Pollen morphology and its taxonomic significance in 22 species of Styrax (Styracaceae) from China

The pollen morphological characters of 22 species of the genus Styrax in China were observed and analyzed by scanning electron microscopy, with the pollen morphology of seven species reported for the first time, providing evidence for taxonomic identifications and clarifying interspecific relationships among some challenging species within this genus. Results showed that: (1) The pollen types of the 22 species were primarily medium- (25–50 μm) and large-sized (50–100 μm) pollen. The pollen was tricolporate type, triangular or triangular-circular in polar view and rhombic or elliptical in equatorial view. (2) The polar axis length (P) was 36.28–55.56 μm, the equatorial axis length (E) was 16.67–36.72 μm, the P/E value was 1.41–2.18, the volume index \sqrt\mathrmP\times \mathrmE was 24.59–45.17, and the pollen germinal furrow length was 26.82–46.87 μm. (3) The pollen exine ornamentation was divided into four types, reticular, verrucate, cerebelloid, or foveolar. (4) According to cluster analysis, the 22 species of Styrax could be divided into five groups. These results show that the pollen morphological traits of Styrax differ among different species and can serve as crucial criteria for species identification and classification.