Abstract:

The application of nitrogen fertilizer is the main driving force for biomass production in artificial systems of agriculture and forestry. However, problems such as the increased cost and substantial pollution of soil, water and atmosphere are resulted from nitrogen leaching. Improving plant nitrogen use efficiency (NUE) is the crucial way to resolve these problems. NUE is a complex trait associated with an interplay between the genetic background and environmental factors, and is the comprehensive reflection of respective efficiencies with uptake of soil nitrogen by plant roots, assimilation and utilization of absorbed nitrogen inside plants, and resorption of nitrogen from senescent tissues or organs. The physiological steps of nitrogen resorption include a protein degradation in senescing organs,amino acid interconversion, and phloem loading of amino acids for remobilization. Phloem loading is thought to be the rate-limiting step of nitrogen resorption from senescing leaves. Nitrogen resorption is always accompanied with either natural senescence of the whole plant or certain tissues/organs determined by a developmental age or precocious senescence induced by environmental stresses. Nitrogen resorption in senescing leaves is an important strategy for perennial woody plants to conserve intrinsic nitrogen and increase stress tolerance through the winter season. The efficiency of nitrogen resorption is not only associated with species, age and functional types (i.e. deciduous or evergreen, broad- or needle-leaved) of woody plants, but also influenced by environmental factors such as geographic habitat, status of soil nitrogen and water supply. Improving efficiency of nitrogen resorption from senescing leaves may alleviate the requirement of trees for nitrogen fertilizer in the next growth season. As studies about mechanisms of leaf senescence and nitrogen resorption in trees are rare at present, characterization of key factors (such as NAC and WRKY family members) regulating leaf senescence, certain transporters responsible for phloem loading of amino acids in senescing leaves and signaling components mediating functions of phytohormones (such as abscisic acid and ethylene) which may promote leaf senescence is desired in woody plants in coming years, thus providing putative technological supports for breeding tree cultivars with the improved NUE through the genetic modification.

Key words: woody plants, leaf senescence, nitrogen resorption, nitrogen use efficiency, regulatory mechanism