These findings demonstrated, for the first time, the technical feasibility of suicidal germination for controlling S. hermonthica. Optimising structure, formulation and application protocol of germination stimulants should be the main goal for further improvement of the technology. © 2016 Society of Chemical Industry.

Summary Germination of Striga hermonthica (Del.) Bentth seeds in response to the strigol analogue GR 24 was strongly influenced by soil moisture prior to, at, or after, stimulation. Exposure of seeds to excessive soil moisture contents (70% w/w) during conditioning resulted in a low response to GR 24. Transfer of Seeds from soil at 70% moisture to a lower moisture level (47%) for 2 days or more improved the response to GR 24 (37–58% germination), white air‐drying restored germination (88%). In Gezira soil the optimum soil moisture contents for maximum response to GR 24 were 44, 38 and 40% during conditioning, stimulation and germination, respectively. The corresponding figures for the sandy soil were 20, 22 and 22%, respectively. Persistence of GR 24 was similar in both soil types, but was strongly influenced by soil moisture content. The stimulant was stable in air‐dry soil, but a rapid loss was encountered in moist soil.

Summary The activity of GR7 and Striga germination stimulant(s) from Euphorbia aegyptiaca Boiss. showed adequate persistence (6–8 days) in acidic soils (pH 5·0–6·3), but residual activity was short (1–3 days) in alkaline soils. The compounds tended to lose activity at a faster rate in the alkaline clay Gezira soil (pH 7·8), than in its sandy equivalent (pH 8·1). In solution, pH had no influence on initial activity, but residual action was reduced more rapidly by alkalinity and high temperature. However, the rate of loss of activity in solution was slower than in soils.

Sorghum is an important food, feed, and industrial crop worldwide. Parasitic weeds of the genus Striga constitute a major constraint to sorghum production, particularly in the drier parts of the world. In this study we analysed the Striga germination stimulants, strigolactones, in the root exudates of 36 sorghum genotypes and assessed Striga germination and infection. Low germination-stimulating activity and low Striga infection correlated with the exudation of low amounts of 5-deoxystrigol and high amounts of orobanchol, whereas susceptibility to Striga and high germination-stimulating activity correlated with high concentrations of 5-deoxystrigol and low concentrations of orobanchol. Marker analysis suggested that similar genetics to those previously described for the resistant sorghum variety SRN39 and the susceptible variety Shanqui Red underlie these differences. This study shows that the strigolactone profile in the root exudate of sorghum has a large impact on the level of Striga infection. High concentrations of 5-deoxystrigol result in high infection, while high concentrations of orobanchol result in low infection. This knowledge should help to optimize the use of low germination stimulant-based resistance to Striga by the selection of sorghum genotypes with strigolactone profiles that favour normal growth and development, but reduce the risk of Striga infection.

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Germination of witchweed (Striga hermonthica [Del.] Benth), an important root parasite on poaceous crops, requires pretreatment ‘conditioning’ in a warm moist environment and a subsequent exposure to a stimulant. The roles of conditioning period, CO2 and a strigol analogue (GR24) in ethylene biosynthesis and germination of the parasite were investigated. Conditioning increased the seeds’ capacity to oxidize exogenous 1‐aminocyclopropane‐1‐carboxylic acid (ACC). Exogenous CO2 increased the seeds capacity to oxidize ACC by 3‐ to 9‐fold. A combination of GR24 and ACC increased ethylene production by more than 3‐fold in comparison with the rates obtained using these compounds separately. Aminoethoxyvinylglycine (AVG) completely inhibited ethylene induction by GR24, but not by ACC. A GR24 treatment, made subsequent to conditioning in GR24, did not induce ethylene. However, seeds conditioned in GR24 and then given 1 mM ACC produced 293 nl l−1 ethylene. ACC oxidase (ACCO) activity in crude extracts was increased by conditioning and CO2. The enzyme displayed an absolute requirement for ascorbate. Absence of exogenous Fe2+ reduced enzyme activity only by 14%. GR24 applied during conditioning reduced germination in response to a subsequent GR24 treatment. ACC was, invariably, less effective in inducing S. hermonthica germination than GR24 even at concentrations which induce more ethylene than concurrent GR24 treatments. The results are consistent with a model in which conditioning removes a restriction on the ethylene biosynthetic pathway in S. hermonthica seeds. GR24 modulates the key enzymes in ethylene biosynthesis. The stimulant suppresses ethylene biosynthesis in unconditioned seeds and promotes it in conditioned ones. Germination of S. hermonthica results from the joint action of GR24 and the ethylene it induces.

For the first time a close association is reported between strigolactone levels analysed under laboratory conditions and Striga infection in the field in sorghum genotypes. These genotypes may be used for further study of this resistance mechanism and for the introgression of the low germination trait in other sorghum varieties to breed for a strigolactone composition with low stimulant activity. The use of such improved varieties in combination with other Striga management tools could possibly alleviate the current Striga problem on the African continent. © 2016 Society of Chemical Industry.

Rice has become a major staple cereal in sub-Saharan Africa. Currently, upland rice cultivation is expanding particularly in rainfed areas where the root parasitic weed Striga hermonthica, a major constraint to cereal production, is endemic. Laboratory, pot, and semi-controlled open air experiments were performed to evaluate resistance of selected rice varieties in Sudan to a resident S. hermonthica population. In the laboratory, 27 varieties were screened for post-attachment resistance using the rhizotron technique. Varieties displaying high post-attachment resistance, Umgar, NERICA5, and NERICA13 together with NERICA4, NERICA18, and Nipponbare, a lowland rice variety, were further evaluated for performance and Striga resistance in pot and semi-controlled open air experiments and for germination inducing activity in a laboratory. In addition, comparative studies on reaction of Umgar, Kosti1 and Kosti2, released varieties for commercial production in Sudan, to the parasite were performed in two pot experiments. In the pot experiments Umgar and NERICA5, consistently, sustained the lowest Striga emergence (<2.2 Striga plants per pot), while NERICA13 and NERICA4 supported 1.8–5.7 and 8.7–16.4 Striga plants per pot, respectively. In an artificially Striga-infested field, number of emergent Striga plants per 10 rice hills, at harvest, was 2.0, 2.0, 4.8, 13.5, 13.3, and 18.3 on Umgar, NERICA5, NERICA13, NERICA4, NERICA18, and Nipponbare, respectively. Striga had no adverse effects on total above-ground parts and panicle dry weight in Umgar and NERICA5. Germination-inducing activity of root exudates, at 14 days after sowing onward, was markedly lower for Umgar than for NERICA5, NERICA13, NERICA4, and NERICA18. Based on these findings, Umgar has both pre and post-attachment resistance to a resident Striga population in Sudan. Kosti1 and Kosti2 did not exhibit Striga-resistance at the same level as Umgar. Further the resistance of NERICA5, a variety reported to be endowed with a broad spectrum resistance to Striga species and ecotypes, at least to one resident Striga population in Sudan was clearly indicated.