| Clines are defined as populations that form a series of variation gradients with dradual change of gene requency or phenotype,resulting from the continuous changes of environmental selection pressure in geographical space.The traits of clines have not only differences in phenotypic quantitative traits,but also genetic differentiation controlled by genes.Whether the changes of phenotype and genotype are synchronized or not is an important research topic of clines.Leymus chinensis is one of the important constructive and dominant species pervasive in the eastern Eurasian steppe.Distinctive variations in leaf color of L.chinensis are often seen in natural steppes.At present,studies concerning the variation in leaf color of L.chinensis mainly focus on the gray-green type and yellow-green type.However,little is known about the typical-green type.Here,we chose the leaf color clines(including the gray-green type,yellow-green type and typical-green type)of L.chinensis on the Songnen plain as the study object,and assessed their genetic diversity under natural conditions and their photosynthetic physiological characteristics,ramet phenotypic characteristics and population quantitative characteristics under common garden conditions,and systematically analyzed their characteristic differences at the molecule,physiology,phenotype,and population levels,and finally revealed the differentiation mechanism of leaf color cline in L.chinensis.The results of this study enrich the research theory of plant clines,provide an important reference for further research on the origin and evolution of different leaf colors of L.chinensis,and consolidate a scientific basis for the cultivation of high-quality varieties and the conservation of germplasm resources in L.chinensis.Below are the major results of the present study:(1)At the molecular level,there were significant differences in genetic diversity of L.chinensis among the three leaf color types under natural conditions.The Shannon-Wiener diversity index and the evenness of both the gray-green type and typical-green type were significantly greater than those of the yellow-green type.The gene flow value between the gray-green type and yellow-green type was the least,while the genetic distance was the greatest and the level of genetic differentiation was the highest.The gene flow value between the typical-green type and yellow-green type was the greatest,while the genetic distance was the least and the level of genetic differentiation was the lowest.In the cluster analysis of genetic characteristics,the gray-green type and yellow-green type were divided into two different groups,whereas most of the typical-green was divided into the yellow-green type group.There was no significant correlation between genetic distances and actual geographic distances in the three leaf color types of L.chinensis.However,the expected heterozygosity of the typical-green type had a significantly positive correlation with total phosphorus content of the soil,and that of the yellow-green type with organic carbon content of the soil.A certain degree of gene flow and the selection of environmental factors are major causes for the genetic differentiation of leaf color cline in L.chinensis.(2)At the physiological level,there were significant differences in photosynthetic physiological characteristics of L.chinensis among the three leaf color types under common garden conditions.The net photosynthetic rate,stomatal conductance,chlorophyll a content,chlorophyll a + b content,carotenoid content,and water content of leaves were significantly higher in the gray-green type than in the typical-green type or yellow-green type.In the "double-peak" curve of the daily change of leaf net photosynthetic rate,the peaks were markedly higher in the gray-green type than in the typical-green type and yellow-green type,and were the lowest in the yellow-green type.The light saturation point and light compensation point of leaves were the highest in the typical-green type,and the lowest in the yellow-green type with significant differences between the two types.Both the CO2 saturation point and the CO2 compensation point of leaves were significantly higher in the yellow-green type than the other two types.The maximum photochemical efficiency of photosystem II,performance index and photosynthetic enzyme activity of leaves were significantly greater in the gray-green type than in the typical-green and yellow-green types.In the cluster analysis of physiological characteristics,three leaf color types were divided into three different groups.On the basis of genetic differentiation,the leaf color cline of L.chinensis has obviously synchronized differentiation in physiology.(3)At the phenotypic level,there were significant differences in phenotypic characteristics of L.chinensis ramets among the three leaf color types under common garden conditions.The total leaf area,ramet height,leaf biomass,stem biomass,ramet biomass and stem biomass allocation of vegetative ramets were all the greatest in the yellow-green type,and the least in the typical-green type with significant differences between the two types.The leaf biomass,inflorescence biomass,spikelet number,floret number and leaf biomass allocation of reproductive ramets were all the greatest in the yellow-green type,and the least in the typical-green type with significant differences between the two types.In the cluster analysis of phenotypic characteristics,vegetative ramets of leaf color cline were not grouped solely according to the leaf color,while reproductive ramets were divided into three different groups completely according to the leaf color.On the basis of genetic differentiation,the leaf color cline of L.chinensis has obviously synchronized differentiation in the phenotype of reproductive ramets.(4)At the population level,there were both a certain level of divergence and consistent dynamics and regularity in quantitative characteristics of L.chinensis populations among the three leaf color types under common garden conditions.The vegetative propagation rate of the yellow-green type was greater than that of the typical-green type and the gray-green type,however,with the process of growing season,the dynamics in vegetative propagative numbers of three leaf color types all well accorded with the law of exponential growth.In ramet composition,there were four age classes in the gray-green type and the typical-green type,and five age classes in the yellow-green type.However,the number and biomass of 1a-ramets all accounted for the largest proportion in the age spectrums of three leaf color types,and the ramets were expansive age structures.In rhizome composition,there were three age classes in the three leaf color types,and the length and biomass of 2a-rhizomes all accounted for the largest proportion in the age spectrums,and the rhizomes were stable age structures.The densities of both bud and juvenile ramet banks were significantly greater in the typical-green type and yellow-green type than those in the gray-green type.However,at the end of the growing season,the bud banks and juvenile ramet banks of leaf color cline were all dominated by rhizome buds and tillering node juvenile ramets,respectively.In the aspect of population survival and development,the leaf color cline of L.chinensis adopts the same ecological strategy.In L.chinensis,this study found that there were different levels of differentiation among the leaf color cline at the molecular,physiological and phenotypic levels.The differentiation level was the highest between the gray-green type and yellow-green type,higher between the gray-green type and typical-green type,and the lowest between the typical-green type and yellow-green type.In natural steppes,the typical-green type was not grouped by itself in genetic clustering,but was mostly divided into the yellow-green type group,indicating that there is leaf color cline in L.chinensis,and both the gray-green type and yellow-green type have been stable ecotypes,while the transitional typical-green type have not been a stable ecotype. |
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