Objective  Physiology of and heavy-metal accumulation in marigold ( Tagetes erecta L.) and peafowl ( T. patula L.) seedlings under cadmium (Cd) stress were studied.

  Method  In a hydroponic test, the seedlings were treated with a Cd concentration of 0, 0.3, 3, 20, 60, 120, 180 or 240 mg·L⁻¹ in the medium. Soluble protein, malondialdehyde, peroxidase, superoxide dismutase, and Cd of the plants were monitored.

  Result  There were apparent effects on the physiological regulatory mechanisms and metal-ion accumulation in the seedlings by the imposed Cd stress at varying concentrations. On marigold, increasing Cd caused the soluble protein in the seedlings to decline constantly, the malondialdehyde to decrease initially but raise then decrease again, the peroxidase activity to gradually increase, and the superoxide dismutase to decrease at first then increase followed by another decline. On peafowl, the increased stress induced a continuous decline on soluble protein, a down-up-down-and-up trend on malondialdehyde, a rise-then-fall on peroxidase, and a roller coaster up-down-up-and-down effect on superoxide dismutase in the seedlings. Cd stress directly raised the metal-uptakes in marigold as well as peafowl. At 180 mg Cd·L⁻¹, the marigold seedlings attained a Cd content of 440.40 mg·kg⁻¹ and the peafowl 478.23 mg·kg⁻¹.

  Conclusion  At medium and low Cd concentrations (e.g., 0.3−120 mg·L⁻¹), little physiological impact was evident on either marigold or peafowl, as the plants could apparently regulate to tolerate the stress. However, under a high Cd stress (e.g., 180 or 240 mg·L⁻¹), significant harms resulted on the plants. Cd would continue to accumulate in the seedlings till the heavy metal concentration in the medium reached the level when the seeds simply failed to germinate. For marigold, peroxidase played a leading role in the Cd stress resistance, whereas, superoxide dismutase did for peafowl. It appeared that Tagetes seeds could be useful for studying phytoremediation.