Studies On The Response Of Antioxidant System Of Hg～(2+)-tolerant Rice Mutant To Hg～(2+) Stress

In this paper, the difference between wild type (Oryza sativa L. subsp. Japonica, cv. Zhonghua 11) and its Hg²⁺-tolerant mutant were investigated. The metabolism of reactive oxygen species, proline, AsA-GSH cycle and activities of antioxidant enzymes under Hg²⁺ stress were studied. The results showed as follows:The research showed that wild type and the mutant exhibited obvious phenotypic difference when exposed to Hg²⁺ stress. Hg concentration in roots of the mutant was higher than that of the wild type, while Hg concentration in leaves and shoots of the wild type were higher than that of the mutant. Hg²⁺ stress decreased the panicle number, grain setting rate, thousand-seed weight and yield per plant of rice. With increasing Hg²⁺ level, the chlorophyll content and photosynthesis rate decreased more rapidly in wild type. The growth and yield of wild type were more inhibited by Hg²⁺ compared with the mutant.At low concentration, Hg²⁺ could stimulate the generation rate of O₂^- and increased the H₂O₂ content of rice leaf. These reactive oxygen species induced the response of antioxidant system. With increasing Hg²⁺ level, the efficiency of AsA-GSH cycle and antioxidant enzymes decreased. The antioxidant system could not eliminate excess reactive oxygen species, thus malondialdehyde content increased, and integrality of cell membrane was destroyed. Under Hg²⁺ stress, the AsA-GSH cycle and antioxidative enzymes were less inhibited in mutant, thus the mutant the reactive oxygen species scavenging activity, which is advantageous to the reduction of Hg²⁺ injury.Proline accumulation was in positively correlation with Hg²⁺ treatment time, which suggested a protective role of this amino acid against Hg²⁺ toxicity. Proline could reduce reactive oxygen species content by sequestering O₂^- or scavenging H₂O₂, thus inhibit Hg²⁺-induced lipid peroxidation. Proline minimized the damage caused by reactive oxygen species and maintained plasmalemmal permselectivity, so that less Hg²⁺ was absorbed by cell. These reactions result in a more reducing cellular environment (higher GSH/GSSG value). Higher GSH level in turn facilitate phytochelatin synthesis and sequestration of Hg(2+)-phytochelatin conjugates into the vacuole, thus reduced the injury of Hg(2+).