SEM observation of pollen in 17 cherry germplasm resources

【Objective】Morphological characteristics of pollen grains of different cherry germplasm resources were observed under a scanning electron microscope. The similarities and differences in pollen among different germplasm resources were analyzed, so as to provide a scientific basis for the identification of different cherry germplasm resources. At present, scanning electron microscopy (SEM) technology has opened up a new perspective for the study on pollen structure with its excellent high resolution and the ability to observe three-dimensional morphology. It can observe the microscopic characteristics of the surface of cherry pollen grains in detail, including complex surface decoration, pore structure and microscopic structure of pollen walls. Therefore, this technology is chosen to observe the morphological characteristics of pollen grains in the experiment.【Methods】Pollen grains of 17 cherry germplasm resources from the Tongzhou Cherry Breeding Base, Forestry Fruit Research Institute ofBeijing, Academy of Agriculture and Forestry Sciences were selected for the experiment, which were as follows: Summit, Sunburst, Caihong, Rainier, Brooks, Mashad Black, Santina, Russian 8, Minie Royal, Royal Lee, Chelan, Katalin, Tieton, 7-2-9, 7-5-30, Duiying and Landing 1. During the blooming period of cherries, the pre- bloom flowers were collected, the anthers were completely peeled off with small tweezers, placed in a sulfuric acid paper box, and irradiated with a desk lamp to help loose powder. Then the dried pollen was collected in a bottle of Xilin, stuffed with rubber plugs, labeled and stored in a refrigerator at -20 ℃ for later use. The experiment was conducted in the electron microscope room of the Institute of Food Science and Technology, CAAS. First, a cotton swab was dipped into a sample of dried pollen grains, and they were gently flicked on the sulphate paper, and appropriate amount of pollen was taken with 1 cm×1 cm double-sided conductive tape, and then affixed to the metal platform and gold- plated in the Eiko IB5 Ion Coater ion spater. Finally, the pollen grain morphology of 17 cherry germplasm resources was observed under the Hitachi SU-8010 scanning electron microscope. The population status of pollen grains was photographed and observed under the field of view of 500×, and 20 pollen grains were selected from each cherry germplasm resource to investigate the uniformity of pollen. Image J was used to measure the polar axis length (P) and equatorial axis length (E) of selected pollen grains respectively and the polar equatorial ratio (P/E) was calculated. The polar erythroid ratio indicates the shape of pollen grains, P/E＞2 indicates that the pollen grains are ultra- elliptic shape, and 1.14＜P/E＜2 indicates that the pollen grains are elliptic shape. The pollen grains of 17 cherry germplasm resources were cluster analyzed and the correlation between pollen uniformity and polar axis length was analyzed. In addition, the individual morphology of pollen grains of 17 cherry germplasm resources was photographed and observed under a field of view of 1500×, and the related traits of pollen grain outer wall pattern were investigated and recorded, which were stripe width, stripe spacing, stripe slope, stripe clarity and stripe density.【Results】Among the 17 cherry germplasm resources, Landing 1 had the highest pollen uniformity (87.34%). The pollen uniformity of Katalin was the lowest (50.04%). Except for Chelan, Katalin, and Tieton, the pollen uniformity of the other 14 cherry germplasm resources was above 70%. Meanwhile, the polar axis length of pollen grains of 17 cherry germplasm resources for testing was 41.53-48.57 μm. The equatorial axis length was 21.30-25.29 μm. The polar declination ratio was 1.67-2.17. The appearance of pollen grains was mostly elliptic shape or ultra-elliptic shape, of which 12 germplasm resources were ultra-elliptic shape. By observing the morphology of pollen grains under a scanning electron microscope, it was found that most of the pollen grains were elliptical on the equatorial plane, and 3 germination channels were observed in the polar axis direction with circular distribution, belonging to N3P4C5 type pollen. These germination grooves extended to both ends of the pollen, but no joint grooves were formed in the polar region. The outer wall pattern was striped, and there were obvious differences in stripe width, spacing, slope, clarity, and outer wall hole density among different germplasm resources.【Conclusion】In the tested cherry germplasm resources, there were 14 pollen uniformity greater than 70%. Concurrently, a significant positive correlation was observed between the pollen viability and the length of the polar axis in 17 cherry germplasm resources. The morphological characteristics of pollen grains can be used as a basis for different cherry germplasm resources. Scanning electron microscopy is crucial for the classification and phylogeny of cherry. Analyzing the morphological characteristics of pollen grains enables the effective distinction of cherry germplasm resources, as well as the revelation of their interrelationships. The study of these characteristics provides a basis for genetic research, and also helps to explore the genetic mechanisms that control pollen morphology. However, there are still few reports on the systematic identification and classification of different cherry germplasm resources based on morphological characteristics of pollen grains, and further studies are needed.