Broomrape Germination Research Articles

Sesquiterpene Lactones Isolated from Centaurea cineraria L. subsp. cineraria Inhibit the Radicle Growth of Broomrape Weeds.

The plant Centaurea cineraria L. subsp. cineraria has been investigated as a potential source of inhibitors of broomrape radicle growth. The latter are weeds that pose a threat to agriculture and for which there are few methods available for the control of infestations. Four sesquiterpene lactones have been isolated from C. cineraria L. subsp. cineraria aerial parts and identified as isocnicin, cnicin, salonitenolide, and 11β,13-dihydrosalonitenolide using spectroscopic, spectrometric, and optical methods. Salonitenolide and 11β,13-dihydrosalonitenolide have been isolated for the first time from this plant. Tested at 1.0-0.1 mM against the broomrape species Phelipanche ramosa, Orobanche minor, Orobanche crenata, and Orobanche cumana, isocnicin, cnicin, and salonitenolide demonstrated remarkable inhibitory activity (over 80% in most of the cases) at the highest concentrations. Structure-activity relationship conclusions indicated the significance of the α,β-unsaturated lactone ring. In addition, the synthetic acetylated derivative of salonitenolide showed the strongest activity among all compounds tested, with inhibitions close to 100% at different concentrations, which has been related to a different lipophilicity and the absence of H-bond donor atoms in its structure. Neither the extracts nor the compounds exhibited the stimulating activity of broomrape germination (induction of suicidal germination). These findings highlight the potential of C. cineraria to produce bioactive compounds for managing parasitic weeds and prompt further studies on its sesquiterpene lactones as tools in developing natural product-based herbicides.

Inhibition of broomrape germination by 2,4-diacetylphloroglucinol produced by environmental Pseudomonas.

Parasitic weeds such as broomrapes (Phelipanche ramosa and Orobanche cumana) cause severe damage to crops and their development must be controlled. Given that phloroglucinol compounds (PGCs) produced by environmental Pseudomonas could be toxic towards certain plants, we assessed the potential herbicidal effect of the bacterial model Pseudomonas ogarae F113, a PGCs-producing bacterium, on parasitic weed. By combining the use of a mutagenesis approach and of pure PGCs, we evaluated the in vitro effect of PGC-produced by P. ogarae F113 on broomrape germination and assessed the protective activity of a PGC-producing bacteria on oilseed rape (Brassica napus) against P. ramosa in non-sterile soils. We showed that the inhibition of the germination depends on the PGCs molecular structure and their concentrations as well as the broomrape species and pathovars. This inhibition caused by the PGCs is irreversible, causing a brown coloration of the broomrape seeds. The inoculation of PGCs-producing bacteria limited the broomrape infection of P. ramosa, without affecting the host growth. Moreover, elemental profiling analysis of oilseed rape revealed that neither F113 nor applied PGCs affected the nutrition capacity of the oilseed rape host. Our study expands the knowledge on plant-beneficial Pseudomonas as weed biocontrol agents and opens new avenues for the development of natural bioherbicides to enhance crop yield.

Assessment of weed root extracts for allelopathic activity against Orobanche and Phelipanche species

Broomrapes (Orobanche and Phelipanche species) are holoparasitic weeds that infect roots of crop hosts from Asteraceae, Brassicaceae, Apiaceae, Fabaceae, and Solanaceae. The parasitic weeds are difficult to control selectively without crop damage once attached to their roots. Identification of natural compounds with herbicidal activity against pre-attached broomrape stages can provide control alternatives. With the aim to identify plant species with efficacy for broomrape control, organic and residual aqueous phase extracts from roots of seventeen weed species common in south Spanish broomrape-infested fields were assessed as potential inducers of suicidal broomrape germination and as inhibitors of broomrape radicle growth. Assessments were carried out in vitro using seeds and seedlings of four noxious broomrape species, Orobanche crenata Forsk., Orobanche cumana Wallr., Orobanche minor Sm. and Phelipanche ramosa (L.) Pomel. While root extracts from all the weed species did not induce suicidal germination on O. crenata seeds, most of the extracts induced germination of P. ramosa except for those obtained from Amaranthus albus L., Amaranthus retroflexus L. and Convolvulus arvensis L. Moderate levels of germination activity were induced in O. cumana and O. minor seeds by some of the root extracts tested, with strongest induction obtained from Heliotropium europaeum L. on O. cumana seeds, and from Silybum marianum (L.) Gaertn. on O. minor seeds. For root extract inhibition of broomrape radicles, the extract from C. arvensis roots strongly inhibited radicles of all the broomrape species. While extracts from the other weed species induced low or negligible inhibition of O. cumana and O. crenata radicle growth, many inhibited P. ramosa and O. minor radicles. Exceptions were root extracts from Datura stramonium L., Heliotropium europaeum L., Malva sylvestris L., Solanum nigrum L. and Urtica dioica L., which did not inhibit P. ramosa radicles, and those from A. retroflexus, Datura stramonium L., Malva sylvestris L., Portulaca oleracea L. and S. nigrum, which did not inhibit O. minor radicles. Among the active organic extracts assessed, those showing promising chemical profiles were selected for future studies to characterize natural compounds with potential herbicidal activity on early stages of broomrape growth.

