Observation of citrus pollen morphology by scanning electron microscopy

Abstract:【Objective】Pollen morphology is often used as references for plant classification for its conservation attribute. Therefor, the observation of citrus pollen morphology can provide some references for citrus classification.【Methods】In full flowering period, the flowers were picked up in the upper part of the crown periphery from robust plants, and the petals and calyx were removed with sterile tweezers.The anthers were placed in an oven at 37 ℃ for more than 24 hours. The pollens were collected and stored at low temperature. The pollen of the Fortunella, Poncirus trifoliata and Citrus were observed by FEI Quanta 200 field emission scanning electron microscopy. A total of six quantitative features, including equatorial axis length, polar axis length, mesh diameter, net width, polar axis equatorial axis ratio, mesh net ridge ratio, were investigated. Data were normalized by NTsys-pc software and clustered. The genetic distance was calculated as the average classification distance, and the UPGMA method was used for the cluster analysis.【Results】The pollens of most species were 4-colpate type. The pollens of C. limom and C. ichangensis were 5-colpate type. The pollen grains (P × E) of the Poncirus trifoliata were (20.40-30.33) μm× (17.91-28.49) μm; the widths of mesh ridge were 0.52-0.58 μ m; and the meshes were mostly polygonal, the mesh sizes were 0.48-0.74 μ m. The pollen grains (P × E) of Fortunella Swing were (23.81-29.58) μ m× (15.73-17.37) μ m; the widths of mesh ridges were 0.79-0.93 μm; and the mesh sizes were 0.66-1.07 μm; The pollen grains (P×E) of Citrus were (20.52-39.14) μm× (14.28-31.93) μm; the widths of mesh ridges were 0.17-1.41 μm; and the mesh sizes were 0.43-1.69 μm; most species of citrus had round or roughly round pollen mesh but the pollen meshes of some species of C. ichangensis and citron were polygonal. Pollen exines of C. grandis were foveolate decoration and those of the other species were reticulate decoration.【Conclusion】The pollens of different citrus species had different morphological characteristics. Pollen morphology differences could provide some basis for the identification of citrus species and taxonomic classification. It seems to be possible that C. mangshanesis and Guposhan wild Zhoupigan are the most primitive wild species of Citrus.