This review deals with higher plants that are parasitic on the roots of other higher plants. These unique organisms in the genera Striga and Orbanche, are both weeds and parasites. As weeds they show great phenotypic plas­ ticity, wide environmental tolerance, prefer permanently disturbed habitats, and are part of a plant guild associated with colonizing or crop complex species. As parasites, they depend upon another vascular plant for food or water, which flows from host to parasites through haustoria. These struc­ tures form a morphological and physiological graft with the roots or other underground parts of the host. The remainder of the root system of these parasites is usually condensed and lacks typical root hairs and root caps. All species of Orobanche and most Striga are obligate parasites-they will not develop at all without a host. Facultative parasites such as most of the parasitic species in the Scrophulariaceae are not nutritionally dependent on a host plant but they are invariably attached to one in nature. Root parasitism is found in the following diverse dicotyledon families of seed plants: Hydnoraceae, Balanophoraceae, Krameriaceae, Lennoaceae, Santalaceae (and segregate and allied families), Scrophulariaceae, and Oro­ banchaceae (88, 176). No parasitic monocots are known and there is only one parasitic gymnosperm, Parasitaxus ustus; this organism is endemic to New Caledonia-it has an unique host-parasite interface involving a graft­ like union of tissue (L. J. Musselman, unpublished).

The Biology of Striga, Orobanche, and other Root-Parasitic Weeds

The plant family Orobanchaceae includes many parasitic weeds that are also impressive invaders and aggressive crop pests with several specialized features (e.g. microscopic seeds, parasitic habits). Although they have provoked several large-scale eradication and control efforts, no global evaluation of their invasive potential is as yet available. We use tools from ecological niche modeling in combination with occurrence records from herbarium specimens to evaluate the global invasive potential of each of 10 species in this assemblage, representing several of the worst global invaders. The invasive potential of these species is considerable, with all tropical and subtropical countries, and most temperate countries, vulnerable to invasions by one or more of them.

Global invasive potential of 10 parasitic witchweeds and related Orobanchaceae.

Brassinolide [2α, 3α, 22R,23R-tetrahydroxy-24S-methyl-B-homo-7-oxa-5-α-cholestan-6-one] and its related compounds, brassinosteroids, applied at the early stages of conditioning shortened the conditioning period required before clover broomrape seeds would germinate after exposure to germination stimulants, such as dl-strigol and natural stimulants from red clover (Trifolium pratense L.) root exudate. Brassinosteroids applied after conditioning increased the rate of the seed germination induced by these stimulants. The inhibitory effect of light on seed germination when it was induced by dl-strigol could be overcome by brassinosteroids. Brassinosteroids also eliminated the inhibitory effects of light and dl-strigol applied at the early stages of conditioning. GA3 was also effective in causing seed conditioning and increased the rate of the germination induced by these stimulants. There was a relationship between brassinosteroids and GA3 in many of the experiments conducted. These findings may have practical implications in increasing the effectiveness of applying germination stimulants in the field to soils for ‘suicidal germination’ of broomrape seeds.

Effects of brassinosteroids on conditioning and germination of clover broomrape (Orobanche minor) seeds

Light conditions provide important information about the best time and place for seedling establishment Photoinhibition of seed germination (PISG), defined as the partial or complete suppression of germination under white light, has been interpreted as a physiological adaptation to avoid germination at or near the soil surface This review is the first report of an all-inclusive, fully quantitative analysis of PISG in seed plants Pertinent data available from the published literature for 301 taxa from 59 families and 27 orders were assessed The association of PISG with several plant and seed traits allowed us to consider the adaptive significance of PISG in relation to plant life histories and the natural environments As no gymnosperm has been found to be truly photoinhibited, it seems that PISG is apomorphic to flowering plants (especially monocots) Seeds of most taxa with PISG have a dark colour and intermediate mass, mostly in the range 1 to 27 mg PISG is absent from humid tropical regions and from cold climates, but it is strongly associated with open, disturbed and dry habitats An intriguing implication of PISG is the formation of a soil-surface seed bank Taken together, these results clearly indicate that PISG is a physiological adaptation to avoid germination on the soil surface, where conditions are not suitable for seedling establishment PISG is probably much more frequent in seed plants than previously thought Thus, laboratory experiments should be conducted under well-characterized light and dark conditions

Photoinhibition of seed germination: occurrence, ecology and phylogeny

Totalling more than 3000 species, parasitic angiosperms are a most diverse group of organisms. Classified as either shoot or root parasites according to site of attachment, plant parasites are further classified, according to the degree of parasitism, into one of two divisions: holoparasitic or hemiparasitic. Within these divisions, angiosperm parasites are either obligate or facultative. Obligate parasites cannot survive to maturity without attaching to a host plant, thus the need to germinate in close proximity to a host first characterizes the specialized germination and seed dispersal strategies of these plants. However, seeds of all species of the root-holoparasitic Orobanchaceae and four genera within the Scrophulariaceae have evolved very specialized germination strategies. Once the seeds have undergone an after-ripening period, they require a prolonged imbibition period at temperatures of approx. 30°C before they acquire the potential to germinate. Minute seeds with very little reserves, they must germinate within millimetres of the host root for successful host attachment and continued survival. To this end, germination only occurs in response to specific chemical germination triggers released from the host root. In Striga hermonthica (Del) Benth., the perception of the host-derived trigger results in stimulation of endogenous ethylene production and it is this ethylene that initiates germination. It is proposed that the chemical triggers released from the host be considered as ‘elicitors’ which function primarily as allelochemicals and that the parasite seeds have become adapted in such a way as to have an absolute requirement for external germination triggers.

Germination of the seeds of parasitic angiosperms

A B S T RA CT Dormancy in seeds of the parasitic phanerogam Aeginetia indica L. can be broken by chemical treatment with sodium hypochlorite, which also helps to control contaminating microflora. A germination medium was developed that suppressed microbial contamination and permitted long-term observation of these slow-germinating seeds. The medium consisted of 10 ppm streptomycin, 10 ppm penicillin, and 10-' M indole-3-acetic acid (IAA) (or other growth regulator) in 1 % water agar. Optimum germination range was 25-30 C. Dormancy could also be broken by exposure on agar for several days at 3-5 C (stratification), or by brief exposures (15 min) to 50 C. Continuous light as low as 0.1 ft-c completely inhibited germination on this growth medium. Brief, intermittent light exposures depressed germination. Germination and growth in vitro of nondormant seed of Aeginetina indica L. can be described in five stages: (1) Germination: expansion of spheroidal cells or nodule at micropylar end of the seed, stimulated

L. J. Sherman

Papers

Factors affecting dormancy, germination, and seedling development of aeginetia indica l. (orobanchaceae)