Embryological observation and reproductive system of invasive alien plant Ageratum conyzoides L.
Alien plant invasions are a serious threat to agricultural production and economic development. Understanding the reproductive characteristics of invasive alien plants can provide a theoretical basis for effective prevention and control. Ageratum conyzoides L., an alien Asteraceae species, has invaded 23 provinces and administrative regions in China, where it has greatly impacted local agriculture and the economy. Here, flow cytometric seed screening (FCSS) and controlled pollination experiments were used to study the reproductive system of A. conyzoides. Cytoembryological development of the ovule and anther was observed by differential interference contrast (DIC) microscopy after the application of whole clearing technology. The FCSS results showed that the seeds were produced by both sexual reproduction and apomixis, i.e., facultative apomixis. There were no significant differences in seed sets between open pollination and bagging treatment, but significant differences were found among seed sets under emasculation and the other two treatments, with relatively high seed sets under open-pollination 88% ± 1.2% and bagging treatments 86.2% ± 1.2%. Thus, A. conyzoides showed a high degree of self-compatibility. Embryo sac development under sexual reproduction was Polygonum-type, whereas the apomixis embryo sac was Hieracium-type, which was initiated by one or two nucellar cells at the chalazal end and development into an eight-nucleate embryo sac without meiosis. Embryo and endosperm were developed autonomously and without pseudogamy. Polyembryos were observed in some ovules. A. conyzoides showed precocious embryo formation, which began before flowering. Pollen germinated within the anther and cleistogamy occurred. Therefore, the reproductive traits of cleistogamy and facultative apomixis ensure A. conyzoides fecundity in different environments and increase the opportunity of naturalization and invasion in new habitats, which is closely related to its strong invasiveness.