Nutritional Quality Response to Different Fertilizers in Young Stems of Rapeseed (Brassica napus L.) at Different Harvesting Stages

The young stem of rapeseed is a highly nutritional vegetable, but there is a lack of information on quality regulation by slow-release fertilizers (SRFs). This study aims to evaluate the effects of SRFs on nutritional contents, including vitamin and sugar profiles and regulatory mechanisms, using enzymatic activity and gene expression analysis. A field experiment was conducted with a split-plot design, in which treatments with two fertilizers (traditional compound fertilizer (TF) and SRF) served as the main plot and two harvesting stages (main stem harvesting (S1) and the first branch harvesting (S2)) served as the sub-plot. The results showed that vitamin E (VE) content under the SRF treatment was 48.31% and 18.44% higher than that under the TF treatment at both stages. The contents of vitamin C (Vc) at the S2 stage and vitamin B6 (VB6) at the S1 stage under the TF treatment were 7.56% and 2.95% higher than under SRF treatments. Water-soluble sugar (WSS) and glucose contents under the SRF treatment were significantly higher than under the TF treatment at both stages, while fructose, trehalose, and sorbitol contents exhibited the opposite trend. The offset effect of the activity of ascorbate oxidase (AAO) and dehydroascorbate reductase (DHAR) between the two fertilizers resulted in a non-significant difference in Vc content at the S1 stage. Under the TF treatment, sucrose phosphate synthase had greater activity as compared to the SRF treatment. The selected key genes involved in vitamin and carbohydrate metabolism were generally in agreement with the changes in enzymatic activity. This study highlights the importance of SRF for the quality formation of young stems of rapeseed as a vegetable.