Preliminary study on pollen vitality, stigma receptivity and distant hybridization between different genera of Chionanthus retusus

Objective This paper aims to explore the pollen viability, stigma receptivity, effective pollination period, and interspecific distant hybridization between the male and hermaphroditic flowers of Chionanthus retusus and provide a foundation for improving flower color and hybrid breeding of Chionanthus retusus.

Method The pollen viability of Chionanthus retusus, Forsythia suspensa and Syringa oblata was determined by pollen in vitro culture method. The stigma receptivity of hermaphroditic flowers of Chionanthus retusus was detected by benzidine-hydrogen peroxide method. Using Chionanthus retusus ‘Xuezaohua’ as the female parent and Forsythia suspensa and Syringa oblata as the male parent, intergeneric hybridization research was carried out by mento pollen pollination method. After pollination, we observed the elongation and growth of pollen tubes in two hybrid combinations and two control combinations using the benzidine blue fluorescence microscopy, as well as examined the characteristics of the resulting fruits and hybrid embryos. After 70 d of pollination, when the embryo began to solidify, hybrid seedlings were cultivated.

Result (1)There was a significant difference in pollen viability between the male flower and hermaphrodite flower. The pollen viability of the male flower (55.91%) was significantly higher than that of the hermaphrodite flower (42.94%). (2) The pollen vitality of fresh Forsythia suspensa pollen vitality was 65.72%, and the pollen viability of Forsythia suspensa stored for 28 d at −20 ℃ was 12.78%. The pollen viability of fresh Syringa oblata pollen vitality was 66.21%, and the pollen viability of Syringa oblata stored for 15 d at −20 ℃ was 26.35%. (3) The stigma receptivity of Chionanthus retusus presented in different flower development stages: full flowering stage > initial flowering stage > final flowering stage > bud stage, and the best pollination time was full flowering stage. (4) Fluorescence observation of pollen tube elongation after mento pollen pollination revealed that the pollen tubes of both hybrid combinations were able to extend into the ovule for fertilization. (5) After 37 d of pollination, embryos began to form in two hybrid combinations, and after 70 d, the solidified embryo structure began to grow rapidly. After 120 d, the fruit matured. Finally, 121 seeds of the mento pollen pollination in Syringa oblata hybrid combination and 63 seeds of the mento pollen pollination in Forsythia suspensa hybrid combination were obtained. (6) After 70 d of pollination, 60 hybrid embryos solidified from each hybrid combination were selected for sterile seedling cultivation, resulting in 18 hybrid seedlings from the mento pollen pollination in Syringa oblata hybrid combination and 14 hybrid seedlings from the mento pollen pollination in Forsythia suspensa hybrid combination.

Conclusion The pollen viability of male flowers in the Chionanthus retusus is significantly higher than that of hermaphroditic flowers. Storing the pollen of Forsythia suspensa and Syringa oblata at −20 ℃ for an extended period ensures the pollination requirements for the Chionanthus retusus. The stigma receptivity of Chionanthus retusus is strongest during the full flowering stage, and pollination using the mento pollen pollination method can effectively ensure the smooth implementation of distant hybridization and lay the foundation for obtaining new varieties of Chionanthus retusus with different flower colors.