Nitrous oxide (N2O) is a key atmospheric greenhouse gas that contributes to global climatic change through radiative warming and depletion of stratospheric ozone. In this report, N2O flux was monitored simultaneously with photosynthetic CO2 and O2 exchanges from intact canopies of 12 wheat seedlings. The rates of N2O-N emitted ranged from <2 pmol⅐m ؊2 ⅐s ؊1 when NH 4 ؉ was the N source, to 25.6 ؎ 1.7 pmol⅐m ؊2 ⅐s ؊1 (mean ؎ SE, n ؍ 13) when the N source was shifted to NO 3 ؊ . Such fluxes are among the smallest reported for any trace gas emitted by a higher plant. Leaf N2O emissions were correlated with leaf nitrate assimilation activity, as measured by using the assimilation quotient, the ratio of CO2 assimilated to O2 evolved. 15 P lants play a critical role in regulating the chemical and physical state of the atmosphere through the exchange of biogenic greenhouse gases. Most notable are plant-atmosphere exchanges of CO 2 , O 2 , and H 2 O, but leaves also emit a variety of carbon-and nitrogen-based trace gases involved in climate alteration processes (1). One such trace gas is nitrous oxide (N 2 O). Plants-either aerenchymous (2) or nonaerenchymous (3, 4)-can serve as conduits for N 2 O between the soil and atmosphere. They transpire significant quantities of N 2 O when its concentration in the soil solution greatly exceeds the solution equilibrium concentration with ambient N 2 O, currently at Ϸ315 nmol⅐mol Ϫ1 (5).The global N 2 O budget is beset by uncertainty, and sources of N 2 O have historically fallen short of the primary sink, photolysis in the upper atmosphere (6-9). The primary biogenic N 2 O sources are from soils (70%) and involve the microbial nitrogen transformations brought about by nitrification and denitrification (6). Although nitrification and denitrification are the major N 2 O sources, several microbial organisms that do not nitrify or denitrify can also produce N 2 O during NO 3 Ϫ assimilation (10, 11). These observations have led to the general hypothesis that any enzymatic nitrogen transformation through the ϩ2 to ϩ1 oxidation state may generate N 2 O (12). One such transformation in higher plants is NO 2 Ϫ assimilation in chloroplasts. Nitrite assimilation in chloroplasts can generate intermediates capable of reacting to produce N 2 O, including NO 2 Ϫ (as HNO 2 ) with hydroxylamine (13) or reaction of NO released during NO 2 Ϫ reduction (14, 15) with ascorbate (16). Nonetheless, early attempts to observe N 2 O production by higher plant tissues were not successful (10) and were probably limited by lack of an analytical method capable of detecting plant N 2 O emission at the exceptionally slow rates reported here. We developed an analytical approach by using cryogenic trapping (17) and gas chromatography coupled to high-precision isotope ratio mass spectrometry (18). This approach resolved leaf N 2 O emissions at more than six orders of magnitude lower than photosynthetic gas exchanges of CO 2 and O 2 (Table 1), placing such fluxes among the smallest ever reported for any trace gas...