Anatomical Structure And Histochemistry Of The Rare Plant Myricaria Laxiflora That Adapts To The Riparian Environment Of The Three Gorges Reservoir

Myricaria laxiflora(Tamaricaceae)was an endangered species,which was narrowly distributed in the riparian zone of the Yangtze River Three Gorges in China.After the Three Gorges Dam construction was completed,M.laxiflora habitats were lost.M.laxiflora remained dormant while completely submerged during summer flood pulsed,and then sprouted in the autumn and winter after the floods receded.Using brightfield and epifluorescence microscopy,we investigated the anatomical and histochemical features that allowed Myricaria laxiflora to tolerate both submerged and terrestrial environments to explain the relationship between this species and the riparian environment of the Three Gorges.The main contents and results of this study were as follows:(1)Structure of the thick adventitious root: The thick adventitious roots possessed the primary structures and secondary structure.The primary structures had protoxylem,metaxylem,endodermis,passage cells,cortex,aerenchyma,hypodermis,and rhizodermis.The endodermis had Casparian bands and almost complete suberin lamellae(only a few passage cells).Cortex possessed four to five layers of cells,and irregular lysigenous aerenchyma.The cortex and hypodermal walls had lignified thickenings.The rhizodermis was still intact.The secondary structure possessed primary xylem,secondary xylem,vascular cambia,cork,endodermis,passage cells,cortex and bark.The vascular cambium inwarded to form secondary xylem,The cork cambium developed outward to form cork.The cork had suberized to form bark.The cortex sloughed off as the bark forms.(2)Structure of the fine adventitious root: The fine adventitious roots possessed the primary structures and secondary structure.The primary structures had protoxylem,metaxylem,endodermis,passage cells,cortex,hypodermis,and rhizodermis.The stele had a diarch protoxylem.The endodermis had Casparian bands and suberin lamellae,passage cells.Cortex possessed one or two layers of cells,and the intercellular space.The cortex and hypodermal walls had lignified thickenings.The rhizodermis,cortex and hypodermis sloughed off.The secondary structure possessed primary xylem and cork.The stele had only primary xylem.The cork possessed suberin lamellae.(3)Stem structure: The stem of M.laxiflora possessed the primary structures and secondary structure.The primary structures had Pith,primary xylem,phloem fibers,cortex,chloroplast,epidermis,and cuticle.The secondary structure possessed pith,secondary xylem,vascular cambia,phloem fibers,cortex,chloroplast,cork,epidermis,and cuticle.Young stems of M.laxiflora possessed a lignified fiber ring,a thick cuticle,and a cortex either with chloroplasts and small aerenchyma,or with one layer of cork cells.In contrast,mature stems had prominent secondary xylem in the center of the stem.The cortical cells had redivided to form suberized cork,and a thick bark.(4)Leaf structure: The anatomical structure of the leaves of M.laxiflora from top to bottom was papillose cuticle,upper epidermis,stomata,palisade tissue,aerenchyma,spongy tissue,palisade tissue,lower epidermis,papillose cuticle.The shoots of M.laxiflora had typical xerophyte features,including small leaves,bi-layer palisade tissues,sunken stomata,a hyaline epidermis and a thick papillose cuticle,which indicated that M.laxiflora was adapted to drought environment.(5)apoplastic barriers: The apoplastic barriers in the roots and shoots of M.laxiflora consisted of the endodermis that had Casparian bands and suberin lamellae,a lignified cortex and hypodermis in the primary structures,and the suberized cork in secondary structure in the adventitious roots;A lignified fiber ring and a thick cuticle at the young stems,and the suberized cork at the mature stems;and the thick papillose cuticle in the shoots.The apoplastic barriers and the aerenchyma of the adventitious root and shoots made M.laxiflora remain dormant while completely submerged during summer flood pulses,and then sprouted in the autumn and winter after the floods recede.Thus,M.laxiflora had typical amphibious plant features.(6)General structure: M.laxiflora had thick adventitious roots,fine adventitious roots,and shoots.The thick adventitious roots possessed four to five layers of cortex cells in the primary structure.In the secondary structure,the cortex sloughed off with the bark.The fine adventitious roots contained one or two layers of cortex cells in the primary structure,and only cork in the secondary structure.Both thick and thin adventitious roots had diarch to tetrarch stele with differentiated protoand metaxylem,a cortex with an endodermis,a hypodermis,and a rhizodermis.The cortex and hypodermal walls had lignified thickenings.Aerenchyma were present in the root cortices.The stem possessed cork and an epidermis,as well as a lignified phloem fiber ring enclosing a central cylinder of bundles internal to the cortex.The leaf had two layers of palisade tissue,sunken stomata,a cuticle with a papillose surface and a hyaline marginal epidermis.In summary,the apoplastic barriers and the aerenchyma of the adventitious root and shoots of M.laxiflora made the plant better adapt to the riparian environment of the Three Gorges.This adaptive feature might help explain the phenomenon that M.laxiflora remained dormant while completely submerged during summer flood pulsed,and then sprouted in the autumn and winter after the floods receded.