Nectaries secrete sweet liquid to attract and feed pollinators, whereas the so-called extrafloral nectaries are not involved iwth pollinators, although they may occur on reproductive parts, but instead in maintaining mutalistic relationships with animnals that provide antiherbivor protection. Locations of extrafloral nectaries vary from species ot species, but most often they occur on the petiole or rachis near the attachment of the leaf or leaflet bade. In many cases, the extrafloral nectary is a modified stipule or a trichome (plant hair) on a stipule; other cases are a modification of a leaf margin or an emergence or trichome on the blade surface. Extrafloral nectaries are also common on the calyx or reproductive axes.
The primary targent of extrafloral nectaries are ants. These plants have evolved in scores of angiosperm families. Most species that have been studied secrete glucose, fructose, and sucrose plus some protein, amino acids, and organic acids. Extrafloral nectar is hypertonic to phloem sap (e.g., sweeter than).
The cells of the extrafloral nectary have densely staining cytoplasm and an inconspicuous vacuole, and the nuclei are large. Characteristically, there are many plasmodesmata connecting these cells through radial and basal cell walls. Cytoplasm stains very densely because it has very high concentrations of ribosomes and mitochondia, along with quantities of rough and smooth endoplasmic reticulum with abundant vesicles. Golgi apparatus tend to be more abundant in developing cells than when mature and secreting.
Phloem usually ends close to the nectary, and is the obvious source of the carbohydrates, but sugars must be transported across a few cells to reach the secretory layer. One current model for secretion by extraflorla nectaries is that there is symplastic flow through plasmodesmata into the secretory cells, and the ER vesicles via exocytosis (fusion with the plasmalemma) release the compounds to the surface. By some accounts, the sugar collects first beneath the cuticle before rupturing occurs.
Experiments done on plants with extrafloral nectaries has dramatically shown how plants guarded by ants have high success, whereas without ants they can succub to very heavy foraging by other animals.
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