Integrative physiological and transcriptomic analyses reveal divergent chilling tolerance mechanisms in contrasting zoysiagrass cultivars and protective effects of exogenous chitosan

Background Chilling stress is one of the major factors affecting the green period and turf quality in zoysiagrass (Zoysia spp.). However, the physiological and molecular mechanisms of differential chilling tolerance among zoysiagrass cultivars remain largely unexplored. Results The objectives of this study were to elucidate differences of chilling tolerance between the two cultivars with different chilling tolerance through integrating physiological and transcriptomic analyses, and to investigate protective effects of exogenous chitosan on the sensitive cultivar. The results showed that the chilling-tolerant cultivar ‘Nannong NT1’ suffered less injury than the sensitive ‘Nannong LG’ under chilling stress, as evidenced by lower electrolyte leakage, reduced reactive oxygen species (ROS) accumulation, and higher survival rate. Transcriptomic analyses further identified that ‘Nannong NT1’ specifically up-regulated carbohydrate and lipid metabolism, while concurrently down-regulating photosynthesis and carbon fixation. In contrast, ‘Nannong LG’ activated carotenoid and flavonoid biosynthesis, and repressed amino acid metabolism and protein export. Additionally, foliar application of chitosan (50 mg·L-¹) significantly alleviated chilling injury in ‘Nannong LG’ by enhancing chlorophyll content, relative water content, osmoprotectant accumulation, and survival rate, while reducing electrolyte leakage and ROS accumulation. At the transcriptional level, chitosan specifically modulated the expression of several critical differentially expressed genes, such as ZsNAC2, ZsNAC3, ZsLecRLK1, ZsLecRLK2, and ZsBAM. Conclusions These findings demonstrate that differential chilling tolerance was attributed to the divergent regulatory and metabolic strategies. Exogenous chitosan offers a complementary approach to enhance chilling tolerance in the sensitive cultivar in zoysiagrass.