Insights into the promoting effects of water-soluble amino acid fertilizers on strawberry fruit quality under nitrogen reduction treatment

•Additional WAAF application under N reduction can improve strawberry fruit quality.•WAAF application shifted the C flux to N metabolism in strawberries.•WAAF application improved P accumulation. Reasonable chemical nitrogen (N) reduction and water-soluble amino acid fertilizers (WAAF) application can mitigate the negative effects of excessive N supply. Here, we reported that a 30% N reduction (T1) led to attenuated plant growth and decreased fruit quality of strawberries, which could be overwhelmingly restored by additional WAAF application (T2). To explore the underlying mechanism, comparative transcriptomic analysis was performed. Results revealed significant expression changes of genes involved in metabolisms of starch and sucrose, ascorbate and aldarate, carbon (C) and N, glyoxylate and dicarboxylate and so on. In consistence with the increased ascorbic acid (AsA) content and sugar/acid ratio, WAAF application upregulated GLDH, SPS and β-GE genes and downregulated APX, ICL and MS genes. Additionally, the differential expression of PK, IDH, GDH and SPX was consistent with the shift from C flux to N metabolism and the improved phosphorus (P) accumulation resulted from WAAF application. Our study will be helpful for understanding the effect of N reduction and WAAF application on strawberry fruit quality.