Responses Of Wheat Seedlings To Salt Combined With Drought Stress And Its Physiological Mechanism

Salt and drought stress are two major limiting factors to crop productivity.In north or northwest China,salt and drought stress often occur simultaneously owing to less rainfall and higher evaporation in winter and spring,which results in higher wheat yield loss.Recently,several studies have indicated that certain crop species exhibit lower growth inhabitation under the combined stress of salt and drought compared with salt and drought stress separately,but less information about adaptation mechanisms of these plants is available.Two wheat cultivars,Yangmai16 and Luohan7(drought-tolerant),were used in a hydroponic experiment to investigate the effects of salt combined with drought stress(SD)on root water uptake capacity,photosynthetic capacity,and lipid membrane metabolism at the seedling stage of wheat.Sodium chloride(NaCl)and polyethylene glycol 6000(PEG)were applied to solution to simulate salt and drought stress,respectively.The main results were summarized as follows:1.Maintaining a higher water absorption capacity and a lower Na+/K+ ratio to increase the ability of photosynthesis photosynthesis were the major contributors for wheat seedlings to adapt salt combined with drought stress.Salt,drought and SD significantly affected the plant biomass,leaf area,root length,root surface area,root volume,chlorophyll content,fluorescence parameters(Fv/Fm or Fv'/Fm'),electron transfer rate,Rubisco content and activity,carboxylation efficiency,and net photosynthetic rate for both wheat cultivars.However,the effects of SD on those were lower than their sole effects.Root hydraulic conductivity and xylem sap intensity under SD were observed to be significantly higher than those for the single stress,which resulted in higher leaf water potential and content under SD than under single stress.The Na+ content and Na+/K+ ratio in leaves and roots under single salt stress were significantly higher lower than that for the SD,although those were significantly higher than those for the control.Compared with single salt and drought stress,SD not only improved the root water uptake capacity and leaf water status,but also decreased the Na+/K+,hence alleviating inhibition of leaf photosynthetic capacity.Overall,maintaining a higher water absorption capacity and photosynthesis were the major contributors for wheat seedlings to adapt SD.Luohan7 and Yangrmai16 responded similarly to salt and SD,whereas Luohan7 showed more obvious tolerance to drought stress than Yangmai 16.2.Under salt combined with drought stress,wheat seedlings could improve the antioxidant capacity and decrease the membrane lipid peroxidation to increase the ability of resisting adversity.The O2·-production rate,H2O2 content,MDA content and relative electrical conductivity in leaf and root of wheat seedlings under SD were significantly lower than those in single stress treated plants though those were significant higher compared to the control.Under 3 stresses conditions,the activities of SOD,POD and CAT in leaf and root of wheat seedlings increased significantly,respectively.The content of IAA and CTK in leaves and roots under SD were significantly higher than single stress,and the ABA content in SD-treated plants were lower than that in single stress,which indicated that SD could enhance the content of IAA and CTK and reduce the content of ABA compared with single stress.Compared with single stress,SD not only reduced the content of ABA and ROS in leaf and root of wheat seedlings,but also improved IAA and CTK content,implying that SD alleviated the membrane lipid and reduced the permeability of the cell membrane damage which contributed to the damage of wheat seedlings under salt and drought stress was less than that of single salt and drought stress.Luohan7 and Yangmai16 responded similarly to salt and SD,whereas Luohan7 showed more obvious tolerance to drought stress than Yangmai 16.3.The growth of wheat seedlings can be quickly recovered to the control level after relieving salt combined with drought stress.Wheat seedling biomass,total root length and root surface area decreased significantly under single salt,single drought and salt combined with drought stress,however,the reduction degree was lowest under S1D1 treatment which could recover quickly after rehydration.Wheat seedling relative water content was lower than control under stress condition while S1D1 treatment kept high relative water content,which made chlorophyll content,Fv/Fm and Fv'/Fm' stable so as to reduce damage of photosynthetic system and maintain high photosynthesis rate.In addition,after relieving stress,relative water content and photosynthetic capacity could recover rapidly until all physiological characteristics were in line with control seven days after relieving stress.Therefore,S1D1 had a little damage to wheat seedling compared with other stress and wheat seedling could recover rapidly after relieving stress in two cultivars,which indicated wheat seedling can adapt to S1D1 in a certain degree.In summary,wheat seedlings could show some adaptability to salt combined with drought stress.Compared with single stress,the SD delayed the membrane lipid peroxidation by improving the antioxidant capacity,improved the leaf water potential and relative water content by increasing the water absorption capacity of root,ameliorated hormone content to increase the ability of resisting adversity which were beneficial for the improvement of photosynthetic capacity and resulted in wheat seedlings to adapt SD.