Abstract: Mercury (Hg) contamination in rice and its resultant health risks to human have received considerable attention. It is urgent to find rice cultivars with low accumulation of Hg. By using hydroponic experiments, the responses of root morphology, activity, physiological characteristics of the three rice cultivars (black rice, red rice, and hybrid rice) were investigated under different levels of Hg stress (0, 0.5, 1.0 and 2.5 mg·L^-1). The accumulation and transportation of Hg within plants were studied as well. Our results showed that Hg concentration higher than 0.5 mg·L^-1 could significantly inhibit root growth, the longest root length and root volume of hybrid rice by 36.8%, 12.6% and 28.6%, respectively. Compared with the control, 0.5 mg·L^-1 Hg stress increased root activity of red rice and hybrid rice by 28.7% and 6.6%, respectively, while 1.0 and 2.5 mg·L^-1 Hg stress could decrease root activity of the three cultivars. Malondialdehyde (MDA) concentration in the three cultivars increased under Hg stress. With the increases of Hg concentration, the activities of peroxidase (POD) and catalase (CAT) in black rice and red rice increased first and then decreased, and reached the maximum value at 0.5 mg·L^-1. The CAT activity of hybrid rice reached the maximum value at 1 mg·L^-1 Hg concentration, which was 69.5% higher than that of control. The highest concentration of total Hg was observed in black rice roots under the 0.5 and 1.0 mg·L^-1 of Hg stress, and in the red rice root when exposed to 2.5 mg·L^-1 Hg stress. The translocation factors of Hg in red rice and black rice were lower than that of hybrid rice. Our results provide a theoretical basis for better understanding the resistance of colored rice to Hg toxicity and for selecting rice cultivars with low Hg accumulation.

Key words: color rice, mercury stress, root activity, antioxidant enzyme, malondialdehyde