Parasitic plant

Type of plant that derives some or all of its nutritional requirements from another living plant

A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the haustorium, which penetrates the host plant, connecting them to the host vasculature – either the xylem, phloem, or both. For example, plants like Striga or Rhinanthus connect only to the xylem, via xylem bridges (xylem feeding). Alternately, plants like Cuscuta and some members of Orobanche connect to both the xylem and phloem of the host. This provides them with the ability to extract resources from the host. These resources can include water, nitrogen, carbon and/or sugars. Parasitic plants are classified depending on the location where the parasitic plant latches onto the host (root or stem), the amount of nutrients it requires, and their photosynthetic capability. Some parasitic plants can locate their host plants by detecting volatile chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plants in approximately 20 families of flowering plants are known. There is a wide range of effects that may occur to a host plant due to the presence of a parasitic plant. Often there is a pattern of stunted growth in hosts especially in hemi parasitic cases, but may also result in higher mortality rates in host plant species following introduction of larger parasitic plant populations.

Parasitic plants germinate in several methods. These can either be chemical or mechanical and the means used by seeds often depends on whether or not the parasites are root parasites or stem parasites. Most parasitic plants need to germinate near their host plants because their seeds are limited in the number of resources necessary to survive without nutrients from their host plants. Resources are limited due in part to the fact that most parasitic plants are not able to use autotrophic nutrition to establish the early stages of seeding. Root parasitic plant seeds tend to use chemical cues for germination. For germination to occur, seeds need to be quite close to the host plant. Stem parasitic plants, unlike most root parasites, germinate using the resources inside their endosperms and can survive for some time. For example, the dodders (Cuscuta spp.) drop their seeds to the ground. These may remain dormant for up to five years before they find a host plant. Using the resources in the seed endosperm, the dodder can germinate. Once germinated, the plant has 6 days to find and establish a connection with its host plant before its resources are exhausted. Once the dodder finds its host, it wraps itself around the host plant's stem. Using adventitious roots, the dodder taps into the host plant's stem with a haustorium, an absorptive organ within the host plant vascular tissue. Dodder makes several of these connections with the host as it moves up the plant.

Some parasitic plants are generalists and parasitize many different species, even several different species at once. Dodder (Cuscuta spp.) and red rattle (Odontites vernus) are generalist parasites. Other parasitic plants are specialists that parasitize a few or just one species. Beech drops (Epifagus virginiana) is a root holoparasite only on American beech (Fagus grandifolia). Rafflesia is a holoparasite on the vine Tetrastigma. Plants such as Pterospora become parasites of mycorrhizal fungi. There is evidence that parasites also practice self discrimination, species of Triphysaria experience reduced haustorium development in the presence of other Triphysaria. The mechanism for self discrimination in parasites is not yet known. Marine parasites occur as a higher proportion of marine flora in temperate rather than tropical waters. While no full explanation for this is available, many of the potential host plants such as kelp and other macroscopic brown algae are generally restricted to temperate areas. Roughly 75% of parasitic red algae infect hosts in the same taxonomic family as themselves, these are given the designation adelphoparasites. Other marine parasites, deemed endozoic, are parasites of marine invertebrates (mollusks, flatworms, sponges) and can be either holoparasitic or hemiparasitic, some retaining the ability to photosynthesize after infection. These are the only parasitic plants that parasitize animal hosts. and varieties of the genus Brassica (e. g. cabbage and broccoli). Yield loss from Orobanche can reach 100% and has caused farmers in some regions of the world to abandon certain staple crops and begin importing others as an alternative. Much research has been devoted to the control of Orobanche and Striga species, which are even more devastating in developing areas of the world, though no method has been found to be entirely successful. A few dozen parasitic plants have occasionally been used as food by people. Western Australian Christmas tree (Nuytsia floribunda) sometimes damages underground cables. It mistakes the cables for host roots and tries to parasitize them using its sclerenchymatic guillotine. Some parasitic plants are destructive while some have positive influences in their communities. Some parasitic plants damage invasive species more than native species. This results in the reduced damage of invasive species in the community. Parasitic plants are major shapers of their community, affecting not just the host species but indirectly affecting others. Competition amongst host species will change due to the parasitic plant. Plant parasitism have been shown to keep invasive species under control and become keystone species in an ecosystem. In many regions, including the Nepal Eastern Himalayas, parasitic plants are used for medicinal and ritual purposes.

About 400 species of flowering plants, plus one gymnosperm (Parasitaxus usta) and one bryophyte (the liverwort Aneura mirabilis), are parasitic on mycorrhizal fungi. This effectively gives these plants the ability to become associated with many of the other plants around them. They are termed myco heterotrophs. Some myco heterotrophs are Indian pipe (Monotropa uniflora), snow plant (Sarcodes sanguinea), underground orchid (Rhizanthella gardneri), bird's nest orchid (Neottia nidus avis), and sugarstick (Allotropa virgata). Within the taxonomic family Ericaceae, known for extensive mycorrhizal relationships, there are the Monotropoids. The Monotropoids include the genera Monotropa, Monotropsis, and Pterospora among others. Myco heterotrophic behavior is commonly accompanied by the loss of chlorophyll.