Abstract: Low temperature is a major limiting factor for the productivity of many thermophilic plants such as cucumber and tomato, they may experience a physiological dysfunction called chilling injury when exposed to nonfreezing temperatures below 12℃ for more than the critical period. However, the development of chilling injury symptoms may be mainly due to the production of activated oxygen species(AOS) (superoxide radicals, singlet oxygen, hydrogen peroxide and hydroxyl radical) that can cause damage to proteins, nucleic acids and cell membranes. Salicylic acid(SA), which is an ubiquitous plant phenolic, has been reported to induce defense responses and increase chilling tolerance in plants. Although some studies found that SA can increase chilling tolerance of cucumber(Cucumis sativa L.) seedlings by promoting the activities of antioxidant enzyme, its molecular mechanism still remains unclearly. In this study, the stress condition of low temperature(day/night 10/5℃, photo flux density 80 μmol·m^-2·s^-1) was stimulated in a light growth chamber, and cucumber ‘Youbo1-5’ was used as the material. Cucumber seedlings were sprayed with deionized water or 1 mmol·L^-1 SA once a day at the 30-day ages. Forty eight hours after treatment, plants were exposed to low temperature stress for 4 days, then were allowed to grow under normal conditions(day/night 25/18℃, photo flux density 300 μmol·m^-2·s^-1) for 2 days. The effects of SA spraying on growth, photosynthetic characteristics and antioxidant enzyme activities were investigated, and the gene expression of antioxidant enzyme was determined by quantitative real-time PCR analysis. The most evident effect of low temperature stress was the marked reduction in plant growth, chlorophyll content, and net photosynthetic rate. Four days after treatment, low temperature stress reduced the specific leaf weight, SPAD value, photosynthetic rate(Pn) and maximal photochemical efficiency of PSII(Fv/Fm) by 18.2%, 30.37%, 64.99% and 4.18% over their respective controls, respectively. However, seedlings pretreated with 1 mmol·L^-1 SA had higher values of above parameters, compared to the water sprayed plant. When plants were treated 1d under low temperature, the activities of superoxide dismutase(SOD), peroxidase(POD), catalase (CAT) decreased by 11.56%, 23.88% and 29.29% over the control, respectively, and increased more or less after 4 days. In contrast, the stressed seedlings pretreated with SA increased the activities of SOD, POD and CAT over the controls during low temperature stresses and recovers stage. The quantitative real-time RT-PCR analysis indicated that the expression of SOD, POD and CAT genes could be induced by SA and their relative expression maintained high values under low temperature stress. We considered that SA(1 mmol·L^-1) enhanced low temperature tolerance in cucumber seedlings may due to the induction of SOD, POD and CAT transcription factor and activations of antioxidative capacity. The study provides theoretical basis to elucidate the molecular mechanism of SA alleviating low temperature stress on cucumber seedlings.

Key words: salicylic acid, cucumber, antioxidant enzyme, gene expression, low temperature