Effects Of Different Heat Wave Patterns On Hydraulic Structure And Xylem Anatomy Of Phoebe Bournei And Schima Superba

In the scenario of global warming,the increasing frequency of extreme climate events such as heat wave,is bound to affect growth and development of plants.However,the effects of complex heat wave patterns（different frequency and interval time）and its accompanying drought on xylem hydraulic function,carbon allocation and effects on growth in plantsremain unclear.Therefore,the main broad-leaved tree species Phoebe bournei and Schima superba in subtropical region were selected as the research objects in this study.We set five different heat wave frequency and interval treatments,including no heat wave（CK）,one heat wave（HW）,two heat wave interval of 7 days（2HW7）,two heat wave interval of 30 days(2HW₃₀)and two heat wave interval of 45 days(2HW₄₅)to study the response of xylem hydraulic structure,xylem anatomical structure,carbon allocation and growth of different broad-leaved tree species to different heat wave modes,and compare the differences of anti-embolization ability and post-embolization repair ability of Phoebe bournei and Schima superba seedlings to different heat wave patterns.Furthermore,it was revealed that the xylem hydraulic structure characteristics,growth and carbon allocation of the main broad-leaved tree species in subtropical region（Phoebe bournei and Schima Superba）were different in response to heat wave stress.The main conclusions are as follows:（1）Heat wave stress significantly affected the hydraulic structure indexes of Phoebe bournei and Schima Superba.A single heat wave event（HW）aggravated the degree of embolization and decreased the water conductivity of the two species.The water conductivity of Phoebe bournei decreased significantly by 9%,while Schima superba had no significant change.The repetitive heat wave significantly affected the hydraulic structure indices of the two species.The response degree of the short interval repetitive heat wave event（2HW7）was high in Phoebe bournei Kh increased significantly by 60% and PLC increased significantly by 206%.Stem embolism of Schima superba decreased with increasing interval of repeated heat wave.（2）The effects of heat wave on the content and distribution of NSC of Phoebe bournei and Schima superba were different in species and organs,which showed that the contents of soluble sugar and NSC of Phoebe bournei stem were increased,while the contents of starch in stem and root of Schima superba were increased.Repeated heat wave had cumulative effect on the NSC content and distribution of Phoebe bournei and Schima superba,but the effect was limited by the interval time and frequency of heat wave,which showed that the NSC content of Phoebe bournei was significantly increased under short interval repeated heat wave,while 2HW₃₀ and 2HW₄₅ had no significant difference with CK.（3）The short-term effects of heat wave on plant carbon allocation were ultimately transformed into the effects on the biomass of plant organs.Heat wave treatment significantly affected the biomass,leaf area,specific leaf area and chlorophyll content of each organ of Phoebe bournei,but there were significant differences in leaf biomass,total biomass and chlorophyll b indexes of Schima Superba.Short interval repeat heat wave 2HW7 promoted the biomass accumulation and NSC storage of Phoebe bournei,and the biomass of leaf,stem,root and leaf area increased by 29%,34%,58% and 44%,but Schima superba had higher leaf biomass,total biomass and specific leaf area under 2HW₃₀.（4）Phoebe bournei and Schima Superba adopted different NSC regulatory mechanisms to alleviate xylic embolism caused by heat wave stress.Phoebe bournei stored photosynthates in the form of starch to ensure the growth of underground parts,while Schima superba stored soluble sugars in leaves to maintain normal growth and water transport.In addition,the stem water conductivity of Phoebe bournei was weak,embolization was severe after heat wave stress,and NSC had little effect on alleviating embolization,while Schima superba had strong water conductivity,and the degree of embolization recovery was closely related to the content of NSC.Based on the above results,in order to ensure their normal growth and life activities under the stress of complex heat wave,Phoebe bournei and Schima superba adopted different regulation strategies.In addition,in the face of repeated heat wave stress at different intervals,there are different elastic regulation mechanisms,not only through the regulation of NSC content in various organs and the distribution of short-term strategies to cope with stress,but also through the long-term homeostasis regulation mechanism to maintain the NSC budget balance in plants to avoid carbon hunger and hydraulic imbalance.