Effects of exogenous salicylic acid on morphology and physiology of pitaya seedlings under low temperature stress

Pitaya (Hylocereus undatus) is a typical tropical cash crop that is not chilling-tolerant. Low temperature stress seriously affects the quality and yield of pitaya. It is an efficient and safe method to apply exogenous natural substances to improve the chilling tolerance of plants. Salicylic acid (SA) not only regulates many physiological and biochemical processes such as seed germination, growth, fruiting and senescence, but also helps plants to cope with biotic and abiotic stresses. However, the role of exogenous SA in chilling tolerance of pitaya has not been reported. To explore the effect of exogenous SA on the morphology structure and physiology of pitaya seedlings under low temperature, the leaves of ‘Zihonglong’ seedlings were sprayed with SA at four concentrations (0.1, 0.3, 0.5, 0.7 mmol·L-1). after spraying for 48 h, seedlings were incubated at 4℃. The change in the morphology and leaf tissue structure of pitaya seedlings were observed on 0, 3, 6, 9 d. Physiological indexes such as relative electrical conductivity, contents of malondialdehyde, contents of osmotic regulators (soluble sugar, soluble protein, proline) and antioxidant enzyme (superoxide dismutase, peroxidase, catalase, glutathione S-transferase) activities in leaves were measured. The results were as follows: (1) Under low temperature stress, pitaya seedlings showed chilling injury of Grade 0, Grade Ⅰ and Grade Ⅱ. The leaf shape, color and cell structure of pitaya with different chilling injury symptoms were quite different. The percentages of Grade Ⅰ and Grade Ⅱ chilling injury of pitaya seedlings treated with SA were significantly reduced. (2) Compared with the low temperature control, SA treatment could reduce the relative conductivity and malondialdehyde contents of pitaya leaves, and increase the contents of osmotic regulators and antioxidant enzyme activities. (3) After comparing with different SA treatments, SA at 0.3 mmol·L-1 had the best effects on relieving chilling injury symptoms, reducing the relative conductivity and malondialdehyde content, increasing the soluble sugar and proline content, and increasing the activities of superoxide dismutase and glutathione S-transferase. The optimal SA concentrations for increasing soluble protein content, peroxidase activity and catalase activity were 0.7, 0.1 and 0.5 mmol·L-1, respectively. In summary, exogenous SA could alleviate chilling injury of pitaya seedlings under 4℃, and has a positive physiological effect on improving chilling tolerance of seedlings, and exogenous SA at 0.3 mmol·L-1 is more suitable.