Abstract:

Aims: Viola philippica and V. prionantha are the plants with typical cleistogamous dimorphic flowers. The same plant can induce two flower types: chasmogamous (CH) flower and cleistogamous (CL) flower. Photoperiod plays an important role in development of dimorphic flower. Viola. philippica is diploid, while V. prionantha is tetraploid. At present, the correlation between chromosome ploidy and morphology remains unclear. 

Methods: Viola philippica and V. prionantha were grown under 10 h, 12 h and 16 h daylight, respectively. Experimental methods such as karyotype analysis and microscopic observation were utilized, which were employed to investigate the flower type changes between diploid V. philippica and tetraploid V. prionantha under different photoperiods. Chromosomal karyotype analysis was performed via carbol fuchsin staining and observed under optical microscope. Dissection and photography were conducted under a stereomicroscope to count the numbers of CH, intermediate CL, and CL flower buds under different photoperiods. Morphological indices of leaves, flower buds and fruits were measured with a vernier caliper. Additionally, the number and size of stomata were counted by the epidermal peeling method under optical microscope. Seeds weight was determined with an electronic balance. 

Results: Viola philippica and V. prionantha have 24 chromosomes (2n = 24) and 48 chromosomes (4n = 48), respectively. Although photoperiods regulated the development of dimorphic flowers in Viola, there were certain differences in the proportion of different flower types under the same photoperiod. Under short daylight (10 h) and medium daylight (12 h) conditions, the ratio of CH flowers in V. prionantha was lower than that in V. philippica. However, under long daylight conditions (16 h), V. prionantha produced only complete CL flowers, while V. philippica still retained a small amount of intermediate CL flowers that were more inclined towards CL flowers. In addition, the nutritional and reproductive organs of tetraploid V. prionantha were significantly larger than those of diploid V. philippica. Moreover, the pore diameter of the stomata of V. prionantha was significantly larger than that of V. philippica, while the pore density was significantly lower than that of V. philippica. These characteristics can serve as auxiliary indicators for the identification of the two species. 

Conclusion: The ploidy difference may have affected the flower type changes of V. philippica and V. prionantha under different photoperiods. The tetraploid gene dose effect in V. prionantha may have strengthened the regulatory response to photoperiodic signals. This study provides a basis for the research on the co-evolution of polyploids and breeding systems, and the identification of Viola species.

Key words: Viola, ploidy, dimorphic flowers, photoperiodic regulation, cytological morphology