| The genus Sorbus L.belongs to the Rosaceae family,subfamily Maloideae,and is one of the most complex genera in Rosaceae.In this study,22 species,three varieties,and one form of Sorbus were selected to measure and describe the morphology of their floral organs.The microstructure of the petal epidermal cells was observed using an optical microscope and scanning electron microscope,and the petal anatomical structure was compared using paraffin sectioning.Based on the morphology and anatomy of the petal epidermal cells,the ecological significance of Sorbus petal and its relationship with environmental adaptation were discussed,in order to provide new scientific evidence for the systematic classification and introduction and cultivation of Sorbus plants.The main research results are as follows:1.Floral organ characteristicsPlants of the Sorbus genus have bisexual flowers,with inflorescence type of terminal compound cyme.Each inflorescence contains 15-531 flowers.The calyx tube is bell-shaped,without hairs,or with white or brown hairs.There are 5 triangular sepals,with acute,slightly blunt or rounded apex,and entire margins or serrated margins.The sepals may be hairless,sparsely hairy or densely hairy.During flowering,the sepals are usually erect,while some taxa may be reflexed.There are 5 petals,usually white,pale yellow,yellow-white or pink-white in color,with entire or slightly notched tips,and mostly clawed at the base.When fully open,the petals are usually hairy on the ventral side.The stamens are arranged in 2 whorls,with 14-28 stamens.The anthers are cruciate and are milk-yellow,light pink,purple-red or pink-purple in color.The carpels are 2-5,fused with the calyx tube.The styles are also 2-5 and the base of the styles is 1/3-1/2 connate or free.The number of carpels is the same as the number of styles,with each carpel containing 2 ovules and borne on the receptacle.The base of the styles and the apex of the ovary are mostly hairy.The morphology of floral organs can distinguish between the single and compound leaf groups of the Sorbus genus and can serve as a basis for classifying subgenera.There are significant differences in the number of flowers in the inflorescence,the diameter of the inflorescence stalk,the open state of the sepals,the length of the stamens and flower stalks,the length of the inflorescence stalk,and the length of the pistil among different groups,which can be used as a basis for dividing subgenera.In addition,the classification status of S.henryi,S.megalocarpa,S.meliosmifolia,S.setschwanensis and S.pogonopetala still needs further study.From the results of floral organ morphology,it can be seen that the characteristics of floral organs show continuity and transitional features among different groups within the genus.The number and size of flowers,as well as the length of stamens and pistils,exhibit synchronous evolution among different groups within the broad Sorbus genus.There is a significant positive correlation(P<0.01)between the length of the stamens and the length of the pistil,the length and width of the sepals,and the length and width of the petals.The number of flowers in the inflorescence of the compound leaf Sorbus is significantly greater than that of the single leaf Photinia,and the flowers are relatively smaller.The stamens,pistils,and flower stalks of the single leaf Sorbus are generally longer than those of the compound leaf Sorbus.The size of the flowers,the length of the stamens and pistils,have strategic significance in ensuring the pollination process.Therefore,it can be seen that the compound leaf Sorbus and single leaf Sorbus have evolved their own pollination strategies.2.Micromorphology of epidermal cellsUsually,the upper epidermal cells on petals are larger than the lower epidermal cells,with a lower density than the lower epidermal cells.The mean basal area of upper epidermal cells ranges from 584.4-261.08 μm2.The area of lower epidermal cells ranges from 431.26-186.88μm2.The density of upper epidermal cells ranges from 38.2-21.03(100×100μm).The density of lower epidermal cells ranges from 51.5-22.87(100×100μm).The differences in the size and density of petal epidermal cells among different groups are significant(P<0.05).The micromorphological changes in petal epidermis are diverse.Epidermal cells are usually polygons,equilateral polygons,or polygon-rectangles,or elongated polygons.The periclinal walls are usually curved,wavy,or occasionally straight.The ornamentation of the cuticle layer shows certain variations,with the anticlinal walls being curved or sinuous,and the cuticle layer being striate or wrinkled.Papillate and non-papillate epidermal cells can be seen on the epidermis.Papillate cells appear near the axis of petals,while the non-papillate cells are found on the abaxial surface of petals.Papillate cells are divided into four types: papillose conical cells with striations(PCS),papillose lobular cells with striations(PLS),papillose knobby with striations(PKS)and papillose skittle with striations(PSS).Reticulate cells include three types: tabular rugose striate cells with obscure anticlinal(TRSi),tabular rugose striate cells with raised anticlinal(TRSii)and tabular rugose to flat cells(TRF).There are significant differences in the morphology of the epidermal cells of the flowers in the Sorbus genus in 24 different taxa,and these identifying characteristics can be used to effectively identify some species in the genus.The results show that S.megalocarpa and S.meliosmifolia have similar epidermal cell morphology in their petals,and therefore,this study supports grouping them together.3.Petal anatomical structure characteristicsThe structure of petals is divided into upper epidermis,lower epidermis,middle tissue,and vascular bundles.There are significant differences in petal thickness among different groups(F=220.812,P<0.05),ranging from 275.9 μm to 77.77 μm.The upper epidermal cells are convex with gaps between adjacent cells,and the distance between cell tops ranges from 14.15 μm to7.25 μm.There is a cuticle layer covering the outer surface of the cells,and the upper epidermal cells are higher than the lower epidermal cells.The thickness of the upper epidermal cells ranges from 26.65 μm to 9.89 μm.The lower epidermal cells are densely and regularly arranged with a thicker cuticle layer covering their surface,and their thickness ranges from 20.54 μm to 10.53μm.The middle tissue is loosely arranged,with elliptical or cylindrical cells of varying sizes and distinct intercellular spaces.The number of layers in the middle tissue varies greatly.The vascular bundles are relatively small and consist of xylem and phloem conduits,each surrounded by a bundle sheath of thin-walled cells.The cross-sectional shape of the upper and lower epidermal cells is relatively similar among different groups,with the upper epidermal cells mainly being papillate or elliptical,and the lower epidermal cells mainly being flattened or nearly circular.The similarities and differences in the external and internal morphological structures of Sorbus petals can provide anatomical evidence for the systematic classification of this genus. |
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