Enhancing Tolerance of Rye (Secale cereale L.) Seedlings to Cadmium Chloride stress and Freeze–Thaw Cycles through Protocatechuic Acid Application

Freeze–thaw (FT) and cadmium (Cd) are the main factors which effect the plants distribution in high altitude. Protocatechuic acid (PCA) used as an antioxidant organic acid, can enhance plant tolerance to abiotic stresses in agricultural environments and promote physiological features and plant growth. The experiment, investigated the response of rye (Secale cereale L.) seedlings to freeze–thaw cycles (FTC) and cadmium (Cd) compound stresses and PCA on these seedlings. An integrated biomarker response (IBR) model was used to comprehensively assess the ecological risk posed by Cd and FTC on rye seedlings. Molecular docking studies exposed Cd interacted with SOD and CAT proteins’ active sites. The results demonstrated that FTC and Cd stresses decreased the net photosynthetic rate (Pn), internal CO₂ (Ci), transpiration rate (Tr) and stomatal conductance (Gs) of rye seedlings. While PCA did not perform optimally in completely counteracting these effects, it did improve the seedlings' resistance to the stresses. Notable increases were observed in the activity of peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) enzymes, as well as in soluble protein (SP) levels. Additionally, there was a decrease in malondialdehyde (MDA) content, which is significant because in plants high levels of MDA can cause oxidation. Thus, the study confirmed that PCA has a protective effect on rye plants under compound stresses (FTC and Cd), reducing physiological damage and improve tolerance. It is recommended that, in agricultural practices, the occurrence of these stresses (freeze–thaw and Cd) be minimized to mitigate physiological stress. If the effect of freeze–thaw and Cd continue it can make the serious problem for agriculture. This study establishes a theoretical foundation for mitigating ecological disasters in plants situated in northern high-latitude regions.