Adaptive Strategy Of Aboveground/Belowground Growth Of Leymus Chinensis To Saline-alkali Stress And Clipping Intensity

Overgrazing and clipping are important human factors restricting the productivity of Songnen meadow steppe in north China.Plants can adapt to grazing and clipping through compensatory growth.Land salinization is another important environmental problem in Songnen meadow steppe.The saline-alkali has a very complex impact on plant growth,and plants can adapt this stress environment by altering the allocation strategy of nitrogen and carbohydrate.However,how saline-alkali induces resources integration between ramets and promotes compensatory growth remains unclear,the underlying mechanisms need to be explored.Leymus chinensis,a common perennial clonal species in Songnen meadow steppe,can well tolerate saline-alkali and shows a positive compensation response after clipping（or grazing）.Therefore,the study on the response mechanism and adaptive strategy of L.chinensis compensatory growth to clipping under saline-alkali conditions has important theoretical and practical significance to improve the compensatory growth theory of grassland plants.In this study,three-year field in-situ experiments were carried out in the artificial L.chinensis grassland from 2015 to 2017.The field in-situ experiment includes three levels of clipping treatment:no clipping,moderate clipping（removing 40%plant height）and heavy clipping（removing 80%plant height）,as well as no saline-alkali addition,moderate saline-alkali addition(the mixed salts are applied three times a year,total adding amount 155.5 g m^-2yr^-1)and heavy saline-alkali addition(the mixed salts are applied three times a year,total adding amount 310.5 g m^-2yr^-1).In 2015,the field micro-environment experiment was conducted on the field in-situ experimental platform in the no saline-alkali addition and moderate saline-alkali addition treatment plots.The¹⁵N injection labeled isotope in belowground was used to track the distribution strategy of newly absorbed nitrogen in different organs of L.chinensis during compensatory growth.At the same time,the greenhouse pot experiment of clonal integration was carried out,and ¹⁵N isotope was used to mark the ramets of L.chinensis with complete rhizome connection,so as to study the direction and intensity of clonal integration between ramets under homogeneous and heterogeneous saline-alkali and/or clipping treatments.We mainly analyzed the changes of morphological indices,such as organs biomass,ramets density and tiller numbers;physiological indices,such as photosynthetic rate,soluble sugar,K⁺and Na⁺ions,¹⁵N recovery ratio and ¹⁵N allocation ratio of L.chinensis in different parts under different saline-alkali and clipping intensity.The main results are as follows:（1）The field experiment results showed that the compensatory growth responses to saline-alkali were different between the aboveground and belowground of L.chinensis.Under no saline-alkali addition treatment,there was no significant difference between the effect of moderate clipping treatment and no clipping treatment on the biomass accumulation,aboveground biomass compensation index was 1.01,resulting in equal-compensatory growth,but belowground biomass accumulation was significantly lower than that of no clipping treatment by 21.1%,and the compensation index was 0.78,resulting in under-compensatory growth.But different from no clipping and moderate clipping treatments,the biomass of aboveground and belowground after heavy clipping was decreased significantly by24.8%and 47.4%respectively,and the compensation indices were only 0.59 and 0.53,resulting in under-compensatory growth.These results suggest that clipping significantly reduces the belowground biomass allocation of L.chinensis under no saline-alkali addition environment,and plants try to recover the compensation of aboveground carbon assimilation function.However,under moderate and heavy saline-alkali addition environment,clipping did not significantly change the biomass accumulation,but it promoted belowground biomass allocation of L.chinensis by21%and 18%respectively.Belowground biomass compensatory growth index was1.42 and 1.02,respectively.These results suggest that under saline-alkali addition environment,L.chinensis need to maintain nutrient absorption function via increasing the growth of roots,showing over-compensatory and equal-compensatory growth.（2）The biomass of L.chinensis was not affected by the interaction of saline-alkali and clipping.No matter what kind of saline-alkali treatment,there was no significant physiological restriction,and no significant change in total soluble sugar on the aboveground of L.chinensis was observed.This indicated that biomass compensatory growth of L.chinensis was not affected by saline-alkali treatment,but significantly affected only by clipping intensity,showing equal-compensatory growth after moderate clipping and under-compensatory growth after heavy clipping.However,unlike the aboveground compensatory growth response,L.chinensis strongly responded to the physiological limitation of saline-alkali production by increasing partial biomass allocation and sugar accumulation changes of organ,leading to a significant belowground compensatory growth.Under no saline-alkali environment,clipping treatments resulted in a significant decrease in total soluble sugar accumulation and different small molecular sugars in belowground,especially total soluble sugar in the root decreased by 33%and 44%,which was an important reason for the under-compensatory growth of belowground.However,moderate saline-alkali addition promoted total soluble sugar accumulation in belowground and changed the response of different sugar components to clipping.Especially,the total soluble sugar content in roots after moderate clipping was significantly 20%higher than that without the clipping treatment,and small molecular sugar contents in roots and stems were also higher than that without clipping,resulting in over-compensatory growth in belowground under moderate saline-alkali addition and moderate clipping treatment.Although heavy saline-alkali addition did not significantly increase total soluble sugar content of roots and stems,it promoted total soluble sugar and small molecular sugar contents in roots.Therefore,after moderate clipping,the total soluble sugar in roots did not decrease significantly compared with no clipping,which led to equal-compensatory growth in belowground.Our results suggest that saline-alkali addition improves the compensatory growth performance of belowground by regulating total soluble sugar and small molecular sugar contents in rhizomes and roots.However,heavy clipping seriously damaged the carbon assimilation organ function of L.chinensis,and resulted in a significant reduction of more than 40%of total soluble sugar contents in roots,showing under-compensatory growth.In all,the results indicated that saline-alkali addition induced root sugar regulation of L.chinensis,which was an important reason for the different aboveground and belowground compensatory growth responses.