Parasitism for plants is a relationship in which one organism uses the nutrients and water of another plant, the host. A holoparasite is a relationship where parasitism is obligatory. A hemiparasite is exemplified by a plant that can live either as a parasite or on its own, hence this plant is a facultative parasite.
Parasitic plants belong to about 15 families of flowering plants. The many species of mistletoes, belonging to Viscaceae and Loranthaceae, comprise about three-quarters of all parasitic species. Several other families of parasitic flowering plants are also well-known, especially the broomrapes (Orobanchaceae) and dodders (Cuscutaceae).
Parasitism has evolved many times in dicotyledons, i.e., there has been convergence toward several types.
The following are some general properties of parasitic plants.
Parasites become established via germination. Seeds land on the host tissue, for stem parasites especially in bird droppings, and germinate after reading a chemical stimulus from the host. A modified lateral root becomes an haustorium; this root is chemotrophic, i.e., responding to a chemical gradient, and contacts the host epidermis. The root then attaches by pushing against the plant and forming a disc, called a hapteron, and secretes a polysaccharide adhesive. The root tip then mechanically penetrates the host, apparently without enzymatic digestion, and establishes a vascular connection by attaching vessels and positioning phloem next to leaky host phloem.
Mistletoes were formerly alleged to receive no host carbohydrates, but substantial heterotropic carbon gain has been measured in mistletoes, even without phloem connections. Direct xylem-to-xylem continuity between host and parasite is not easy to demonstrate. Mistletoes often exhibit high transpiration rates during the day, through stomates and cracks in the epidermis. Losing substantial water from the leaves and stems of the parasite results in a steep water potential gradient, favorable to drawing water into the mistletoe plant. Nitrogen is supplied to the parasite in the xylem stream, and the high transpiration rates, hence, high water demands appear instead to represent a nitrogen-gathering mechanism for the mistletoe.
Typical thick, fleshy root parasites generally lack any adaptations to restrict water loss from their achlorophyllous stems and leaves, because they tend to lack the waxy coating, cuticle.
Dodder and mistletoes are serious problems for plants. Dodder is weedy and can cover woody plants and damage certain economically important crop plants. Mistletoe can become so abundant on a tree, that most of the foliage is of the parasite not the host. In general, experts generally state that parasitic plants rarely, perhaps never, kill the host plant, so that the host and parasite live unhappily together is some balance.
These are three families, all having opposite leaves, which in some species may be photosynthetic. In the temperate Northern Hemisphere, the three commonly seen genera of Viscaceae, which have nonshowy flowers but white or red fruits, are Phoradendron, Viscum, and the dwarf mistletoes of Arceuthobium. Loranthaceae is a tropical family with showy yellow, orange, or red tubular flowers, including Nuytsia floribunda, a tree of Australia that parasitizes roots of grasses. A remarkable endoparasitic loranth is Tristerix aphylla, which attacks a columnar cactus in Chile. Many species of mistletoes appear to have photosynthetic leaves, and thereby may be hemiparasites, but for most physiological studies of leaves have not been attempted.
Root parasites belong to a number of families and are either nonphotosynthetic or photosynthetic. Broomrapes (either classified as family Orobanchaceae or subfamily Orobanchoideae of Scrophulariaceae) have nonphotosynthetic fleshy stems, e.g., the genera Orobanche, Conopholis, and Boschniakia. Other nonphotosynthetic root parasites are found in the families Lennoaceae, Hydnoraceae, Rafflesiaceae e.g., Mitrostemon and Bdallophyton), and Balanophoraceae. Parasitic species in Santalaceae (the sandalwood family) and Krameriaceae (the ratony family) have photosynthetic leaves.
In temperate and some tropical regions of the world are parasitic vines of the Cuscutaceae (or the morning glory family, Convolvulaceae) called dodder, Cuscuta. Stems of dodder are orange to yellow or white. In tropical regions is the vine genus Cassytha of the laurel family (Lauraceae).
These are often cryptic until they flower and fruit. Stem and root parasites occur in the family Rafflesiaceae. In California is the rare Pilostyles thurberi, which bursts from stems of Psorothamnus, a leguminous woody desert plant. In Borneo is the plant with the largest flower, Rafflesia arnoldii, an endoparasite which erupts through the bark of trees and lianas in the grape family (Vitaceae). The nonphotosynthetic vines technically are also stem parasites.
Especially in the family Scrophulariaceae are some common hemiparasites, such as Indian paintbrush and owl's clover (Castilleja), lousewort (Pedicularis), and bird's beak (Cordylanthus). These have green, photosynthetic leaves, but a substantial portion of the parasite's carbon is derived from the host plant, parasitized from the roots.
The most recent flora of California listed 45 species of parasitic dicotyledons that are native or introduced to the state, including the introduced European mistletoe, Viscum album. In Los Angeles County, species of two groups are most commonly seen: the twining vines of dodder (genus Cuscuta) and the woody mistletoes of the genus Phoradendron. In coniferous forests you may find the golden-colored dwarf mistletoes of the genus Arceuthobium (12 species). Much more difficult to find are the secretive root parasites of the Orobanchaceae and Lennoaceae, which are only rarely seen, and only botanists are ever likely, if persistent enough, to find Pilostyles thurberi, very uncommon stem parasite of a leguminous desert shrub. The flora also lists about 70 species of hemiparasites occurring in California, all but one species belonging to Family Scrophulariaceae. Of the hemiparasites, owl's clover and Indian paintbrush (Castilleja) are very common herbs. This combined list of parasites and hemiparasites comprises less than 2% of the total species in California, but collectively they provide a fascinating life style.
There are five species of Phoradendron that can be easily observed during casual walks and drives through Southern California.