Parasitic plant
Alternately, plants like Cuscuta and some members of Orobanche connect to both the xylem and phloem of the host.
Parasitic plants are characterized as follows:[5] For hemiparasites, one from each of the three sets of terms can be applied to the same species, e.g. Holoparasites are always obligate so only two terms are needed, e.g. Plants usually considered holoparasites include broomrape, dodder, Rafflesia, and the Hydnoraceae.
Plants usually considered hemiparasites include Castilleja, mistletoe, Western Australian Christmas tree, and yellow rattle.
Parasitic behavior evolved in angiosperms roughly 12-13 times independently, a classic example of convergent evolution.
Later evolution led to the development of terminal or primary haustoria at the tip of the juvenile radicle, seen in obligate hemiparasitic species within Striga.
Lastly, holoparasitic plants, always forms of obligate parasites, evolved over the loss of photosynthesis, seen in the genus Orobanche.
[8] The most specialized forms of holoparasitic plants are the four families Rafflesiaceae, Cytinaceae, Mitrastemonaceae and Apodanthaceae, lineages which independently has evolved further into endoparasites that, except for the flowers, spend their entire life cycle within the tissue of their host.
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.
[13][14] For example, the seeds of witchweed (Striga asiatica) need to be within 3 to 4 millimeters (mm) of its host to receive chemical signals in the soil to trigger germination.
These chemical cues are a variety of compounds that are unstable and rapidly degraded in soil and are present within a radius of a few meters of the plant exuding them.
Scientists used volatiles from tomato plants (α-pinene, β-myrcene, and β-phellandrene) to test the reaction of C. pentagona and found that the stem orients itself in the direction of the odor.
[14] Some studies suggest that by using light reflecting from nearby plants dodders can select hosts with higher sugar because of the levels of chlorophyll in the leaves.
The third hurdle is the host's ability to create a toxic environment at the location where the parasitic plant attaches.
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.
Species of Striga alone are estimated to cost billions of dollars a year in crop yield loss annually, infesting over 50 million hectares of cultivated land within sub-Saharan Africa alone.
Striga can infest both grasses and grains, including corn, rice and sorghum, some of the most important food crops.
Orobanche also threatens a wide range of important crops, including peas, chickpeas, tomatoes, carrots, lettuce,[20] 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.
[29] In many regions, including the Nepal Eastern Himalayas, parasitic plants are used for medicinal and ritual purposes.
[30] About 400 species of flowering plants, plus one gymnosperm (Parasitaxus usta) and one bryophyte (the liverwort Aneura mirabilis), are parasitic on mycorrhizal fungi.
