Effects Of Drought Stress On Growth And Grain Traits Of Soybeans

The source-sink relationship of soybean will change with the progress of growth and development.Drought stress during vegetative growth and reproductive growth period can seriously affect the growth of soybean by regulating the source-sink relationship,and eventually lead to the decline of yield and quality.Therefore,it is of great significance for soybean production management and yield improvement to study the effect of drought stress in different growth periods on soybean source-sink relationship and its regulation mechanism.In this study,three soybean cultivars with similar growth period(Shennong 17,Shennong 8,and Shennong 12)were used for pot experiment,and the soybean plants were treated with drought at seedling stage and reproductive growth stage,respectively.This study was used to evaluate effects of drought stress on photosynthetic physiology,sucrose distribution and metabolism,nitrogen accumulation and metabolism,antioxidant capacity,dry matter accumulation and distribution,and yield formation of soybean during different growth periods,to analyze soybean grain transcriptome under drought stress,and to elucidate the soybean molecular and physiological mechanism in response to drought stress.1.Under drought stress,the reduction in shoot biomass was more pronounced than the reduction of biomass in the root of two soybean cultivars,resulting in higher root/shoot(R/S)ratio.Drought stress can increase the contents of soluble sugar and sucrose in leaves and decrease the content of starch.However,soluble sugar,sucrose and starch contents were significantly increased in the root system.Drought stress can significantly increase the activities of enzymes related to sucrose metabolism in soybean leaves and roots,including sucrose phosphate synthase(SPS),sucrose synthase(Su Sy),?-amylase and ?-amylase,and up-regulate the expression level of genes related to sugar metabolism enzyme activity.Furthermore,the expression levels of sucrose transporter genes(Gm SUC2,Gm SWEET6,and Gm SWEET15)in leaves and roots of soybean seedlings were up-regulated(by 2.92-10.79 fold)under drought stress.Drought stress induced the increase of Sn RK1 and HXK gene expression level of soybean sugar signal sensor.In conclusion,our results highlighted that the increase in R/S ratio caused by the changes of sugar allocation,metabolism,and transport under drought stress contributed towards drought resistance of soybean.2.The net photosynthetic rate,stomatal conductance,transpiration rate and chlorophyll content of leaves of the three soybean cultivars decreased significantly under drought stress during the reproductive growth period.The antioxidant enzymes activities of leaves changed significantly with the growth period.During the early stage of drought stress treatment(15-30 days after drought stress),the activities of superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT)in leaves increased compared with control.However,under long-term drought stress(45 days after flowering),drought stress significantly increased the activities of POD and CAT in leaves but decreased SOD activity when compared with control.Malondialdehyde content in soybean leaves significantly increased by 22.25 %-90.11 % under drought stress during the reproductive growth period.These results suggested that drought stress during reproductive growth period resulted in limited photosynthetic capacity of leaves,weakened reactive oxygen scavenging capacity,and cell components and basic metabolism damage.3.The results showed that drought stress significantly reduced the biomass accumulation in soybean shoot.Drought stress had no significantly affect on grain weight at the early stage of grain development(15 days after drought stress),but significantly reduced grain weight at the late growth stage(30-45 days after drought stress).The results showed that drought stress significantly reduced the biomass of soybean stems,roots and seeds,and reduced the distribution of biomass to soybean seeds.4.The results of nitrogen distribution and metabolism in soybean leaves showed that drought stress resulted in the decrease of total nitrogen content but the increase of nitrate nitrogen content.Under drought stress,the activities of key enzymes of nitrate reductase(NR),nitrite reductase(Ni R),glutamine synthase(GS),and glutamate synthase(GOGAT)in leaves significantly decreased,and the expression levels of enzymes related to nitrogen metabolism(Gm NR,Gm Ni R,Gm GS,and Gm GOGAT)were significantly down-regulated.The contents of free amino acid,proline and soluble sugar increased significantly under drought stress.Correlation analysis showed that there was a significant correlation between soybean grain weight and nitrogen metabolism parameters(P < 0.05).These results indicated that soybean plants respond to drought stress by regulating nitrogen metabolism and nitrogen assimilation product distribution.5.The distribution and metabolism of soybean photosynthetic products in leaves and grains were studied.The results showed that the content of starch decreased and the contents of sucrose,glucose and fructose increased under drought stress.Drought stress significantly increased the activities of key enzymes of SPS,Su Sy,acid invertase(AI)and neutral invertase(NI)in leaves,and up-regulated the expression levels of genes related to sucrose metabolism(Gm SPS1,Gm Su Sy2 and Gm A-INV)in leaves.In the early seed development stages(15 day after drought stress),drought stress increased the sucrose content in soybean grain,enhanced the activities of enzymes related to sucrose metabolism,up-regulated expression levels of sugar metabolism related genes and sucrose transporters genes(Gm SUC2,Gm SWEET12,and Gm SWEET21),while the starch,glucose and fructose contents were not significantly affected.During the late stage of seed development(30-45 days after drought stress),drought stress increased sucrose content in seeds,while decreased starch,glucose and fructose contents,and decreased activities of key enzymes in sucrose metabolism,the expression levels of genes related to sucrose metabolism and sucrose transport.Drought stress disrupted the balance of sucrose metabolism and transport in soybean leaves and seeds during the reproductive growth stage,leading to the decrease of soybean seed weight.6.Transcriptome analysis of soybean seeds showed that there were 8416 differentially expressed genes under drought stress treatment.GO and KEGG enrichment analysis suggested that under drought stress,early developing soybean seeds promoted cell growth might by regulating DNA replication,cell division,and cell wall synthesis,activated mismatch repair and base-removal repair systems to correct the mismatch resulting from DNA replication,and induced multiple defense mechanisms to clear oxidative damage caused by drought stress.