Mechanism of short-term nitrogen deficiency at flowering stage improving content of vitamin C in cucumber

Abstract: AsA (L-ascorbic acid)，also known as vitamin C, is an abundant antioxidant in plants. The previous study found that the AsA level of cucumber was higher under the short-term nitrogen deficiency than control（without nitrogen deficiency）. However, the regulatory mechanism is not explicit. In this study, the mechanism of short-term nitrogen deficiency at the flowering stage improving AsA content of cucumber was studied. The effect of relative enzyme activities and AsA-GSH recycling-related genes expression profiles, as well as the hydrogen peroxide (H2O2), malondialdehyde (MDA) and AsA contents were studied in hydroponic cucumber under short-term nitrogen deficiency，respectively. Based on this, the mechanism of AsA content increasing in cucumber under short-term nitrogen deficiency was studied. The results showed that under nitrogen deficiency, the content of MDA and H2O2 increased by 1- and 0.8-fold in the fruits of T1 (without nitrogen at flowing stage) compared with CK(nitrogen content of 12 mmol/L at flowing stage) on the 7th day after nitrogen deficiency treatment, respectively. The superoxide dismutase (SOD) activity, peroxidase (POD) activity and catalase (CAT) activity in the fruits of T1 increased by 60%, 21% and 74% respectively on the 7th day. Moreover, the total AsA (T-AsA) content in fruits and leaves were increased by 70% and 17% on the 5th day, respectively. The expression levels of APX-1, APX-3, APX-4, APX-5, AO-4, GR-1 and GR-2 were significantly up-regulated on the 1st day in leaves of T1 under short-term nitrogen deficiency. In the fruits of T1, transcripts of AO-1, MDHAR-1, MDAHR-3, GR-2 and APX-5 were higher than that of the control on the 1st day. The APX activity of T1 fruits increased by 51.6% on the 5th day. The MDHAR activity in leaves and fruits of T1 increased by 59.6% and 24.8% on the 3rd day, respectively. And the DHAR activity in fruits of T1 increased by 109% on the 3rd day. It can be seen that short-term nitrogen deficiency at flowering stage promoted the accumulation of T-AsA in cucumber fruits and leaves, the activities of related enzymes in AsA-GSH recycling pathway increased, and the expression level of genes in AsA-GSH recycling pathway were up-regulated.