant. Leaf N2O emissions wer e correlated with leaf nitrate assimilation activity, as measured by using the assimilation quotient, the ratio of CO2 assimilated to O-2 evolved. N-1 5 isotopic signatures on N2O emitted from leaves supported direct N2O produ ction by plant NO3- assimilation and not N2O produced by microorganisms on root surfaces and emitted in the transpiration stream. In vitro production of N2O by both intact chloroplasts and nitrite reductase, but not by nitrat e reductase, indicated that N2O produced by leaves occurred during photoass imilation of NO2- in the chloroplast. Given the large quantities of NO3- as similated by plants in the terrestrial biosphere, these observations sugges t that formation of N2O during NO2- photoassimilation could be an important global biogenic N2O source.