Physiological Response Mechanism Of Water Use Efficiency To Water Stress In Typical Greening Tree Species In Beijing

In this paper,four typical greening tree species in Beijing(P.davidiana,R.pseudoacacia,P.tabulaeformis,and P.orientalis)are selected to conduct simulated natural drought and rewatering experiment.We use the stable carbon isotope 13 C technology to analyze the distribution pattern of soluble sugars in various organs of each species under drought and rewatering.Combining with the physiological indicators of plants under different water conditions and the response law of water use efficiency(WUE),response mechanisms of different tree species to drought stress and compensation effect after rewatering are explored.Our findings could provide theoretical basis for the full use of limited water resources in Beijing's urban areas to improve the survival rate of green tree species.The main results of this study are as follows:1.Photosynthetic physiological adaptation strategies to soil moisture variability differed among the four typical tree species undergoing the drought-return process.Specifically,there is a significant difference(P<0.01)between the photosynthetic physiological parameters of the four species.The rate of water consumption of deciduous species in the initial phase of drought is faster than that of evergreen species,as evidenced by a much greater reduction in soil volume water content(SWC)of deciduous species(50.79%)than that of evergreen species(20.60%)on day 2 of drought.During drought,the photosynthetic capacity of deciduous tree species,acacia,is most significantly reduced,followed by poplar,but drought do not significantly affect the photosynthetic capacity of evergreen tree species,oil pine and cypress(P>0.05).The net photosynthetic rate(Pn),stomatal conductance(Gs)and transpiration rate(E)of the deciduous species poplar and acacia show a decreasing trend,the intercellular CO2 concentration(Ci)show an "n" trend and the Pn,Gs,E and Ci of the evergreen species oil pine and cypress show an "n" trend.After water restoration,the earliest compensatory effect(day 2)occur in SWC of poplar,and SWC of all other tree species return to control levels by day 5.The photosynthetic parameters of the deciduous species poplar(day 2)and acacia(2 to 18 days)gradually recovered,and the photosynthetic index of all four species show overcompensation.It can be seen that water is an important factor affecting the physiological indicators of photosynthesis in different plants.2.Changes in PSII photochemical reaction centers and photosynthetic mechanism electron transfer differed between plants undergoing drought-replete water processes.The specific manifestation is the variation in plant chlorophyll fluorescence parameters.In general,the chlorophyll fluorescence parameters of different tree species vary widely:The PSII reaction centers of the evergreen cypress and oil pine species are more open and have higher primary light energy conversion efficiency and photosynthetic electron transfer activity.During drought,deciduous species suffer more damage to photosynthetic bodies than evergreen species:The OJIP curves of each species are altered by continuous water stress,with the O-J phase increasing and the I-P phase decreasing,as evidenced by the fluorescence parameters Fo,Vj and Vi increasing and Fm decreasing.In mid-drought(6 to 8 days)deciduous species,the PSII photochemical reaction center is somewhat disrupted and leaf PSII electron transfer is inhibited.Fluorescence parameters of the evergreen species show some adaptation to the initial drought,and the photosynthetic mechanism is not significantly impaired in the later stages.After water restoration,each fluorescence parameter of oil pine recovers fastest or do not decrease significantly during drought,and each fluorescence parameter of other species returned to control level in 2 to 5 days.It can be seen that moisture is an important factor affecting the proper functioning of plant photosynthetic bodies.3.The distribution pattern of soluble sugar ?13C in each organ of different tree species differed during the drought-emergence process.Under field water-holding conditions,leaves and roots of deciduous species have higher ?13C values than evergreen species and branchlets and trunk have lower ?13C values than evergreen species.During the drought,the ?13C values of each organ of poplar and lateral cypress show an overall decreasing trend,while only leaf ?13C values of acacia and oil pine show a decreasing trend.Other organs show a fluctuating increase in ?13C values,as the soluble sugar ?13C values of poplar [(-24.95±0.43)‰],acacia [(-23.71±0.52)‰] and oil pine [(-24.6±0.47)‰] are greatest in fine roots,the soluble sugar ?13C of lateral cypress is greatest in the main trunk [(-23.84±0.69)‰],the soluble sugar ?13C of deciduous tree species show the least in the main trunk and evergreen species show the least in leaves and thick roots.After rehydration,soluble sugars of poplar are preferentially distributed to the fine root portion(day 2),followed by the photosynthetic organ leaves and the coarse roots and trunk that function as water transporters(day 8),with branchlets the slowest(day 18).Acacia are preferentially distributed to the leaves,branchlets,and trunk(day 2),followed by the roots(days 5 ~ 18).Oil pine is preferentially distributed to the leaves(day 3),followed by the trunk and roots(day 10),without affecting branchlets.Lateral cypress is preferentially allocated to twigs and coarse roots(day 3),followed by leaves and fine roots(day 10),with little effect on the main stem.4.There are differences in the response strategies of different tree species' water use efficiency to changes in soil moisture during the drought-return process.The short term and instantaneous water use efficiency of the four species is significantly different between species(P<0.05)under field water holding conditions.All deciduous species have greater short-term water use efficiency than evergreen species.The instantaneous and short-term water-use efficiency of the four species varied in an "n" trend during the drought.Among them,the maximum short-term water use efficiency is 3.90 mmol/mol,3.26 mmol/mol,4.55 mmol/mol and 5.28 mmol/mol for poplar,acacia,oil pine and sagebrush,and the maximum instantaneous water use efficiency is 77.91 mmol/mol,59.56 mmol/mol,75.17 mmol/mol and 95.79 mmol/mol for the four species,respectively.After water restoration,the WUE of deciduous species show compensatory effects on days 5 ~ 8,and the WUE of oil pine is not affected by drought on day 3.The effects of deciduous poplar and acacia E and Ci on WUE are significant(P<0.05),and those of evergreen cypress and oil pine Pn,Gs,Ci and E on WUE are significant(P<0.05)during natural drought-post-drought rewetting.In summary,physiological indicators,photosynthetic mechanisms,and water use of deciduous poplar and acacia species are more affected by soil moisture variability than evergreen pine and cypress species under drought rehydration treatment.Among them,acacia is most affected by soil moisture,and each species showed different degrees of compensatory effects after rehydration.This study combines plant physiological indicators to investigate the response mechanism of different plant water use efficiency(WUE)to drought stress and the compensatory effect of water restoration,and provides theoretical support for the scientific planning of greening tree species in urban areas of Beijing under arid climate.