Responses To Soil Drought-Rehydration On Photosynthetic Characteristics Of Lonicera Japonica Seedlings

Lonicera japonica Thunb.is an important soil and water conservation plants and medicinal economy tree species,and has been planted on a large scale in Yimeng Mountainous area.Lonicera japonica contributes a lot to the ecological restoration and economic development of Yimeng Mountainous area.However,drought and poor site conditions coupled with seasonal droughts in Yimeng mountain area have seriously restricted the growth and productivity of Lonicera japonica.When soil water deficit and rehydration occur,Lonicera japonica adapts and resists the stress through a series of ecological and physiological processes and mechanisms,and the resilience process after rehydration also closely connects with former soil drought stress.Therefore,intensive study including intrinsic process,external morphology,and responding mechanisms is essential by quantizing the relationship between photosynthetic physiological parameters of Lonicera japonica and its soil moisture content.So Lonicera japonica can be carried out scientific and efficient water management.However,studies regarding the effects of severe drought on photosynthetic characteristics of Lonicera japonica are scarce.The responses of the growth,photosynthetic light-response processes,and chlorophyll fluorescence of Lonicera japonica to severe drought remain unclear.There are few studies on the decline of photosynthetic characteristics of Lonicera japonica under severe drought and its resilience after rehydration,which blocks risk diagnose,early warning,and establishment of water management system to severe drought.By using the potted one-year-old seedlings of Lonicera japonica as material,which is one of native tree species in Yimeng Mountainous area,we arranged the experiments,measured and observed the soil water content,photosynthesis,fluorescence,and spectrum parameters,enzymatic activity indices and morphology characteristics of different soil water treatments of Lonicera japonica from March 2016 to October 2016 in the greenhouse of the Forestry Experimental Station of Shandong Agricultural University,Tai'an City,Shandong Province,China(36°11? N and 117°08? E).We use the net photosynthetic rate(Pn)and water use efficiency(WUE)as evaluation indices of photosynthetic “productivity” and “efficiency” to the soil water content,and use time to the photosynthetic parameter minimum value or its maximum value as evaluation index of “velocity” of photosynthesis reduction or its resilience of Lonicera japonica.Based on the experimental data and observed results,we first analyzed responses of photosynthetic characteristics of Lonicera japonica to multi-level soil water ranges from saturated soil water content to soil water content of net photosynthetic rate(Pn)of Lonicera japonica to zero(Pn =0,which Lonicera japonica begin wilting),and then the photosynthetic characteristic parameters decline and resilience under severe soil water stress and rehydration.As a result,the range of severe soil water was determined,and the decline and resilience mechanisms of photosynthetic characteristics under different soil water stresses and rehydration were revealed.Therefore we formed the grading and evaluation systems of soil water availability,explored the decline and resilience processes,mechanisms,and thresholds of photosynthetic characteristics under severe soil drought and rehydration,discussed the coupling effect of drought degree and duration on photosynthetic characteristics.Thus we obtained the water threshold value of decline and resilience of photosynthetic characteristics based on “velocity-productivity-efficiency” under severe soil drought and rehydration,and finally we established the dynamic process system of the decline and resilience of photosynthetic characteristics of Lonicera japonica under soil drought and rehydration.Our study could offer scientific basis for the diagnosis and early risk warning on severe drought stress to Lonicera japonica.The main achievements of this thesis are as follows:1)The photosynthesis gas exchange parameters,chlorophyll(Chl)fluorescence parameters,spectrum parameters,and enzymatic activity indices show threshold response characteristics to relative soil water content(RSWC),and soil water will restrain the photosynthesis of Lonicera japonica if RSWC was too high or too low.According to inhibited photosynthetic characteristics,soil water ranges are divided into water range of severe water stress(RSWC?29.71%),it of moderate water stress(29.71%<RSWC<55.07%),it of mild water stress(55.07%?RSWC<70.54% or RSWC>83.50%),and it of no water stress(70.54%?RSWC?83.50%).According to threshold value of classification,soil water is divided into non-productivity and non-efficiency water(RSWC?13.82%),low productivity and efficiency water(13.82 < RSWC ? 