Ammonia exchange between rice [Oryza sativa] leaf blades and the atmosphere: Effect of broadcast urea and changes in xylem sap and leaf apoplastic ammonium concentrations

To elucidate the effects of broadcast urea on ammonia (NH3) exchange between the atmosphere and rice, we investigated the NH3 exchange flux between rice leaf blades and the atmosphere, xylem sap ammonium (NH4sup(+)) concentration, leaf apoplastic NH4sup(+) concentration and pH, and determined the stomatal NH3 compensation point. Paddy rice (Oryza sativa L. cv. Nipponbare) cultivation using experimental pots was conducted in the open air. Three treatments, no nitrogen (NN), standard nitrogen (SN) and high nitrogen (HN), were prepared for two supplemental fertilizations. Urea with 0, 30 and 60 kg N/ha for the NN, SN and HN treatments, respectively, was broadcast at panicle initiation, and urea with 0, 20 and 40 kg N/ha for the NN, SN and HN treatments, respectively, was broadcast at heading. The NH3 exchange fluxes between the rice leaf blades and the atmosphere (SN treatment) measured using a dynamic chamber technique showed net deposition in general; however, net emission from the old leaves occurred 1 day after the application at heading. In contrast, the xylem sap NH4sup(+) concentrations increased markedly 1 day after both applications, which suggests direct transportation of NH4sup(+) from the rice roots to the above-ground parts. The applications resulted in no obvious increase in the leaf apoplastic NH4sup(+) concentrations. The relationship between the NH4sup(+) concentration in the xylem sap and that in the leaf apoplast was uncertain, although the NH4sup(+) in the xylem sap came from the roots and the NH4sup(+) in the apoplast might be affected by the stomatal deposition of NH3. The stomatal NH3 compensation point of rice was estimated to be 0.1-4.1 nmol/mol air (20degC). The direction and intensity of the exchange flux through the stomata, interpreted on the basis of the temperature-corrected NH3 compensation point, agreed with the observed exchange flux between the rice leaf blades and the atmosphere.