Toxicity Of Cadmium On Roots Of Different Genotypes Of Rice And The Regulation Of Calcium

Cadmium(Cd)is highly toxic and easy to migrate in soil.Rice is a kind of Cd-enriched crop,and has genotype differences in Cd tolerance,absorption and accumulation.Rice Cd pollution has become a focus of social attention.Calcium(Ca),as an essential nutrient element of plants,participates in the regulation of plant cell metabolism and signal transduction,which can effectively alleviate the toxicity of Cd to plants.Studying the toxic effect of Cd on different genotypes of rice and the regulation of Ca,using the difference of tolerance to Cd among different genotypes of rice and the interaction between Cd and Ca,can provide theoretical basis for alleviating Cd toxicity in rice.In this study,rice cultivars with low Cd accumulation(Nipponbare,Japonica)and high Cd accumulation(9311,Indica)were used as experimental materials.The toxicity of Cd on rice roots and the tolerance of rice roots to Cd were studied by hydroponic experiments with different Cd concentration.Furthermore,under Cd stress,different Ca concentrations were set to study the mechanism of Ca regulation on Cd toxicity in rice roots.The results are as follows:(1)The toxicity of Cd to Rice Roots and the antioxidant Stress of Rice RootsCd treatment caused certain toxicity to rice roots,while the toxicity of Nipponbare's roots was weaker than that of 9311's.The root biomass,total root length,root volume,root surface area and root tip number of rice decreased with the increase of Cd treatment concentration,while the ROS content,root membrane peroxidation level and root tip plasma membrane damage increased with the increase of Cd treatment concentration.Cd treatment stimulated the antioxidant stress of rice roots.Under Cd stress,superoxide dismutase(SOD),peroxidase(POD),ascorbate peroxidase(APX),glutathione reductase(GR)and catalase(CAT)in rice roots showed a certain stress response.With the increase of Cd concentration,SOD activity in Nipponbare's roots increased continuously,while that in 9311 increased first and then decreased.POD,APX and GR activities in rice roots increased with the increase of Cd concentration,while CAT activity decreased with the increase of Cd concentration.Under Cd stress,the reduced ascorbic acid/dehydroascorbic acid(ASA/DHA)values of rice roots decreased with the increase of Cd concentration,GSH/GSSG value of Nipponbare's roots increased first and then decreased with the increase of Cd concentration,while 9311 showed a continuous downward trend.The contents of NPT,Pro and soluble protein in rice roots increased with the increase of Cd concentration.The activity of antioxidant enzymes and the content of antioxidant substances in Nipponbare's roots were higher than those in 9311's roots,showing greater antioxidant stress ability.(2)Effects of Ca on Cd Toxicity and Antioxidant System in Rice RootsCa addition could alleviate the toxicity of Cd to rice roots,while Ca deficiency treatment showed the opposite effect.Under Cd stress,root biomass,total root length,root volume,root surface area and root tip number of rice roots increased with the increase of Ca concentration.ROS content,membrane peroxidation level and root tip plasma membrane damage of rice roots decreased with the increase of Ca concentration.Under Cd stress,the addition of Ca alleviated the toxicity of Cd,and the antioxidant effect of rice roots was weakened,whereas Ca deficiency treatment was the opposite.Under Cd stress,SOD,POD,APX and GR activities of rice roots decreased with the increase of Ca concentration,while CAT activities increased with the increase of Ca concentration.The ASA/DHA and GSH/GSSG values of different genotypes of rice roots increased with the increase of Ca concentration,while the contents of NPT,Pro and soluble protein decreased with the increase of Ca concentration.(3)Effects of Ca on Root Absorption and Transport of Cd in Rice under Cd StressAdding Ca reduced the uptake of Cd by rice roots,but promoted the transportation of Cd to the shoots,and the vacuole segregation of root system was weakened,whereas Ca deficiency was the opposite.Under Cd stress,Cd content in rice roots decreased with the increase of Ca concentration,while Cd content in shoots increased with the increase of Ca concentration.The Cd transfer coefficient and the Cd content in xylem sap of rice increased with the increase of Ca concentration.Under Cd stress,the relative expression of CAL1 in Nipponbare's roots increased gradually with the increase of Ca concentration,while the relative expression of OsHMA2 had no significant difference with the increase of Ca concentration.The relative expression of CAL1 and OsHMA2 in 9311's roots increased with the increase of Ca concentration,while the relative expression of OsHMA3 and OsNramp5 in rice roots decreased with the increase of Ca concentration.In conclusion,under Cd stress,rice roots were poisoned to a certain extent,and rice roots showed a certain degree of antioxidant stress.The antioxidant stress ability of Nipponbare's roots was stronger than that of 9311's roots,so the toxicity of Nipponbare's roots was weaker than that of 9311's roots.Ca addition could reduce the uptake of Cd by rice roots,so the oxidative toxicity of roots was weakened,and the antioxidant stress of roots was weakened,but the opposite was true in the absence of Ca.