Annals of Botany 70: 325-331, 1992 Ultrastructural Aspects of the Nectary Spur of Limodorum abortivum (L) Sw. (Orchidaceae) A. CRISTINA S. FIGUEIREDO and M. SALOME PAIS Departamento de Biologia Vegetal, Faculdade de Ciencias de Lisboa, Bloco C2, Campo Grande 1700 Lisboa, Portugal Accepted: 23 April 1992 The infrastructure of the nectary spur of Limodorum abortivum (L) Sw. was examined before and after anthesis. In cross section the nectary spur shows an internal epidermal layer of thin-walled cells bordering the secretory cavity and 10-12 layers of parenchyma cells. The ultrastructure of the secretory cells suggests the involvement of ER, Golgi and plastids in nectar secretion. The nectar accumulated in the sub-cuticular space is released into the nectariferous cavity by rupture of the outer layer of the cuticle. Key words: Limodorum abortivum (L) Sw., Orchidaceae, nectary spur, nectar secretion, ultrastructure, anthesis, endoplasmic reticulum, dictyosomes, plastids. INTRODUCTION The ecological interest of nectar and its taxonomic impli- cations have led to a number of studies dealing with its composition and role in insect attraction, as well as the physiology and ultrastructure of nectar secreting cells (Findlay, 1988). The anatomy and ontogeny of nectaries vary broadly depending upon taxa. From an ultrastructural viewpoint, ER may be absent in nectariferous cells (Eriksson, 1977; Mauseth, 1982) or it may appear as the predominant organelle (Rachmilevitz and Fahn, 1973; Dumas, 1975; Wergin et ai, 1975; Baker, Hall and Thorpe, 1978; Eleftheriou and Hall, 1983; Sawidis, Eleftheriou and Tsekos, 1989). Although the involvement of dictyosomes in nectar secretion is still a matter of controversy, since they may show different degrees of activity (Heinrich, 1975; Schnepf and Benner, 1978; Von Teichmann and Robbertese, 1979; Meyberg and Kristen, 1981; Sawidis et ai, 1989), some authors have suggested that both Golgi and ER can be involved in this process (Eyme, 1966; Figier, 1968; Rachmilevitz and Fahn, 1975). According to other authors, plastids also seem to participate in nectar secretion, either in the synthesis of precursors or in the finalization of nectar components (Rachmilevitz and Fahn, 1973; Dumas, 1975; Pais and Chaves das Neves, 1980). Since the nectaries of different taxa show different ultrastructural features there is some controversy concerning the mechanism of pre-nectar production and transport to nectariferous cells and from there to the exterior (Fahn, 1988). Different models have been proposed to explain nectar secretion (Schnepf, 1977; Fahn, 1979; Pate et al., 1985; Kronestedt et al., 1986; Robards and Stark, 1988). Structural observations in some species have led some authors to define the nectar secretion as eccrine, while in other species, it has been classified as granulocrine (Fahn, 0305-7364/92/100325 + 07 S08.00/0 1979). Based on structural and physiological data some authors concluded that, in Abutilon and Hibiscus, prenectar is loaded into the secretory reticulum (Kronestedt et al., 1986; Robards and Stark, 1988; Sawidis et al., 1989). Increase in hydrostatic pressure within this compartment leads to the opening of 'sphincters' which connect the cisternal space of the secretory reticulum to the outside of the plasmalemma, the nectar being exuded afterwards by transient pores. According to this model there is no need for high transmembrane fluxes or rates of vesicle fusion, but it implies an active loading of prenectar into the secretory reticulum, which could be supported by sucrose hydrolysis (Kronestedt et al., 1986; Robards and Stark, 1988; Sawidis el al., 1989). As in many other families, Orchidaceae have both floral and extrafloral nectaries. Cup-like shaped nectaries (Pais, 1987) or long spurs produced from the internal portion of the labellum are known (Pijl and Dodson, 1966). Limodorum abortivum is a saprophytic orchid with an inflorescence of four to 20 flowers. Each flower has a long, thin and curved nectary spur. L. abortivum has been classified as belonging to a fairly primitive group within the evolutionary sequence for orchids, since its nectar contains mainly sucrose and a low total amino acid concentration, when compared to other open nectary orchid species (Pais and Chaves das Neves, 1986). Continuing our work on this orchid we present, in this paper, a study on the ultrastructure of L. abortivum floral nectary tissue during nectar secretion. MATERIALS AND METHODS Plant material Flowers of Limodorum abortivum were collected at Cotovia (Sesimbra, Portugal). A voucher specimen has been de- posited in the Herbarium of the Instituto Botanic da Faculdade de Ciencias de Lisboa under no. LISU: 160274. © 1992 Annals of Botany Company Downloaded from https://academic.oup.com/aob/article/70/4/325/181154 by guest on 14 October 2021