Nitrogen and phosphorus supply strongly reduced the control efficacy of maize against sunflower broomrape

ABSTRACT Heavy infestation by the root parasitic sunflower broomrape over wide regions constitutes a major biological constraint to sunflower production. The use of maize as a trap crop to induce sunflower broomrape germination is considered a cost-effective method for controlling sunflower broomrape. This study was designed to examine the activity of maize to induce sunflower broomrape seeds germination under varying levels of nitrogen (N) and phosphorus (P) fertilization. Orthogonal experiments were conducted during two cropping seasons with four maize cultivars under four levels of N and P application. A significant reduction in sunflower broomrape germination induced by maize root and rhizosphere soil extracts upon N and P application was observed in both 2014 and 2015, with P having the strongest effect on the broomrape germination inducing activity of maize root extracts. Structural equation modelling analysis revealed that N and P supplementation not only directly affected the germination-inducing capacity towards sunflower broomrape of maize root extracts but also had an indirect effect through the impact on abscisic acid contents. This study may contribute to a better understanding of the connection between soil fertility and the production and release of germination stimulants in maize and that can lead to improved control strategies.

Costunolide Influences Germ Tube Orientation in Sunflower Broomrape - A First Step Toward Understanding Chemotropism.

Orobanche cumana WALLR. is a host-specific root parasite of cultivated sunflowers with increasing economic importance in Europe, North Africa, and parts of Asia. While sesquiterpene lactones (STLs) released from sunflower roots were identified as natural germination stimulants of O. cumana seeds in the soil, the chemical nature of the signals guiding the emerging germ tube toward the host root has remained unknown hitherto. Thus, we designed a bioassay that allowed the observation of broomrape germination and subsequent germ tube development in the presence of substances with putative chemotropic activity. Root exudates and sunflower oil extracts, both containing STLs in micromolar concentrations, caused the positive chemotropic orientation of germ tubes. A similar positive chemotropic effect was achieved with costunolide, one of the four STLs of sunflower present in the exudate and oil extracts. In contrast, GR24, a synthetic strigolactone (SL) with germination-inducing activity on O. cumana seeds, showed no effect on the germ tube orientation. The effect of costunolide was concentration-dependent and within the range of its natural micromolar occurrence in roots. We assume that an STL gradient is responsible for the stronger inhibition of elongation growth on the host-facing flank of the germ tube compared with the far side flank. This would confer a double role of STLs from sunflower root exudates in the sunflower–broomrape interaction, namely, as germination stimulants and as chemotropic signals.

Assessing the Performance of Maize (Zea mays L.) as Trap Crops for the Management of Sunflower Broomrape (Orobanche cumana Wallr.)

Sunflower broomrape (Orobanche cumana Wallr.) is a weedy root parasite that causes huge damage to sunflower (Helianthus annuus L.). Trap crop can stimulate parasitic seed germination without attachment in trap crop, and it was considered as economic methods for controlling the parasite. In this study, co-culture and pot experiments were conducted to assess the allelopathic activity of maize (Zea mays L.) to stimulate broomrape seeds germination and to evaluate the performance of maize rotation for reducing sunflower broomrape infection. All the tested maize cultivars could induce sunflower broomrape germination, and the most active maize cultivar was N314, which induced approximately 40% broomrape seeds germination. Rotation of maize significantly reduced broomrape infection on subsequent sunflower and increased sunflower biomass. After cultivated N314 for 3 years, broomrape attachment on sunflower was the lowest (0.8 attachment plant−1), and the dry mass of sunflower shoot (28.7 g plant−1) was approximately twice higher than the sunflower without previous crop. The effectiveness of broomrape management was significantly different among various maize cultivars. Meanwhile, the differences among cultivars were decreased with the years of maize cultivated prolonged. Our results confirmed that maize was significantly effective in reducing the sunflower damage caused by sunflower broomrape and suggested that maize rotation could be included in the integrated management of sunflower broomrape.

