Arceuthobium

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The genus Arceuthobium, commonly called dwarf mistletoes, is a genus of 42[1] species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia, Europe, and Africa.[2] Of the 42 species that have been recognized, 39 and 21 of these are endemic to North America and the United States, respectively. They all have very reduced shoots and leaves (mostly reduced to scales) with the bulk of the plant living under the host's bark. Recently the number of species within the genus has been reduced to 26 as a result of more detailed genetic analysis.[1]

Description

File:Arceuthobium vaginatum subsp. cryptopodum.jpg
Female plant of A. vaginatum susbp. cryptopodum on Ponderosa pine

Arceuthobium species are dioecious, individual plants being either male or female. The fruit is unusual in that it builds up hydrostatic pressure internally when ripe and shoots the single sticky seed up to speeds nearly 50 miles per hour (80 km/h), an example of rapid plant movement. The lodgepole pine dwarf mistletoe, Arceuthobium americanum, has been found to explosively-disperse its seeds through thermogenesis.[3] Dwarf mistletoe seeds are enveloped in a hygroscopic, glue-like substance called viscin. Many fail to land on a suitable host's shoot, but some succeed, and in this way they are spread through the forests as a pest front. The spread of dwarf mistletoes in forest stands is greatest from the overstory to the understory, owing to gravity. Advantageous stand conditions for the spread of the parasite include an uneven-aged stand structure with severely infected hosts in dominant and codominant crown classes, species composition dominated by the primary host, and tree densities between 175 and 500 trees/ha.

There are also several species from Europe and Asia. In particular, Arceuthobium minutissimum, which lives primarily on Pinus wallichiana in the Himalayas, produces the smallest shoots of any member of the genus.Lua error in package.lua at line 80: module 'Module:Footnotes/anchor_id_list' not found. It is also the smallest known dicotyledonous plant.[4]

Effects of parasitism

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In western forest ecosystems of North America, numerous dwarf mistletoe species are considered to be serious forest-borne disease agents. Severe dwarf mistletoe infection can result in a reduction in tree growth, premature tree mortality, reduced seed and cone development, and reduced wood quality, and increases the susceptibility of the host tree to pathogen and/or insect attack. Most of the commercially important conifers in western North America are parasitized by one or more dwarf mistletoes.

The interaction between dwarf mistletoes and their host can be generalized as a source-to-sink relationship. Dwarf mistletoes derive the majority of their nutrition from the host's vascular tissues. Dwarf mistletoes have a root-like endophytic system, composed of primary and secondary haustoria, which invade, but do not injure, both the xylem and phloem of the host. Because this root-like endophytic system is not soilborne, dwarf mistletoes are dependent solely on their host for water. Along a xylem-to-xylem link, dwarf mistletoes draw water from their host using the difference in water potential between parasite and host. The greater transpiration rate of the dwarf mistletoe produces a lower water potential, allowing water to flow from host to parasite. The water gradient or transpiration stream is consistently maintained, even when the host is under moderate water deficit.

In addition to host-water dependence, dwarf mistletoes must acquire carbohydrate and mineral nutrition from their hosts. Dwarf mistletoes have both chlorophyll a and chlorophyll b and the necessary mechanisms for photosynthesis, but chlorophyll concentrations in dwarf mistletoes are approximately 1/5 to 1/10 of those found in their host's foliage, and dwarf mistletoes have low photosynthetic rates as measured by the rate of carbon fixation. The principal carbohydrate transported from the host to dwarf mistletoe is sucrose. Because dwarf mistletoes are phloem-deficient, they draw carbohydrates from their hosts by connections to the host phloem and ray parenchyma. The rate of carbohydrate transport varies by season, but dwarf mistletoes continuously draw carbohydrates from their hosts throughout the year.

Dwarf mistletoe rating system

A standardized system called the Hawksworth 6-class dwarf mistletoe rating (DMR) system has been devised to determine how much dwarf mistletoe has infected a tree. To use this system, the living part of the tree crown needs to be broken up into 3 sections, (top, middle, bottom). Each section is then rated either 0, 1, or 2 with a score of 0 being assigned for a uninfected section, 1 for light infection with fewer than half of the branches infected, or 2 for heavy infection with over half of the branches infected. The 3 numbers are then added together to give the total rating for the tree.[5]Template:R/superscript

Species

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File:Arceuthobium occidentale on Pinus sabiniana.jpg
Arceuthobium occidentale shoots on Pinus sabiniana branches, Mount Diablo, California. 25 December 2008

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See also

References

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  1. ^ a b Page Module:Citation/CS1/styles.css has no content.Nickrent, D.L.; M.A. García; M.P. Martín; R.L. Mathiasen (2004). "A phylogeny of all species of Arceuthobium (Viscaceae) using nuclear and chloroplast DNA sequences". American Journal of Botany. 91 (1): 125–138. Bibcode:2004AmJB...91..125N. doi:10.3732/ajb.91.1.125. PMID 21653369. S2CID 13745441.
  2. ^ Page Module:Citation/CS1/styles.css has no content.O'Neill, A.R.; Rana, S.K. (2019). "An ethnobotanical analysis of parasitic plants (Parijibi) in the Nepal Himalaya". Journal of Ethnobiology and Ethnomedicine. 12 (14): 14. doi:10.1186/s13002-016-0086-y. PMC 4765049. PMID 26912113.
  3. ^ [1], Rolena A.J. deBruyn, Mark Paetkau, Kelly A. Ross, David V. Godfrey & Cynthia Ross Friedman. Thermogenesis-triggered seed dispersal in dwarf mistletoe.
  4. ^ Page Module:Citation/CS1/styles.css has no content.Datta, R. M. (February 1951). "Occurrence of a Hermaphrodite Flower in Arceuthobium minutissimum Hook.f., the Smallest Known Dicotyledonous Plant". Nature. 167 (4240): 203–204. Bibcode:1951Natur.167..203D. doi:10.1038/167203a0. ISSN 1476-4687. PMID 14806428. S2CID 4181798. Retrieved 16 April 2022.
  5. ^ Page Module:Citation/CS1/styles.css has no content.Hoffman, James T. (May 2004). "Management guide for dwarf mistletoe" (PDF). United States Department of Agriculture, Forest Service. Archived from the original (PDF) on November 23, 2015. Retrieved 25 July 2016.

Further reading

  • Page Module:Citation/CS1/styles.css has no content.Hawksworth, Frank G.; Wiens, Delbert (March 1996). Dwarf Mistletoes: Biology, Pathology, and Systematics (PDF) (Report). Washington, DC: United States Department of Agriculture, Forest Service, Agricultural Handbook 709. Retrieved 8 May 2026.
  • Kenaley, S.C., R.L. Mathiasen, & C.M. Daugherty. 2006. Selection of dwarf mistletoe-infected ponderosa pines by Ips species (Coleoptera: Scolytidae) in northern Arizona. WNAN 66:279-284.
  • Mathiasen, R.L. 1996. Dwarf mistletoes in forest canopies. Northwest. Sci. 70:61-71.

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