Alleviation Effects Of Exogenous Melatonin On Cadmium Toxicity In Rice

It is now widely recognized that a great deal of paddy soil has been contaminated by the heavy metal cadmium,owing to the industrial waste and misuse of chemical fertilizers in recent years.It has a direct effect on rice germination and growth,which finally threat the grain security and the health of our human beings.Melatonin,sometimes called “MT” in short,plays an important role in plant's resistance to abiotic stress.The addition of appropriate concentration of melatonin can significantly improve the tolerance of plants to abiotic stress,cold stress and heat stress,for instance.At the same time,one group of scientists has proposed that melatonin in plants showed an obvious change when it was under heavy metal stress,which implied the crucial role of endogenous melatonin in plant heavy metal stress resistance and the possible role of exogenous melatonin in relieving the heavy metal stress in plant.In this research,germination experiment and hydroponic experiment were conducted to investigate the effect of treatment with exogenous melatonin on rice germination,growth inhibition and Cd accumulation and translocation under Cd stress.And Compared the change of the malondialdehyde(MDA)content,superoxide dismutase(SOD)activity,catalase(CAT)activity and peroxidase(POD)activity under different treatment at the same time.Following results were obtained.1.Inhibition of Cd stress on rice seeding germination and growthThrough germination experiment and hydroponic experiment,we investigated the inhibition of Cd stress on rice seeding germination and growth.In the germination experiment,the result showed that 10 ?mol·L-1 Cd had an obviously inhibition on rice seeding germination and growth inhibition rate reached 50% under 100 ?mol·L-1Cd stress.In the hydroponic experiment,we found that 5 ?mol·L-1Cd had a significantly bad effect on rice seeding growth,and the inhibition rate reached 50% when the concentration of Cd was 20 ?mol·L-1.2.Effects of exogenous melatonin on seed germination of rice under Cd stress The empirical results showed that the addition of 10 ?mol·L-1exogenous melatonin markedly enhanced the germination of rice seedings under 10 ?mol·L-1Cd stress by 7.8%.Addition of exogenous melatonin clearly increased the biomass of shoots and roots under Cd stress.Compared with the same stress treatment,addition of 10 ?mol·L-1 melatonin could increase the total length of rice roots by 103.5% when the Cd stress concentration was 100 ?mol·L-1.The addition of exogenous melatonin could effectively alleviate membrane lipid peroxidation caused by Cd stress by improving the activity of antioxidant enzymes.When the Cd stress concentration is 10 ?mol·L-1,addition of 10 ?mol·L-1 melatonin could obviously enhanced the activity of antioxidant enzymes,such as POD,SOD and CAT by 21.3%,16.8% and 63.0%,which finally resulted in decreasing of MDA content by 12.8%.3.Effect of exogenous melatonin on growth,Cd accumulation and translocation in rice seedingsAddition of exogenous melatonin markedly alleviated the inhibition of rice seedings growth under Cd stress.At Cd concentration of 5 ?mol·L-1,20 ?mol·L-1 melatonin could increase the chlorophyll a and chlorophyll b content by 26.9% and 34.4%.The addition of exogenous melatonin obviously decreased the Cd-translocation efficiency and Cd content of rice seedling.When the Cd concentration was 5 ?mol·L-1,addition of 20 ?mol·L-1 exogenous melatonin could decrease the Cd-translocation efficiency and accumulation of shoots by 24.4% and 42.0%.It also decreased the Cd content in roots and shoots by 16.9% and 48.4%.The result also indicate that exogenous melatonin enhanced the proportion of NaCl extractable Cd,which reduced their ability to move to the aerial part and decreased the Cd-translocation efficiency as well.At Cd concentration of 5 ?mol·L-1,addition of 20 ?mol·L-1 exogenous MT increased the ethanol extractable Cd by 29.6%,decreased the proportion of soluble Cd by 17.8%,and increased the proportion of NaCl extractable Cd by 15.8% at the same time.To sum up,these results suggest that application of exogenous melatonin could effectively alleviate the toxic effect of Cd on rice and in some ways turn the Cd with better mobility to Cd with poor mobility,which in turn cause the decreasing of the Cd-translocation efficiency finally.