Seed morphoanatomy of the genus Passiflora L. (Passifloraceae) by scanning electron microscopy and light microscopy

Morphoanatomical analysis of seeds contributes to knowledge of the development of seedlings and identification of species, as well as supporting conservation studies. The conservation of the species belonging to the Passiflora genus is crucial due to of the threats to the genetic resources of these species. Thus, the objective of this study was to morphoanatomically characterize Passiflora seeds, verify possible injuries to the tissues after cryopreservation and thus contribute to the conservation strategies of the species of this genus. Initially, seeds of Passiflora coccinea, P. edulis, P. gibertii, P. maliformis, P. morifolia, P. setacea, P. suberosa, and P. tenuifila collected from the Passion Fruit Active Germplasm Bank of the Embrapa Cassava and Fruits research unit (Embrapa Mandioca e Fruticultura) were analyzed. Then, their length, width and thickness, shape of the base and tip, and ornamentations present on the body and edge of the seeds were evaluated. The seeds of the species were placed in cryotubes and immersed in liquid nitrogen to assess possible cryoinjuries. The tegument and tissues of the seeds were examined by scanning electron microscopy. The seeds had varied biometric data, with average values of 4.63 mm for length, 3.28 mm for width, and 1.51 mm for thickness. Six ornamentation types were observed: reticulate for the species P. coccinea; finely reticulate for P. edulis; foveolate reticulate for P. gibertii and P. setacea; alveolate reticulate for P. maliformis and P. tenuifila; coarsely reticulate for P. morifolia; and falsifoveolate reticulate for P. suberosa. Some seeds suffered tegument cracks due to the freezing in liquid nitrogen, but without physiological damages to the embryo and endosperm. The cryopreservation of the seeds in the presence of the tegument significantly reduced the cryoinjuries caused to the embryo. Cryopreservation can be promising for long-term conservation of passion fruit seeds.