The Combined Effects Of Elevated Atmospheric CO₂ And Cadmium Exposure On Flavonoids In The Rhizosphere Soil Of Robinia Pseudoacacia L. Seedlings

In recent years,elevated atmospheric CO₂ in global and the resulting greenhouse effect have received extensive attention.At the same time,with the rapid development of industrialization and urbanization,soil heavy metal pollution is increasingly widespread.Environmental changes such as elevated CO₂ and soil cadmium（Cd）pollution may affect the secretion characteristics of roots by affecting the production of flavonoids in plants,thereby affecting the accumulation of flavonoids in rhizosphere soils.Flavonoids in the rhizosphere soils are important for formation and stability of organic and organic-inorganic composite colloids and activation and transformation of heavy metals in soils,have allelopathic effect on microorganisms in the rhizosphere,and impact the function of rhizosphere microecosystem,etc..Under the background of coexistence of elevated CO₂ and soil Cd pollution,the effects of the two coupling on the plant flavonoids have made some progress,but the researchs on the effects of flavonoids in rhizosphere soils are still rare.Here,we investigated the combined effects of elevated CO₂ and Cd-contaminated soils on the source and accumulation of flavonoids in the rhizosphere soil of Robinia pseudoacacia L.seedlings.The conclusions are as follows:1.Under the condition of elevated CO₂,the content of C and N in the stems decreased compared to the control and reached significant（p<0.05）levels at 135 and 45 days,respectively.And elevated CO₂ significantly（p<0.01）promoted the accumulation of stem biomass at 90 and 135 days of seedling growth.Under Cd exposure alone,the changes of C and N contents and the C/N ratio in the stems had no obvious regularity,and the Cd content in the stems decreased gradually with the increase of seedling growth time under the same Cd treatment.Under elevated CO₂ combined with Cd exposure,elevated CO₂ significantly（p<0.05）decreased the N content in the stems under Cd exposure,and the biomass and C/N ratio of stem significantly（p<0.05）increased compared to Cd exposure alone.In addition,elevated CO₂ significantly（p<0.01）increased the accumulation of Cd in the stems under Cd2 exposure at 45 days,while the accumulation of Cd decreased significantly（p<0.01）at90 and 135 days.2.Elevated CO₂ promoted the accumulation of total N in the rhizosphere soils,and the total C content only increased significantly（p<0.01）at 90 days and decreased significantly compared to the control（p<0.05）at 45 and 135 days.And the C/N ratio decreased and reached a significant（p<0.05）level at 135 days.Under Cd exposure alone,the accumulation of total C and N in the rhizosphere soils increased compared to the control at 45 and 90 days,and increased with the increase of soil Cd concentration.Under elevated CO₂ combined with Cd exposure,with the extension of seedling growth time,elevated CO₂ promoted the accumulation of total C and total N in the rhizosphere soils under Cd exposure at 90 and 135days,but the effect on total Cd content in the rhizosphere soils was not significant.3.Elevated CO₂ had a significant（p<0.01）effect on total flavonoids and quercetin in the roots,but the effect on total flavonoids in the stems was not significant.At 45 days of seedling growth,elevated CO₂ significantly（p<0.01）promoted the accumulation of quercetin in the roots,while the total flavonoids content in the roots decreased significantly（p<0.01）compared to the control.At 90 days of seedling growth,the response of total flavonoids and quercetin in the roots to elevated CO₂ increased consistently,but decreased consistently at 135 days.Under Cd exposure alone,the content of total flavonoids in stems showed an overall increasing trend with the increase of soil Cd concentration at 45 and 90days,but decreased with the increase of Cd concentration at 135 days.In general,Cd pollution promoted the accumulation of total flavonoids and quercetin in roots,except that the total flavonoids in the roots decreased significantly（p<0.01）compared to the control at 45days.Under elevated CO₂ combined with Cd exposure,elevated CO₂ significantly（p<0.01）increased the accumulation of total flavonoids in the roots under Cd exposure at 45 days,while the accumulation of quercetin always decreased significantly（p<0.01）compared to the Cd treatment alone.In addition,the response of total flavonoids in the stems to elevated CO₂under Cd1 and Cd2 treatments at 45 days was opposite,while it showed a consistent increase at 135 days.The Pearson correlation analysis indicated that the accumulation of total flavonoids in the stems was significant positively correlated（p<0.05）with the C/N ratio and Cd content in stems,while it was significant negatively correlated（p<0.01）with N content in the stems and total flavonoids in the leaves.The accumulation of total flavonoids and quercetin in the roots was significant positively correlated（p<0.01）with C and Cd contents in the roots and significant negatively correlated（p<0.05）with the p H of rhizosphere soils.In addition,the accumulation of quercetin in the roots was also significant positively correlated（p<0.05）with quercetin in the leaves.4.Elevated CO₂ significantly（p<0.01）improved quercetin accumulation in the rhizosphere soils,but the stimulation of CO₂ on total flavonoids accumulation reduced with time.Under Cd exposure alone,the accumulation of total flavonoids and quercetin in the rhizosphere soils increased relative to the control at 45 and 90 days,and it increased with increasing Cd levels,but decreased at 135 days with increasing Cd levels.Under elevated CO₂ combined with Cd exposure,elevated CO₂ significantly（p<0.01）decreased the accumulation of total flavonoids and quercetin under Cd2 exposure at 45 and 90 days,but the accumulation increased significantly（p<0.01）at 135 days.The Pearson correlation analysis indicated that the accumulation of total flavonoids and quercetin in the rhizosphere soils was significant positively correlated（p<0.05）with total flavonoids in the stems and the C、C/N ratio and Cd content in the roots and significant negatively correlated（p<0.01）with N content in the roots.In addition,quercetin content was significant positively correlated（p<0.05）with quercetin in the roots.