Morpho-anatomical structure and development of fruit in Asyneuma canescens (Саmpanulaceae)

Abstract

The most usual fruit type in the family Саmpanulaсеае is an inferior multi-seeded capsule. In Саmpanula and related genera, A. Kolakovsky determined a new type of capsule, characterized by a specialized organ, axicorn. Some types of axicorn capsules were determined depending on the way of dehiscence. In Campanula, capsule dehiscence is called axicorn-fissuricidal mode, while in Asyneuma – axicorn-valvate mode with scaliformis valves. The precise differences between slit and valve so far have not been identified. In this connection, we performed the study of the inner fruit structure and dehiscence in Asyneuma canescens intending to compare the results with those for Саmpanula species. Anatomical fruit structure was studied under a light microscope on temporary preparations of transversal and longitudinal sections. For the first time, the obtained data provide precise characteristics of the inner fruit morphology, the anatomy of the fruit wall, and dehiscence mode in A. canescens. The survey revealed predominance in the ovary of a synascidiate zone with axile placentation. In the anatomical structure of the fruit wall, there we found a parenchymatous mesocarpium, non-lignified exo- and endocarpium, and lignified elements of fruit – axicorns, located in the small upper part of the septa. Fruit dehiscence in A. canescens occurs in two steps. First, during early flower development, narrow-oval dips are formed on the septum radii; during the fruiting period, a hippocrepiform slit at the lower margin of each dip develops. At this time axicorns detach from the central column of the ovary following a curved direction, meaning the formation of the septifragal slit. During the second stage, the longitudinal slits are formed from the lowest point of the hippocrepiform slit to the fruit base. These slits facilitate seed release from the capsule. Fruit in A. canescens we define as an inferior trilocular syncarpous capsule with two-stage, hippocrepiform and laminar dehiscence. Our study confirms resemblance of anatomical structure and dehiscence mode of fruit in A. canescens and species of the section Rapunculus of the genus Cаmpanula with erect capsules. We consider it inexpedient to accept the new fruit type for A. canescens, because differences compared with species of Cаmpanula are derivative and concern the small size of the axicorn slit and the appearance of additional slits only.