Effects Of Elevated Atmospheric CO₂ Concentration On Physiological Characteristics And Metabolomics Of Stems And Leaves In Foxtail Millet

It has become an indisputable fact that the global CO2 concentration rises,which has a significant impact on agricultural activities.Previous studies focused on the response of C3 crops to elevated CO2 concentration,but the response mechanism of C4 crops to elevated CO2 concentration are unclear.In this experiment,the C4 crop foxtail millet(Yugu 1)was used as experimental material to study the effects of elevated CO2 concentration(600 μmol·mol-1)on physiological characteristics,gene expression and metabolomics of stems and leaves in foxtail millet,and to explore the response mechanism of foxtail millet to elevated CO2 concentration.The main test results are as follows:1.The increase of atmospheric CO2 concentration significantly increased photosynthetic pigment content and photosynthetic capacity in millet leaves,but had no significant effect on stomatal density and stomatal conductance.2.With the increase of CO2 concentration,the contents of total lignin and hemicellulose in millet stem was increased by 14.57% and 57.89%,and the contents of guaiacyl lignin(G),syringyl lignin(S)and p-hydroxyphenyl lignin(H)in millet stem was significantly increased.The contents of xylose,glucose and galactose in millet stem was significantly increased,while the contents of cellulose and pectin was decreased by 45.36% and 35.50%,respectively.3.The increase of CO2 concentration improved the antioxidant capacity and osmotic adjustment capacity of millet stems and leaves.Elevated CO2 concentration promoted the accumulation of proline,free phenol,bound phenol and total phenol in leaves and stems in millet,leading to the decrease of active oxygen and malondialdehyde.Elevated CO2 concentration increased the activity of peroxidase(POD)by11.69%,and significantly increased the expressions of related genes Si ZFP182,Si P5 CR and Si SOD1 by193.20%,148.83% and 20.71%,respectively.4.Elevated atmospheric CO2 concentration promoted carbon metabolism of millet.With the increase of CO2 concentration,the content of soluble sugar,fructose(reducing sugar)and starch in leaves and starch in stems of millet was significantly increased,while the content of soluble sugar and fructose(reducing sugar)in stem was significantly decreased.The wax content and the expression of genes related wax content both were increased by the increase of CO2 concentration.5.The increase of atmospheric CO2 concentration promoted nitrogen metabolism of millet.Elevated CO2 concentration significantly increased nitrate nitrogen content in leaves and stems,inhibited soluble protein accumulation in leaves and stems,decreased free amino acid content in leaves and increased it in stems.Elevated CO2 concentration increased nitrate reductase activity in leaves by 52.33%.6.The effect of increasing CO2 concentration on source flow in millet was less than that of single source or flow.The increase of CO2 concentration was beneficial to promote the change of metabolic pathway among different tissues of millet,and the effect on leaves was higher than that of stems.The metabolic pathways related to carbon and nitrogen metabolism were starch and sucrose metabolism,cysteine and methionine metabolism,alanine-aspartate-glutamic acid metabolism,and the substances significantly affected were L-aspartic acid,L-asparagine,L-methionine,sucrose and citric acid.7.The lipid components(long chain phosphatidylinositol and thiorhamnose diglyceride)of cell membrane in millet was changed with the increase of CO2 concentration.Elevated CO2 concentration changed the metabolic pathway of phosphonate and phosphinate,possibly changed cell structure and the cell resistance to environment.The change of glucose and lipid content might affect the signal recognition and transmission.To sum up,with the increase of CO2 concentration,foxtail millet can improve the antioxidant capacity and osmotic adjustment capacity of leaves and stems by regulating the related metabolites,metabolic pathways and genes in leaves and stems,thus promoting photosynthetic carbon assimilation and growth to adapt to the increase of atmospheric CO2 concentration.