Remobilization of vegetative nitrogen to developing grain in wheat (Triticum aestivum L.)

A substantial percentage of the grain nitrogen (N) in cereal crops originates from the remobilization of N stored in vegetative tissues before anthesis. In wheat (Triticum aestivum L.), the percentage can reach 90%. All vegetative plant parts, including the leaves, stem, sheath, chaff and root, reserve N prior to anthesis and function as sources of nutrients during the grain filling phase. The developing grain per se accumulates nutrients as a sink. Nitrogen remobilization (NR) may be initiated immediately after anthesis, as indicated by decreases in the N or protein contents of the flag leaves. Senescence, a process that involves high macromolecular degradation for grain filling, begins approximately 8-16 days after anthesis. The resulting large amount of N compounds, mainly amino acids, will be remobilized to the grain. Precise programming of the timing and duration of NR and senescence is important. A large body of evidence has demonstrated that NR and senescence are under genetic control. Functional Gpc-1 genes, in combination with the NAC and WRKY transcription factors and their important targets, are involved in early senescence and enhance the grain protein content (GPC). NR and senescence are also regulated by a delicate balance between sink strength and source capacity, environmental factors and field managements. A better understanding of all these steps will reveal potential strategies to further increase GPC and processing quality and to minimize environmental contamination due to excess fertilizer run-off and associated expenses.