Morphological characteristics of male reproductive organs in some walnut (Juglans regia L.) genotypes

•Significant differences among genotypes for catkin, anther and pollen traits were found.•Numbers of catkins per shoot and node were higher in protandrous genotypes.•Catkin length was higher in genotypes with later time of pollen shedding.•Pollen is medium-sized, suboblate, pantoporate, with microechinate exine ornamentation.•Pollen size, shape, and microechini density can be used in genotype identification. Morphological characteristics of male reproductive organs (catkins, male flowers, anthers, and pollen) were studied in six walnut genotypes. Morphology and ultrastructure of pollen grains were studied using scanning electron microscopy (SEM) technique. Numbers of catkins per shoot and node ranged 2.35–5.61, and 0.29–0.73, respectively. They were higher in protandrous genotypes, compared with protogynous ones. The average values for catkin characteristics for all genotypes were: length 120 mm, width 15 mm, and the number of male flowers in catkin 120. Catkin length was higher in genotypes with the later time of pollen shedding. The average number of stamens in a male flower for all genotypes was 19, while average anther dimensions were: length 2.32 mm, width 0.96 mm, and thickness 0.75 mm. Significant differences between genotypes were found for all catkin and anther characteristics, except for catkin width. Pollen grains of all examined walnut genotypes were isopolar and pantoporate. They are classified as medium-large in size, with the average length of the polar axis (P) 36.84–39.88 μm, and the average length of the equatorial axis (E) 43.12–46.37 μm. The average P/E ratio varied between 0.85 and 0.88 and, based on this, pollen shape was identified as suboblate. All studied genotypes had microechinate exine ornamentation. The number of microechini per an area of 100 μm−2 of the exine surface varied between 22 and 25. Results obtained by SEM examination indicated that some pollen characteristics, such as size, shape, and microechini density can potentially be used in the identification of walnut genotypes.