AOBPreview originally published online on June 10, 2005
Annals of Botany 2005 96(2):217-227; doi:10.1093/aob/mci182
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Nectar-secreting Floral Stomata in Maxillaria anceps Ames & C. Schweinf. (Orchidaceae)
SKA21 School of Earth, Ocean and Planetary Sciences, Cardiff University, PO Box 914, Park Place, Cardiff CF10 3YE, UK, 2 Department of Botany, Agricultural University, Akademicka 15, 20-950 Lublin, Poland and3 Swansea Botanical Complex, Singleton Park, Swansea SA2 9DU, UK
* For correspondence.
Received: 7 March 2005 Returned for revision: 31 March 2005 Accepted: 28 April 2005 Published electronically: 10 June 2005
• Background and Aims Although it was generally assumed that Maxillaria spp. do not produce nectar, in recent years, nectar has been reported for a number of these orchids. Nevertheless, our current understanding of nectary structure and nectar secretion in Maxillaria is based solely on M. coccinea (Jacq.) L.O. Williams ex Hodge, which, since it shows many features characteristic of ornithophilous flowers, is atypical of this largely entomophilous genus. The aim of the present paper is to describe, for the first time, nectar secretion in a presumed entomophilous species of Maxillaria.
• Methods The structure of the nectary of M. anceps Ames & C. Schweinf., nectar composition and the process of nectar secretion were investigated using light microscopy, scanning electron microscopy, transmission electron microscopy, histochemistry, refractometry and high performance liquid chromatography.
• Key Results and Conclusions Nectar appears as droplets that are exuded by modified stomata borne upon the labellar callus and collects upon the labellum and at the base of the column-foot. Although such stomata are known to occur in a number of angiosperm families, this is the first time for them to be observed in orchids. The callus consists largely of parenchyma with raphides and is supplied by eight to ten collateral bundles. This tissue, together with the single-layered epidermis, seemingly contains terpenoids. During the bud stage, the callus cells contain an organelle complement consistent with secretory cells whereas by day 4 of anthesis, much of the cell is occupied by a vacuole. The nectar is sucrose-dominant but also contains low concentrations of glucose, fructose, free amino acids and possibly terpenoids. The high sugar concentration (approx. 66 %) is consistent with melittophily and may indicate that, like the majority of Maxillaria spp., M. anceps is visited by stingless bees (Meliponini).
Key words: Entomophily, HPLC, light microscopy, Maxillaria, melittophily, nectar, nectary, refractometry, SEM, stomata, TEM, terpenoids, ultrastructure
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