Leptospermum

Leptospermum /ˌlɛptəˈspɜːrməm, -toʊ-/[2][3] is a genus of shrubs and small trees in the myrtle family Myrtaceae commonly known as tea trees, although this name is sometimes also used for some species of Melaleuca. Most species are endemic to Australia, with the greatest diversity in the south of the continent but some are native to other parts of the world, including New Zealand and Southeast Asia. Leptospermums all have five conspicuous petals and five groups of stamens which alternate with the petals. There is a single style in the centre of the flower and the fruit is a woody capsule. The first formal description of a leptospermum was published in 1776 by the German botanists Johann Reinhold Forster and his son Johann Georg Adam Forster, but an unambiguous definition of individual species in the genus was not achieved until 1979. Leptospermums grow in a wide range of habitats but are most commonly found in moist, low-nutrient soils. They have important uses in horticulture, in the production of honey and in floristry.

Leptospermum
Leptospermum squarrosum
Scientific classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Rosids
Order: Myrtales
Family: Myrtaceae
Tribe: Leptospermeae
Genus: Leptospermum
J.R.Forster & G.Forster[1]
Synonyms[1]

Description

Plants in the genus Leptospermum range in size from prostrate shrubs to small trees and have fibrous, flaky or papery bark. The leaves are arranged alternately and are relatively small, rigid and often aromatic when crushed. The flowers may be solitary or in groups and have bracteoles and sepals which in most species fall off as the flower opens. There are five spreading, conspicuous petals which are white, pink or red. There are many stamens which are usually shorter than the petals and in five groups opposite the stamens, although they often appear to not be grouped. A simple style usually arises from a small depression in the ovary which has from three to five sections in most species, each section containing a few to many ovules. The fruit is a woody capsule which opens at the top to release the seeds, although in some species this does not occur until the plant, or the part of it, dies.[4][5][6]

Taxonomy and naming

The first formal description of a leptospermum was published by Johann Reinhold Forster and Johann Georg Adam Forster in their 1776 book, Characteres Generum Plantarum.[7][8] In 1876, George Bentham described twenty species, but noted the difficulty of discriminating between species. ("The species are very difficult to discriminate.") Of the species he named, only ten remain as valid.[4][9]

In 1979, Barbara Briggs and Laurie Johnson published a classification of the family Myrtaceae in the Journal of the Journal of the Linnean Society of New South Wales. Although there have been revisions to their groupings, their paper allowed a systematic examination of species in the genus Leptospermum.[10] In 1989, Joy Thompson published a complete revision of the genus.[11] In 2000, O'Brien et al. published yet another revision, using matK-based evidence to suggest that Leptospermum is polyphyletic, and should be split into persistent, Western non-persistent, and Eastern non-persistent fruiting plants, with Leptospermum spinescens as an outlier.[12] However, neither phylogeny has been universally accepted.[13] Current estimates recognize about ninety species of Leptospermum.[4]

The common name tea tree derives from the practice of early Australian settlers who soaked the leaves of several species in boiling water to make a herbal tea.[14]

Distribution and habitat

Most Leptospermum species are endemic to Australia where most are found in southern areas of the country. They are most common in moist nutrient-poor soils although they sometimes occupy other situations. Leptospermum laevigatum is usually found growing on beach sand and L. riparium growing in Tasmanian rainforest on the edges of rivers. Leptospermum amboinense extends from Queensland to Southeast Asia and three species, L. javanicum, L. parviflorum and L. recurvum are endemic to southeast Asia. L. recurvum is only found on Mount Kinabalu in Sabah. Leptospermum scoparium is one of the most widespread in the genus and occurs in New South Wales, Victoria, Tasmania and New Zealand, where it is one of the most widespread and important native shrub species.[4][11][15][16]

Ecology

In Australia, Leptospermum species are sometimes used as food plants by the larvae of hepialid moths of the genus Aenetus, including A. lewinii and A. ligniveren. These burrow horizontally into the trunk then vertically down.

Uses

Use in horticulture

Most Leptospermum species make desirable garden plants. The hardiest species (L. lanigerum, L. liversidgei, L. polygalifolium, L. rupestre, L. scoparium) are hardy to about −8 °C (18 °F) to −10 °C (14 °F); others are sensitive to frost. They tolerate most soils, but many supplies specify ericaceous (i.e. lime-free) compost with good drainage and full sun. Established plants are drought tolerant. They are often found as hedge plants on the west coast of the United States, and some species are popular for cultivation as bonsai. Many cultivars exist.

Use in floristry

These flowers are also grown in double cultivars and are used in floral designs. However, they do not last when out of water and the single flowers do not last when wired. The 'Pacific Beauty' (Leptospermum poolgalifolium) is a useful flower to use in large church-service bowls and function arrangements, however use of Leptospermum in corporate designs is less desirable as they dry and drop when subjected to heating and air conditioning.

