Ornamental plant species may vary substantially in their tolerance response to heavy metals. The aim of this research was to check chrysanthemum cultivars, namely Donglin Ruixue (C), Yellow (F), Red pocket (G), and New 9714 (I), which are commonly used as landscape plants to determine their levels of cadmium (Cd) tolerance at different cadmium concentrations through hydroponic cultures. Chrysanthemum cultivars were treated with five different Cd concentrations (0, 10, 20, 50, and 100 mg L−1) and different physiological, enzymatic, and ultra-structure traits were taken under consideration in vitro. The results showed that cadmium concentration significantly inhibited the total chlorophyll content, chlorophyll a, chlorophyll b, and carotenoid content. Chlorophyll contents were significantly reduced at higher Cd concentrations in all cultivars, but the reduction rates were higher in cultivar F (59.49%), G (40.41%), I (44.97%), and C (33.86%). Similarly, the chlorophyll b reduction was higher than that of chlorophyll a in I (73.33%), followed by G (58.06%), F (61.66%), and C (32.43%), under Cd stress conditions. Additionally, the relative conductivity was recorded in cultivars C (146.48%), F (223.66%), G (165.96%), and I (154.92%), respectively, at 100 mg L−1 Cd concentrations. Likewise, MDA was significantly increased with high Cd stress, at 155.56, 325.27, 173.91, and 322.18%, in C, F, G, and I cultivars at 100 mg L−1, but it was promoted with a greater increase in F and I cultivars. Similarly, SOD and CAT activities were increased with the increase in Cd stress, but reduced in F and I cultivars at higher stress levels of 100 mg L−1. In the same way, POD activity was significantly higher in the C and G cultivars. Additionally, ultrastructure changes also occurred with the increase in the Cd stress, i.e., 20 mg L−1 to 100 mg L−1, and these changes caused alterations in cell organelles, including in the chloroplast, grana, lamella, thylakoid, and stroma. They also caused noticeable damage to mitochondria at higher Cd concentrations. It was concluded that the higher levels of antioxidative defense of the C and G cultivars of chrysanthemum indicated their ability to tolerate high Cd stress conditions. These could, therefore, be used for their phytoremediation potential in Cd-contaminated areas.

Terpenoids are important secondary metabolites in plants. The synthesis of terpenoids involves a key enzyme, 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR). In an aromatic ornamental plant, Chrysanthemum indicum var. aromaticum, terpenoids were found to be an important component in leaf secretions. To study the regulatory mechanism of terpenoid synthesis in C. indicum var. aromaticum, we cloned the DXR gene from C. indicum var. aromaticum, named CiDXR, based on transcriptome data. CiDXR is 1419 bp, encoding a polypeptide chain of 472 amino acids. Comparative and bioinformatic analysis showed that CiDXR was highly similar to DXRs from other plant species. The protein product of CiDXR was analyzed for physical and chemical properties, and secondary and tertiary structures. Methyl jasmonate (MeJA) is an effective exogenous elicitor that can impact the synthesis of secondary metabolites by regulating the expression of a series of specific key enzymes. In our study, MeJA treatment was used to study the relationship between CiDXR expression and terpenoid metabolism. The expression pattern of CiDXR was analyzed by quantitative real-time polymerase chain reaction. Leaves expressed the highest level of CiDXR mRNA. In addition, CiDXR expression was found to be activated by MeJA, up to 20-fold at 24 h post induction. The MeJA treatment significantly affected the relative contents of the secretion products, and the induction effect was maximal at 24 h, consistent with the expression of CiDXR. This study shows that CiDXR may play a key regulatory role in the control of terpenoid synthesis in C. indicum var. aromaticum.