| Canopy seed bank is a very unique type of seed bank formed by desert annual plants to adapt to the extreme environment of desert.Heteracia szovitsii is an annual plant of the genus Heteracia in the Asteraceae widely distributed in eastern Europe,Central Asia,Iran and Turkey.This species only distributed in the desert or semi-desert in northern Xinjiang in China,which is an important part of the plant community in this region,and has essential ecological significance for maintaining the species diversity and stability of the desert ecosystem.Through field observation,it was found that the stem of this species did not lodge after achene maturation,while the central achene disperse to the soil surface,the peripheral achene and intermediate achene of the inflorescence did not disperse.In this study,the research method of seed physiology and ecology was used to study the anatomical structure of stem and achene base,achene dispersal dynamics,seed dormancy and germination.The main results are as follows:(1)Role of anatomy: compared with Epilasia hemilasia(plants of the Asteraceae having lodging stems;control),the stem of Heteracia szovitsii was not lodging after the achene matures due to the close to arrangement of vascular bundles,thick phloem fiber,xylem and vessel wall,perimedullary zone and no pith cavity;The anatomical structure of the abscission zone of polymorphic fruit was obviously different.Among them,the parenchyma cells at the base of the peripheral achene and intermediate achene did not dedifferentiate and did not form abscission layer.While the parenchyma cells at the base of the central achene are dedifferentiated,and the abscission layer is initially formed.Therefore,central achene was dispersed earlier,peripheral achene and intermediate achene were dispersed later,forming canopy seed bank with longer duration.(2)Dispersal dynamics: during the dispersion phenological observation period(spring and summer of the following year),the remainder percentage of achenes in each part of the plant(upper,middle and lower)on the crown showed as follows: peripheral achene>intermediate achene>central achene.And the peripheral achene(20.66%,29.00% and 33.19%)and the intermediate achene(27.50%,32.5% and 39.69%)still remained in the canopy at the end of the experiment,forming a canopy seed bank lasting more than one year.(3)Dormancy characteristics: there was physiological dormancy(PD)in all polymorphic fruit of this species.The degree of dormancy was as follows: peripheral achene(intermediate PD)> intermediate achene(intermediate PD)> central achene(non-deep PD).The time of seed vitality of polymorphic fruit in the field was as follows: peripheral achene(47 months after sowing)> intermediate achene(40 months after sowing)> central achene(16 months after sowing).It shows a kind of temporal dispersal strategy,which can spread the risk effectively.(4)Dormancy/germination characteristics in seed bank: in [canopy-soil seed bank](treatment ?;Simulating the transformation process of achene from canopy to soil seed bank in natural habitat)and[soil-soil seed bank](treatment ?;Control,the achene is always buried in the soil),the germination percentage of polymorphic fruit was always as follows: central achene>intermediate achene>peripheral achene.The germination percentage of polymorphic fruit in the stage of planting canopy seed bank in treatment ? was lower than that in treatment ?.At the stage of ? soil seed bank,the germination percentage of peripheral achenes and intermediate achenes was not significantly increased as compared with that at the stage of canopy seed bank(P>0.05),but the germination percentage of central achenes(10 and 24 months)was significantly increased(P<0.05).At the end of the experiment,the seed viability of polymorphic fruit treatment ? was significantly higher than that of treatment ?(P<0.05).The results showed that canopy seed bank extended the viability of seeds and beneficial to the continuation of population.In addition,during dormancy and germination of polymorphic fruit,the soluble sugar content increased at first and then decreased the starch content,while the protein content decreased gradually.Compared with peripheral achenes and intermediate achenes,CAT activity significantly affected the germination rate of central achenes(P<0.05).The structural cause of the formation of the canopy seed bank is mainly due to the strong mechanical supporting capacity of the stem and the incomplete dedifferentiation of parenchyma cells at the base of the achene(peripheral achene and intermediate achene).Because of the differences in remain time,dormancy/germination and vitality in the canopy seed bank,the polymorphic fruit also showed a“bet-hedging” adaptation strategy to its desert extreme environment.The peripheral achene has the characteristics of longer time in the canopy,deeper dormancy and longer viability.It can achieve the purpose of maintaining the population,which represents a “low risk” strategy.Central achene has the characteristics of shorter time in the canopy,low dormancy degree and shorter survival time,which can achieve the purpose of rapid population expansion,which represents a “high-risk” strategy.The time in the canopy,degree of dormancy,and viability of intermediate achene were between peripheral achene and central achene,representing a “lmedium risk” adaptation strategy. |
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