Honey production

The nectar from the flowers is harvested by bees; this is used to make Leptospermum honey. Honey produced from Australian Leptospermum polygalifolium, also known as jelly bush or the lemon-scented tea tree, has been found to contain up to 1750 mg/kg of 'methylglyoxal' (MGO), an antibacterial compound.[17] However, after neutralization of this compound, the honey retains bactericidal activity.[18] Methylglyoxal thus does not appear to be the main contributor to the antimicrobial and antibacterial activities.[19]

The nectar of L. scoparium is collected by bees and the honey produced is collected and marketed as Manuka honey.[20]

Species

The following is a list of species accepted by the Australian Plant Census as at March 2020,[1] apart from two species only occurring outside Australia that are accepted by the Royal Botanic Gardens, Kew:[21]

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References
  1. "Leptospermum". Australian Plant Census. Retrieved 19 March 2020.
  2. "Leptospermum". Dictionary.com Unabridged. Random House. Retrieved 2016-01-21.
  3. Sunset Western Garden Book, 1995:606–607
  4. Wrigley, John W.; Fagg, Murray (1993). Bottlebrushes, paperbarks & tea trees, and all other plants in the Leptospermum alliance (1181–183 ed.). Pymble, N.S.W.: Angus & Robertson. ISBN 978-0207168673.
  5. "Genus Leptospermum". Royal Botanic Gardens Sydney: plantnet. Retrieved 21 February 2017.
  6. "Leptospermum and its Relatives - Background". Australian Native Plants Society (Australia). Retrieved 21 February 2017.
  7. "Leptospermum". APNI. Retrieved 21 February 2017.
  8. Forster, Johann Reinhold; Forster, Johann Georg Adam (1776). Characteres Generum Plantarum. London: Prostant apud B. White, T. Cadell, & P. Elmsly. p. 71. Retrieved 21 February 2017.
  9. Bentham, George (1867). "Orders XLVIII. Myrtaceae- LXII. Compositae". Flora Australiensis. 3: 100–111. Retrieved 21 February 2017.
  10. Briggs, Barbara Gillian (1979). "Evolution in the Myrtaceae - Evidence from inflorescence structure". Proceedings of the Linnean Society of New South Wales. 102 (4): 157–256. Retrieved 21 February 2017.
  11. Thompson, Joy (1989). "A revision of the genus Leptospermum (Myrtaceae)". Telopea. 3 (3): 301–449. doi:10.7751/telopea19894902.
  12. O'Brien, Marcelle M.; Quinn, Christopher J.; Wilson, Peter G. (2000). "Molecular systematics of the Leptospermum suballiance (Myrtaceae)". Australian Journal of Botany. 48 (5): 621. doi:10.1071/bt99021. ISSN 0067-1924.
  13. Thornhill, Andrew H.; Ho, Simon Y.W.; Külheim, Carsten; Crisp, Michael D. (December 2015). "Interpreting the modern distribution of Myrtaceae using a dated molecular phylogeny". Molecular Phylogenetics and Evolution. 93: 29–43. doi:10.1016/j.ympev.2015.07.007. ISSN 1055-7903. PMID 26211451.
  14. "Leptospermum - family Myrtaceae Commonly known as "teatrees"". Australian National Botanic Garden. Retrieved 21 February 2017.
  15. "Leptospermum scoparium". Royal Botanic Gardens Sydney; plantnet. Retrieved 21 February 2017.
  16. Stephens, Jonathan M.C.; Molan, Peter C.; Clarkson, Bruce D. (January 2005). "A review of Leptospermum scoparium (Myrtaceae) in New Zealand". New Zealand Journal of Botany. 43 (2): 431–449. doi:10.1080/0028825X.2005.9512966.
  17. Native honey a sweet antibacterial Archived 2011-03-06 at the Wayback Machine, Australian Geographic, March 3, 2011.
  18. Kwakman PHS; te Velde AA; de Boer L; Vandenbroucke-Grauls CMJE; Zaat SAJ (2011). "Two major medicinal honeys have different mechanisms of bactericidal activity". PLoS ONE. 6 (3): e17709. Bibcode:2011PLoSO...617709K. doi:10.1371/journal.pone.0017709. PMC 3048876. PMID 21394213.
  19. Molan, P. (2008). "An explanation of why the MGO level in manuka honey does not show the antibacterial activity". New Zealand BeeKeeper. 16 (4): 11–13.
  20. "Growing and harvesting Mānuka honey". New Zealand Ministry for Primary Industries. Retrieved 5 December 2019.
  21. Kew World Checklist of Selected Plant Families
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