Can nectary structure in Laeliinae promote or constrain nectar secretion?

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 narrow-tubular 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.