Nitrogen fertilization increases ammonium accumulation during senescence of barley leaves

Abstract

Leaf senescence is a developmental process characterized by two events: (1) dismantling of the photosynthetic apparatus and, (2) nitrogen remobilization to other parts of the plant. Ammonium accumulation during senescence of barley leaves was produced by chloroplast proteins degradation. Although most of the ammonium is remobilized as amino acids from senescing leaves, a minor part is lost as NH3emitted across stomata. The amount of ammonia emitted depends on the amount of NH3accumulated in the substomatal cavity, which is continuously re-supplied with NH4 1+from the cytoplasm of neighboring cells. Ammonia accumulation in tissues could increase the possibility of loss of N as NH3emitted. In this report we analyzed the effect of N fertilization on nitrogen metabolism during senescence of barley leaves during the vegetative and reproductive stages of development. During senescence of barley leaves protein degradation was accompanied by transient ammonium accumulation at both stages of development. The peak of ammonium occurred immediately after major protein degradation in all samples analyzed, thereafter levels of ammonium clearly decreased. N accumulated as ammonium during senescence of barley leaves represented a high percentage of protein-N, i.e., approximately 16 % in primary leaves and 23 % in flag leaves. A significant increase of ammonium peak concentration was observed when doses of N fertilizer increased, mainly at the reproductive stage, where the percentage of N accumulated as ammonium reached near 35 % of protein-N at that stage. Vascular cytosolic glutamine synthetase (Hv GS1_1) transcript levels were upregulated during senescence of the flag leaf, but they were downregulated by increases in N availability. These results suggest that the decreases of ammonia levels after its peak may be more closely related to NH3emission than to N re-assimilation by GS.