Low Temperature Rather Than Nitrogen Application Mainly Modulates the Floral Initiation of Different Ecotypes of Rapeseed (<i>Brassica napus</i> L.)

Rapeseed has formed three ecological types, namely winter, semi-winter, and spring during the long domestication process. Different ecotypes have different low-temperature requirements for floral initiation. Rapeseed growth has a large demand for nitrogen, and the amount of nitrogen application significantly impacts the number of flowering. Meanwhile, the time of floral imitation determined the quantity of floral bud, the final number of pods, and yield of rapeseed. Therefore, it is of great significance to understand the regulation of temperature and nitrogen on floral initiation. This experiment selected representative semi-winter and spring rapeseed varieties to study the leaf’s soluble sugar and protein concentration under different nitrogen supplies and the transcriptome reactions to vernalization for spring and semi-winter varieties rapeseed in transition to floral initiation. The results showed that the soluble sugar content and carbon-to-nitrogen ratio changed due to the different growth processes and nitrogen application rates. The increase of the sugar content to a peak could be regarded as the signal to start floral initiation. Reducing the nitrogen application rate increased the peak of sugar content, but the effect on the appearance time of the peak was not obvious. Under normal (20–25 °C) and low temperatures (10–15 °C), the floral initiation time of spring variety “1358” showed no difference, nor did expression of hub gene <i>SOC1</i>, which is involved in the flowering regulation network. The semi-winter variety “Zhongshuang No. 11” did not commence floral initiation under normal temperature because of the lacking of vernalization requirement. Low temperature promoted the floral initiation of semi-winter variety mainly through the <i>FLC</i>, <i>SOC1</i>, and <i>LFY</i> signaling pathways, and the gibberellin also played a positive factor in this process. In essence, the present study provides valuable information on the gene expression differences of vernalization-driven floral transition for spring and semi-winter ecotypes of rapeseed when the photoperiod is not an unlimited factor.