Objective To provide some references for the studies on genetic breeding, pollen germplasm preservation and pollination biology of Rhododendron longipedicellatum by investigating the pollen grains morphology and its characteristics.

Method  The pollen grains of R. longipedicellatum at different development stages including alabastrum intumescence stage, petal loose stage and petal full bloom stage were observed by light microscope (LM) and scanning electron microscope (SEM). The pollen viability in the process of flowering was detected by TTC staining method. The effects of 10 g·L^-1 agar and different concentrations of sucrose, H₃BO₃ and CaCl₂ on R. longipedicellatum pollen grains germination were studied by L₂₅(5³) orthogonal test. Simultaneously, the authors also studied the changes of pollen germination rate under different storage temperature.

Result  The pollen grains mature gradually from the alabastrum intumescence to the petal full bloom stage, and are arranged in tetrahedral tetrad. The tetrad diameter ranges from 43.0 μm to 65.4 μm with an average 51.3 μm. A single pollen is subspheroidal with tricolporate aperture, and viscin threads were seen on the pollen grains surface. The pollen grains exine sculpture is uniform granular in SEM, and the granulate around the colpus are more compact. Exine is composed with two layers in LM. The pollen viability maintained a high level over single flowering period, which was the highest (92.18%) at 9:00 on the first day of anthesis, and still remained relatively high (48.5%) until the end of flowering time (on the 9 days after flowering). The highest germination rate of R. longipedicellatum pollen grains at 9:00 on the first day of anthesis was 90.26% when cultured in the medium for 10 g·L^-1 agar + 100 g·L^-1 sucrose + 200 mg·L^-1 H₃BO₃ + 0 mg·L^-1 CaCl₂. Range analysis showed that the important order of influence on pollen germination rate was sucrose, CaCl₂ and H₃BO₃, a certain concentration of sucrose and H₃BO₃ had a good promote sprouting effect, while the addition of CaCl₂ significantly inhibited the pollen grains germination. Furthermore, the appropriate low temperature was helpful to storage of R. longipedicellatum pollen, the germination rate maintained a certain level after 48 days storage with -18℃.

Conclusion  The special pollen grain exine sculpture of R. longipedicellatum supports the rationality of the division of its systematic position by predecessors. These findings of high pollen viability, suitable solid germination medium and storage temperature provide a reliable theoretical basis for hybridization breeding of R. longipedicellatum as the male parent.