| In recent years,Influenced by global climate change and natural seasonal drought,extreme drought emerge frequently in south China,especially in dry season(October to next march),eucalyptus face the adversity of the long term drought stress,there appeared the natural phenomenon about artificial forest of eucalyptus production decreased and trees dead.These phenomena have aroused wide concern in the society.However,the physiological mechanism of tree growth restriction and death caused by drought is not clear.There are two hypotheses about the causes of drought death in plants:hydraulic failure hypothesis and the carbon starvation hypothesis.The two hypotheses can reveal certain phenomena for different tree species but also have limitations.According to the latest research,the mechanism of drought death of different types of trees is different,so it is of great value to carry out the research on the drought death mechanism of various tree species in time.This experiment used Eucalyptus urophylla×E.grandis DH32-29 annual seedlings as the experiment material,set up two groups of the water(CK)and dry(D)and two re-water(Fa,Fb)group,set up four groups of sampling time point(background,zero photosynthesis,50% leaf death,100% of leaf death),through the comparative analysis of eucalyptus physiological characteristics under drought,photosynthetic characteristics,hydraulic conductivity,non-structural carbohydrates,explore the eucalyptus growth restriction under different moisture and the physiological mechanism of death.The results of this study are as follows:(1)Diameter growth of Eucalyptus urophylla×E.grandis seedling under drought stress significantly suppressed,height growth is not affected,when net photosynthetic falling to 0(soil moisture content 53.85% of field capacity)growth stops,the leaves begancurl,light green,seedling survival after water,recovery are mainly the upper blade.Drought39 d(soil moisture content 33.00% of field capacity),50% of leaf death,for the most part of dead leaves of Eucalyptus urophylla×E.grandis are lower leaves,after water seedling survival,upper leaves a small amount of recovery,mainly for the stem base of initiation of peak green leaves grow.The 54 d(soil moisture content of 28.95 percent of the water content in the field)was severe drought stress,and the leaves all died,and the rehydrating experiment plant died.(2)In the early stage of drought,the photosynthetic rate began to decline,and with the aggravation of drought stress,the photosynthetic rate decreased more and more obvious.The decrease of transpiration rate was more average,and instantaneous water use efficiency was higher and stable in the early stage of drought.In drought 24 d,the leaves of eucalyptus trees were net photosynthetic 0.(3)The hydraulic conductivity decreased with the rising of severe drought stress,until 100% the blade dies,a large number of pipeline causes cavitation and embolism,xylem dysfunction,hydraulic imbalance,plant death.(4)The drought on soluble sugar and starch concentration of Eucalyptus urophylla×E.grandis leaf,stem and root effect is apparent,the increasing trend of soluble sugar concentration,starch concentration is declining.Plants and large molecules such as hydrolyzed starch to soluble sugar and other small molecules to reduce cell water potential.The Eucalyptus urophylla×E.grandis has maintained a higher concentration of NSC under different degrees of drought stress,and is used to maintain osmotic regulation and resistance to adversity,there is no obvious decline in NSC content.(5)The NSC was mainly used to repair cavitation and embolism after re-water when the photosynthetic is 0and 50% of the leaves died.However in 100% of leaf death,irreversible damage of drought on plant hydraulic structure,severe cavitation and embolism repair failure due to drought,dehydration plant is also death.In the case of the NSC surplus,the hydraulic failure is the main reason for the recovery of the Eucalyptus urophylla×E.grandis. |
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