Preliminary study for testing the significance of butyl–iso–butyl phthalate in ‎controlling faba bean broomrape

Due to deleterious impacts of broomrapes against the host plants, its combating is ‎essential for sustaining crop productivity. Thus, two–year field trail was conducted at ‎El Nubaria experimental farm, National Research Centre, Egypt, on two faba bean ‎genotypes (Misr–3 and Sakha–1) to investigate the importance of butyl–iso–butyl ‎phthalate as an activator for broomrape germination in comparison to glyphosate and ‎unweeded. Findings revealed that glyphosate in 2015/16 and 2016/17 seasons in ‎addition to butyl–iso–butyl phthalate in 2016/17 season caused significant reductions ‎in broomrape biomass and numbers of broomrape–infected faba bean plants plot−1 ‎compared to the unweeded. Plots of Misr–3 genotype treated with butyl–iso–butyl ‎phthalate in both season as well as plots of Misr–3 or Sakha–1 sprayed by glyphosate ‎in the first season achieved the maximum seed yield of faba bean. Since butyl–iso–‎butyl phthalate exhibits suicidal germination of broomrape seeds, it can be exploited ‎as an effective and helpful tool in integrated management programs of broomrape in ‎faba bean fields. ‎ Due to deleterious impacts of broomrapes against the host plants, its combating is ‎essential for sustaining crop productivity. Thus, two–year field trail was conducted on ‎two faba bean genotypes (Misr–3 and Sakha–1) to investigate the importance of ‎butyl–iso–butyl phthalate as an activator for broomrape germination in comparison to ‎glyphosate and unweeded. The trail design was a strip–plot in completely randomized ‎block design in six replications. Genotypes occupied the vertical main plots as well as ‎broomrape control treatments distributed in horizontal ones. Results showed that ‎broomrape biomass and numbers of broomrape–infected faba bean plants plot−1 ‎markedly responded to weed control and genotype and their interaction in both ‎seasons, except numbers of broomrape–infected plants of faba bean genotypes plot−1 ‎in 2016/17 season. Also, findings revealed that glyphosate in 2015/16 and 2016/17 ‎seasons in addition to butyl–iso–butyl phthalate in 2016/17 season caused significant ‎reductions in broomrape biomass and numbers of broomrape–infected faba bean ‎plants plot−1 compared to the unweeded. In plots of Misr–3 or Sakha–1, glyphosate ‎was the potent practice for reducing broomrape biomass and numbers of broomrape–‎infected faba bean plants plot−1 in 2015/16 and 2016/17 seasons, with no significant ‎differences with butyl–iso–butyl phthalate application in 2016/17 season. There were ‎no noticeable variations between weeded practices and genotypes and their interaction ‎on total dry weight of faba bean plants estimated at 105 DAS, in both seasons, except ‎glyphosate x Misr–3 interaction in the first season only. Plots of Misr–3 genotype ‎treated by butyl–iso–butyl phthalate in both season as well as plots of Misr–3 or ‎Sakha–1 sprayed by glyphosate in the first season achieved the maximum seed yield ‎of faba bean. Since butyl–iso–butyl phthalate causing suicidal germination of ‎broomrapes seeds, it can be exploited as an effective and helpful tool in broomrape ‎management programs in faba bean fields. Genotype; Orobanche spp; Parasitic ‎weeds; Suicidal germination; Vicia faba ‎

Effects of wheat-canola intercropping on Phelipanche aegyptiaca parasitism

AbstractParasitic weeds especiallyPhelipanche aegyptiacadecrease severely the production of canola. This study evaluated the effect of intercropping different wheat genotypes with canola onPhelipanche aegyptiacagrowth. Ten wild wheat genotypes with different ploidy levels including TRI11712, TRI19322, TRI18664, TRI19652, TRI565, TRI15593, TRI12911, TRI11554, TRI17606, TRI7259P and seven cultivated bread wheats, namely: Falat, Chamran, Alamut, Baiat, Kavir, Sepahan, Alvand in addition to a canola cultivar called Zarfam were studied. The results revealed that intercropping of canola with wheat could significantly reduce broomrape growth depending on the type of wheat genotype. A significant genetic variation of allelopathic activity in wheat was observed, indicating the contribution of multiple genes conferring the allelopathic trait. TRI565 and TRI12911, TRI15593, TRI18664, TRI19652, TRI17606, TRI19322, and TRI7259 genotypes showed strong inhibitory effects and can be considered as potential allelopathic genotypes to suppress broomrape. The inhibitory potential of wild wheat genotypes was stronger than cultivated wheat genotypes. Alamut, Baiat, Alvand, Sepahan, and TRI11712 possessed strong stimulatory effects on broomrape germination. Such genotypes may be valuable as trap crops for depleting the Egyptian broomrape seed bank.

