Response Characteristics Of Highland Barley Seedlings To Freeze-thaw Stress Under The Background Of Soil Salinization

The temperature difference between day and night in early spring and late autumn in the Qinghai Tibet Plateau is large,which often produces freeze-thaw stress on plants and soil,and the problem of soil salinization in this area is prominent.Freeze-thaw and soil salinization,as two main constraints,will have a negative impact on the production of the local crop highland barley（Hordeum vulgare L.var.nudum Hook.f.）.By clarifying the climate change and the occurrence of freezing and thawing in the sowing period of highland barley,the certainty of the occurrence of freezing and thawing can be clarified,which can be used as the basis for the setting of indoor experimental temperature.Through the study of the changes of physiological indexes of highland barley in the face of compound stress,it can provide a theoretical basis for reducing the impact and loss of freezing and thawing on agriculture in the Qinghai Tibet Plateau under the background of soil salinization.Based on the surface temperature data of 39 meteorological observation stations in the highland barley planting planning area of Qinghai Tibet Plateau from 1981 to2019,taking 0℃as the threshold,the day when the daily maximum temperature is greater than 0℃and the daily minimum temperature is less than 0℃is defined as the freeze-thaw day（FTD）.After data integration and statistics,the results show that FTD occurs in the whole planning area from March to April,that is,in the early stage of highland barley planting.The minimum temperature（-5℃）during the freezing and thawing cycle in the indoor experiment is set by calculating the monthly average minimum temperature（0.1℃）in March at 39 meteorological stations.In order to explore the response characteristics of highland barley seedlings to freeze-thaw stress under the background of soil salinization,highland barley hydroponic culture experiment was selected in this study.After 1/2 concentration Hoagland nutrient solution was added with 60 mmol/L Na HCO₃,it reached the alkaline soil standard（p H=8.02）,and the concentration of neutral salt Na Cl was 60mmol/L;The temperature change rate in the freeze-thaw cycle（FT）stage is-0.5℃/12 min.three samples are taken,and the corresponding sampling temperatures are T1:10℃,T2:-5℃and T3:10℃.The physiological and ecological responses of highland barley seedlings were studied.The experimental results showed that both saline alkali combined stress（S-A）and freeze-thaw cycle cooling process（T1-T2）could significantly increase the content of malondialdehyde（MDA）in highland barley seedlings.The combined stress of alkaline salt and freeze-thaw resulted in the increase in relative conductivity（RC）of green tree seedlings.The content of soluble protein in CK（FT）and S（FT）highland barley seedlings under freeze-thaw stress increased first and then decreased;The soluble protein content of A（FT）in alkaline salt stress group and S-A（FT）in salt alkali compound stress group continued to increase until the temperature rose at T3.Both saline alkali combined stress and freeze-thaw cycle cooling process（T1-T2）will lead to the decrease of pod enzyme activity in highland barley seedlings.In the temperature recovery stage of the freeze-thaw cycle（T2-T3）,the POD activity in S（FT）group rose to the initial level,the activity in CK（FT）group and a（FT）group almost maintained at the lowest level of temperature,and it in S-A（FT）group continued to decline.Under alkaline salt stress,the net photosynthetic rate（Pn）of highland barley seedlings decreased,and the freezing and thawing cycle cooling process（T1-T2）also caused a significant decrease in Pn of CK（FT）,S（FT）and A（FT）.The Pn of S（FT）group and S-A（FT）group reached the lowest value at T2.During the freeze-thaw cycle,the transpiration rate（E）of highland barley seedlings increased first and then decreased.The intercellular carbon dioxide concentration（Ci）of highland barley seedlings increased with the increase in stress degree.The stomatal conductance（gs）of highland barley seedlings decreased to the lowest value at-5℃;In the temperature rise stage of the freeze-thaw cycle（T2-T3）,gs of S（FT）,A（FT）and S-A（FT）in freeze-thaw group increased slowly.Water use efficiency（WUE）of S（FT）and S-A（FT）decreased significantly under freeze-thaw stress.Highland barley seedlings are resistant to saline alkali stress to a certain extent.The freeze-thaw cycle will aggravate the damage of soil salinization to highland barley seedlings and affect the biofilm permeability,osmotic regulation and photosynthesis of seedlings.In order to better reduce the impact and loss of soil salinization and freeze-thaw disasters on agriculture in the Qinghai Tibet Plateau,the saline alkali soil should be improved in advance to inhibit the synergistic effect of saline alkali and freeze-thaw combined stress.