Ammonia exchange between rice 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 (NH₄⁺) concentration, leaf apoplastic NH₄ ⁺ 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^-1 for the NN, SN and HN treatments, respectively, was broadcast at panicle initiation, and urea with 0, 20 and 40 kg N ha^-1 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 NH₄⁺ concentrations increased markedly 1 day after both applications, which suggests direct transportation of NH₄⁺ from the rice roots to the above-ground parts. The applications resulted in no obvious increase in the leaf apoplastic NH₄⁺ concentrations. The relationship between the NH₄⁺ concentration in the xylem sap and that in the leaf apoplast was uncertain, although the NH₄⁺ in the xylem sap came from the roots and the NH₄⁺ 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^-1 air (20°C). 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.