Abstract
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.