Can nectary structure in Laeliinae promote or constrain nectar secretion?

(2025) 25:772 Stpiczyńska et al. BMC Plant Biology https://doi.org/10.1186/s12870-025-06810-5 BMC Plant Biology Open Access RESEARCH Can nectary structure in Laeliinae promote or constrain nectar secretion? Małgorzata Stpiczyńska1* , Emerson R. Pansarin2, Kevin L. Davies3, Bartosz J. Płachno4 , Mateusz Wrazidlo5 , Klaudia Stodolska1 and Patryk Czortek6    Abstract The orchid subtribe Laeliinae has an assemblage of morphologically diverse taxa. The diversity in floral morphology of its members can be explained in terms of pollination ecology in that this subtribe contains both entomophilous and ornithophilous species. Given the wide range of pollinators, one would expect to find considerable differences in morphology of the floral nectaries. Fully developed nectaries appeared to be entirely non-functional in some taxa. The aim of this work was to compare the micromorphology of the inner nectary spur in selected representatives of Laeliinae in order to ascertain which structural features improve or reduce nectar secretion, and thereby contribute towards the evolutionary success of this subtribe. Here, we investigate the nectary structure of 48 species representing the genera Prosthechea, Encyclia, Epidendrum and Dinema. Of these, the nectary of Encyclia was of the narrowtubular form (cuniculus-type), that of Prosthechea and Dinema was short and sac-like, whereas both nectary types were present in Epidendrum, the former type being the more common. Whereas the nectary of Dinema contained nectar, this was either absent or present in nectaries of the other three genera. Statistical analyses of the morphological and micromorphological characters of the nectary revealed that the probability of nectar being present was lower for the long, tubular nectaries (e.g. Encyclia and Epidendrum), whereas most Prosthechea spp. investigated, as well as Dinema, possessed sac-like, functional nectaries. Also, all investigated taxa, irrespective of the presence of nectar, shared a thick cuticle and thick epidermal and subepidermal cell walls (in the secretory layer). Analyses also showed that the probability of nectar being present increased with an increase in the thickness of the secretory layer. Furthermore, there was also a greater probability of the epidermal cells lining functional nectaries having a smooth cuticle. The occurrence, or otherwise, of nectar may indicate that the secretory capacity of this group of orchids is plastic, and not limited by structural constraints, thus allowing for the relatively easy turning on and off of the secretory process. Keywords Orchidaceae, Orchids, Nectaries, Micromorphology *Correspondence: Małgorzata Stpiczyńska Full list of author information is available at the end of the article © The Author(s) 2025. Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/. Stpiczyńska et al. BMC Plant Biology (2025) 25:772 Introduction The subtribe Laeliinae Benth. has a Neotropical distribution. Neotropical orchids display great diversity with regard to their habitats, morphology and pollination ecology. Of these orchids, the mega-diverse subtribe Laeliinae, which includes the genera Epidendrum L., Encyclia Hook., Prosthechea Knowles & Westc. and Dinema Lindl., shows a wide range of flower morphology and floral rewards that is reflected in the diversity of its pollinators. Although recent molecular investigations provide further data on the phylogeny and floral characteristics of Laeliinae [1–12], the occurrence of rewards and morphological adaptations to pollinators have generally been neglected. There are, however, exceptions to this, such as studies of Encyclia mapuerae (Huber) Brade & Pabst [13], Epidendrum densiflorum Hook. [14] and Cattleya cernua (Lindl.) Van den Berg [5, 15], for which reward production, spur structure and pollination mechanisms are described in detail. Based on an analysis by [16], the presence of floral nectar in both temperate and tropical orchids can double their reproductive success. Nectar is the most common floral food-reward in Orchidaceae, the rewardless state being regarded as an ancestral character in this enormous family, though seemingly not so in Laeliinae, where the ancestral state, as represented by Dinema, Encyclia and Epidendrum, is nectariferous [11, 12, 17]. In Orchidaceae, nectar secretion has been repeatedly gained and lost in several lineages [17–22]. For example, independent nectary evolution in Disa P.J. Bergius [22] involved both repeated recapitulation of the secretory epidermis, and the acquiring of stomatal nectaries. In angiosperms, floral nectaries can be associated with any floral structure [23]. In eudicots, CRABS CLAW (CRC), a YABBY‐like transcription factor (involved also in gynoecium development), is critical for the initiation and regulation of floral nectary development, but in Aquilegia L., the development of nectary spurs was instead found to involve the gene STYLISH [24–26]. Moreover, variation in spur length and shape can be hormonally controlled by mechanisms of cell proliferation and cell expansion, resulting in a range of final spur morphologies [25]. To date, no analogous gene programs for nectary initiation are known for monocots. However, [24] reported that the gene CvSWEET9 is necessary for nectar formation and secretion in the dicot Cleome violacea L. In Orchidaceae, floral nectar may be secreted by cells located on the adaxial surface of the labellum, as in Maxillaria Ruiz & Pav. and Bulbophyllum Thouars [27, 28] but floral spurs are also common in this family [23, 29]. Labellar spurs may be formed by fusion of the labellum in its basal parts, as in many Orchidiinae [19], or Page 2 of 16 alternatively, it may involve the column-foot and sepals (e.g. Dendrobium Sw.). However, in the majority of Laeliinae spp., the floral spur is concealed within the flower (inner spur), and is formed by fusion of the basal part of the labellum, the column and the ovary. This inner floral spur is termed the cuniculus, and occurs as a long and (...truncated)