Response Of Juglans Mandshurica Seedlings To Chilling Stress At Different Initial Growth Temperatures

Northeast China is located in the middle and high latitudes at the eastern end of Eurasia,which belongs to the sensitive area of climate change response.It is also the heavy and frequent area of low-temperature disasters in China.Climate warming leads to the delay of the initial frost period,the advance of the final frost period,the early development of trees and the postponement of dormancy,which makes the trees in the early growth stage vulnerable to extreme low temperature disasters caused by climate fluctuations in spring and autumn.Based on this,this paper takes the annual seedlings of Juglans mandshurica,the main constructive species in the eastern mountainous area of Liaoning Province,as the test material,uses simulated natural cooling to create different low-temperature stress environments,and discusses the response of forest seedlings growing under different initial ambient temperatures to chilling stress.The results will provide basic data and scientific basis for studying the development dynamics of early regeneration of temperate secondary forest ecosystem under the condition of climate warming.The main results are as follows:(1)Chilling stress gradually reduced the biomass of roots,stems and leaves of Juglans mandshurica seedlings grown at different initial environmental temperatures(T1,T2 and T3),but the change range of leaf biomass was small.At the stage of normal temperature and low temperature treatment of 4℃,the change of root biomass of Juglans mandshurica seedlings showed T2 treatment(initial temperature of 15 ℃)> T1 treatment(initial temperature of10℃)> T3 treatment(initial temperature of 20℃);For the stem,the changes of biomass under normal temperature,4℃ and 2℃ were T3 treatment > T2 treatment > T1 treatment;For leaves,there was no significant difference in biomass among T1,T2 and T3 at room temperature and 4℃(P > 0.05).When the low temperature treatment was 2℃～-4℃,the biomass of roots,stems and leaves of Juglans mandshurica seedlings were significantly different among T1,T2 and T3 treatments(P < 0.05).In addition,when the stress temperature was-4℃,the seedlings of Juglans mandshurica under T3 treatment had died.(2)Under low temperature stress,the content of non-structural carbon(NSC)in the roots and stems of Juglans mandshurica seedlings grown at different initial environmental temperatures showed a gradual increase trend.The increase range of NSC content in the roots of Juglans mandshurica seedlings under T2 and T3 treatment(supplementary increase amount)was significantly greater than that under T1 treatment(supplementary increase amount).Under T1 treatment,the content of NSC in leaves increased first and then decreased with the decrease of stress temperature,and reached the maximum at 0 ℃.Under the treatment of T2 and T3,the content of NSC in leaves decreased with the decrease of temperature.Therefore,cooling promoted the accumulation of NSC content in roots and stems of Juglans mandshurica seedlings,and increased the content of starch and soluble sugar in varying degrees.(3)The responses of photosynthetic characteristic parameters and chlorophyll fluorescence parameters of Juglans mandshurica seedlings grown at different initial temperatures to low temperature stress were different.Among them,net photosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate(Tr),primary light energy conversion rate(Fv/Fm)and electron transfer quantum efficiency(ΦPSⅡ),apparent electron transfer rate(ETR)and non photochemical quenching(NPQ)showed an obvious downward trend with the aggravation of low temperature stress under T1,T2 and T3 treatment.When the stress temperature dropped to about 0 ℃,the photosynthetic and fluorescence parameters of the treatment group with higher initial ambient temperature(T2 and T3)were significantly lower than those of the treatment group with lower initial ambient temperature(T1).Therefore,the coupling of higher initial temperature and lower stress temperature makes the PSⅡ reaction center irreversibly inactivated or destroyed,which leads to the significant reduction of light energy conversion and the intensification of leaf photoinhibition.In conclusion,this study reveals the response mechanism of the early growth of the seedlings of the main constructive species in the mountainous area of Eastern Liaoning to low temperature stress after climate warming.The results can provide a basic theory for the study of the development dynamics of the early regeneration of temperate secondary forest ecosystem under the condition of climate warming,and also provide a scientific basis for the early formulation of secondary forest management countermeasures to deal with climate change in the future.