In vitro Post-germination Growth and Development of Embryos of Alectra (Scrophulariaceae)

In vivo, seeds of the obligate root parasite Alectra vogelii Benth. (Scrophulariaceae) germinate only after being soaked in water for a period of time (pretreatment) followed by stimulation by certain factors exuded from a host root. Germinated seedlings do not develop beyond radicle emergence, and finally die, unless their radicles make contact with and penetrate into a host root conductive system. In vitro, germinated embryos obtained by exposing sterilized and pretreated seeds to root exudate of Vigna unguiculata were aseptically cultured on Knop's, White's and Murashige and Skoog's media. The embryos grew into seedlings with shoots and roots on a medium containing mineral salts and sucrose, but not on mineral salts alone. Seedling performance was generally not improved when the mineral salts-sucrose media were supplemented with vitamins. Shoot extension growth was better on Murashige and Skoog's mineral salts-sucrose medium than on Knop's or White's medium. However, seedling development was greatly boosted when cultivated on White's minerals salts-sucrose medium supplemented with coconut milk. Seedlings turned green on transfer to light but did not flower. The successful culture of these embryos and seedlings on a simple, chemically defined medium of mineral salts and sugar suggests that these nutrient components are the minimal external requirements for stimulation and support of normal seedling growth. These may be obtained in vivo by the parasite's tapping of the host root conductive system.     

Facultative Hemi-parasitism in Buchnera hispida Buch.- Ham. ex D. Don

Some aspects of growth in pot cultures of Buchnera hispida seedlings,and variations in their performance in the presence and absenceof a host, Sorghum vulgare, were investigated. It is shown thatB. hispida, planted singly or in groups, can complete its lifecycle in the absence of a foreign host but when one is available,it attacks the host through haustorial connections, and growsmore vigorously. There is no effect on the time of floweringor on seed viability. Anatomical studies of the haustorial connectionbetween host and parasite reveal that actual union of tissuesbetween the two roots occurs at the haustorial bridge. The host-parasiteinteraction often results in the death of the host Sorghum.The potential danger posed by B. hispida to cereal crops inthe tropics is discussed.     

Structure and Development of the Primary Haustorium in Alectra vogelii Benth. (Scrophulariaceae)

Anatomical observations were made on the structure and developmentof the primary haustorium of Alectra vogelii. Its developmentinvolves a mutual aggressive growth of both the host and parasitetissues resulting in the formation of a very large and complextuberous organ. One of the host tissues whose growth is stimulatedby parasite infection is the pericycle whose cells divide repeatedlyand grow around and within the parasite haustorial cortex. Fromvarious points of the proliferating host pericycle, roots becomeinitiated and eventually the entire surface of the haustoriumbecomes covered with these roots. We have referred to them as‘haustorial roots’, a term which we have re-examinedand redefined. True xylary connections are established not onlybetween the parasite and the host root but also between theparasite and these ‘haustorial roots’. The uniquedevelopment of primary haustorium and ‘haustorial roots’in A. vogelii is discussed in relation to the development ofprimary haustoria in other root parasites.     

Initiation, Development and Structure of the Primary Haustorim in Striga gesnerioides (Scrophulariaceae)

A light-microscopic study is reported on the initiation, establishmentand structure of the primary haustorium of Striga gesnerioideson the host, cowpea (Vigna unguiculata). The radicular apexof the germinated parasite seed dissolves its way through thehost root cortex to the stele. Thus, it is converted into aprimary haustorium. Some of the haustorial front-line cellsin contact with the host endodermis penetrate into the steleand make contact with the xylem vessels. Differentiation ofthese haustorial cells into xylem vessels occurs and extendsbackwards through the median axial region of the haustorialtract in the host cortex to connect with the conductive xylemof the radicle outside the host root. Subsequently the parasite'splumule develops into a leafy shoot. On penetrating the steleof the host, the haustorium stimulates cell division in thehost pericycle whose triggered proliferation together with expansionof the parasite haustorial tissues result in the formation ofa large, tuberous primary haustorium. At various points of thehost-parasite interface, differentiation of xylem elements occurs,presumably maximizing nutrient transfer from host to parasite.In spite of this, many proliferated host cells at the interfaceremain apparently meristematic showing densely-stained cytoplasmand prominent nuclei.     

Photocontrol of seed germination in the hemiparasite Buchnera hispida (Scrophulariaceae)

Abstract Buchnera hispida, a facultative root parasite of grasses and graminaceous crops, has a light requirement for germination. Studies were carried out on the effects of varying photoperiods with or without preceding dark incubation, on seed germination. Buchnera seeds showed long-day behaviour, since they germinated at all photoperiods including continuous light, and longer photoperiods were more effective in triggering seed germination than shorter photoperiods. Also, effects of red and far-red light indicated that the phytochrome system is operative in the light-induced germination of Buchnera. Although dark incubation in water before illumination was not absolutely necessary for germination, it caused the seeds to respond more rapidly to light. The longer the time of the dark incubation the more responsive the seeds were to photoperiod except when 15 min light was given. The effectiveness of a preceding dark incubation in making Buchnera seeds sensitive to rapid light action was completely inhibited at 4°C. This is in agreement with the hypothesis that a reaction partner of the far-red absorbing form of phytochrome is produced during dark incubation of Buchnera seeds. Such an intermediate has also been reported in some positively photoblastic seeds of non-parasitic flowering plants.     

Seed Germination in Buchnera hispida Buch.--Ham. ex D. Don

The effects of host-produced germination stimulant, light and2-chloroethylphosphonic acid (CEPA) on the germination of seedsof Buchnera hispida collected from Sorghum farms in Bornu Provinceof the North Eastern State of Nigeria were investigated. Lightinduced the germination of about 30 per cent of the seeds inthe absence of host-produced germination stimulant. There waslittle or no germination in the dark. Neither water pretreatmentnor the host stimulant was required for germination. GenerallyCEPA had little or no effect on germination. Light-stimulatedgermination required an induction period of 6 to 8 days in light.This induction period could be shortened if seeds were incubatedin water for a period in the dark prior to transfer to light.The results of our experiments indicate that an endogenous germinationmetabolite is synthesized and accumulated in light. Also thereis evidence to suggest that some steps leading to its formationcan be accomplished in the dark.     

Bleach-induced Germination and Breakage of Dormancy of Seeds of Alectra vogelii

The effects of solutions of sodium hypochlorite and calcium hypochlorite (bleach) on dormancy and germination of Alectra vogelii seeds were investigated. Dry (non-pretreated) and 10-day water-pretreated seeds were exposed to various bleach concentrations, as well as to the host (Vigna unguiculata) root exudate. The 15-month-old Alectra seeds used were partially dormant in that only 40% or less of the 10-day pretreated seeds could be stimulated to germinate by the standard host root exudate. Comparable percentage germination of nonpretreated seeds was achieved with certain concentrations of bleach and the halogens, chlorine and bromine. Bleach and the halogens served not only as germination stimulant for pretreated Alectra seeds but appropriate concentrations induced also high percentage (70–90%) germination indicating a breakage of seed dormancy as well. The activity of the bleach in stimulating high percentage germination could be significantly reduced when the optimally bleach-treated seeds were rinsed daily with water during the germination period. Vigna root exudate, but not bleach, was shown to act as root stimulant for Alectra seedlings. The possible mechanism of the bleach effect on germination and dormancy of Alectra seeds is discussed.