Abstract:

【Objective】Chilling stress is a significant abiotic factor that leads to reduction in yield and quality of cucumber plants cultivated in solar greenhouse during winter. Cold-induced stress memory, as a flexible behavior in response to environmental changes, plays an important role in plant acclimation to adverse conditions. To elucidate the regulatory mechanism of cold circulation-induced stress memory on the chilling tolerance in cucumber, and provide technical guidance for enhancing the adaptation of cucumber to low temperature in solar-greenhouse. 【Method】Jinyou 35 cucumber plants were used as experimental materials, and were treated in an artificial climate chamber. There were four treatments: No cold induction treatment (C0R0), single cold induction (C1R1: 24 h treatment at a day/night temperature of 8 ℃/8 ℃ followed by a recovery period of 48 h at 25 ℃/18 ℃), repeated cold circulation induction twice (C2R2: same conditions as C1R1, cycle twice), and repeated cold circulation induction three times (C3R3: same conditions as C1R1 treatment, cycle three times). Subsequently, the seedlings were exposed to chilling stress condition (8 ℃/5 ℃). The C0R0 treatment at normal temperature (25 ℃/18 ℃) was used as the control. Following a 48 h exposure to chilling stress, the functional leaves were sampled for analysis. 【Result】The results showed that cold circulation induction significantly alleviated the symptoms of chilling injury, markedly reduced the levels of malondialdehyde (MDA), electrolyte leakage (EL), chilling injury index (CI), as well as the accumulation of hydrogen peroxide (H₂O₂) and superoxide anion ($\mathrm{O}_2^{\bar{.}}$) in cucumber seedlings, compared with the C0R0. The cold circulation induction treatments exhibited a significant increase in the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR) and their mRNA expression levels, relative to the C0R0 treatment under chilling stress. Among the cold circulation induction treatments, C2R2 exhibited the mildest symptoms of chilling injury. After exposure to 8 ℃/5 ℃ for 48 h, the CI of C2R2 seedlings decreased by 30.6% compared to that of the C0R0. Moreover, the MDA content, EL, as well as the accumulation of H₂O₂ and $\mathrm{O}_2^{\bar{.}}$ in C2R2 seedlings were lower by 39.4%, 29.8%, 34.0% and 48.0%, respectively compared with those observed in C0R0 seedlings. However, the activities of SOD, POD, APX and GR of C2R2 treatment were 48.3%, 185.1%, 34.8% and 50.4% higher, with corresponding increases in mRNA expression by 0.94-, 3.45-, 1.76- and 1.17-fold, respectively, compared to the C0R0 seedlings. The induction of cold circulation also significantly upregulated the expression levels of cold- responsive genes CsICE1, CsCBF1, CsCOR47 and heat shock protein CsHSPs. Overexpression of CsHSP26.5 and CsHSP17.6C distinctly enhanced the chilling tolerance of cucumber induced by cold circulation induction. 【Conclusion】Stress memory induced by cold circulation can enhance the chilling tolerance of cucumber seedlings, with the most optimal effect observed after two cycles. The main mechanisms are: (1) Cold circulation induction increase the activity of antioxidant system and decrease the accumulation of reactive oxygen species (ROS) under chilling stress, and consequently alleviate the oxidative damage in cucumber caused by chilling stress; (2) Cold circulation induction upregulate the expression of cold- responsive genes and activates of their function, thereby improving the chilling tolerance of cucumber seedlings; (3) CsHSPs are involve in the regulation of stress memory induced by cold circulation to the chilling tolerance in cucumber. The stress memory induced following two cycles of cold circulation in cucumber plants was between 14 d and 21 d.

Key words: cold circulation induction, stress memory, antioxidant system, cold response genes, CsHSPs, cucumber