The effect of nitrogen application and planting density on the radiation use efficiency and the stem lignin metabolism in rapeseed (Brassica napus L.)

Using the rapeseed Huayouza 62 as experimental material, yield and lodging related traits were evaluated under various nitrogen applications [120,240 and 360kgha−1 (N1–N3, respectively)] and planting densities [15×104, 30×104 and 45×104 plants ha−1 (D1–D3, respectively)]. Results showed: (1) the increase of nitrogen fertilizer elevated the leaf (and silique) area index (LAI) and light interception rate (LIR), significantly affecting the bud stage. In addition, the radiation use efficiency (RUE) for different growth stages and biomass accumulation were increased, which improved yield by facilitating formation of siliques in both single plants and populations. The increase of planting density significantly elevated the population LAI and LIR in seedlings, and the RUE in various growth stages, increased population biomass accumulation, which may have also improved yield by promoting the formation of siliques in the population. The correlation between LIR/LAI and yield was the highest at the bud stage, while the strongest correlation between RUE and yield was at silique development. (2) More nitrogen increased the lodging angle, base lodging index (LIL) and the upper lodging index (LIU), resulting in elevated whole plant lodging risk. With increases in nitrogen fertilizer, expression of phenylalanine ammonia-lyase (PAL) of lignin biosynthesis decreased in the upper stem, leading to lower lignin content and lodging resistance. However, higher plant density reduced the lodging angle and LIU, but increased LIL. The increase in planting density induced the expression of PAL, cinnamoyl-CoA reductase 1 (CCR1), 4-coumarate: CoA ligase (4CL) in the stem, which showed their highest expression in the N2D3 treatment, leading to higher lignin content and lodging resistance. (3) In this study, N2D3 achieved relative high yield and the lowest lodging index, which will meet the requirements for mechanized harvest.