Striga spp. are obligate root hemiparasites that constrain cereal production in sub-Saharan Africa. Although purple witchweed [Striga hermonthica (Delile) Benth.] and Asiatic witchweed [Striga asiatica (L.) Kuntze] infect all cereal crops, maize (Zea mays L.) is particularly vulnerable to their infestations. A sustainable control strategy for Striga would be to breed crops with host-based resistance as part of an integrated management plan. In maize, the open-pollinated variety Kakamega Striga-tolerant population of the year 1994 (‘KSTP 94’) has been popularized as a Striga-tolerant/resistant variety. This resistance was earlier reported to result from production of low amounts of sorgomol, a less potent strigolactone. To determine whether KSTP 94 harbors postattachment resistance, we used a soil-free assay based on observation chambers called rhizotrons. We found that the size of Striga seedlings attached to ‘CML 144’ (a susceptible maize inbred line) were 2.5-fold longer than those on KSTP 94. In addition, KSTP 94 had significantly fewer Striga attachments, which corresponded to significantly lower biomass (2.6-fold) compared with CML 144. Histological analysis revealed that the low Striga growth and development while infecting KSTP 94 was due the parasite’s inability to penetrate the host’s endodermis and make effective xylem–xylem connections. We therefore conclude that in addition to preattachment resistance, KSTP 94 exhibits postattachment resistance to S. hermonthica and could therefore be a good genetic source for postattachment resistance breeding.

Four synthetic multiring analogs of strigol (0.1 mM) were effective as germination stimulants of dormant, unchilled seeds of shepherdspurse (Capsella bursa-pastoris (L.) Medik. # CAPBP). The effect was concentration dependent and exceeded that produced by 0.1 mM gibberellic acid (GA). The analog treatments resulted in normal, unpigmented seedlings after 7 days, whereas GA treatment led to distorted, nongeotropic, yellow seedlings. The strigol analogs were also more effective in inducing shepherdspurse germination than 10 mM nitrate or 5.0 mM thiourea (ineffective in the dark) and 5.0 mM thiourea plus 4.5 mM ascorbate (germination stimulated, but seedling growth, inhibited). Chilling at 4 C for 11 days reduced the effectiveness of the strigol analogs, but not that of gibberellic acid and thiourea plus ascorbate. Ascorbate acts synergistically with thiourea in promoting shepherdspurse germination. Strigol and epistrigol had no effect on either chilled or unchilled shepherdspurse seeds. The stimulation of dormant shepherdspurse seeds by the synthetic strigol analogs is the first demonstration that these compounds have bioregulatory activity in dormant seeds of genera other than Striga.

Twelve analogs of the strigol D-ring (butenolides) were evaluated at 0.1 mM as germination regulators of 26 weed and crop seeds. In general, the butenolides either inhibited or did not affect monocot germination, although the hydroxybutenolide stimulated germination of dormant cheat and nondormant perennial ryegrass and sorghum seeds. The response rate for dicots was greater but still primarily inhibitory. Dormant lettuce seeds (both light-sensitive and light-insensitive), Palmer amaranth, and redroot pigweed showed several significant responses, both inhibitory and stimulatory. Some of the butenolides that had not been previously assayed against witchweed were tested as 10-4 to 10-11 M dilution series. The allyloxybutenolide demonstrated activity comparable to strigol (30 to 60% germination at 10-6 to 10-11 M); the sec-butyloxybutenolide was somewhat less active (20 to 40% germination) but still effective at 10-9 M. In general, the butenolide analogs of strigol were less effective in stimulating seed germination than the multiring analogs.

Production of maize in western Kenya is severely constrained by the parasitic weed striga. Although productivity of maize can be improved through adoption of improved varieties, adoption of such varieties remains low in the region, as the majority of smallholder farmers still grow unimproved open-pollinated varieties (landraces). The performance of two improved hybrid varieties was evaluated against six landraces in striga-infested soils in western Kenya. The varieties were planted in plots under natural striga infestation and were supplemented with pot experiments under artificial infestation. Striga emergence was lower in landraces than in the hybrid varieties in both field and pot experiments. Similarly, height of maize plants at harvest and grain yields were higher in the landraces than in the hybrids. After three continuous cropping seasons, in all treatments, striga seedbank density increased two to seven times. Seedbank increase was higher with hybrids and two of the landraces, ‘Rachar' and ‘Endere'. These results provide an insight into the potential role landraces could play in efforts toward an integrated management approach for striga in smallholder cropping systems. They also highlight the need to develop hybrid maize lines with local adaptation to biotic constraints, specifically striga.

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 rootholoparasitic 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 responseto 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 consideredas ‘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.

Seed germination of the hemiparasitic angiosperm Striga hermonthica is elicited by compounds present in the root exudates of the host plant. A variety of chemicals can substitute for the host-derived signal although the exact mechanism through which these act is unknown. In the present study, thidiazuron (TDZ), a cytokinin-active urea derivative was found to stimulate germination. This thidiazuron-induced germination was shown to be mediated by endogenous ethylene, by a similar mechanism to germination induced by host-root exudate and the synthetic stimulant GR-24. Comparing the effects of TDZ, host-root exudate, GR-24 and the ethylene precursor 1-aminocyclopropane-1-car-boxylic (ACC) suggests that ethylene production increases as a result of increased ACC synthesis and is consistent with a model for Striga seed germination in which host-derived signals and other stimulants act by eliciting the synthesis of ethylene via a stimulation of ACC synthase activity. All germination stimulants tested trigger a rapid increase in O2 uptake by conditioned seeds. The patterns of O2 uptake following TDZ or GR-24 treatments show similarities and three distinct phases of respiration are apparent. In contrast, stimulation of O2 uptake by ethylene results in a different pattern, with no distinct phases obvious following the initial burst of activity.

