Anatomy Of Petal Development In The Berberidaceae Family

Angiosperms are the latest group during the seed plant evolution,and flowers are the key innovative character.Petals,as the latest flower organs in flower,have rich diversity in morphological,micromorphology and nectary structure.Petals play an important role in attracting pollinators.Many studies focused on the petal origin and evolution,molecular mechanisms and diversity.Petals in Ranunculales which is a basic group of eudicots have a nectaries structures and can produce nectar,thus named nectary-leaves,and is an important material for studying the origin,evolution and diversity of petals.Berberidaceae,as one of the core groups of Ranunculales,which has petals in various shape,either simple flakes without nectaries and complex structures with nectaries and accessory structures.In this study,seven species belonging to seven different genera of Berberidaceae were investigated,and the genera are selected from different systematic position of the phylogenetic tree of Berberidaceae._The scanning electron microscopy,histology,and transmission electron microscopy were used to study the petal development,micromorphology,structure,and ultrastructure.The results showed that:Petal development:in Berberidaceae,the development of petals was divided into the following five stages:Stage 1,broad oval petal primordium initiation;Stage 2,delayed development of petals;Stage 3,re-developed into an oval shape;Stage 4,the differentiation of petals;stage 5,reached the mature stage.In Berberidaceae,petal primordium and stamen primordium occurred independently.In different species,different morphology of mature petals was mainly caused by the differences in stage 4.In Nandina domestica and Diphylleia sinensis,the petals only elongated and widened to reached an oblong shape;in Caulophyllum robustum,the top of the petals became thicker and the base was extended to form a fan shape;in Berberis poiretii and Mahonia bealei,the base of the petals formed two glands,top of petals formed bilobed apices,finally formed an obovate shape;in Epimedium sagittatum,the middle of petals appeared capsules and deepened,finally formed saccate petals.Petal structure:in Berberidaceae,the petal structure was similar.In Nandina domestica and Diphylleia sinensis,the nectary-less petals consisted of upper and lower epidermis,5-8 layers of parenchyma cells,and 4-6 vascular bundles.In Caulophyllum robustum,Berberis poiretii,Mahonia bealei and Epimedium sagittatum,the petals which had nectary,the part without secretion function was composed of upper and lower epidermis,3-12 layers of parenchyma cells,and 1-3 vascular bundles;the nectary was composed of secretory epidermis,3-20 layers of secreted parenchyma cells and vascular tissue.The location of nectary was different in these species,the layers of secreted parenchymal cells and the involvement degree of vascular tissue were different.(1)Nectary loctation:In Caulophyllum robustum,the nectary tissue being located at the thickened area of the top of the petals;in Berberis poiretii and Mahonia bealei,the nectary tissue being located on the glands of the base of petals;in Epimedium sagittatum,the nectary tissue being located at the inner tip of the capsule.(2)Vascular tissue of nectary:Except nectary tissue of Berberis poiretii,which had vessel elements,in the other three species,the vascular tissue only consisted of sieve tube elements.In Caulophyllum robustum and Berberis poiretii,sieve tube elements were distributed between parenchymal cells and epidermis;in Mahonia bealei and Epimedium sagittatum,sieve tube elements were distributed between parenchymal cells.Petal micromorphology:The mature petal epidermal cells of Berberidaceae were distinguished 11 types based on their size,morphology,function and characteristics,3 of which were secretory epidermal cells,which were slim rectangular cells,papillose knobby cells and polygonal cells with papillae.The cells at the base of the petals of Berberidaceae were mostly rectangular,and the cells at the main veins were mostly elongated.In Caulophyllum robustum and Diphylleia sinensis,the cell types were the least,only two types;in Berberis poiretii,Mahonia bealei and Epimedium sagittatum,there were more cell types,each with four types;Ultrastructure of nectary:in Berberidaceae,secretory cells were generally small and densely arranged,with thick cytoplasm;cuticle were existed outside the epidermal cells,in which microchannels were distributed.A large number of mitochondria and plastids were distributed in epidermal cells.There were more Golgi present in Caulophyllum robustum.The secreted epidermal cells of Berberis poiretii,Mahonia bealei and Epimedium sagittatum,had the phenomenon of cell wall ingrowths,and there had plasmodesmata between the cells.There were abundant plastids,mitochondria,and rough endoplasmic reticulum in secreted parenchymal cells.There were intercellular spaces and plasmodesmata between cells.In Caulophyllum robustum,parenchyma cells were irregular in shape.The cell wall of the sieve tube elements between parenchyma cells was significantly thickened,and plasmodesmata existed with the companion cells.Petals in Berberidaceae are originated from stamen according to result in this study:the petals of Berberidaceae have single trace,the shape of the primordia is similar to that of the stamen primordia,they occured independently and no petal-stamen common primordium was found.In Berberidaceae,analysis of morphological characters based on phylogenetic tree showed that the ancestral character of the petals were simple flaky structures without nectaries,and later evolved into different,more complex petal types with nectaries,like fan-shaped with thicken edge,obovate with a pair of glands at the base or funnel-shaped and saccate.The results provided a basis for further exploring the diversity and evolution of petals in Ranunculus.