Winter oilseed rape is a key oil crop in temperate climate regions of the world. Yield of seeds depends mostly on the nitrogen management throughout the season. The N status in oilseed rape canopy was studied in 2008, 2009 and 2010. A one factorial experiment, run in order to verify the initial hypothesis, consisted of the following six treatments: absolute control (AC), NP, NPK, NPK + MgS with 1/3 rate of the total planned dose applied in spring (NPKMgS1), NPKMgS with 1.0 dose in autumn (NPKMgMgS2), NPK+MgS with 2/3 of the dose in autumn + 1/3 in spring (NPKMgS3). Plant samples were taken at three stages: i) full rosette (BBCH 30), ii) the onset of flowering (BBCH 61), iii) maturity (BBCH 89). A complete plant sample was divided according to the growth stage into main plant organs such as leaves, stems, straw and seeds. The biomass yield, nitrogen concentration and content were determined in each part of the plant. The study revealed two strategies of dry matter and nitrogen accumulation by oilseed rape throughout the season. In 2008 and - to some extent - in 2010, the plants showed the yield formation strategy of a relatively slow but continuous increase in the nitrogen and biomass accumulation during the season. This strategy resulted in a high seed density in pods on the main branch. The value of this yield component significantly depended on the nitrogen content in leaves at the onset of flowering. Another strategy became apparent in 2009. It consisted in a very high rate of nitrogen accumulation during the vegetative growth, resulting in high leaf biomass at the onset of flowering. This growth pattern, as corroborated by the seed yield results, was not as effective as the former one, mainly because  of a lower seed density, especially in pods on the secondary branch, due to the excessive nitrogen content in leaves at the onset of flowering. Irrespective of the nitrogen management strategy followed by the oilseed rape canopy, the best predictor of the final yield was the nitrogen content in seeds. In conclusion, any growth factor leading to a nitrogen sink decrease, such as reduced plant density and/or disturbed N management throughout the season, can be considered as adverse for the seed yield.

19.3.2014