CONICET | Buscador de Institutos y Recursos Humanos

Leaf senescence is a developmental process characterized by two events: 1-completedegradation of chloroplasts and, 2-nitrogen remobilization to other parts of the plant.During senescence chloroplast proteins are degraded into amino acids andammonium. Most of the ammonium is re-assimilated into amino acids for export fromthe senescing leaf, whereas a minor part is lost as NH3 emitted from the leaves. Theamount of ammonia emitted depends on the amount of NH3 accumulated in thesubstomatal cavity of mesophyll cells, which is continuously re-supplied with NH41+from the cytoplasm. Ammonia accumulation in tissues could increase the possibility ofloss of N as NH3 emitted. In this report we analyzed the effect of N fertilization onnitrogen metabolism during senescence of barley leaves during the vegetative andreproductive stages of development. During senescence of barley leaves proteindegradation was accompanied by transient ammonia accumulation at both stages ofdevelopment. The peak of ammonium occurred immediately after major proteindegradation in all samples analyzed, thereafter levels of ammonium clearly decreased.N accumulated as ammonium during senescence of barley leaves represented a highpercentage of protein-N, i.e., approximately 5% in primary leaves and 16% in flagleaves. A significant increase of ammonium peak concentration was observed whendoses of N fertilizer increased, mainly at the reproductive stage, where the percentageof N accumulated as ammonium reached near 35% of protein-N. Vascular cytosolicglutamine synthetase (Hv GS1_1) transcript levels were up regulated duringsenescence of the flag leaf, but they were down regulated by increases in Navailability. These results suggest that the decreases of ammonia levels after its peakmay be more closely related to NH3 emission than to N remobilization by GS.