Effects Of Elevated CO₂ Concentration On Rice Plant Growth And Rice Nutritional Quality Under Soil Cadmium Pollution

At present,the global climate is changing and the atmospheric CO₂concentration is gradually increasing.The increase of atmospheric CO₂concentration has a great impact on ecological environment and agricultural production.The problem of heavy metals in soil is becoming more and more serious,which threatens our ecosystem and human health.This study took the two environmental problems of the coexistence of atmospheric CO₂concentration and heavy metal pollution as the breakthrough point,and choose on two rice varieties（Xiangwanxian No.13 and Shenyou No.957）as the research object,using open top chamber（OTC）,setting up two CO₂concentration（400 ppm,600 ppm）,three cadmium pollution levels（0,2mg/kg,10mg/kg）on rice growth,photosynthetic characteristics and quality safety study,the results are as follows:（1）In this experiment,under different concentrations of Cd contamination,the plants grew normally,the leaves were of normal color,and no toxic symptoms were observed.Under normal CO₂concentration（400ppm）,Cd treatment at different concentrations had no effect on root biomass,stem and leaf biomass and yield of Xiangwanxian No.13;Cd treatment had no significant effect on the stem and leaf biomass and yield of Shenyou No.957,but Cd pollution（Cd2,Cd10）inhibited the root biomass,which decreased by 24.1%and 29.0%.The increase of CO₂concentration（600ppm）increased the stem and leaf dry weight of Xianwanxian No.13 and Shenyou No.957,but had little effect on the biomass of the roots and yield of these two varieties of rice.With the increase of CO₂concentration,the proportion of increase of stem and leaf dry weight in different Cd concentrations（CK,Cd2,Cd10）was29.3%,25.6%and 24.8%;The increase of dry weight of stem and leaf of Shenyou No.957was 35.1%,19.9%and 17.5%.Two varieties of rice,Xiangwanxian No.13 and Shenyou No.957,have strong tolerance to Cd.The increasing effect of CO₂concentration on the stem and leaf biomass of the two rice varieties was the largest in the Cd control group（CK）,and the increasing effect was weakened with the increase of soil Cd concentration.（2）Cd pollution and increased CO₂concentration had no effect on chlorophyll content in leaves of two varieties of rice,Xiangwanxian No.13 and Shenyou No.957.The Cd pollution treatment had no effect on the net photosynthetic rate,intercellular CO₂concentration and water use efficiency of the leaves of Xiangwan No.13,but reduced the stomatal conductance and transpiration rate of the leaves.Cd contamination had no effect on net photosynthetic rate,intercellular CO₂concentration and water use efficiency,stomatal conductance and transpiration rate of rice leaves of Shenyou No.957.The increase of CO₂concentration increased the net photosynthetic rate of the leaves of lower Xiangwanxian No.13 under different Cd pollution concentrations（CK,Cd2,Cd10）by 41%,66.8%and86.1%,respectively,and the net photosynthetic rate of the leaves of Shenyou No.957 was28.3%,8.4%and 28.2%,respectively.The increase of CO₂concentration significantly increased the CO₂concentration and water use efficiency of the leaves of Xiangwan No.13and Shenyou No.957.The increase of CO₂concentration reduced stomatal conductance and transpiration rate of Shenyou No.957 leaves,but had no effect on stomatal conductance and transpiration rate of Xianwanxian No.13.The increase of CO₂concentration increased the net photosynthetic rate of the leaves of the two varieties of rice,which in turn increased the above-ground stem and leaf biomass.（3）The accumulation of Cd in each part of two rice varieties Xiangwanxian No.13and Shenyou 957 was consistent;its content was as follows:root>stem>leaf>rice.The content of Cd in each part of rice increased with the increase of the concentration of Cd,and the content of Cd in root increased the most.In this experiment,under different Cd concentrations,the content of Cd in all parts of Xiangwanxian No.13 rice was lower than that in all parts of Shenyou No.957.At different Cd concentrations,the Cd content in rice of Xiangwanxian No.13 was lower than the national food safety standard,while that of Shenyou No.957 rice was 0.29 mg/kg when treated with high Cd concentration（Cd10）,exceeding the national food safety standard.The effect of rising CO₂concentration on Cd content of rice varies with different varieties and parts.Under high Cd pollution（Cd10）,with the increase of CO₂concentration,the content of Cd in the stem of Xiangwanxian No.13 decreased,but the content of Cd in the rice increased,indicating that the increase of CO₂increased the safety risk of rice contamination of Xiangwanxian No.13,but it was still lower than the national standard value of food safety.Under high Cd pollution（Cd10）,the increase of CO₂concentration significantly decreased the content of Cd in stem and leaf of Shenyou No.957,but there was no significant change in the content of Cd in rice.The effect of rising CO₂concentration on the safety risk of Cd content in rice varies from rice variety to rice variety.（4）Cd pollution reduced the content of most mineral elements in rice.Cd contamination reduced the content of Fe in the roots,P、Ca、Mg、Zn content in the stems,P、Ca、Mg content in the leaf,K、Ca、Mg、Fe、Mn content in rice of Xiangwanxian No.13;reduced Ca、Mg、Fe、Mn in root,Ca、Cu、Zn in stem and Ca in leaf of Shenyou No.957.However,the content of Mg and Mn in root of Xiangwanxian No.13 increased with high Cd concentration（Cd10）.The increase of CO₂concentration reduced the content of mineral elements in rice under the pollution of Cd.The increase of CO₂concentration reduced the content of K,Cu and Zn in the root,Cu and Zn in the stem,and Ca and Mn in the rice of Xiangwanxian No.13,while the increase of CO₂concentration reduced the content of Zn in the root,Zn in the stem,Fe in the leaf and Mn in the rice of Shenyou No.957.The effects of Cd pollution and rising CO₂concentration on the content of mineral elements in rice vary with the types of mineral elements,rice varieties and different organs of rice.Cd pollution had different effects on the protein content of the two varieties of rice.Cd pollution had no effect on the protein content of the rice of Xiangwanxian No.13,while Cd pollution reduced the protein content of the rice of Shenyou No.957.The increase of CO₂concentration had no effect on the protein content of the two rice varieties.