Defoliation affects seed yield but not N uptake and growth rate in two oilseed rape cultivars differing in post-flowering N uptake

Abstract Genotypic variation in nitrogen efficiency of winter oilseed rape is closely related to variation in post-flowering N uptake. In this study the hypothesis was tested that assimilate supply from the leaves is decisive for a high N uptake during reproductive growth. Two winter oilseed rape cultivars differing in N efficiency were cultivated in a three-year field experiment at three N rates, ranging from deficiency to optimal N supply. In sub-plots, 50% of the leaves were removed at the beginning of flowering. Seed yield, total biomass, N uptake and yield components were assessed. Defoliation caused substantial yield decrease at all N rates and for both cultivars. This was not related to decreased growth rates or post-flowering N uptake, but mainly to the reduction in total N by leaf removal. In addition, harvest index and seed number per area were reduced by the defoliation treatment. The results demonstrate that leaves are important for yield formation in oilseed rape. The hypothesis, that assimilate supply from the leaves enhances N uptake during reproductive growth could not be confirmed by this experiment. Instead, the importance of the leaves for yield may be due to assimilate remobilization from the leaves during reproductive growth.