From flower to fruit: The origin of the trilocular ovary and fruit development in Phragmipedium longifolium (Warsz. & Rchb.f.) Rolfe (Orchidaceae: Cypripedioideae).

While carpels in Orchidaceae are predominantly unilocular, trilocular carpels can also occur, mainly in branches separated during the early diversification of the family. Variation in septa structure in the trilocular ovaries of the anthetic flower of orchids suggests distinct ontogenetic processes. In general, Orchidaceae fruits are capsules dehiscent by longitudinal slits, however, the number of valves, the pericarp composition, and the form of dehiscence can vary. To date, little is known about the development of unilocular capsules in Orchidaceae, and even less is understood about the development of trilocular fruits and the potential mechanisms of seed dispersal. Our aim was, therefore, to evaluate development of the ovary of the trilocular fruit and the form of dehiscence in fruits of Phragmipedium longifolium (Warsz. & Rchb.f.) Rolfe, a terrestrial species with dehiscent trilocular capsules. Using light microscopy and scanning electron microscopy, we discovered that septum formation in ovaries occurs in the flower bud stage, after differentiation of the floral parts. Septum is formed mainly by the installation of an intercalary meristem at the apex of each placenta. After pollination and during fruit development, there are few structural changes in the ovary, besides lignification process of the pericarp. The rupture and lysis of cells of the dehiscence line and dehydration of the mesocarp result in fruit dehiscence. The ovary of P. longifolium therefore has post-genital septum formation. Fruit dehiscence occurs by a mixed mechanism of cell rupture and lysis and fruit dehydration, evidenced for the first time in Orchidaceae.