Mechanism Of Fulvic Acid Regulation Of Photosynthetic Capacity And Drought Resistance And Yield Increase In Oat Under Drought Stress

Oat is a dominant characteristic crop in Inner Mongolia,with the planting area and total yield ranking first in the country.It is mainly cultivated in dry land,and drought and water scarcity are the main factors limiting oat yield.Photosynthesis is one of the most sensitive physiological processes in crop response to drought stress,and its ability is crucial for the formation of crop grain yield.Therefore,how to increase yield by improving the photosynthetic capacity of oat has become an urgent problem to be solved in oat production.Our team’s previous research results showed that humic acid substances have a significant effect on crop drought resistance and yield increase,but the regulatory mechanism of spraying humic acid on oat photosynthetic capacity under drought stress is still unclear.Therefore,this study focuses on the issue of drought affecting crop photosynthetic capacity and resulting in reduced yield.Water sensitive variety’Bayou 9’and drought resistant variety’Yanke 2’were used as test materials,and a combination of pot and field experiments was conducted to study the effects of spraying fulvic acid on oat drought resistance,leaf growth,photosynthetic capacity,and yield at different leaf positions under drought stress.Combined with transcriptome and metabolome analysis,Identify the key differential genes and main metabolic pathways involved in the regulation of drought resistance and yield increase in oat by fulvic acid under drought stress,clarify the physiological reasons for the differences in photosynthetic potential of oat leaves at different leaf positions affected by fulvic acid under drought conditions,reveal the key physiological pathways and drought resistance and yield increase mechanisms of fulvic acid regulation in oat under drought conditions,and provide scientific basis for improving drought resistance and yield increase in oat.1.Under drought stress,spraying fulvic acid enhances the antioxidant defense ability of oat leaves,reduces the degree of membrane lipid peroxidation,and is beneficial for improving the drought resistance of oat.Under drought stress,spraying fulvic acid significantly increased the activities of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,phenylalanine ammonia lyase（PAL）,glutathione peroxidase（GPX）,ascorbic acid peroxidase（APX）,glutathione reductase（GR）,and dehydroascorbic acid reductase（DHAR）in oat leaves,and increased the activities of reducing ascorbic acid（As A）Reducing glutathione（GSH）content,reducing oxidative ascorbic acid（DHA）,maintaining high As A/DHA,GSH/GSSG ratios,promoting As A-GSH cycling,thereby improving the plant’s ability to clear ROS,reducing the accumulation of O₂^·-and H₂O₂content,reducing membrane lipid peroxidation,MDA content and LOX activity,increasing the relative water content of oat leaves,and enhancing the drought resistance of oat plants.2.Spraying fulvic acid effectively alleviated the inhibition of drought stress on the photosynthesis of oat leaves,increased the content of photosynthetic pigments in the leaves,promoted the growth and development of oats,and increased their yield.Under drought stress,spraying fulvic acid significantly increased the net photosynthetic rate（P_n）,transpiration rate（T_r）,stomatal conductance（g_s）of flag and second leaf of oat,decreased the intercellular CO₂ concentration（C_i）,significantly increased the content of chlorophyll a（Chl a）and carotenoids（Car）,promoted the accumulation of dry matter in oat leaves,expanded leaf area,and increased oat grain yield,with an increase of 21.87%in drought resistant varieties,The increase in drought sensitive varieties was 29.80%.3.Fulvic acid can alleviate the degree of photoinhibition of PSII under drought stress,protect the activity of PSII reaction centers and the stability of electron transfer chains,thereby improving the photosynthetic performance of oats.Under drought stress,spraying fulvic acid significantly increased the maximum fluorescence（F_m）,capture excitation energy efficiency（F_v’/F_m’）,maximum light energy conversion efficiency（F_v/F_m）,photochemical quenching coefficient（q P）,electron transfer rate（ETR）,and actual light energy conversion efficiency of flag and second leaf leaves(Φ_PSII)significantly increased the absorption of light energy per unit area（ABS/CS_m）and the captured light energy（TR_o/CS_m）,as well as the reduction energy（ET_o/CS_m）in the electron transfer chain of QA^-,the electron transfer ability per unit reaction center（ET_o/RC）,and the quantum yield of electron transfer in the electron transfer body（S_m）on the PSII receptor side(φ_Eo)reduces the maximum rate at which QA is reduced（M_o）,reduces the thermal dissipation per unit area（DI_o/CS_m）and the energy dissipated（DI_o/RC）,increases the reduction frequency of QA（N）,and increases the probability of electron transfer to electron acceptors downstream of the electron transfer chain（Ψ_o）Thus,the photosynthetic performance index(PI_abs)and comprehensive performance parameters(PI_total)were improved.Among them,the PIabs and PItotal increases of Yanke 2 flag leaf and inverted 2 leaf were 18.69%,21.29%and15.68%,9.87%,respectively.The PI_abs