Improved nitrogen efficiency in winter oilseed rape hybrid compared with the parental lines under contrasting nitrogen supply

Abstract

Winter oilseed rape (Brassica napus L.) is distinguished by poor nitrogen (N) efficiency but heavy N balance surplus. Cultivation of oilseed rape hybrids has been suggested as an important strategy to enhance the N efficiency and to reduce the large N balance surpluses in this crop, but the underlying physiological mechanism remains not well understood. In this study, seed yield and N efficiency parameters of an oilseed rape hybrid and its parental lines were investigated under contrasting N fertilization levels over three site-years. Seed yield-N uptake relationships were analyzed by quantile regression fitting a logarithmic model to both the 99th yield quantile and the whole data set. The maximum achievable yield (YieldMAX) of the hybrid was greater than the parental lines at a wide range of N uptake levels; in addition, the gap in yield between YieldMAX and actual yields for the hybrid was smaller than for its parents, indicating higher yield stability against environmental stresses. At both low (0 kg N ha−1) and high N (180 kg N ha−1) supply, the hybrid accumulated more N, which contributed to the yield heterosis. The higher seed yield of the hybrid was also related to the higher N utilization efficiency at the high N rate, which was attributed primarily to the improved N harvest index (NHI), but there was no genotypic difference in seed N concentration. Further analysis of seed N origins highlighted the importance of leaf and stem N remobilization other than post-flowering N uptake for genotypic variation in NHI at high N supply. These results suggest heterosis in seed yield and stability as a consequence of higher maximum yield attainable at a wide range of N uptake levels and smaller yield gaps. A higher capacity of N uptake contributed to the yield heterosis regardless of N levels. Additionally, efficient leaf and stem N remobilization also contributed the yield heterosis at high N supply.