Effects of exogenous calcium and calcium inhibitor on physiological characteristics of winter turnip rape (Brassica rapa) under low temperature stress

Low temperature is one of the environmental factors that restrict the growth and geographical distribution of Brassica. To investigate the effects of exogenous calcium and calcium inhibitor on the ability of winter turnip rapeseed (Brassica rapa L.) to withstand low temperatures (4℃), we used a strong cold-resistant variety Longyou 7 (L7) and a weak cold-resistant variety Longyou 99 (L99) as the materials. The seedlings were treated with CaCl₂ (20 mmol·L^-1) and calcium inhibitor LaCl₃ (10 mmol·L^-1) at 0 h (CK), 6 h, 12 h, 24 h and 48 h after 4℃ treatments. Physiological characteristics, Ca²⁺ flux and Ca²⁺ concentration in roots after treatments were analyzed. Results illustrated that under 4℃ treatment, activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) increased by both CK and exogenous CaCl₂ treatments. Contents of soluble protein (SP) and proline (Pro) increased, while contents of malondialdehyde (MDA) decreased, resulting in reduced membrane lipid peroxidation. But enzyme activity decreased and MDA content increased following treatment with exogenous LaCl₃. The rate of Ca²⁺ flow showed a higher uptake in L7 roots compared with L99. L99 showed Ca²⁺ efflux with a rate of 30.21 pmol‧cm^-2‧s^-1, whereas L7 showed short efflux then returned to influx. Calcium ion content in roots decreased in both cultivars after CaCl₂ treatment. Results of RNA-seq revealed that genes were differentially expressed in response to low temperatures, hormones, photosystem II, chloroplasts, DNA replication, ribosomal RNA processing, and translation. This study found significant expression genes related to cellular signal transduction (MAPK signaling pathway) and material metabolism (nitrogen metabolism, glycerol ester metabolism).It was also analyzed by WGCNA that two modules had the strongest correlation with physiological indicators. Eight candidate genes were identified among MAPK signaling pathway and the two modules.