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Raphide

Plant chemical defense From Wikipedia, the free encyclopedia

Raphide
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Raphides (/ˈræfɪdiz/ RAF-id-eez; singular raphide /ˈreɪfaɪd/ RAY-fyde or raphis) are needle-shaped crystals of calcium oxalate monohydrate (prismatic monoclinic crystals) or calcium carbonate as aragonite (dipyramidal orthorhombic crystals), found in more than 200 families of plants. Both ends are needle-like, but raphides tend to be blunt at one end and sharp at the other.

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Raphides in Epipremnum Devil's ivy (600× magnification)
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Calcium oxalate in plants

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Raphides in Hypoestes phyllostachya, the polka dot plant

Many plants accumulate calcium oxalate crystals in response to surplus calcium, which is found throughout the natural environment. The crystals are produced in a variety of shapes. The crystal morphology depends on the taxonomic group of the plant. In one study of over 100 species, it was found that calcium oxalate accounted for 6.3% of plant dry weight.[1] Crystal morphology and the distribution of raphides (in roots or leaves or tubers etc.) is similar in some taxa but different in others leaving possible opportunities for plant key characteristics and systematic identification; mucilage in raphide containing cells makes light microscopy difficult, though. Little is known about the mechanisms of sequestration or indeed the reason for the accumulation of raphides but it is most likely as a defense mechanism against herbivory. It has also been suggested that in some cases raphides may help form plant skeletal structures. Raphides typically occur in parenchyma cells in aerial organs, especially the leaves, and are generally confined to the mesophyll. As the leaf area increases, so does the number of raphides, the process starting in even young leaves. The first indication that the cell will contain crystals is shown when the cells enlarge with a larger nucleus.[2]

Raphides are found in specialized plant cells or crystal chambers called idioblasts.[3] Electron micrographs have shown that raphide needle crystals are normally four sided or H-shaped (with a groove down both sides) or with a hexagonal cross section and some are barbed. Wattendorf (1976) suggested that all circular sectioned raphides, as visible in a light microscope, are probably hexagonal in cross section[4] The hexagonal crystals reported by Wattendorf in Agave americana were apparently calcium oxalate.[5]

Microscopy using polarized light shows bright opalescence with raphides.

Plants like Tradescantia pallida also accumulate calcium oxalate crystals in response to heavy metals stress.[citation needed]

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Harmful effects

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Raphides can produce severe toxic reactions by facilitating the passage of toxin through the herbivore's skin when the tissue containing the raphides also contains toxins. The lethal dose to mice is around 15 mg/kg.[6] Raphides seem to be a defense mechanism against plant consuming animals,[7] as they are likely to tear and harm the soft tissues of the throat or esophagus of an animal chewing on the plant's leaves.[8][9] The venomous process is in two stages: mechanical pricking and injection of harmful protease. Typically ingestion of plants containing raphides, like those common in certain houseplants, can cause immediate numbing followed shortly by painful foot swelling, vesicle formation and swelling throat, accompanied by painful stinging and burning to the mouth and throat with symptoms occurring for up to two weeks.[10] Airway assessment and management are of the highest priority, as are extensive irrigation and analgesics in eye exposure.

Raphides cannot normally be destroyed by boiling; that requires an acidic environment or chemical solvents like ether. Plants containing large amounts of raphides are generally acrid and unpalatable. However, it is not always possible to detect the presence of raphides through taste alone. In some tubers such as the Indian turnip which contain large quantities of raphides, the roots are not unpalatable when cooked because the raphides are bound within a matrix of starch which prevents the tongue from sensing their presence. Some other plants store raphides in mucilaginous environments and also do not taste acrid.[3]

There is some doubt about the role of raphides in defense against herbivory. Insects are only rarely affected by them, and no more than by sand. Ruminants are not harmed by them, because certain bacteria in their rumen help to degrade oxalate.[11]

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Plants containing raphides

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Common names vary. The following list is incomplete. Raphides are found in many species in the families Araceae and Commelinaceae, but are also found in a few species in a number of other families.[12]

Actinidiaceae:

Amaranthaceae:

Amaryllidaceae:[12]

Araceae:[12][13]

Araliaceae:

Arecaceae:[12]

Aspleniaceae:

Asparagaceae:[12]

Asphodelaceae:[12]

Bromeliaceae:

Commelinaceae:[12]

Onagraceae:

Orchidaceae:[12]

  • very widespread

Piperaceae:[12]

  • Manekia incurva (leaf)
  • Manekia sydowii (leaf)
  • Peperomia fagerlindii (leaf)
  • Peperomia incana (leaf)
  • Peperomia longespicata (leaf)
  • Peperomia macrostachya (leaf)
  • Peperomia serpens (leaf)
  • Peperomia vinasiana (leaf)
  • Piper augustum (leaf)
  • Piper betle (leaf)
  • Piper caldense (leaf, stem)
  • Piper cavendishioides (leaf)
  • Piper chrysostachyum (leaf)
  • Piper cinereum (leaf)
  • Piper confusum (leaf)
  • Piper dotanum (leaf)
  • Piper eriopodon (leaf)
  • Piper filistium (leaf)
  • Piper glabrescens (leaf)
  • Piper hispidum (leaf)
  • Piper lapathifolium (leaf)
  • Piper leucophyllum (leaf)
  • Piper mikanianum (leaf)
  • Piper munchanum (leaf)
  • Piper corrugatum (leaf)
  • Piper peltatum (leaf)
  • Piper pseudovariabile (leaf)
  • Piper sancti-felicis (leaf)
  • Piper umbellatum (leaf)
  • Piper urdanetanum (leaf)
  • Piper oxystachyum (leaf)
  • Zippelia begoniifolia (leaf)

Poaceae:

Polygonaceae:

Rubiaceae:

Vitaceae:

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See also

References

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