Fruit nectar or prefruit nectar? Implication of the postfloral nectar of <i>Dendrobium chrysotoxum</i> (Orchidaceae)

Postfloral nectar presenting on reproductive organs after pollination generally functions in plant defense to protect fruits and seeds, but the mechanisms for postfloral nectary development and nectar secretion remain largely unknown. We report the first case of postfloral nectar production in the genus Dendrobium (Orchidaceae). D. chrysotoxum produced postfloral nectar on the surface of the inferior ovary after pistil pollination. The features of postfloral nectar secretion were revealed through anatomical and histochemical approaches, transcriptome and metabolome analysis, pollination experiments, and field observations. A key role of stigmatic pollen germination in postfloral nectar induction was demonstrated as well as the ant attraction effect of the postfloral nectar. Unexpectedly, the secretion of postfloral nectar ceased before ovule fertilization, making it more like 'prefruit nectar' than 'fruit nectar' (generally referred to the postfloral nectar occurring on organs that used to be floral ovaries) since the ovaries had not yet become fruits. The clarification of pollen germination in postfloral nectar induction and the provoking thought to define postfloral nectar based on the fertilization status provide new insights into the understanding of postfloral nectar.