Convolvulaceae In Meyler's Side Effects of Drugs (Sixteenth Edition), 2016

K 5/(5) [3,4/(3,4)] C (5) [(3,4)] A (5) [3,4] G (2) [(3–5)], superior.
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The Convolvulaceae consist of herbaceous to woody vines, less commonly herbs, shrubs, or rarely trees. Some family members are achlorophyllous and parasitic (e.g., Cuscuta). The roots are haustorial in parasitic taxa. The stems of viney members are dextrorse (twining clockwise when moving away, like the grooves of a typical “right-handed” screw). The leaves are simple, undivided to divided, spiral, exstipulate, reduced and scalelike in Cuscuta. The inflorescence is a head, dichasium, or of solitary flowers, bracteate, of usually two, often accrescent bracts. The flowers are bisexual, actinomorphic, and hypogynous. The perianth is dichlamydeous. The calyx has 5 [3,4], sepals or lobes. The corolla is sympetalous, often infundibular, with 5 [3,4] lobes, with usually involute (plicate) aestivation (imbricate in Cuscuta). The stamens are 5 [3,4], filaments often unequal in length, the stamens epipetalous. Anthers are longitudinal in dehiscence. The gynoecium is syncarpous with a superior ovary, 2 [3–5] carpels, and 1–several locules (as many as carpels). The style(s) are solitary to as many as carpels. Placentation is basal; ovules are anatropous, unitegmic, 2 per carpel [rarely ∞]. Nectaries are present, consisting of an annular disk around base of ovary. The fruit is a capsule (loculicidal, circumscissile, or irregularly dehiscing), berry, drupe, or nut. The seeds are endospermous. Internal phloem (inner to the xylem) is present in many family members.

