Response Of Seedling Regeneration Of Three Dominant Tree Species In Taihang Mountains To Drought Stress And Screening Of Drought Resistance Indicators

With its rich forest resources,the Taihang Mountains play an important ecological service function and are a natural ecological barrier in northern China.However,the regeneration and survival of trees within the mountains are facing great challenges due to the changing trend of warm and dry climate in northern China.In this context,it is important to characterise the response of tree seed germination and seedling photosynthetic physiology to drought stress,to select the physiological indicators of seeds and seedlings that are more sensitive to drought,and to screen tree species that are better adapted to drought conditions in order to alleviate water stress and drought trends in the Taihang Mountains.In this study,three dominant tree species in the region,Catalpa ovata,Ailanthus altissima and Robinia pseudoacacia,were used as target species to determine the photosynthetic and fluorescence characteristics of seedlings in response to different drought stress gradients by setting different soil moisture gradients.Different concentrations of polyethylene glycol（PEG-6000）solutions were used to simulate the changes of seed germination indicators under different drought stress gradients.Finally,the preferred sensitive indicators for assessing the drought resistance of seeds and seedlings were selected to provide a comprehensive evaluation of the drought resistance of the three tree species,and the results of the study were as follows.（1）The germination vigor,final germination rate,germination index and vitality index of C.ovata,A.altissima and R.pseudoacacia seeds decreased with increasing concentration of polyethylene glycol（PEG-6000）solution:the germination vigor of C.ovata,A.altissima seeds decreased to 0 at 10%PEG-6000 solution concentration,and R.pseudoacacia seeds decreased by 54.2%compared to the control（pure water）;compared to the other two species,the final germination rate,germination index and vitality index of R.pseudoacacia seeds decreased the least compared to the control.It is noteworthy that the 5%PEG-6000 solution slightly promoted the germinal length of C.ovata and R.pseudoacacia seeds,and the fresh weight of C.ovata and A.altissima seeds.It is worth noting that 5%PEG-6000 solution slightly promoted the germinal length of C.ovata and R.pseudoacacia seeds and the fresh weight of C.ovata and A.altissima seeds,while 10%PEG-6000 solution concentration inhibited the seeds of all three tree species.（2）The net photosynthetic rate（Pn）,transpiration rate（Tr）and stomatal conductance（Gs）of seedlings of the three species decreased with increasing drought stress,while intercellular CO₂concentration（Ci）of C.ovata and A.altissima showed a gradual increase and decrease respectively,while Ci of R.pseudoacacia showed a decrease followed by an increase.The trend of stomatal limitation values（Ls）of the three species was opposite to the trend of Ci as drought stress increased.Light drought stress caused a 2.2%decrease in leaf water use efficiency（WUE）of C.ovata compared to CK（full irrigated）,while A.altissima and R.pseudoacacia increased by4.5%and 4.0%,respectively;WUE of C.ovata increased by 3.8%,A.altissima decreased by 10.1%and R.pseudoacacia decreased insignificantly under severe drought stress compared to the control.（3）Drought stress caused a gradual increase in initial fluorescence（Fo）and a gradual decrease in maximum fluorescence（Fm）,actual photochemical efficiency（ΦPS Ⅱ）and electron transfer rate（ETR）in the three tree species.The ETR was most sensitive to drought stress,with both mild and severe drought stress leading to significantly lower ETR in all three tree species compared to CK（P<0.01）.The maximum photochemical efficiency（Fv/Fm）and light energy capture efficiency（Fv’/Fm’）of C.ovata and A.altissima were both ranked under different drought treatments:CK>mild drought>severe drought,while Fv/Fm and Fv’/Fm’of R.pseudoacacia showed:mild drought>CK>severe drought.The photochemical quenching coefficients（qP）of C.ovata and R.pseudoacacia showed a decreasing and then increasing trend with increasing drought stress,while A.altissima showed a gradually decreasing change.The non-photochemical quenching coefficient（NPQ）of C.ovata showed the opposite trend to its qP,increasing by 3.1%and decreasing by2.1%compared to CK,while the NPQ of A.altissima and R.pseudoacacia gradually increased,rising by 10.7%（P<0.01）and 5.2%compared to CK under severe drought stress,respectively.（4）Principal component analysis（PCA）was applied to select germination vigor and germinal length as the preferred indicators of seed drought resistance,and ETR,Ci,NPQ and Fo as the preferred indicators of seedling drought resistance,and the three tree species were ranked in order of drought resistance by the fuzzy subordinate function method:R.pseudoacacia>A.altissima>C.ovata.In summary,the germination and seed vigor of R.pseudoacacia seeds under moderate drought stress were higher than those of C.ovata and A.altissima,and the photosynthesis of seedlings was less inhibited by drought stress than that of the other two species,and a combination of non-stomatal and stomatal factors was activated in time to resist stress.In addition,changes in fluorescence parameters show that R.pseudoacacia have a strong heat dissipation capacity,which allows them to consume and release excess light energy to reduce the damage to the photosynthetic system,and to allocate most of the absorbed light energy to photosynthesis to accumulate nutrients for plant growth when light energy use decreases.Therefore,it is considered that R.pseudoacacia should be the preferred tree species for forest regeneration and plantation in Taihang mountain area.