## 4 Discussion
### Response of Leafy Mustard to Chilling Stress
Under low-temperature stress, a large amount of ROS accumulate and cause membrane lipid peroxidation, leading to an increase in the content of the membrane lipid peroxidation product malondialdehydedehyde (MDA). MDA reflects the degree of cell membrane damage and is an important indicator of the degree of plant oxidative stress [35]. In this study, the MDA content of mustard showed an increase under 5 \({}^{\circ}\)C, with a range of 0.73 to 68.92 nmol/g and an average value 1.04 times greater than that of the control. These results are consistent with those of [PERSON] et al. [14]. Chilling stress can increase the content of ROS and free radicals in mustard leaves and enhance membrane lipid peroxidation, leading to increased cell membrane permeability, electrolyte leakage, and damage to the cell membrane system, resulting in the accumulation of a large amount of MDA [36]. Excessive accumulation of ROS in plants under low-temperature stress leads to oxidative damage in plant cells [37]. Plants remove excess ROS by regulating their own enzymatic reaction system (SOD, CAT, and POD), where SOD catalyses the disproportionation of superoxide anion radicals to produce O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\); POD works synergistically with CAT to decompose H\({}_{2}\)O\({}_{2}\) into H\({}_{2}\)O and O\({}_{2}\), thereby reducing the toxic effects of hydrogen peroxide on cells [38]. In this study, the SOD and CAT activities of most mustard cultivars were increased, whereas POD activity were decreased. The results suggest that chilling stress activates the antioxidant enzyme system to scavenge excess reactive oxygen radicals in the cells by increasing enzyme activity, thus reducing the damage caused to the cells and improving the chilling tolerance of mustard [39]. SOD and CAT activities play major roles in the ability of mustard to protect against oxidative damage caused by chilling stress, indicating that most mustard has a strong ability to scavenge ROS. Chlorophyll is one of the most important physiological indicators of chilling tolerance in plants. High levels of polyunsaturated fatty acids in chloroplast lipids help to maintain plant survival and normal formation of chloroplast membranes under low-temperature stress [40]. In this study, more than half of the mustard cultivars exhibited an increase in chlorophyll content at 5 \({}^{\circ}\)C, and the chlorophyll content of the different mustard cultivars ranged from 0.37 to 2.65 mg/g; this finding may be due to changes in endogenous hormone levels, information transfer, and plant physiological responses under short-term low-temperature stress. Plants construct a low-temperature defence system to actively increase chlorophyll content and prevent the reduction in photosynthesis and energy production caused by the gradual decrease in chlorophyll content [18]. Chlorophyll fluorescence technology can be used to determine the degree of harm caused by chilling stress in plants [41]. Chlorophyll fluorescence parameters can be used to evaluate the function of photosynthetic mechanisms and the response of plants to low-temperature stress. Chlorophyll fluorescence parameters reflect the photosynthetic potential of plants, their ability to convert light energy into chemical energy, and their level of photosynthetic activity. In this study, the Fv/Fm, Fv\({}^{\prime}\)/Fm\({}^{\prime}\), \(\upmu\)PSII, and qP exhibited an decrease in most mustard cultivars under 5 \({}^{\circ}\)C treatment, indicating reductions in plant photosynthetic capacity, the efficiency of the photosynthetic electron transfer process, and photosynthetic activity, which is consistent with the results of [PERSON] et al. [18]. Under low-temperature conditions, the metabolism of leaf cells is severely inhibited mainly due to the effects of chilling stress on reducing carbon dioxide fixation or delaying the photosynthetic cycle as well as changes in sugar formation and distribution, which reduce the ability of plants to recover from stress damage [41].