（3）The results of clonal integration isotope labeling experiment showed that in the control treatment,the mother ramet of L.chinensis transferred a large ¹⁵N amount（22.5%）to the daughter ramet for supporting its growth,indicating that there was clonal integration among the clonal ramets of L.chinensis in the homogeneous stress-free environment.When both the mother ramet and daughter ramet were clipping and/or saline-alkali treatment（homogeneous stress treatments）,the ¹⁵N transfer ratio was significantly reduced to 8.5%-14.6%,indicating that the homogeneous stress environment reduced the integration intensity between ramets,and the ramets adopted the"relatively selfish"clonal integration strategy.However,when only the daughter ramets were treated with saline-alkali（heterogeneous treatment）,the photosynthetic rate of daughter ramet increased,and promoted the growth and ¹⁵N recovery ratio of the mother ramet,indicating that L.chinensis can overcome saline-alkali stress by"division of labor".While only clipping the daughter ramet also inhibited the biomass of the mother ramet,and the ¹⁵N transfer ratio between ramets increased,indicating that ramets could share resources by clonal integration to overcome the damage by clipping.The heterogeneous saline-alkali and clipping treatment resulted in a significant decrease in biomass of the daughter ramet,and had a little effect on the total biomass of the mother ramet.However,the aboveground and belowground biomass allocation of mother ramets was changed,and there was still existing 16.7%¹⁵N transfer ratio between ramets,indicating that clonal integration still existed even if the daughter ramet was subjected to the dual stresses of saline-alkali and clipping.These results indicated that clonal ramets can adopt different"mutual assistance"strategies to adapt to different heterogeneous stresses,which was key reason why L.chinensis can become a dominant population in Songnen meadow steppe.（4）As the main nitrogen pool,and more than 60%of the newly absorbed nitrogen was always distributed to the aboveground for plant growth and not affected by the saline-alkali addition and clipping treatment.The increase of partial aboveground nitrogen allocation contributed to the improvement of carbon assimilation function of L.chinensis.Therefore,whatever growing under saline-alkali addition conditions or not,moderate clipping always leads to over-compensatory growth of leaves and stems.Saline-alkali addition promoted the accumulation and carbon allocation to roots,improved nitrogen absorption,and then contributed to belowground compensatory growth.Therefore,under saline-alkali addition environment,moderate clipping promoted L.chinensis to showed a positive compensatory growth response,and the ramets density and rhizomes number were significantly increased,compensated for the loss of carbon assimilation caused by clipping,ensuring the supply of assimilated substrate during resource absorption,and finally promoted the over-compensatory growth.However,under saline-alkali addition environment,heavy clipping significantly reduced the total ¹⁵N recovery of L.chinensis by 20.1%.Only stem base had high nitrogen distribution,showing a"conservative"nitrogen distribution strategy,leading to the occurrence of under-compensatory growth.These results suggest that saline-alkali addition influences compensatory growth strategy by altering nitrogen distribution in different organs of L.chinensis.In summary,the resource allocation strategy of L.chinensis was mainly affected by clipping intensity.Moderate clipping promoted the nitrogen allocation to the aboveground of L.chinensis,which provided energy support for the restoration of aboveground carbon assimilation function.While saline-alkali addition improved the compensatory growth performance of belowground by regulating the belowground sugar distribution after moderate clipping.However,heavy clipping always led to under-compensatory growth of L.chinensis.Plants took a"conservative"resource isolation strategy,and allocated resources to the stem base that were not vulnerable to stresses,which would help the long-term survival of L.chinensis.In addition,clonal integration always exists between clonal ramets,which is an important mechanism for L.chinensis to the adaptation to saline alkali stress and clipping intensity.Generally,homogeneous stress reduce the integration intensity between ramets,and plants adopt a"selfish"integration strategy.When subjected to heterogeneous stress,plant ramets weigh"cost-benefit",and adopt different"mutual assistance"integration strategies.Our results suggest that different saline-alkali addition change the functional balance between aboveground and belowrground organs of L.chinensis after clipping,and then affect resources allocation strategy and clonal integration strategy between ramets,resulting in different compensatory growth between aboveground and belowground.Therefore,under the moderate saline-alkali addition L.chinensis grassland,moderate grazing or clipping may not lead to a negative impact on the grassland,instead it can help to restore L.chinensis production from the whole plant perspectives.This study further enriches and improves the compensatory growth theory of grassland plants,and provides a scientific basis for the restoration of saline-alkali grassland and the sustainable utilization.