| Canker is an important stem disease of Populus and one of China's major forest diseases,which seriously affects the development of Populus plantation industry and ecological public welfare forest construction in China.In the past,researches on Populus canker diseases mainly focused on the field of pathology,but the physiological response mechanism of Populus in the interaction between Populus and canker diseases was unclear.The stability of carbon balance and hydraulic structure is the basis of plant survival.Therefore,from the perspective of carbon and water metabolism balance,the research focused on the mechanism of Populus death caused by Botryosphaeria dothidea and Valsa sordida.This study laid a foundation for the control and breeding of canker diseases.P.bolleana is an important farmland shelterbelt,fast growing and high-yield forest,windbreak and sand fixation forest and good tree species in the north of China.We use punching-inoculation,girdling-inoculation and microtrauma inoculation systems to evaluate the gas-exchange and hydraulic parameters,concentrations of nonstructural carbohydrates(NSCs),and symptoms of P.bolleana with Botryosphaeria or Valsa canker disease.The main results are as follows:1.Punching-inoculation systems: 17,24,33 days after inoculation of Botryosphaeria canker;24,33 days after inoculation of Valsa canker,the net photosynthetic rate of P.bolleana decreased by 11% and 7% on average.During this period,the stomatal limit value was significantly lower and the intercellular carbon dioxide concentration was significantly higher.This indicated that the main reason for the decrease of photosynthetic rate was non-stomatal restriction.2.Girdling-inoculation system:(1)Canker diseases induce carbon starvation by inhibiting photosynthesis.The net photosynthetic rate decreased by 54%-75% and 53%-76% with Botryosphaeria and Valsa canker disease.(2)In the middle stage,carbon starvation of stem is an important cause of poplar canopy death.Girdling-inoculation leads to the reduction of the starch and soluble sugar content below the infection site.The contents of starch and soluble sugar in the lower part of infection site were 61%,68%,59%,65% of those in the control group,respectively.Meanwhile,canker girdling significantly reduced the organic content of poplar roots.After Botryosphaeria,Valsa canker girdling,the content of starch and soluble sugar content in the root of P.bolleana was 53%,56%,50%,52% of the control,respectively.(3)In addition,it was found that in the middle stage of canker diseases,pathogenic fungi destroyed the cambium of poplar,resulting in the functional dysfunction.(4)Girdlinginoculation significantly reduced the water conductivity of stem and the water content of stem above girdling-inoculation.The results showed that although canker diseases did not lead to hydraulic failure,girdling-inoculation caused by canker diseases hindered the upward transport of water.3.Microtrauma inoculation system:(1)Microtrauma inoculation reduced the net photosynthetic rate of P.bolleana by 90%.Lead to the decrease of starch and soluble sugar below the infection site.The contents of starch and soluble sugar below the microtrauma inoculation site were 71% and 83% that of the control group,respectively.The content of starch and soluble sugar in root was 25% and 88% of that in control.It was further confirmed that Botryosphaeria reduced the synthesis of organic carbon by inhibiting photosynthesis,and cut the phloem of stem through girdling,which hindered the downward transport of photosynthetic products and induced carbon starvation.(2)In the early stage,there was no significant change in the water conduction loss rate of P.bolleana,while in the later stage,the water conduction loss rate was significantly reduced,indicating that Botryosphaeria did not lead to the formation of air cavitation embolism in the xylem of P.bolleana.4.In our study,transpiration rate and midday water potential of P.bolleana were significantly lower and higher than those of control.Canker girdling have not cause cavitation embolism in poplar trees.The results showed that the infection of canker did not lead to hydraulic failure of poplar.In summary,pathogenic fungi reduce the photosynthetic rate of poplars,leading to reduced organic synthesis.However,phloem girdling caused by canker impedes the downward transport of organic carbon and decrease of organic content in the root.And under the stress of canker,there was no hydraulic failure in poplar.These results suggest that canker diseases cause poplar mortality by inducing carbon starvation in the xylem,not by hydraulic failure. |
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