Enhancement of Cold Tolerance by Drought Stress in Pitaya (Hylocereus undatus)

Pitaya (Hylocereus undatus) is a typical Crassulacean Acid Metabolism (CAM) plant with strong drought tolerance but high sensitivity to low temperatures. In this study, the responses of pitaya cultivated in the karst areas of Guizhou Province in southwest China to drought and low temperature were examined in winter seasons. The stems of ‘Zihonglong’ pitaya were used as materials to investigate the physiological responses to cold temperatures of pitaya stems under different water conditions, so as to understand the effects of drought stress on the response to low temperatures. The results showed that the severity of chilling injury in pitaya stems was influenced by cold degree and duration and temperature variation. Under sustained low-temperature conditions, the lower the temperature and the longer the duration, the more severe the chilling injury, particularly at 4 °C and below. Drastic temperature rise after exposure to low temperature of 5 °C aggravated the damage, especially when the temperature rise exceeded 10 °C. Compared to normally irrigated plants, those subjected to drought pretreatment exhibited milder chilling injury and higher survival rates under a temperature shift from 5 to 20 °C. The drought-treated pitaya stems had significantly lower membrane leakage and malondialdehyde (MDA) and reactive oxygen species (ROS) contents compared with the well-watered control under different temperature increases starting from 5 °C. Drought significantly reduced soluble sugars and soluble proteins but increased proline under a temperature shift from 5 to 20 °C. It significantly enhanced the activities of catalase (CAT) and ascorbate peroxidase (APX) under temperature shifts from 5 to 10 or 20 °C, but had no significant effect on peroxidase (POD) and superoxide dismutase (SOD). Drought also significantly increased ascorbic acid (ASA) content but significantly reduced glutathione (GSH). It is concluded that a drastic post-cold temperature rise causes more severe damage than the cold temperature itself. Drought pretreatment increases the chilling tolerance of pitaya stems. This effect involves an enhanced ASA-GSH cycle, which strengthens ROS scavenging and prevents membrane damage.