29.71%),middle productivity and efficiency water(29.71%<RSWC?30.64% or RSWC>93.39%),middle productivity and high efficiency water(30.64%<RSWC?51.53%),high productivity and efficiency water(51.53<RSWC?83.76%),and high productivity and middle efficiency water(83.76<RSWC?93.39)using Pn and WUE as evaluation indices.When the dominating reason of the photosynthesis decrease is from stomatal limitation to non-stomatal limitation,the soil water(RSWC)is called RSWCSL-NSL,and when Pn of Lonicera japonica is to zero,the soil water(RSWC)is called RSWCPn=0.Water range of severe water stress(RSWC?29.71%)includ non-productivity and non-efficiency water(RSWC?13.82%),low productivity and efficiency water(13.82<RSWC?29.71%),and RSWCSL-NSL and RSWCPn=0 are called its key threshold value.2)Responses to soil water stress on photosynthetic characteristics of Lonicera japonica are closely related to the coupling effect of drought degree and duration.Under sustained soil water stress(30 days)of the two water threshold value(RSWCSL-NSL,RSWCPn=0),the Pn,WUE,transpiration rate(Tr),stomatal conductance(Gs),stomatal limitation(Ls),apparent quantum yield(?),light-saturated net photosynthetic rate(Pnmax),light saturation points(LSP),dark respiration rate(Rd),maximal fluorescence yield of the dark-adapted state(Fm),potential quantum efficiency of PSII(Fv/Fm),effective quantum efficiency of PSII(?PS?),electron transport rate(ETR),non-photochemical quenching(NPQ),photochemical quenching coefficient(qP),water content index(WI),chlorophyll normalized index(ChlNDI),normalized color index(NDCI),and superoxide dismutase(SOD),peroxidase(POD),and catalase(CAT)activities decreased significantly.Furthermore,the parameters,such as the intercellular CO2 concentration(Ci),light compensation point(LCP),minimal fluorescence yield of the dark-adapted state(Fo),photochemical reflectance index(PRI),and malondialdehyde(MDA)content increased significantly.The decrease of photosynthesis was dominant by non-stomatal limitation,leaves of Lonicera japonica begin to wither and fall,the photosynthetic apparatus were destroyed irreversibly,and photosynthate was severely reduced.According to the processes and characteristics of Pn and WUE responding to the lasting time of water thresholds under severe drought,the severe soil drought grading about the decline of photosynthetic characteristics based on “velocity-productivity-efficiency” under severe soil drought was done,and the critical thresholds in the decline of photosynthetic characteristics were determined,which were the drought of RSWCSL-NSL and RSWCPn=0 lasting 25 d,and 5d,respectively.When soil drought degree exceeds these thresholds,both Pn and WUE of Lonicera japonica are at the lowest level(zero value).3)The resilience of photosynthetic characteristics of Lonicera japonica is closely related to both RSWC threshold values(RSWCSL-NSL,RSWCPn=0)of severe drought stress and its stress lasting time before rehydration.The decline of photosynthetic characteristics of Lonicera japonica under severe soil drought remarkably influences its resilience after rehydration.After rehydration the photosynthesis gas exchange parameters,Chl fluorescence parameters,spectrum parameters,and enzymatic activity indices of Lonicera japonica recovered fast in former 15 days,and then comed to a steady state or recovered slowly.According to the resilience of photosynthetic characteristics,wilting state under the influence of drought stress of RSWCPn=0 can be divided into temporary wilting,which drought stress of RSWCPn=0 last 0 day and permanent wilting,which drought stress of RSWCPn=0 last over 10 days.According to the resilience of photosynthesis,fluorescence fluorescence,spectrum parameters,enzymatic activity indices and morphology characteristics based on “velocity – productivity-efficiency” after rehydration,the critical thresholds of resilience of Lonicera japonica were determined.The first threshold is severe drought treatment which RSWCSL-NSL last 0d,and when soil drought is milder than the threshold,both Pn and WUE can recover to the contrast level in 15 days.The second threshold is severe drought treatments,which RSWCSL-NSL last 10 d,and RSWCPn=0 last 0d,respectively,and when soil drought is milder than this threshold but more severe than the first threshold,Pn and WUE can recover to the level above the average and below the contrast value.The third threshold is severe drought treatment which RSWCSL-NSL last 20 d,and RSWCPn=0 last 10 d,respectively,and when soil drought is more severe than the threshold,Pn and WUE cann't recover to the average level.