| Cultivation of salt-tolerant agricultural crops is an important way and a feasible measure for the development of saline-alkali land.Many types of salt-tolerant food crops and cash crops have been cultivated in China,but the planting cost is high and the economic performance is low.So it is still in the experimental stage.Compared with food crops,vegetables have higher economic value and are a more promising crops for developing saline-alkali land.At present,the salt-tolerant vegetables obtained by traditional breeding or genetic engineering breeding are unsatisfactory in salt-tolerant effect,and the yield of these vegetables is low under salt-tolerant condition.So they are promoted and applied only on a small scale.Chinese cabbage has a shorter growth cycle and higher yield,and it produces a special taste after salt treatment.On the basis of previous research,our laboratory propose a scheme of planting Chinese cabbage in summer when the salinity of saline-alkali land decreased due to rainfall and exploitation of saline-alkali land.However,salt stress and high temperature stress exist simultaneously in summer.In order to clarify whether Chinese cabbage can adapt to saline-thermal cross stress,this study took salt-sensitive ‘38 Kuaicai(38KC)'and salt-tolerant ‘Tianjin Qingmaye(QMY)' as research materials..Firstly,Chinese cabbage was treated with different concentrations of salt(0-200 m M NaCl),and then the Chinese cabbage was subjected to high temperature stress(25,30,35,40 ?)after salt adaptation.The research results can provide a basis for the feasibility of cultivating Chinese cabbage in saline-alkali land in summer.The main experimental results are as follows.1.Effect of saline-thermal cross stress on germination ability of Chinese cabbage seeds:Under the same NaCl concentration,the germination percentage of "QMY" and "38 KC" showed a decreasing trend with the increase of temperature.At the same temperature,the germination rate decreased with increasing temperature.At 25?-30?,seeds at 100 m M and200 m M NaCl salinity could germinate.However,the germination rate of Chinese cabbage seeds at 200 m M NaCl salinity was only 4%-21% of that in the control group.Thus the germination rate needed for cultivation could only be guaranteed under the low salt concentration(100 m M NaCl).With the continuous increase of ambient temperature,the germination rate of seeds decreased gradually under the two salinity conditions.The germination rate gradually decreased and was significantly lower than the control.At 40?,the control group could still germinate,but the seeds under salt stress did not germinate.These results indicated that both salt and heat stress significantly inhibited seed germination,while cross stress increased the inhibition of seed germination.Two kinds of Chinese cabbage seeds are sensitive to salt stress and heat cross stress in germination stage,and the germination rate of direct sowing in saline-alkali land is too low to be cultivated in saline-alkali land by sowing.2.Comparison of salt tolerance and heat resistance of two kinds of Chinese cabbage seedlings: both kinds of Chinese cabbage seedlings grew for a long time at 200 m M NaCl concentration.Under 100-200 m M NaCl stress,the chlorophyll content of "QMY" had no significant change compared with the control group(P<0.05),the chlorophyll content of "38KC" was significantly decreased at 200 m M.The stability of cytoplasmic membrane of "QMY" was also significantly better than that of "38 KC".The salt tolerance of "QMY" was better than that of "38 KC".Under the high temperature of 35 ?-40?,the chlorophyll content of both varieties of Chinese cabbage decreased,and the chlorophyll content of "38KC" decreased more than that of "QMY" and the increase of plasma membrane permeability was also greater than that of "QMY"."QMY" heat resistance was better than "38 KC" to reduce more,"QMY" relative conductivity change is not obvious,"38 KC" at 30? when a significant increase,with the temperature continues to rise no longer change.The cell membrane of "38 KC" was more easily damaged by salt and heat,and its salt tolerance and heat resistance were slightly worse than that of "QMY",indicating that "QMY" had higher resistance to salt and heat than that of "38 KC".3.The tolerance of the two kinds of Chinese cabbage to saline-thermal cross stress:under 35-40? high temperature stress,the chlorophyll content of the salt-adapted(100-200 m M NaCl)leaves decreased more than that of the non-salt-treated leaves,and the plasma membrane permeability was also more stable than that of the non-salt-treated leaves.The photosynthetic activity of salt adaptation Chinese cabbage leaves at high temperature was significantly higher than that of the control group.For example,at 40?,the photosynthetic rate of two kinds of Chinese cabbage leaves treated with 100 m M NaCl could reach 2.65-4.67 times that of the control group.Under 40? and high temperature stress,the PSII photochemical efficiency,light energy utilization efficiency,electron transfer efficiency,photosynthetic performance index and photosynthetic driving force of salt adaptation leaves of two kinds of Chinese cabbage were significantly higher than those of the control.Under mild high temperature stress,the decrease of the above photoreactivity in salt-adapted leaves of two kinds of Chinese cabbage was also less than that in non-salt-treated leaves.These results proved that salt adaptation to heat resistance of Chinese cabbage significantly improved.The adaptability of the leaves to saline-thermal cross stress was better than that of the leaves of "38 KC".The net photosynthetic rate of the salt-adapted leaves was higher than that of the non-salt-treated leaves under 30-40? stress.The salt-adapted leaves of "38 KC" showed better heat resistance under 40? stress.4.Two kinds of Chinese cabbage on the thermal intercross stress of salt adaptation: salt stress could lead to Chinese cabbage leaf water content decreased,but the two kinds of Chinese cabbage in 30 to 40 ? high temperature stress could keep stable leaf water content,leaf protein content increased slightly,even salt to leaf proline and soluble sugar content were higher than the salt handling leaves,cabbage to osmotic adjustment under in the hot intercross stress of salt.Saline-thermal cross stress could lead to metabolic disorder of Chinese cabbage,increase the content of MDA and hypersonic anion,and cause membrane lipid peroxidation and protein carbonylation.The protein carbonylation level of "38 KC" was significantly higher than that of "38 KC" under saline-thermal cross stress,indicating that the stress damage of "38 KC" was greater than that of "QMY".Under salt-heat stress,the activities of CAT and POD of "QMY" increased with the increase of salt treatment concentration and heat treatment temperature.The activities of SOD,POD and CAT and the content of VC in salt-adapted leaves were not higher than those in non-salt-treated leaves under high temperature stress.Therefore,antioxidant activity may not be the main mechanism of Chinese cabbage's adaptation to saline-thermal cross stress.In order to further explore Chinese cabbage on the thermal intercross stress of salt metabolism adaptation mechanism,we used GC-MS Untargeted Metabolomics to analyze the thermal double salt the metabolic changes of "QMY",under the thermal double salt,has identified more than 200 kinds of metabolites,and the control group(25 ?,0 m M NaCl),found that there are 53 kinds of metabolites significantly raised or lowered,Including substances such as amino acids,organic acids,amines and organic compounds.Six compounds were accumulated under salt-heat stress,including sucrose,phosphoenolpyruvate,citric acid,malonic acid and L-proline.They may play a crucial role in the response of Chinese cabbage to saline-heat cross stress.The results of this experiment prove that the Chinese cabbage could not be planted directly in the saline-alkali land in summer,so it was suggested to transplant seedlings.After salt adaptation,the heat resistance of the two kinds of Chinese cabbage to high temperature stress was enhanced,especially the adaptability of salt-tolerant variety green hemp leaf salt to saline-thermal cross stress was significantly improved.The Chinese cabbage can maintain water well and perform effective osmotic regulation under saline-thermal cross stress,Among them,glycylproline,proline,acetic acid and other compounds may play an important role in osmotic protection.The above research provided a reference for the saline-alkali cultivation methods of Chinese cabbage in summer,and also expanded the understanding of the adaptation mechanism of plant saline-thermal cross stress. |
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