Effect Of Rhizosphere CO₂ On Starch And Soluble Sugar Accumulation Of Melon Seedling Root

In modern agricultural production,soil CO₂ concentration is often too high due to improper cultivation and management.Rhizosphere CO₂（RZ CO₂）directly affects root growth and physiological function,thereby affecting the overall growth of plants.Melon,as a favorite and rhizosphere gas sensitive crop,is often affected by high concentrations of CO₂ in soil during its growth.In this study,the rhizosphere CO₂ automatic control system was used to ventilate the roots of melon’Yumeiren’seedlings under the condition of aerosol culture with0.2%CO₂ concentration in normal soil as control.Through the analysis of root morphology index,starch and sugar content of melon seedlings under 10 rhizosphere CO₂ concentrations,it was clear that the sixth day of treatment was the key turning point,and the transcriptomics analysis of root cell whole gene was carried out.The main results were as follows:1.Compared with CK（0.2%）,the total root length,total surface area,total volume,root tip number and root activity of melon seedlings under 0.3%～1.2%rhizosphere CO₂treatment for 0～6 days were promoted to varying degrees.In the actual production,the supplement of rhizosphere CO₂ concentration in the range of 0.7%～1.2%was lower than that of 6 days（3 days were appropriate）or the concentration was 0.3%～0.6%,and the supplement of 6～9 days was beneficial to the root morphology of melon seedlings,but it was significantly reduced after 9 days of treatment,and increased significantly with the increase of treatment concentration.2.The high concentration of CO₂ can accelerate the accumulation and release of starch granules in root tip cells.The release time of starch granules in root tip cells was different with different concentrations of CO₂ treatment.1.1%CO₂ concentration began to release in 3days,0.4%CO₂ concentration began to release in 12 days.The higher the concentration,the faster the accumulation and release of starch granules.3.Compared with the control,0.3%～1.2%CO₂ concentration in rhizosphere could significantly increase starch content in 0～9 days and total soluble sugar content in 0～6 days.Then,with the extension of treatment time and the increase of CO₂ concentration,starch content and total soluble sugar content gradually decreased.With the increase of treatment concentration,starch content and total soluble sugar content in roots increased first and then decreased compared with the control.This change trend was the same as the root phenotype.4.Compared with the control,the activities of sucrose-decomposing enzymes in root cells of melon seedlings treated with 0.4%and 1.1%CO₂ for 3 days were significantly increased,and the activities of sucrose phosphate synthase（SPS）were significantly decreased after 3 days of treatment.The activities of trehalose hexaphosphate synthase（TPS）and trehalose synthase（TS）related to trehalose synthesis increased significantly from 0 to 6days,and decreased significantly after 6 days,which was consistent with the change trend of trehalose content.The activities of enzymes related to starch hydrolysis were significantly higher than those of the control after 9 days of treatment,resulting in the gradual decrease of starch content,and the significance was positively correlated with the treatment concentration.5.The transcriptome results of 0.4%and 1.1%rhizosphere CO₂ treatment on the 6th day showed that T1/CK involved 82 KEGG metabolic pathways,and the genes in T2/CK were involved in 107 metabolic pathways,and 170 genes in T2/T1 were annotated to 74KEGG metabolic pathways.The KEGG pathway with the highest DEG enrichment was found in’starch and sucrose metabolism’,’plant hormone signal transduction’and’phenylpropanoid biosynthesis’.At the same time,the cell wall and plasma membrane metabolism in GO cluster analysis were significantly enriched,indicating that high CO₂ stress in the rhizosphere affects the carbohydrate metabolism process of melon at the gene level to stimulate the defense stress response of seedlings,accelerate root cell aging and the hydrolysis of plasma membrane around the root tip,and then cause the change of starch and sugar content in the root system.6.The expression levels of sugar metabolism-related genes and sucrose non-enzymatic protein kinase 1 gene Cm Sn FK1 in the roots of melon seedlings treated with CO₂concentration above 0.4%for 3～6 days were significantly increased.Except genes related to sucrose decomposition,they were significantly inhibited after 9 days of treatment,which was consistent with the change trend of enzyme activities related to sugar metabolism in roots.This provides a basis for the study of sugar signaling defense mechanism under high CO₂stress in melon rhizosphere.