1. Introduction: Fresh cucumber fruits are perishable and their quality is impacted by abiotic stresses. During postharvest life, due to internal and external factors, chemical and physical changes occur in fresh fruits, which may result in serious losses in nutritional quality. Chilling injury (CI) as an abiotic stress during storage of vegetables, also mostly increases susceptibility to decay, leads to economic losses. Salicylic acid (SA), a natural and safe phenolic compound, has been found to generate a wide range of metabolic and physiological responses in plants and exhibited a high potential in controlling postharvest losses of horticultural crops. The mechanisms of SA treatment in alleviating CI were extensively, which could be attributed to enhancing membrane integrity and antioxidant system activity. The goal of this study were to examine the effects of SA on controlling postharvest CI of cucumber fruit, with investigate the chlorophyll fluorescence by SA in reducing damage on cell membranes integrity in relation to chilling injury of the cucumber fruit during cold storage preservation.
2. Materials and Methods: This study is a factorial experiment in a completely randomized design in three replications in private greenhouse located in Marand city in mid-August 2017. The factors of this experiment was the different concentrations of salicylic acid (0, 0.25, 0.5 and 1 mM) and distilled water during (0, 6 and 12) days of storage, pre-storage and post-storage on greenhouse cucumber of Nagin variety were studied on fruit weight loss, total soluble solids (TSS), amount of fluorescence chlorophyll, ion leakage, rate of frost damage. Traits were evaluated before, between and after storage. In order to investigate the effects of storage of fruits, they were transferred to the refrigerator, and after 12 days of storage with comparison of the fruits of treated plants with control fruit, Effects of pre-harvest salicylic acid foliar application on Cucumbers were evaluated for exposure to chilling.
3. Results and Discussion: The results showed that different salicylic acid levels were effective in reducing fruit weight, and the highest weight loss (6.64%) was observed in distilled water treatment. The effect of SA treatment in reducing weight loss of fruits might be due to the fact that SA suppressed the transpiration of cucumber fruit by closing stomata of the treated fruit. In this experiment fruit treated with SA developed significantly lower chilling injury than control fruits. The suppression effect of salicylic acid treatment in SA treated cucumber fruit might be due to delaying the ripening process and lowering the incidence of chilling injury. The highest soluble solids (3.25 Brix) were achieved in the middle part of storage in 0.25 mM of SA treatment fruits and the lowest TSS (1.38 Brix) was observed before storage in 0.5 mM SA. The highest levels of Fv/Fm (0.81) was observed in middle of the storage (0) and control treatment and the lowest (0.67) in post storage (1 mM). Therefore, storage had a negative effect on the chlorophyll fluorescence index. A significant decrease in Fv/Fm, implying suppressed photochemical activity, for cucumbers stored at chilling temperatures of 4°C and 7°C, compared to those at 10°C and 13°C.The highest level of chilling damage in control treatment (distilled water) and the lowest was in the level of 1 mM salicylic acid. The highest level of ion leakage was in the treatment with distilled water, and the lowest level was at the level of 0.5 mM salicylic acid. An increase in ion leakage indicates leakiness of ions due to a loss of membrane integrity. This is an inherent feature of plants which are exposed to stresses such as low temperature.
4. Conclusion: Cold storage preservation induced chilling injury in cucumber fruit. The results showed that pretreatment with salicylic acid at a concentration of 0.5 mM can be commercially effective to improve the parameters of weight and performance, growth and protection from stress. The use of salicylic acid, which is known as an internal signal molecule, is mainly discussed in the tolerance of natural stresses in plants, can increase the plant's resistance to stresses and improve the growth and development of plants. These results suggested that SA as a postharvest tool could effectively alleviate chilling injury and enhance chilling tolerance of cucumber fruit. However, further studies are needed to explore the commercial use of SA in alleviating CI of cucumber fruit.