Variation Patterns Of Flower Morphology And Developmental Anatomy Of Nectaries In Eranthis

Flowers are an essential innovative character of Angiospermae.The rich and diverse flower structure adapts to different living environments and pollination media,mainly reflected in types,symmetry,fusion,phyllotaxis,and floral organ number.The petal is the latest organ in the evolution of flowers,and its diversity adapts to pollinators.Petals in Ranunculaceae,which is located at the base of eudicots also known as nectary leaves,retain a large number of original characteristics.Thus,it is an important material to study the diversity and ecological functions of petals.Eranthis Salisb.belongs to Cimicifugeae of Ranunculaceae.The floral structure is vibrant in terms of floral organs.Petals vary within the genus;they are complex bilobed structures,with slender stalks,upper lips,bifurcated lower lips,nectary grooves,two pseudonectaries and other special structures.It is an ideal material for studying flower variation pattern,elaborate petal development,nectary structure and function.In this study,four species of different branches in the phylogenetic tree of Eranthis were selected,including Eranthis stellata Maxim.,Eranthis lobulata W.T.Wang,Eranthis pinnatifida Maxim,and Eranthis hyemalis(L.)Salisbury.Through mathematical statistics analysis,scanning electron microscopy,paraffin sectioning,semithin sectioning,transmission electron microscopy and character reconstruction,the variation in flower structure style,petal development,epidermis micromorphology,mature structure,ultrastructure and petal character evolution were studied.Moreover,Nigella damascena L.was chosen as an outgroup to compare the functional differentiation of petal structures,nectaries and pseudonectaries in two genera.The results show that:(1)Floral organ number and morphology:the number of floral organs and the total number of floral organs in Eranthis are unstable,and there is a positive correlation between the number of floral organs;the number of stamens varies in a wide range and has the highest correlation with the total number of floral organs.It also accounts for the highest total number of floral organs,which means that the stamen is the main contributor to the total number of floral organs;the number of perianth organs conforms to the modified error function(ERF)model.The sepals of Eranthis are oval and lanceolate with smooth edges,while the variant sepals have two,three and four lobes at the apex;the number and division degree of lower lip lobes,the number of pseudonectaries and the morphology of upper lip edge have all changed,among which the number of lower lip lobes increased to 3 is the most common,followed by four and deep upper lip lobes.(2)Petal development:the developmental progress of petals in Eranthis can be divided into five stages:petal primordium generation(stage 1),depression and protrusion in the centre of the petal primordium(stage 2),protrusion healing(stage 3),accessory structure generation(stage 4)and petal maturation(stage 5).The early development stages are similar.In stage 4,some species did not form protrusions at the lower or upper lip,there was no pseudonectary in the mature structure,which led to differentiation and finally led to petal morphological structure diversity.(3)Petal epidermal cells:the epidermal cells of mature petals can be divided into 10 types according to their size,shape,location and function.There is one type of secretory epidermal cells(type 5).In those species with pseudonectaries,there are three types of cells in the region:flat cells with smooth and irregular surfaces(type 1),papillary cells with wax ornamentation(type 6)and polygonal cells with smooth surfaces(type 8),which have a certain reflective ability.The petal cell types of Eranthis stellata are the most complex(5 types),and the cell types of Eranthis hyemalis are the simplest,with only three types.(4)Petal structure:the mature petal structure of Eranthis is two-lip shaped,and the nectary is located at the base of the saccate nectary groove,which is composed of one layer of secretory epidermal cells,9-13 layers of secretory parenchyma cells and vascular tissue.Except for Eranthis hyemalis,the other three species have pseudonectaries.The pseudonectaries of Eranthis stellata and Eranthis lobulata are located in the middle of the lower lip,and the pseudonectaries of Eranthis pinnatifida are located in the top of the upper lip and lower lip.The pseudonectary of Eranthis stellata is hemispherical with an obvious three-dimensional structure,while that of Eranthis lobulata and Eranthis pinnatifida is slightly expanded.The pseudonectary is composed of 1 layer of epidermal cells,6-12 layers of parenchyma cells and vascular tissue.The number of layers of pseudonectary cells is the largest in Eranthis stellata,and the vascular tissue is located on the dorsal side of the pseudonectary.The upper lip of the petal is the thinnest,approximately 3-5 layers of cells.(5)The ultrastructure of the petal nectary and pseudonectary:the secretory cells of Eranthis are small,and there are a large number of mitochondria,endoplasmic reticulum,Golgi apparatus and small vacuoles with different shapes of crytals were observed in cells.The outer tangential wall of the secretory epidermal cells is thickened and there are abundant microchannels in the cuticle,with secretory coverings.A large amount of honey juice is stored in the intercellular space,and the plasmodesmata between the cells are developed;sieve tubes are all over between secreting parenchyma cells and extending to the epidermis,the vessel element are mainly located under the nectaries,and the nectaries secrete nectar mainly through microchannels or cell disruption.The epidermal cells of pseudonectaries are different from those of the secretory zone.There was no secretion accumulation outside the cuticle.The cytoplasm is not dense,the vacuoles are large,and there are many starch granules;the intercellular spaces of parenchyma cells with fewer organelles have no honey juice.Taking Nigella as the outer group,it was found that the outer tangential wall of the epidermal cells in the secretory area of Nigella was obviously thickened.Cell wall ingrowth was observed in both epidermal cells and parenchyma cells.(6)Reconstruction of the ancestral traits of petals:after reconstructing the ancestral state of Eranthis,it is found that the ancestral state of Eranthis is a structure with petals and nectaries but no pseudonectaries.Pseudonectaries in the genus have undergone an evolutionary process from missing to appearing,increasing,and not obvious to obvious.