For the first time, spruce shoots (Picea abies [L.] Karst.) were fumigated in vivo with 13N-labelled NO2 with the aim of elucidating the mechanism of NO2− trapping in the apoplast of the substomatal cavity. Uptake by the needles could be monitored on-line, and a quantitative analysis of the activity records delivered a deposition velocity in agreement with the common dry deposition estimates and ruled out rapid export processes. A fast extraction procedure was applied which revealed that NO2 did not produce any detectable traces of nitrite. In needles in which the enzymes of nitrate reduction were not induced by prior fumigation with NO2, incorporation of NO2 was partially inhibited as compared to the fully induced shoots which took up and assimilated NO2 as expected from a constant influx. The only labelled inorganic species found in the extracts was nitrate (60%), whereas the rest of the label (40%) was assimilated organic nitrogen.A quantitative analysis of the data shows that the reaction of NO2 in the apoplast yields at least three times more nitrate than nitrite, so that the existing models about the apoplastic trapping reaction, disproportionation or antioxidant scavenging, which both postulate substantial production of nitrite, have to be reconsidere