and PI_total increases of Ba You 9 flag leaf and inverted 2 leaf were 23.12%,62.88%and 19.77%,67.15%,respectively4.Under drought stress,spraying fulvic acid effectively alleviates the inhibition of carbon assimilation process by drought stress,induces the metabolic ability of carbohydrates in oat leaves,is conducive to the smooth progress of photosynthesis,and enhances the drought resistance of oats.Under drought stress,spraying fulvic acid significantly increased the activities of aldolase（FBA）,ribose 1,5-diphosphate carboxylase（Rubisco）,Rubisco activator（RCA）,1,7-diphosphate sesquidrogenase（SBPase）,sucrose synthase（SS）,sucrose phosphate synthase（SPS）,acid invertase（AI）,and neutral invertase（NI）in flag leaves of oat,as well as sucrose,glucose,and fructose content.5.Under drought stress,fulvic acid induces differential gene expression in oat leaves.Through transcriptomic analysis of oat flag leaves under drought stress regulated by spraying fulvic acid,it was found that compared to spraying water,sensitive varieties identified 652 DEGs（475 upregulated and 177 downregulated）,mainly involving phenylpropanoid biosynthesis,glutathione metabolism,MAPK signaling pathway-plant,amino sugar and nucleotide sugar metabolism.Drought resistant varieties identified 390DEGs（190 upregulated and 200 downregulated）,It mainly involves pathways such as phenylpropanoid biosynthesis,MAPK signaling pathways in plants,and plant hormone signal transduction.GO functional enrichment analysis found that most of the DEGs in both varieties were annotated as"cellular components",with the most significant common entry being the extracellular domain,while the most significant entry in drought resistant varieties was the extracellular body,and the most significant entry in sensitive varieties was the nucleus.6.Under drought stress,fulvic acid induces the accumulation of differential metabolites in oat leaves.By analyzing the non targeted metabolomics of oat under drought stress regulated by spraying fulvic acid,it was found that compared with spraying water,a total of 571 metabolites were detected in sensitive varieties with significant differences（329 upregulated and 242 downregulated）,mainly involving pathways such as arginine biosynthesis,aminoacyl t RNA biosynthesis,acetaldehyde and dicarboxylate metabolism,and pyrimidine metabolism,Drought resistant varieties detected significant differences in 316 metabolites（123 upregulated and 193 downregulated）,mainly involving sphingolipids metabolism,linoleic acid metabolism,cyanamide metabolism,and lysine degradation pathways.7.Differential expression genes and differential metabolite KEGG enrichment analysis showed that under drought stress,spraying fulvic acid induced the expression of genes related to pentose phosphate metabolism,amino acid metabolism,photosynthetic carbon fixation pathway,phenylpropanoid biosynthesis pathway,and significant accumulation of metabolites in oat leaves,thereby enhancing the drought resistance of oat.In summary,under drought stress,fulvic acid can promote the synthesis of photosynthetic pigments in oat leaves,maintain the stability of PSII reaction centers,promote electron transfer rate,increase PSII photochemical activity,accelerate the process of photosynthesis carbon assimilation,regulate the feedback inhibition of sucrose production,and improve the ability to clear ROS produced during photosynthesis,thereby slowing down the damage and photoinhibition of photosynthetic reaction centers caused by drought stress,Maintaining the photosynthesis process of oat under drought stress promotes its growth,development,and yield increase.Under field experimental conditions,spraying fulvic acid is beneficial for improving the photosynthesis and yield of dryland oat plants,and has a good effect on drought sensitive variety Bayou 9.8.Under field experimental conditions,spraying fulvic acid can increase the photosynthetic rate and chlorophyll content of dryland oat,promote dry matter accumulation,and increase yield.Spraying fulvic acid for two years significantly increased the photosynthetic rate,stomatal conductance,transpiration rate,and SPAD value of oat leaves during the jointing,heading,and filling stages,reduced intercellular CO₂concentration,promoted dry matter accumulation,increased yield,and increased economic benefits of oat production.Among them,drought resistant varieties increased yield by an average of 10.76%and an increase of 771.93 yuan hm^-2 in two years,while drought sensitive varieties increased yield by an average of 13.58%and an increase of 936.08 yuan hm^-2 in two years.In summary,under drought stress,fulvic acid can promote the synthesis of photosynthetic pigments in oat leaves,alleviate the damage to PSII reaction centers and photoinhibition caused by drought stress,accelerate the process of photosynthesis carbon assimilation,regulate carbohydrate metabolism ability,enhance the ability to eliminate ROS produced during photosynthesis,maintain the photosynthesis process of oat under drought stress,and promote the growth,development,and yield increase of oat.Under field experimental conditions,spraying fulvic acid is beneficial for improving the photosynthesis and yield of dryland oat plants,and has a good effect on the water sensitive variety Bayou 9.