Abstract:

Aims: Dimorphic cleistogamy, characterized by the coexistence of open chasmogamous (CH) and closed cleistogamous (CL) flowers within a single plant, presents a unique mating system. Viola monbeigii and V. dissecta exhibit this dimorphic cleistogamy, producing CH flowers in early spring and CL flowers in summer and early autumn. However, the role of photoperiods in regulating the development of CH and CL flowers in these Viola species remains unclear. Therefore, this study aims to investigate the influence of photoperiods on the development of CH and CL flowers in Viola monbeigii and V. dissecta. Additionally, the study seeks to elucidate the main morphological distinctions corresponding to the different development stages of CH and CL flowers in these two Viola species.
Methods: Viola monbeigii and V. dissecta were cultivated under controlled photoperiods of 10, 12, and 16 hours. The morphology of flower buds was meticulously observed using a stereomicroscope. Flowering days and the proportions of flowering induction, chasmogamous (CH), intermediate cleistogamous (inCL), and cleistogamous (CL) flowers were assessed under each of the 10, 12, and 16-hour photoperiods. Additionally, a stereomicroscope was employed to discern the principal morphological disparities corresponding to the different developmental stages of the CH and CL flowers in both Viola species.
Results: (1) Both V. monbeigii and V. dissecta exhibited the development of both chasmogamous (CH) and cleistogamous (CL) flowers. Key morphological disparities between CH and CL flowers included variations in petals and stamen size and number, the presence or absence of nectaries, filament length, and stigma curvature. CH flowers typically featured five large, showy petals with a spur at the base of the lowest petal; five stamens formed a cone surrounding the pistil, and nectar glands on the lowest two stamens that noticeably extended into the spur formed by the lowest petal. The pistil was erect and higher than the stamens. In contrast, CL flowers possessed two stamens without nectar glands, longer filaments, and underdeveloped petals and stamens. All of the remaining stamens and five petals were undeveloped. Typically, the stigma of the pistil is curved toward the two most developed stamens. In our experiment, certain conditions led to the development of intermediate cleistogamous (inCL) flowers, which displayed variable characteristics, with 1-3 poorly developed petals and 2-5 developed stamens, devoid of nectar glands, and with longer filaments. (2) Photoperiods exerted a regulatory influence on the development of CH and CL flowers in both V. monbeigii and V. dissecta. CL and inCL flowers predominately formed under long daylight hours (16-hour daylight), while under 10- and 12-hour daylight, simultaneous development of CH and inCL flowers occurred. (3) Morphological distinctions between CH and CL flowers in V. monbeigii and V. dissecta were discernable from the initial developmental stage after the formation of four floral organ primordia. These differences became more pronounced as flower development progressed, indicating a gradual divergence in floral morphology between the two flower types.
Conclusion: This study elucidated the impact of photoperiods on the development of chasmogamous CH and cleistogamous CL flowers in V. monbeigii and V. dissecta while delineating significant morphological disparities between these flower types in the two Viola species. By comprehensively exploring the mechanistic underpinnings of photoperiod-induced dimorphic flower development in V. monbeigii and V. dissecta, this research contributes to a deeper understanding of CH and CL flower development in Viola. Furthermore, it furnishes a theoretical foundation for investigating the adaptive evolution of dimorphic cleistogamous plants.

Key words: photoperiod, regulation, flower development, Viola monbeigii, Viola dissecta