| After the completion and operation of the Three Gorges Reservoir(TGR),there is a hydro-fluctuation belt on both sides of the reservoir with a maximum range of 30 m and a coastline of 5578 km due to the specificseasonal water management of “storage in winter and drainage in summer”.Species experience periodic stress of “submerged in winter and drought in summer” each year in this belt of the reservoir.Most of them gradually died due to the long-term periodic submergence,resulting in the decrease of ecological service functions.In order to solve these problems,many studies proposed that the construction of plantation was a sustainable measure to improve the ecological environment of the extinction zone in the water-level-fluctuating zone.In previous studies,a number of flood-tolerant candidate plants were selected by simulation tests,and these plants improved the eco-environment in the TGR after the construction of plantation.In order to manage plantation more effectively,the researches on characteristics of flood-tolerant plants to submergence in situ have been paid more attention.The root metabolism is closely related to plant's survival under submergence.Non-structural carbohydrates(NSC)are the energy source for plants under submergence,and organic acids are one of the important secondary metabolites,both play an important role in their tolerance to submergence.To explore the response characteristics of the primary metabolites NSC and secondary metabolites of oxalic acid,tartaric acid and malic acid to winter submergence in the TGR,in this study,the experimental plot was located in the water-level-fluctuating zone of the Ruxi River in Zhong County of Chongqing,and the suitable woody plants Taxodium distichum and Salix matsudana were studied.The experimental pot seedlings were placed on three different elevation gradients,175 m above sea level(ASL,control group,SS),170 m(moderate submergence treatment group,MS),and 165 m(deep submergence treatment group,DS),respectively,before the impoundment in September 2015.After suffering from submergence in winter,the experimental seedlings were collected and the contents of non-structural carbohydrates(NSCs),organic acids such as oxalic acid,tartaric acid and malic acid in plant roots were exaimed.The results as follows:1.Response of NSC contents in roots of T.distichum and S.matsudana to winter submergenceT.distichum grew well under moderate submergence condition of 170 m ASL by maintaining higher contents of soluble sugar,starch and NSC in roots after suffering winter submergence.Although the height of T.distichum and S.matsudana,the contents of soluble sugar and NSC in roots of T.distichum significantly decreased by deep submergence at 165 m ASL.It was an important mechanism for its tolerance to deep winter submergence by maintaining the root starch content as the control.Compared with T.distichum,S.matsudana maintained higher contents of soluble sugar,starch and NSC after experiencing moderate and deep submergence.The results indicated that S.matsudana showed a positive response to submergence with higher NSC content in taproots and lateral roots.In addition,there were strong positive correlations between the contents of soluble sugar and starch in taproot and lateral root of T.distichum and S.matsudana.2.Response of oxalic,tartaric and malic acids in roots of T.distichum and S.matsudana to winter submergenceThe results of this study showed that T.distichum and S.matsudana positively responded to winter submergence by maintaining high contents of oxalic,tartaric and malic acid in the roots.The metabolism of oxalic acid in lateral roots,tartaric acid in taproots and lateral roots of T.distichum were more positive than that of S.matsudana in moderate submergence treatment at 170 m ASL,while the metabolism of oxalic acid in lateral roots,tartaric acid in taproots and lateral roots of S.matsudana were more positive than that of T.distichum in deep submergence treatment.Furthermore,the contents of malic acid in taproots and lateral roots of S.matsudana were higher than that of T.distichum at different winter submergence conditions,which showed that there was difference between two species in organic acid response to winter submergence.Moreover,there were signif icant correlation between the contents of oxalic acid,tartaric acid,malic acid,and soluble sugar,starch,NSC in the roots of T.distichum and S.matsudan,respectively,which showed a linkage between primary and secondary metabolites of roots in response to winter submergence in TGR.3.Dynamic characteristics of oxalic,tartaric and malic acids in roots of T.distichum and S.matsudana during recovery periodThe dynamic characteristics of the three organic acids in the roots of T.distichum and S.matsudana during the recovery period were useful to understand the long-term adaptation mechanism of the two species in the hydro-fluctuation zone of the TGR.The results indicated that the contents of oxalic,tartaric and malic acid in roots of two species showed significant changes with the recovery time.During the experiment period,the contents of taproot oxalate,tartaric acid in taproot and lateral root and malic acid in lateral root of T.distichum,oxalic acid in taproot and lateral root,tartaric acid in lateral root of S.matsudana in moderate and deep submergence treatments showed similar trends,i.e.increased in the de-submergence period,decreased in the growth period,and then no signif icant changes.On the contrary,the contents of tartaric acid in taproot,malic acid in taproot and lateral root of S.matsudana increased in the growing period.However,both T.distichum and S.matsudana maintained higher contents of oxalic,tartaric and malic acid than that before flooding after growing during the recovery period,which could positively respond to the next submergence.In general,the response of tartaric and malic acid in roots of S.matsudana was more positively to soil moisture stress in the TGR when compared with T.distichum.In summary,the survival rates of two species were 100% during the experiment period.The metabolism of NSC and oxalic acid,tartaric acid and malic acid of roots made a great contribution during this process.T.distichum responded to moderate winter submergence by maintaining higher contents of soluble sugar,starch and NSC,oxalic,tartaric and malic acid of root.Moreover,it also maintained starch content as the control and higher contents of oxalic,tartaric and malic acid to deep submergence.However,there were higher contents of soluble sugar,starch and NSC,oxalic,tartaric and malic acid in roots of S.matsudana in moderate and deep submergence treatments.The correlations between soluble sugar and starch,NSC and organic ac id indicated that the primary and secondary metabolites were closely linked to each other in response to winter submergence in the TGR.T.distichum and S.matsudana tended to tolerant winter submergence with “quiescence” strategy of saving energy,i.e.strengthened the reserve of starch and NSC content in the root while maintaining higher organic acid metabolism for supplyying energy.The two species maintained organic acid metabolism as the level before flooding,to ensure the sustainable growth in this belt of the reservoir. |
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