Effects Of Root-zone CO₂ Enrichment On Root Growth And Of Endogenous Hormones Content Of Oriental Melon（Cucumis Melo L.）

Respiration of plant root,activities of microorganisms and decomposition of organic matter and poor soil aeration all lead to the enrichment of CO₂in soil.Melon（Cucumis melo L.）has higher requirements for rhizosphere gas environment,and its root system is sensitive to changes in root-zone CO₂concentration.When the root-zone CO₂concentration is too high,the growth and development,yield and quality of melon will be affected.Endogenous hormones are closely related to the growth and development of plants.At present,the mechanism of the effect of root-zone CO₂enrichment on plant endogenous hormones is still unclear.In the experiment,the oriental melon’Yumeiren’was used as the research material,the root-zone CO₂automatic control system was used,and the conventional root-zone CO₂concentration（0.037%）was used as the control（CK）,the root growth,auxin（IAA）,cytokinin（CTK）,gibberellin（GA）,abscisic acid（ABA）and the ratios between hormones,as well as the main enzymes activity of endogenous hormones synthesis and catabolism,the expression characteristics of related genes in 0.5%（T1）and 1.0%（T2）root-zone CO₂enrichment conditions of oriental melon root in the’4 leaves and 1 heart’period were investigated,the main results were as follows:1.The root-zone CO₂enrichment affected the root growth,endogenous hormones content and hormones ratios of melon root,and the higher the CO₂concentration,the greater the effects.1 6 days after root-zone CO₂enrichment,root length,root activity and root morphological parameters such as total root tip numbers of melon were promoted,and were significantly higher than those of CK on the 3rd day of treatment,and the differences gradually decreased with the prolongation of treatment time.On the 9th day of treatment,the root growth indexes were significantly lower than those of CK,and the differences between T1 and T2 treatments were significant.2 6 days after root-zone CO₂enrichment,the content of IAA,CTK,GA and ABA was higher than that of CK,and the hormones content of T2 treatment was higher than that of T1treatment,the ratios of IAA/ABA,CTK/ABA,GA/ABA,and（IAA+CTK+GA）/ABA were increased.With the prolongation of treatment time,the content of IAA,CTK and GA gradually decreased,while the ABA content increased gradually and decreased slightly on the15th day,and the ratios between hormones decreased.On the 9th day of treatment,the IAA content in T1 and T2 treatments was significantly lower than that of CK,the CTK content in T1 treatment was not significantly different from that of CK,the CTK content in T2 treatment was significantly lower than that of CK,while the ABA content in T1 and T2 treatments was significantly higher than that of CK.It indicated that short-term（within 6 d）root-zone CO₂enrichment promoted the accumulation of endogenous hormones of melon root,which was beneficial to the root growth.With the prolongation of treatment time,the content of growth-promoting hormone decreased,the growth and morphogenesis of root were inhibited.2.The q RT-PCR of the differential genes and the analysis enzymes activity of relating to the hormones metabolism screened by transcriptome sequencing showed that root-zone CO₂enrichment regulated the synthesis and catabolism of hormones at the transcriptional level,and affectd the hormones content of melon root.1 Transcriptome sequencing analysis of melon root tips on the 9th day of CO₂enrichment showed that 1280 and 1042 genes reached significant levels in T1/CK and T2/CK,respectively,of which 1067 and 830 DEGs were up-regulated,213 and 212 DEGs were down-regulated.The numbers of DEGs annotated in the KEGG database for T1/CK and T2/CK were 335 and 303,respectively,involving a total of 91 metabolic pathways,among them,the number of up-regulated DEGs was the largest in the"phenylpropane biosynthesis"pathway,and the number of down-regulated DEGs was the largest in the"carbon metabolism","amino acid biosynthesis"and"plant hormone signal transduction"pathways,respectively.In the KEGG metabolic pathway,IAA,CTK,GA and ABA metabolism were included in tryptophan metabolism,zeatin biosynthesis,diterpenoid biosynthesis and carotenoid biosynthesis,respectively,and screened out the DEGs related to the metabolism of four hormones,there were 6,2,4 and 2.2 6 days after root-zone CO₂enrichment,IAA,CTK and GA synthesis ability was stronger than the catabolism ability,resulting in an increase in IAA,CTK and GA content in root.IAA synthesis-related enzymes activity such as tryptophan transaminase（TAA）and flavin monooxygenase（FMO）was enhanced,and the expression of synthesis-related differential genes such as Cm TAA1 and Cm YUC1 was up-regulated,the activity of indoleacetic acid oxidase（IAAO）of IAA catabolism-related enzyme was significantly decreased on the 3rd day of treatment,resulting in an increase in IAA content.The activity of isopentenyl transferase（IPT）of CTK synthesis-related enzyme was enhanced,and the expression of Cm IPT3 was up-regulated,the activity of cytokinin oxidase（CKX）of the catabolism-related enzyme was decreased,and the expression of Cm CKX was down-regulated,resulting in an increase in CTK content compared with CK.GA synthesis-related enzyme activity such as endo-kaurene oxidase（KO）was enhanced,the expression of Cm KO,Cm KAO2 and Cm GA3ox1 was all up-regulated,the activity of GA2 oxidase（GA2ox）of catabolism-related enzyme was decreased,and the expression of Cm GA2ox1 was down-regulated,resulting in an increase in GA content.3 Under the long-term（after 9 days）root-zone CO₂enrichment,the activity of IAA,CTK and GA synthesis-related enzyme gradually decreased,the expression of synthesis-related gene was down-regulated,and the catabolism-related enzyme was gradually increased,and the expression of corresponding genes was up-regulated.The synthesis ability of hormones was less than the catabolism ability,which led to the decrease in IAA,CTK and GA content in root compared with CK.4 12 days after root-zone CO₂enrichment,the activity of aldehyde oxidase（AO）of ABA synthesis-related enzyme was enhanced,the expression of Cm AO1 and Cm AO2 was up-regulated,and the activity of ABA 8’-hydroxylase（ABA 8’-OH）of the catabolism-related enzyme was decreased,resulting in the increase in ABA content in root.On the 15th day of treatment,the expression of ABA synthesis-related gene was down-regulated,the activity of catabolism-related enzyme was enhanced,resulting in a slight decrease in ABA content compared with CK.To sum up,within 15 days of T1 and T2 treatments,the content of IAA,CTK,GA and ABA of melon root first increased and then decreased,and the synthesis and catabolism of four endogenous hormones were affected to varying degrees at the transcriptional level,which in turn affected root growth,and the higher the CO₂concentration,the earlier the effects,the greater the effects on hormones content and root growth.6 days after root-zone CO₂enrichment,the synthesis ability of hormones was greater than the catabolism ability,and root growth was promoted.With the prolongation of enrichment time（after 9 days）,the synthesis ability of IAA,CTK and GA gradually decreased,and the catabolism ability increased,resulting in the hormone content lower than that of CK,and the root growth was inhibited.