Wheat leaves emit nitrous oxide during nitrate assimilation

Abstract

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 x m(-2) x s(-1) when NH(4)(+) was the N source, to 25.6 +/- 1.7 pmol x m(-2) x s(-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 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(3)(-) 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(2)(-) in the chloroplast. Given the large quantities of NO(3)(-) assimilated by plants in the terrestrial biosphere, these observations suggest that formation of N(2)O during NO(2)(-) photoassimilation could be an important global biogenic N(2)O source.

Figure 1

The relationship between N₂O emission from wheat leaves (pmol N₂O-N m⁻²⋅s⁻¹) and (A) the change in assimilatory quotient (ΔAQ) when the nitrogen source was shifted from ¹⁵NH to ¹⁵NO, or (B) N₂O production in the rhizosphere (pmol N₂O-N g⁻¹⋅s⁻¹). Shown are the regression lines and the R² statistic. The rates of N₂O emission were normalized by using the log₁₀ transformation.

Figure 2

N₂O production in vitro from (A) intact chloroplasts (nmol N₂O-N g⁻¹ chlorophyll s⁻¹), (B) nitrate reductase (nmol N₂O-N g⁻¹ protein s⁻¹), and (C) nitrite reductase (nmol N₂O-N g⁻¹ protein s⁻¹). Intact chloroplasts, nitrate reductase, and nitrite reductase were extracted from fully expanded leaves of 2- to 3-week-old wheat plants (T. aestivum L. cv. Veery 10).