Physiological Responses Of Metasequoia Glyptostroboides Seedlings To Flooding And Drought Stress

With the rapid and continuous changes of global climate,the precipitation pattern has been influenced,causing extreme events such as floods and droughts in local areas,and resulting in the loss of ecosystem health and productivity.Plants response to water stress though a series physiological and biochemical reactions.However,there is a lack of researches on the physiological response mechanism of woody plants under flooding and drought stress at present.In this study,the physiological response mechanisms were investigated using Metasequoia glyptostroboides seedlings cultivated in pots under flooding and drought stress.We set different soil water gradient treatments（low water content（12%）,normal water content（25%）,high water content（31%）,complete submergence）,as well as recovery of flooding（25%）,recovery of high water content（31%-25%）and post-drought rewatering（12%-25%）to explore the changes of hormones,antioxidant system,osmotic adjustment,photosynthesis and fluorescence systems in M.glyptostroboides seedlings.The main results are as follows:1)Under soil flooding stress（seedlings were complete submerged）,ethylene content began to increase on 7^th day firstly,and reached 2687.02 nmolg^-1 on 28^th day,which was 51.77%higher than that under normal water content treatment.Then the activity of SOD increased continuously on the 11^th day,and reached 727.61 U·g^-1 on the 28^th day which was 2.9 times higher than that of the normal water content.POD activity began to increase on the 14^th day and reached 1532.59 U·g^-1 on the 28^th day,which was 4.2 times higher than that of normal water content.As for fluorescence and photosynthesis systems,the photochemical quenching coefficient and net photosynthetic rate decreased between 11^th-14^thday relative to those of normal water content.The average chemical quenching coefficient was 0.25 on the 14^th day,which was 45.65%lower than that of normal water content.The average net photosynthetic rate was 1.11μmol·m^-2·s^-1 on the 14^th day,which was 68.10%lower than that under normal water content.The leaves were rotten after 17 days treatment.There were no differences between the indexes under recovery treatment and normal water content after 6 days of complete submergence（P>0.05）.2)Under soil drought stress（seedlings were treated with low water content（12%））,both abscisic acid and ethylene responded at first.The contents of abscisic acid and ethylene began to increase at 6^th hour of the 6^th day,and grew to 58.59%and 64.96%on the 14^th day,respectively,compared with those of normal water content.POD and GR in the antioxidant system are the main response substances.The activities of them began to increase on the 9^th day of drought stress,and reached 2.4 times and 9.03 times of those under normal water content on the 21^th day,respectively.The proline content began to increase on the 7^th day,and reached 13.51 times of that under normal water content on the 21^th day.The photosynthetic system was also affected on the drought stress,which decreased from 9^th to 11^th days after stress.The average net photosynthetic rate was 0.50μmol·m^-2·s^-1 on the 11^th day,decreasing by 88.37%of that under normal water content.After 14 days treatment,the plant leaves had shrunk and wilted.All indexes of treatment with the rewatering after 6 days from drought showed no significant differences compared to those under normal water content.3)Under the flooding stress（complete submergence of seedlings）,net photosynthetic rate,transpiration rate,water potential and POD were significantly related to water use efficiency（P<0.05）,and net photosynthetic rate,transpiration rate,stomatal conductance were significantly related to the apparent carboxylation efficiency（P<0.05）.Under the drought stress（12%soil water content）,net photosynthetic rate,transpiration rate,stomatal conductance（P<0.05）was significantly related to the water use efficiency,while net photosynthetic rate was significantly related to apparent carboxylation efficiency（P<0.001）.4)With the comparison of the physiological responses between flooding stress and drought stress,the response hormone was ethylene under flooding stress,while both abscisic acid and ethylene were responded in drought stress.Moreover,the response time of hormone under flooding stress was 18 hours later than that under drought stress.As for the antioxidant system,SOD and POD responded to flooding stress,while POD and GR responded to drought stress.The response times of antioxidant enzymes were2 days earlier under drought stress than that under flooding stress.For the osmotic adjustment material,proline content increased only under drought stress.Since the plants were dying with the continuous stress,the photosynthetic fluorescence indexes decreased after treatment of several days.The decline of net photosynthetic rate under flooding stress occurred 2-3 days later than that of drought stress.