Barley Seedlings In Response To The Combined Stresses Of Artemisinin,Drought And NaHCO₃ Under Freeze-thaw Conditions

The Qinghai-Tibetan Plateau（QTP）was known for the characteristics of long sunshine hours and changeable temperature between day and night,easily causing freeze-thaw stress to plants growing in this region.Meanwhile,due to the climate change,soil salinization is often accompanied by drought stress.Additionally,it is reported that Artemisia annua is widely distributed in Southwestern China,and one of its allelopathic substance,artemisinin,is ubiquitous in the QTP and causes allelopathy to surrounding plants.Highland barley is one of the main economic crops in QTP,where complex terrain and climate could make barley exposed to multiple environmental stresses.Therefore,much important practical and theoretical significance has been attached to study the physiological response of highland barley to combined stress of freeze-thaw,alkaline salt stress,drought stress and allelopathic stress.In this indoor simulation study,highland barley（Beiqing No.3）was selected as the experimental material to investigate its physiological response mechanism to alkaline salt and drought stress in the conditions of freeze-thaw stress.Through indoor simulation experiment,highland barley seedlings were treated with alkaline salt stress（60 mmol/L NaHCO₃ solution）for 7d,then drought stress（20%PEG-6000 solution）being performed and treated for 2d,after which the seedlings were put into a high-low-temperature test chamber to carry out a freeze-thaw cycle for a period of 14h.The freezing and thawing temperature conditions were set to decrease from 14.5℃to-5℃and then increase to 10℃,the rate of temperature change was 2.5℃/h.A total of seven samples were taken every 2h.The corresponding sampling temperatures were 10℃,5℃,0℃,-5℃,0℃,5℃and 10℃.Physiological response of highland barley to freeze-thaw,alkaline salt and drought stress can be revealed by analyzing the relative water content（RWC）,soluble protein（SP）content,malondialdehyde（MDA）content,superoxide dismutase（SOD）and catalase（CAT）activity changes in highland barley leaves.The results showed that the RWC of barley seedlings leaves under combined stresses of alkaline salt and drought was the lowest among 8 treatment groups（62.29%～77.59%）,MDA content,SP content and SOD activity were slightly higher than that of the control group,while CAT activity was lower,and generally,the effects of combined stresses were more serious than that of single stress.Freeze-thaw stress exacerbates the effects of alkaline salt and drought stress.During the freeze-thaw cycle,RWC shows a downward trend,while SP content first decreased and then increased.However,MDA content decreased slowly resulted from SOD and CAT scavenging reactive oxygen species.In order to explore the physiological response of highland barley to the combined stress of alkaline salt and artemisinin under freeze-thaw,highland barley Beiqing No.3was chosen as the research material and treated with alkaline salt stress（60mmol/L NaHCO₃ solution）and allelopathic stress（20mg/L artemisinin solution）,simultaneously.The indoor simulated freeze-thaw experiment（10℃/-5℃）was carried out after 9d growth of the seedlings.Samples were taken at 2h,8h and 14h respectively,and the corresponding sampling temperatures were 10℃,-5℃ and 10℃.The changes of relative electrical conductivity（RC）,SP content,MDA content,SOD activity and water use efficiency（WUE）of highland barley leaves were analyzed to reveal the physiological adaptation characteristics of highland barley under single or combined stress of freeze-thaw,alkaline salt and artemisinin stresses.The experiment results showed that compared with the control group,the contents of RC,SP and MDA in the leaves of seedlings increased significantly,while SOD activity and WUE decreased significantly.During the freeze-thaw cycle,the RC of seedlings leaves in compound stress showed a significant increasing trend,while the SP,MDA content and SOD activity increased first and then decreased,and WUE declined greatly during the thawing phase（127.6%～189.9%）.The results proved that as an allelopathy stress factor,artemisinin caused great damage to seedlings,and there was a significant synergistic relationship with freeze-thaw stress in many aspects.