Floral nectaries from Limodorum abortivum (L) Sw and Epipactis atropurpurea Rafin (Orchidaceae): ultrastructural changes in plastids during the secretory processM.S. Pais and A.C.S. Figueiredo
Departamento de Biologia Vegetal, Faculdade de Ciências de Lisboa, Bioco C2, Campo Grande, 1700 Lisbon, Portugal
Abstract - Ultrastructural features of the floral nectaries of 2 orchid species, Limodorum abortivum (L) Sw and Epipactis atropurpurea Rafin, were compared. In particular, ultrastructural changes were followed in the plastids throughout the process of nectar secretion. There is evidence that plastids play an important role in this process. Before secretion begins, plastids of epidermal and subepidermal cells of the nectaries of both species, accumulated large quantities of starch. However, during the phase of nectar secretion, the plastids were found to contain little if any, starch, suggesting that starch degradation had augmented nectar sugar production. Plastoglobuli were evident during all developmental stages. The tubular reticulum found in plastids of pre-secretory nectaries was still evident during secretion, but was then found to contain an unknown, dense osmiophilic material. The floral nectary of L abortivum is sucrose dominant, whereas that of Eatropurpurea is hexose rich. Therefore, the differences in nectar-carbohydrate composition are apparently not explainable by the similar ultrastructural changes of their floral nectaries. Further investigation is required to determine the biochemical basis for the disparity in nectar sugar composition between these 2 species.
Key words: Epipactis atropurpurea Rafin / Limodorum abortivum (L) Sw / nectary / orchid / plastid / starch