Comparative Study On Anatomical Structure And Histochemistry Of Invasive Plant Water Hyacinth That Adapts To The Floating Habitat

Water hyacinth(Eichhornia crassipes),a free-flfloating aquatic macrophyte belonging to the family Pontederiaceae,is invasive in tropical and subtropical areas worldwide.Using brightfifield and epiflfluorescence microscopy,we investigated the anatomical structures and histochemical features allowing this species to flfloat freely,and to successfully invade aquatic environments.Sections were stained with Sudan red7B(SR7B)to visualize suberin in the cell walls,berberine hemisulfate–aniline blue(BAB)to visualize Casparian bands and lignin in the cell walls,and toluidine blue O(TBO)to visualize general structures.The main results of the study are as follows:(1)The anatomical structure of water hyacinth by staining methods of cell wall histochemistry.The gross morphology of the water hyacinth is characterized by short vertical rhizomes,horizontal rhizomes,pedicels,leaves with swollen petioles,and adventitious roots with lampbrush-like fifine lateral Roots.We found that the primary structures of the dense,fifine lateral roots of the water hyacinth were anatomically and histochemically similar to the thicker adventitious roots.In water hyacinth,all adventitious roots had an endodermis as is common in angiosperms.In addition,the hypodermis and the cortex of the fifine lateral roots were each one cell-layer thick.Generally,the Casparian bands in the root endodermis contain suberin and lignin,however,in the water hyacinth they were composed only of lignin,even at the mature stage.the water hyacinth fifine roots has a uniseriate lignifified cortex and hypodermis.The hypodermis(between aerenchyma and rhizodermis)of water hyacinth included a sclerenchyma ring.the multi-layered hypodermis,which has Casparian bands and suberin or lignin,became a multiseriate exodermis at maturity.the water hyacinth stems have spacious central cylinders,have a thick cuticle,a lignifified or suberized peripheral mechanical ring with an exodermis,and either an endodermis or a sclerenchyma ring(lignifified or suberized)delimiting a central cylinder.The petioles of the water hyacinth also have a cuticle,Water hyacinth blades have a thin cuticle,palisade tissue near the upper epidermis,and two rows of vascular bundles in the parenchyma.(2)The anatomical structure characteristics of water hyacinth that can grow floating.The air spaces in the fifine adventitious roots of the water hyacinth,which included schizogenous intercellular spaces even at the mature stage.Typically,radial lysigenous aerenchyma appears in the middle cortex at maturity and releases raphides in water hyacinth thick roots.Water hyacinth shoots had typical spacious schizo-lysigenous aerenchyma,with many raphides,in the cortex and central pith.In the water hyacinth shoots,protoxylem lysigenous intercellular spaces led to emergence of proto-xylem lacuna.The free-flfloating lifestyle of the water hyacinth might be enabled by the dense fifine roots,the lignifified cortex and the hypodermis,all of which absorb ions.In addition,the lysigenous aerenchyma in the roots and the schizo-lysigenous aerenchyma in the shoots might serve to aerate the plant to improve survival under anoxic conditions.Our results thus indicate that water hyacinth possesses anatomical traits that facilitate adaptation to and invasion of aquatic environments.(3)The barrier structure of water hyacinth intrusion resistance in different water environment.We identifified several apoplastic barriers in water hyacinth,including an endodermis with lignin,a lignifified cortex and hypodermis,and a thin cuticle.These results are consistent with previous studies of the barriers to block oxygen solute transport between plants and environments.In the water hyacinth roots,the non-suberized endodermis,as well as the lignifified cortex and hypodermis,might have evolved to support ions absorption in oligotrophic aquatic environments.these structures effiffifficiently remove nitrogen,phos-phorus,heavy metals,and organic contaminants from polluted waters.These air sapces can store oxygen on the one hand to adapt to the hypoxic environment,on the other hand,increase the buoyancy and supporting force of the water hyacinth,so that the water hyacinth can grow floating.The apoplastic barrier structures controls the exchange of water,ions and oxygen in the internal and external environments,ensuring that water hyacinth invades different waters with harsh environments.A large number of air spaces and apoplast barrier structures in water hyacinth plants are important structural features that enable them to adapt to floating habitats,our study elucidated the anatomical and histochemical features that allow water hyacinth to thrive in aquatic environments.