In vitro techniques are essential for Striga research and the development of appropriate control methods. In the laboratory, pre-screening of non-host or false-host plants of Striga for trap cropping or the screening of hosts for resistance involves visual evaluation of Striga seed germination that may be stimulated by plant parts or root exudates. This technique is presently laborious because the small Striga radicles are colourless. A number of solutions were evaluated to visualize the radicles of Striga hermonthica to obtain a reliable, simple and fast staining method yielding good contrast for visual observation, with readily available, inexpensive and minimally toxic dyes and staining solutions. The inks Brilliant Blue (Pelikan), Blue (Geha) and Brilliant Green (Pelikan), in either vinegar or lactic acid, produced radicles with excellent contrast, whereas radicles stained with Brilliant Red (Pelikan), Black (Sheaffer), Brilliant Black (Pelikan) gave good contrast. Striga radicles stained with Aniline Blue in vinegar or lactic acid, or Coomassie Brilliant Blue R250 in lactic acid, showed excellent contrast. Radicles stained with Ink Black (Sheaffer), Cotton Brown, or Rubin S in either vinegar or lactic acid also showed good contrast. With water as the diluent, only Lactophenol Blue showed excellent contrast. For health, safety and environmental concerns, availability and staining time, Blue ink in household vinegar (5% acetic acid) appears to be an excellent dye for Striga radicles and could be used in routine Striga germination assays.

The effects of preconditioning temperature and preconditioning period on the sensitivity of parasitic weed seeds to the synthetic germination stimulant GR24 were studied under laboratory and field conditions. The temperature during preconditioning of Orobanche cumana and Striga hermonthica seeds strongly affected the responsiveness of the seeds to the applied germination stimulant. Preconditioning at an optimal temperature (21°C for O. cumana and 30°C for S. hermonthica) rapidly released dormancy and increased the sensitivity to GR24 by several orders of magnitude. After reaching maximum sensitivity, prolonged preconditioning rapidly induced secondary dormancy, i.e. decreased sensitivity of O. cumana and S. hermonthica to GR24. The rapid change in sensitivity of preconditioned seeds to germination stimulants during prolonged preconditioning was particularly visible at low concentrations of GR24. GR24 at higher concentrations (0.1 and 1 mg l1) usually induced high germination of both species, regardless of the preconditioning period. The striking similarities between the response of parasitic weed seeds to GR24, described here, and results in the literature on non-parasitic wild plant seeds are discussed. Our results show that parasitic weed seeds are highly sensitive to the germination stimulant for a short period of time only, and then enter into secondary dormancy relatively quickly. The similar germination pattern of S. hermonthica seeds preconditioned for prolonged periods of time under laboratory and field conditions suggests that the mechanism observed is of ecological significance.

Seven new species and one new subspecies in Scrophulariaceae from the Flora of Bhutan area are described. They include two new species of Euphrasia, E. chumbica R.R. Mill sp. nov. (Chumbi, Sikkim and C Bhutan) and E. melanosticta R.R. Mill sp. nov. (Sikkim, N Bhutan); one new species of Kickxia, K. papillosa R.R. Mill sp. nov. (E Bhutan); one new species of Lagotis, L. chumbica R.R. Mill sp. nov. (Chumbi); one new species of Neopicrorhiza, N. minima R.R. Mill sp. nov. (N Bhutan); two new species of Scrophularia, S. cooperi R.R. Mill sp. nov. (C Bhutan) and S. subsessilis R.R. Mill sp. nov. (N Bhutan); and a new subspecies of Ellisiophyllum pinnatum (Wall.) Makino, subsp. bhutanense R.R. Mill subsp. nov. (Bhutan). Alectra avensis (Benth.) Merr., Bacopa monnieri (L.) Pennell, Scrophularia himalayensis Royle, Striga asiatica (L.) Kuntze and S. densiflora Benth. are all recorded from Bhutan and/or Sikkim for the first time. A brief argument is given for the continued use of hyphenated fel-terrae as the correct spelling of that epithet in Picria.

The circadian activity of adults of Smicronyx guineanus Voss (Coleóptera: Curculionidae) was studied in semi-field conditions. Pairs of weevils were observed on a 24-h cycle starting from 0800 to the following day at 0600 hrs for 21 days in cages containing Striga hermonthica (Del.) Benth (Scrophulariaceae) plants parasitising sorghum. Each female and male was observed every second hour. Occurrences of the various behaviours exhibited by the weevil were recorded. In decreasing frequency, the weevils were observed to be standing still, feeding, mating, walking, moving and foraging. In addition, the difference in time spent on the different parts of the host plant—classified into four groups: A- stem, leaves and bud; B- corolla; C- calyx; and D- sorghum, petiole, soil, capsule and cage—was highly significant (P = 0.0001,10 couples/days, n = 21 days). The weevil adults spent 46.8% of their time on the Striga inflorescence (bud, corolla and calyx). Striga plants were also stratified with respect to distance from the base and categories were designated as upper, central and lower parts. Smicronyx adults spent 85.2% of their time in the upper stratum, 9.8% in the central stratum and 5% of their occurrences were in the lower third of the Striga shoot. They were more active during daytime, suggesting that the best period of day to sample the weevils in the field is from 0700 to 1100 or from 1600 to 1800 hrs.