Pollen morphology of Clerodendrum L. (Lamiaceae) from China and its systematic implications

Pollen morphology of 26 taxa of Clerodendrum, as well as one species of Volkameria from China, was investigated through a scanning electron microscope (SEM). Pollen grains of Clerodendrum are monads, radiosymmetric and tricolpate, with medium or large size. The equatorial view of the pollen grains is spheroidal or subprolate and the polar view is (sub) circular or rounded triangular. The colpus membrane of the investigated taxa is sunken (rarely even). Five varying pollen types are delimited on the basis of exine sculpturing: (1) spine-tectum perforatum; (2) spine-tectum imperforatum; (3) microspine-tectum perforatum; (4) microspine-tectum imperforatum; and (5) obtuser spine. The results indicate that Clerodendrum is closely related to several genera in Lamiaceae, including Aegiphila, Amasonia, Kalaharia, Tetraclea, Volkameria, Oxera, Faradaya, and Hosea, as supported by previous phylogenic studies. Additionally, the conventional infrageneric classification of Clerodendrum based on inflorescence and leaf characters is not supported by the results. However, the palynological data can be used to identify some closely related species with similar external characteristics. In conclusion, the investigation of pollen morphology not only contributes novel data from palynology for Clerodendrum but also provides a basis for future comprehensive classification of this genus.

Automated summary

The pollen morphology of Clerodendrum, including 26 taxa and one species of Volkameria from China, was investigated using a scanning electron microscope. The results showed that Clerodendrum pollen grains are monads, radiosymmetric, and tricolpate, with five different types based on exine sculpturing. The study supported the close relationship between Clerodendrum and several genera in the Lamiaceae family, and suggested that traditional classification based on inflorescence and leaf characters is not accurate. This investigation contributes novel data for the classification of Clerodendrum based on pollen morphology.