| As the ecological environment,social economy,and human health of the invaded areas are seriously threatened,biological invasion has been paid more attention by governments,international organizations,and academia,becoming one of the three major environmental problems in the world today.Xanthium spinosum L.is a worldwide weed native to South America.It is widely distributed in central and southern Europe and Pacific northwest regions.In 1932,the wild naturalized population was first discovered in Dancheng County,Henan Province,and now it is widely distributed in north and northwest China.Because of its strong ecological adaptability and high seed-bearing capacity,X.spinosum often becomes the dominant species in the community;it spreads rapidly because it is easily attached to the fur of cattle,sheep,and other animals,causing serious harm to the local agriculture,forestry,animal husbandry,and biodiversity.It has now been included in the "List of Quarantined Harmful Organisms of Imported Plants of the People’s Republic of China".Many scholars at home and abroad have carried out relevant studies on X.spinosum.However,there is no report on its invasion mechanism.Many studies have shown that reproductive characteristics are generally considered as crucial determinants of invasion success.Therefore,by comparing the reproductive characteristics of X.spinosum with those of native plant Xanthium sibiricum Patr.,this study aims to explore the mechanism,especially the reproductive ecology,for the successful colonization and spread of X.spinosum,to prevent its rapid expansion and make contributions to the ecosystem stability,and protection of biodiversity and the traditional Chinese medicine plant,X.sibiricum.The main conclusions of this study are as follows:1.By comparing the flowering phenology of X.spinosum and native plant X.sibiricum,it was found that the duration of flowering period of female and male flowers of X.spinosum was about one month longer than that of X.sibiricum.At the individual level,the male and female flowering periods of X.spinosum(80 and 85 d,respectively)were 31 and 25 d longer than those of X.sibiricum(49 and 60 d,respectively),respectively.At the population level,the male and female flowering periods of X.spinosum(91 and 100 d,respectively)were 26 and 27 d longer than those of X.sibiricum(65 and 73 d,respectively),respectively.Long flowering period can reduce the risk of pollination failure caused by bad weather and other external conditions,and significantly improve the probability of successful pollination and fertilization of X.spinosum.Moreover,the flowering time of X.spinosum and X.sibiricum overlapped to a higher degree.At the individual level,the male florescences of X.spinosum and X.sibiricum overlapped for 42 days,the female florescences of X.spinosum and X.sibiricum overlapped for 55 days,and the male florescence of X.spinosum and female florescence of X.sibiricum overlapped for 59 days.At the population level,the male florescences of X.spinosum and X.sibiricum overlapped for 58 days,the female florescences of X.spinosum and X.sibiricum overlapped for 69 days,and the male florescence of X.spinosum and female florescence of X.sibiricum overlapped for 71 days.High overlap in flowering time is likely to result in cross-breeding between the two species,leading to the reduction or extinction of X.sibiricum,or even change its evolutionary direction.2.A long time observation found that X.spinosum and X.sibiricum were wind-pollinated.Results showed that the pollen dispersal of X.spinosum and X.sibiricum showed a single peak curve pattern: the number of pollen grains dispersed increased first and then decreased with the increase of distance.Under the 0.89 m/s wind speed,the farthest distance of X.spinosum pollen dispersing with wind was 34 m,and that of X.sibiricum pollen dispersing with wind was 40 m.However,the total amount of pollen dispersed for X.spinosum was 22.87% more than that for X.sibiricum.Especially in the range of 0-8 m,the amount of pollen dispersed for X.spinosum was 67.5% more than that for X.sibiricum.Because X.spinosum always distributes in high density in the community,the large amount of pollen dispersed by X.spinosum will significantly increase the probability of successful outcrossing in its population,help to produce more outcrossing seeds and improve the environmental adaptability of its offspring.3.The duration of stigma receptivity of X.spinosum(10 d)was 1 d longer than that of X.sibiricum.Within one day,the pollen viability of X.spinosum was 1 h longer than X.sibiricum.After 14:00,the pollen viability of X.spinosum was stronger than that of X.sibiricum.The pollen viability of X.spinosum could be maintained at 75% or 99% from 08:00 a.m.to 20:00 p.m..In addition,the total number of pollen grains per plant of X.spinosum was 26,504,809 ± 3,704,981,which was 1.46 times of that of X.sibiricum(18,158,808.86 ± 4,654,836),and the average number of ovules per plant of X.spinosum was 21,516 ± 2,010,which was 14.72 times of that of X.sibiricum(1,462 ± 94).Meanwhile,the stigma surface area and pollen cross-sectional area of X.spinosum and X.sibiricum showed that the stigma surface area of X.spinosum was about 2% smaller than that of X.sibiricum,but the pollen cross-sectional area of X.spinosum was about 14.5% smaller than that of X.sibiricum.Therefore,the ratio of stigma surface area to pollen cross-sectional area(S/C)of X.spinosum was 12.3% higher than that of X.sibiricum,increasing the probability of pollen grains captured by stigma of X.spinosum and successful pollination and fertilization.X.spinosum can not only maintain a higher pollen viability for a relatively long time,but also maintain a longer stigma receptivity duration,and higher average number of pollen grains and ovules per plant and S/C value.These characteristics greatly increase the probability of successful pollination and fertilization of X.spinosum,and significantly increase the reproductive pressure in the early stage of invasion,which is of great significance to improve the invasiveness of the alien plant.4.The ratio of dry weight of reproductive organs(fruits and male inflorescences)of X.sibiricum to its total dry weight was 47.12%,while that of X.spinosum(55.39%)was 17.55% higher than that of X.sibiricum,indicating that X.spinosum invested more than half of its resources in reproduction,which can significantly increase the probability of successful reproduction and its invasive ability.Moreover,the average number of seeds per plant of X.sibiricum was only 1404,while that of X.spinosum was as high as 21232.The high seed yield can help X.spinosum colonize and spread rapidly in invaded areas.5.The seed set rates for selfing and outcrossing treatments of X.spinosum was as high as 97% and 80%,which were higher than those of X.sibiricum,indicating that X.spinosum can be outcrossed and selfed and has certain advantages over X.sibiricum.This flexible mating system significantly enhanced the probability of reproductive success and ensured as many seeds as possible produced.The seed set rate for selfing treatment of X.spinosum was significantly higher than that of outcrossing treatment;selfing can reduce the interference of external factors on reproduction,so as to achieve the goal of rapid and successful reproduction.The seed set rate for outcrossing treatment of X.spinosum up to 80% was of positive significance for avoiding self-decline and improving the fitness of its offspring.Apomixis could not be found in X.spinosum and X.sibiricum.In conclusion,compared with native plant X.sibiricum,the invasive plant X.spinosum has obvious advantages in flowering phenology,pollination,stigma receptivity,pollen viability,ratio of stigma surface area to pollen grain cross-sectional area,reproductive allocation,breeding system,and other reproductive biological characteristics.It has high sexual reproductive capacity,which may be one of the important reasons for its successful invasion. |
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