Contribution of Nitrogen Uptake and Retranslocation during Reproductive Growth to the Nitrogen Efficiency of Winter Oilseed Rape Cultivars (Brassica napus L. Differing in Leaf Senescence
Genotypic variation in N efficiency defined as high grain yield under limited nitrogen (N) supply of winter oilseed rape line cultivars has been predominantly attributed to N uptake efficiency (NUPT) through maintained N uptake during reproductive growth related to functional stay green. For investigating the role of stay green, N retranslocation and N uptake during the reproductive phase for grain yield formation, two line cultivars differing in N starvation induced leaf senescence were grown in a field experiment without mineral N (N0) and with 160 kg N*ha 1 (N160) Through frequent harvests from full flowering until maturity N uptake, N utilization and apparent N remobilization from vegetative plant parts to the pods could be calculated. NUPT proved being more important than N utilization efficiency (NUE) for grain yield formation under N limiting (N0) conditions. For cultivar differences in N efficiency, particularly N uptake during flowering (NUPT) and biomass allocation efficiency (HI) to the grains, were decisive. Both crop traits were related to delayed senescence of the older leaves. Remobilization of N particularly from stems and leaves was more important for pod N accumulation than N uptake after full flowering. Pod walls (high N concentrations) and stems (high biomass) mainly contributed to the crop residue N at maturity. Decreasing the crop inherent high N budget surplus of winter oilseed rape requires increasing the low N remobilization efficiency particularly of pod wall N to the grains. Addressing this conclusion, multi year and location field experiments with an extended range of cultivars including hybrids are desirable.