Physiological Mechanisms Of Sorghum Responses To Rhizospheric Non-unifrom Salinity Distribution

Soil salinity is a key factor influencing productions of crops.However,the distribution of soil salinity is heterogeneous.There is still no report about the influence of heterogeneous salinity on the growth of sorghum.In the present study,the root of sorghum was divided into two equal portions with a split-root method and treated with different concentrations of Na Cl solutions.Three treatments were set: non-saline control group,non-uniform salinity treated group and uniform salinity treated group.Growth patterns,physiological reactions,and short-term transcriptomic responses of sorghum under heterogeneous distribution of salinity across rhizosphere were studied,and mechanisms of the recovered growth of sorghum exposed to non-uniform root zone salt stress were illustrated.Major findings are summarized below:1.Compared with seedlings under uniform salt stress,fresh and dry weights of those under non-uniform salt stress were significantly increased.Fresh and dry weights of seedlings under non-uniform salt stress were determined by root weighted mean.Root length,root volume,root surface area,root tip number,and root folk number of the low-saline and non-saline treated root side of seedings exposed to non-uniform salt stress were considerably greater than those of high-saline root side and saline root side.The morphology of whole root was improved,therefore fresh and dry weights of non-saline root side were much higher than those of the high-saline root side and saline root.Fresh weight,dry weight,plant height as well as stem diameter of the seedlings under non-uniform salinity condition across all growth stages were significantly increased.Moreover,leaf area was significantly increased.These results showed that the influence of salt tress on growth and development of sorghum was diminished in non-uniform salinity condition.2.SPAD values of leaf,photosynthesis parameters,such as net photosynthate(Pn),stomatal conductance(Gs),transpiration rate(Tr),and fluorescence parameters of ?PS?,Fv/Fm,and electron transfer rate(ETR)were significantly affected by salt stress.However,photosynthesis performance under non-uniform salinity condition was significantly improved,which was reflected in the increase of SPAD values of leaf,photosynthesis parameters and fluorescence parameters.Some parameters of them were significantly increased.The significant increase of photosynthates reduced the damage of salt stress to yield and quality of sorghum.3.Under non-uniform salt stress,Na~+ concentration and Na~+/K~+ ratio were significantly decreased,K~+ concentration was significantly increased.Na~+ concentration and Na~+/K~+ ratio in the side of non or low salt stress were lower than that in the side of salt or high salt stress,while K~+ content was higher than in the side of salt or high salt stress.The content of Na~+ in aerial part of a sorghum plant in non-uniform salt stress was significantly lower than that in uniform salt stress and the content of K~+ was significantly higher than that in uniform salt stress.The content of Na~+ in non saline or low saline side was significantly higher than that in uniform salt stress side.Na~+/K~+ ratio was decreased by accumulating more K~+.Na~+ accumulation and root-shoot ratio under non-uniform salt stress was higher than that under uniform salt stress,which suggesting that Na~+ content in root under non-uniform salt stress was significantly higher that of the aerial parts and could alleviate damage of salt stress to the aerial parts.In non or low saline root side,water uptake is promoted by the accumulation of plenty of Na~+.The compensatory water uptake in the low-saline root side facilitates nutrient uptake as well,which effectively relieves the growth inhibition by salt stress.4.At all growth stages under heterogeneous salinity condition,Leaf MDA has smaller increase,the activities of anti-oxidative enzymes superoxide dismutase(SOD),ascorbic acid peroxidase(APX),peroxidase(POD),catalase(CAT)and glutathionine peroxidase(GPX),the concentrations of anti-oxidants ascorbic acid(ASA)and reduce glutathionine(GSH)and osmoprotectants proline(PRO)and soluble sugar(SS)were largely increased compared with the counterparts under uniform salinity condition.However,different anti-oxidative enzymes and osmoprotectants have differential increases at different growth stages.5.Transcriptomics research showed that totals of 2608 and 1305 differentially expressed genes(DEGs)in the leaves and roots,respectively,were identified by RNA-seq.The amount of DEGs in leaf under uniform(100 m M and 100 m M Na Cl)salinity conditions outnumbered that under non-uniform(Na Cl-free and 200 m M Na Cl).For DEGs in root,the number is larger in high-saline root side than that in low-saline root side and that in root under uniform salt stress condition.Expression of genes associated with photosynthesis,Na~+ compartmentation,phytohormone metabolism,anti-oxidative enzymes,and salt stress responsive transcriptional factors(TF)was up-regulated in leaf under non-uniform salinity condition.In root,expression levels of various genes coding for aquaporins as well as nutrient transporters were enhanced.