Effects Of High Temperature Stress On Physiology And Secondary Metabolism Of Dendrobium Nobile And The Alleviating Effect Of Exogenous Calcium

Dendrobium nobile Lindl It is a Dendrobium plant of orchid family and likes a warm,cool and humid growth environment.In view of the high ornamental and medicinal value of Dendrobium nobile and the increasing market demand,many places in China,including tropical areas（such as Hainan,etc.）,have begun to introduce and cultivate it.However,the research on the tolerance of Dendrobium nobile to high temperature and exogenous mitigation of high temperature has not been reported at home and abroad.Taking Dendrobium nobile as experimental material,the optimum growth temperature of 25℃was selected as control,and 30℃,35 and 40℃were set as high temperature treatment group.The changes of appearance,physiology and biochemistry and leaf anatomical structure of Dendrobium nobile under high temperature stress were systematically studied.Based on the results of high temperature stress test,the appropriate temperature and treatment period were selected for exogenous calcium mitigation test（0mmol/L,5mmol/L,10mmol/L,15mmol/L,20mmol/L）.It provides a scientific basis for large-scale cultivation of Dendrobium nobile in tropical areas.The results are as follows:（1）From the morphological changes of Dendrobium nobile under high temperature stress,compared with the control at 25℃,the treatment at 30℃had no significant effect on its growth and development.However,under the treatment of 35℃and 40℃,the leaf length,leaf width,plant height,biomass and leaf relative water content（RWC）showed a significant downward trend,and the number of yellow leaves and fallen leaves increased significantly with the increase of temperature.It can be seen that when the temperature reaches 35℃or above,it will have a significant impact on the appearance of Dendrobium nobile.（2）From the changes of physiological characteristics of Dendrobium nobile under high temperature stress,compared with the control,SOD,POD,CAT,ASA and GSH increased significantly in individual treatment periods under 30℃treatment（P<0.05）,and the changes of other indexes were not significant.REC,MDA,SOD,POD,pro,SS,SP,ASA and GSH increased significantly at 35℃,Chla,Chlb,Chla+b,Car,CAT and GSH decreased significantly at 40℃（P<0.05）.Most physiological and biochemical indexes can resume growth after treatment at 35℃,but the plants of Dendrobium nobile treated at 40℃have no obvious recovery effect,indicating that the continuous high temperature treatment at 40℃may cause irreversible damage to the plants.（3）Compared with the control,the contents of flavonoids and polyphenols of secondary metabolites of Dendrobium nobile under high temperature stress increased significantly when treated at 30℃for 20 days（P<0.05）.The contents of flavonoids and polyphenols increased significantly at 35℃（P<0.05）.Under the treatment of 40℃,the contents of flavonoids and polyphenols increased first and then decreased（P<0.05）,indicating that continuous high temperature stress will reduce the accumulation of effective components in Dendrobium nobile.（4）Compared with the control,the mesophyll cells of Dendrobium nobile treated at30℃accumulated a large number of giant starch particles,resulting in the expansion and deformation of chloroplasts.At 35℃,a large number of osmiophilic particles appeared,and the cell nuclear membrane was distorted.At40℃,some chloroplasts disintegrated and cytoplasm wall separated,and the nucleus and cytoplasm flowed out.It can be seen that the damage degree of plant mesophyll cells is directly proportional to the stress temperature.（5）After exogenous application of CaCl₂,the growth of leaf length,leaf width,plant height,dry and fresh matter accumulation,RWC,Chla,Chlb,Chla+b,Car,SOD,POD,CAT,pro,SS,SP,ASA and GSH increased significantly（P<0.05）,while the yellow leaf rate,defoliation rate,REC and MDA decreased significantly（P<0.05）.Both 10mmol/L and15mmol/L treatments could significantly increase the content and accumulation of flavonoids and polyphenols in plants.Under 20mmol/L treatment,the plant growth indexes,RWC value and the contents of photosynthetic pigments,ASA,GSH and polyphenols decreased significantly（P<0.05）.It can be seen that only appropriate concentration of CaCl₂ pretreatment can reduce the effect of high temperature on Dendrobium nobile to a certain extent.（6）The results showed that the remission effect of 10 mmol/L CaCl₂ Treatment was the best,followed by 15 mmol/L CaCl₂ Treatment.In conclusion,Dendrobium nobile showed good tolerance at 30℃.When the temperature reaches 35℃,it will cause obvious damage to the plant,but after 14 days of culture at room temperature,the plant can often return to the control level or close to the control.The damage of 40℃treatment to the plant was the most serious,so that the plant could not recover 14 days after the stress was relieved.It can be seen that the damage of this treatment to the plant is irreversible.Therefore,artificial intervention,such as the application of exogenous mitigation agents,is needed to reduce the damage of high temperature to Dendrobium nobile.