Weed Classification

Robert L. Zimdahl, in Fundamentals of Weed Science (Fifth Edition), 2018

3.4 Parasitic Weeds

Parasitic weeds are often placed in other sections of weed science texts. They are here because theirs is a particular and peculiar habitat. Phanerogamic parasites (“phanerogamic” comes from the Greek words phaneros = visible and gamos = marriage) include more than 3000 species distributed among 17 families. Only eight families include important parasitic weeds. The economically important species that damage crop and forest plants are all dicotyledons from five families (Table 3.4) (Sauerborn, 1991). Parasitic weeds from four families will be discussed briefly. Readers who want more detailed information are directed to Parker and Riches (1993).
The Cuscutaceae, which are dodders, are noxious in all US states except Alaska and are distributed throughout the world's agricultural regions. The Composite list of Weeds published by the Weed Science Society of America includes 14 species plus three synonyms.³ A mature dodder plant, a true parasite, is a long, fine, yellow branching stem. A single stem of field dodder, one of the most important species, can grow up to 10 cm in 1 day. It is nonspecific regarding hosts. It coils, twines, and parasitizes many plants. Dodder flowers and reproduces by small, sticky seeds. Haustoria penetrate a host's cortex to the cambium and the fine stems dodder (tremble) when the wind blows. Dodder seed emerges from as deep as 4 ft in soil as a rootless, leafless seedling. The fine, yellow stem, which is 1–3 in. long, emerges as an arch, straightens, and slowly rotates in a counterclockwise direction (circumnutation) until it contacts another plant, which must be within about 1 1/4 in. Seeds have sufficient resources to search for a host for 4–9 days after which the emerged plant dies (Sauerborn, 1991). After contact and attachment, the seed (soil) connection withers and dodder plants live as obligate stem parasites. Westwood (http://wssa.net/2016/08/what-plants-sense-and-say-may-impact-the-future-of-weed-control/) adverts that dodder may tell host plants precisely how to lower their defenses so they can be more readily attacked and produce the nutrients dodder needs to thrive. It is a provocative hypothesis that may lead to new innovative management strategies. The US witchweed eradication program has been one of the world's greatest success stories in parasitic weed control (Tasker and Westwood, 2012).
Witchweed is one of three weedy, hemiparasitic species of the Scrophulariaceae in the world. It damages crop plants before they are even visible above ground. There are 35 species of Striga: 23 are found in Africa and at least 11 parasitize crops (Parker and Riches, 1993). Native to Asia and Africa, it is a US Federal noxious weed that was first identified in the United States, in North Carolina, in 1956 (Werth et al., 1984). Two important species are Striga hermonthica (purple or giant witchweed), which parasitizes sorghum, millet, and corn in Africa, and Striga gesnerioides (cowpea witchweed), the only one that parasitizes dicots. It is important to cowpeas and groundnut in East and West Africa and Asia. It is a root parasite on corn, sorghum, and other grasses in Africa, India, and the far East. Plants are normally 6–12 in. tall, but can grow up to 24 in. Leaves are linear and about 1 in. long. Flowers are less than 1/2 in. in diameter, occur in or on loose spikes, and can vary from white to yellow, red, or purple. Flowers produce a swollen seed pod that contains thousands of microscopic seeds (0.2 × 0.3 mm) per pod. Placed end to end, 1000–1500 seeds would be only 1 foot long. They survive up to 14 years in soil and one plant can produce up to 58,000 seeds.
The desert locust (Schistocerca gregaria) gains a great deal of publicity when it swarms in Africa. Massive efforts are made to combat it, but over the years and in any 1 year, witchweeds cause more crop losses in Africa than the desert locust. The genus has the narrowest host range of the important parasitic weeds and a narrower range of distribution than dodder. In 2014–15 it was limited to five counties in North Carolina and two in South Carolina. The successful eradication program, funded by the US Department of Agriculture (USDA), which was begun in 1957, has reduced the infestation from 450,000 ac to five North Carolina counties, 82 farms, 118 fields, and 1140.9 ac to two counties, and 15 farms, 18 fields, and only 130.3 ac in South Carolina.
Witchweeds are widely distributed in the world's tropical and subtropical regions and occur naturally in parts of Africa, Asia, and Australia. Secretions from corn (and some other grass) roots encourage germination of witchweed seed. After parasitization, corn is stunted, yellow, and wilted because of loss of nutrients and water. Many weeds, including crabgrass, serve as alternate hosts. It easily parasitizes corn because its 90- to 120-day life cycle is similar to corn's. One corn plant can support, but usually does not, up to 500 witchweed plants. Witchweed seed will not germinate in soil in the absence of a host-excreted stimulant. It may be induced to germinate with the artificial stimulant, ethylene gas. It was introduced to a corn field in North Carolina in July 1956 (Werth et al., 1984) and was recognized by a graduate student from India who had seen its effects on sorghum production in India. The USDA, via Animal Plant Health Inspection Service (APHIS) regulations, has had witchweed under quarantine in North and South Carolina since 1957 to prevent its spread throughout the United States. The quarantine has eliminated over 99% of the more than 432,000 ac that were infested within the eastern Carolinas (Iverson et al., 2011; Tasker and Westwood, 2012). South Carolina released the last acres from quarantine in 2009. Eventual eradication is predicted.
Plant parasites such as witchweed have not been controlled in susceptible crops with herbicides or weed management methods before damage occurred. The USDA/APHIS program has emphasized early detection, isolation, and quarantined areas. Crop seed coating with the benzoate herbicide pyrithiobac or the imidazolinone herbicide imazapyr offers promise for controlling witchweed in Africa (Kanampiu et al., 2003; Ransom et al., 2012). High herbicide levels can be localized on or near seed of acetolactate synthase–resistant maize. Imazapyr was optimal for seed dressings to prevent damage from witchweed, which emerged on untreated maize 6–12 weeks after planting. There was almost no emergence for 3 months on imazapyr-dressed maize seed (Kanampiu, 2001). Hand pulling escaped shoots reduced infestation and depleted the seed bank. Seed treatment gave a three- to fourfold increase over no control. The best control has been achieved with integration of improved, adapted varieties with cultural methods. Kanampiu (2008) suggests that herbicide seed treatment is a “stop-gap” measure until genetic resistance becomes available. Other herbicides offer some promise (Kabambe et al., 2008).
The most important parasite in the Loranthaceae is mistletoe. Mistletoes occur in two families: the Loranthaceae and the Viscaceae. Some taxonomists combine both families in the Loranthaceae. Dwarf mistletoe is a photosynthetic, flowering plant that parasitizes ponderosa pine in the southwestern United States. It occurs on the trunk and branches as a dense tangle of short, brown to yellow-brown stems. Seeds are dispersed by birds or by explosion of seed pods and expulsion of sticky seeds that may adhere to adjacent trees. Seeds that burst from pods can travel up to 60 miles per hour over 45 ft. The seeds are usually dispersed in August or early September in southwestern United States.
The Orobanchaceae (from Latin, orobos = bitter vetch and Latin, anchein = to strangle), or broomrapes, include over 100 species, five of which are important, obligate root holoparasites (lacking all chlorophyll) that attack carrots, broad beans, tomatoes, sunflowers, red clover, and several other important, small acreage crops in more than 58 countries (Parker and Riches, 1993; Sauerborn, 1991). The broomrapes have the broadest host range of the parasitic families. They cause major yield losses and often complete loss of some crops in many developing countries where control is not possible. They are the most important weed of cool-season food legumes (e.g., cowpea, fava bean⁴). Broomrape is found in California but is not important in most of the United States. It is important in Southern and Eastern Europe, West Asia, and North Africa. Seed of some species live in soil for up to 10 years. One plant can produce up to 200,000 small seeds; 1 g contains up to 150,000 seeds. Similar to witchweed, seed germination of Orobanche is stimulated by secretions from the host's root or from roots of nonhost plants. Pepper has shown some promise as a trap-crop for two Orobanche species (Hershenhorn et al., 1996). Germination will not occur in the absence of host-excreted chemical stimulants. Most damage from root parasites occurs before the parasite emerges and only 10%–30% of attached parasites emerge (Sauerborn, 1991).
An important aspect of parasitic weeds is the current inability to manage them with other than sophisticated chemical technology or extended fallow periods. It has been noted that as little as 100 g glyphosate/hectare (a sublethal dose) applied three times after rimsulfuron (a sulfonylurea herbicide) selectively reduced broomrape shoot numbers in potato (Haidar et al., 2005). Parasitic mechanisms and control were reviewed by Joel et al. (2013). Many of the world's people live in areas where food is scarce and agricultural technology is not modern or is absent or unaffordable. These are the places where parasitic weeds cause the greatest yield losses. Fields have been taken out of production and production area of some crops has been reduced severely owing to parasitic weeds.
Chris Parker, a retired weed scientist in the United Kingdom, prepares and distributes Haustorium, an online review of worldwide research on parasitic weeds. It is published by the International Parasitic Plant Society (http://www.parasiticplants.org) and distributed by Parker, the editor
The Convolvulaceae have a mostly worldwide distribution. Economic importance includes cultivated ornamentals such as Convolvulus, Ipomoea, and Jacquemontia; Ipomoea batatas is the sweet potato, source of the starchy storage root. See Staples and Brummit (2007) for general information and Stefanovic et al. (2002, 2003), Stefanovic and Olmstead (2004), and Garcia et al. (2014) for phylogenetic studies.
The Convolvulaceae are distinctive in being often dextrorse -twining vines, less commonly shrubs or trees, usually with internal phloem, with simple, spiral leaves, and actinomorphic, sympetalous flowers, corollas typically with involute aestivation and often infundibular.