Effects of nitrogen and phosphorus on the regulation of nonstructural carbohydrate accumulation, translocation and the yield formation of oilseed flax

•The N and P influenced the starch and soluble sugar concentration and content at the growth stage of the oilseed flax.•Budding and post-anthesis were the critical stages for seeds and capsules formation and development of the oilseed flax.•Leaves and buds soluble sugar concentration at budding and anthesis regulate the seeds and capsules number, respectively.