Extracts from cotton over the whole growing season induce Orobanche cumana (sunflower broomrape) germination with significant cultivar interactions

Extracts from cotton over the whole growing season induce Orobanche cumana (sunflower broomrape) germination with significant cultivar interactions

GENETIC ANALYSIS OF RESISTANCE TO OROBANCHE CRENATA (FORSK.) IN A PEA (PISUM SATIVUM L.) LOW-STRIGOLACTONE LINE

Crenate broomrape ( Orobanche crenata Forsk.) is a root parasitic weed severely threatening the cultivation of pea ( Pisum sativum L.) in Mediterranean agricultural settings. We recently characterized a pea line highly resistant to O . crenata , named ROR12. Analysis of root exudates indi- cated that ROR12 resistance is possibly due to lower exu- dation of strigolactones, carotenoid-derived compounds acting in the rhyzosphere as broomrape germination stim- ulants. Here, we provide a first genetic characterization of ROR12 resistance. Two segregating F 2 populations origi- nating from the cross between ROR12 and two susceptible commercial pea cultivars were assayed for response to O . crenata , together with the respective parental lines and F 1 generations. Susceptibility of F 1 individuals indicated that resistance is recessively inherited. In F 2, significant devia- tion from Gaussian distribution suggested that broomrape resistance is not a quantitative trait, as it was previously hypothesized, but is rather controlled by one or a few ma- jor loci. Molecular analysis with RAPD and SSR markers identified a genomic region on pea linkage group 3 associ- ated with the resistant phenotype. Finally, transcriptional evidence was provided, supporting the hypothesis that ROR12 resistance is due to reduced strigolactone biosyn- thesis. Overall, this study provides insights on the control of broomrape resistance and lays the foundation for future work addressing the fine-mapping and cloning of genes controlling strigolactone biosynthesis and/or broomrape resistance.

Characterization of Low-Strigolactone Germplasm in Pea (Pisum sativum L.) Resistant to Crenate Broomrape (Orobanche crenata Forsk.).

Crenate broomrape (Orobanche crenata Forsk.) is a devastating parasitic weed threatening the cultivation of legumes around the Mediterranean and in the Middle East. So far, only moderate levels of resistance were reported to occur in pea (Pisum sativum L.) natural germplasm, and most commercial cultivars are prone to severe infestation. Here, we describe the selection of a pea line highly resistant to O. crenata, following the screening of local genetic resources. Time series observations show that delayed emergence of the parasite is an important parameter associated with broomrape resistance. High performance liquid chromatography connected to tandem mass spectrometry analysis and in vitro broomrape germination bioassays suggest that the resistance mechanism might involve the reduced secretion of strigolactones, plant hormones exuded by roots and acting as signaling molecules for the germination of parasitic weeds. Two years of replicated trials in noninfested fields indicate that the resistance is devoid of pleiotropic effects on yield, in contrast to pea experimental mutants impaired in strigolactone biosynthesis and, thus, is suitable for use in breeding programs.

Conditioning duration and agents involved in broomrape seeds responding to germination stimulants

Weedy broomrape species, such as sunflower broomrape (Orobanche cumana Wallr.) and Egyptian broomrape [Phelipanche aegyptiaca Pers. (syn. O. aegyptiaca)], require a period of pre-conditioning before they can respond to germination stimulants. Thus, the sensitivity of weedy broomrape seeds to germination stimulants could be an important factor for broomrape control. In this study, the influence of conditioning agents, conditioning period (0–21 days) and germination stimulants on the germination of sunflower broomrape and Egyptian broomrape seeds was analyzed. Without conditioning, the sunflower and Egyptian broomrape seeds exhibited negligible germination responses to the stimulants. The germination rate of the broomrape seeds increased rapidly with conditioning period and reached a maximum under a conditioning period of 4–10 days; further prolonged conditioning resulted in a decrease in the germination rate. Gibberellic acid (GA3) could not only break the dormancy of the sunflower and Egyptian broomrape seeds but also maintained the high sensitivity of these seeds even after 21 days of conditioning. Furthermore, 100 µM of GA3 induced the germination of the Egyptian broomrape seeds. The stimulants that induced Egyptian broomrape germination were ranked in decreasing order as GR24 (76.8 %), strigol (76.1 %), tobacco root exudates (49.5 %), dehydrocostus lactones (DCL, 39.2 %), and maize root exudates (18 %). In contrast, GA3 did not directly induce sunflower broomrape seed germination, which responded to strigol (62.8 %) > maize root exudates (58.2 %) > GR24 (57.9 %) > tobacco root exudates (41.6 %) > DCL (41.3 %). These results indicate specialized recognition of germination stimulants by sunflower and Egyptian broomrape. This study may contribute to a better understanding of parasitic weed germination and may lead to improved control strategies.

Effectiveness of ten commercial maize cultivars in inducing Egyptian broomrape germination

Egyptian broomrape (EB), Phelipanche aegyptiaca, is a devastating root parasite, causing enormous crop losses around the world. Maize has the potential to influence the growth of other plants through releasing certain allelochemicals and is able to induce germination of at least three broomrape species. To determine whether maize could be used as a trap crop for EB, 10 maize cultivars were tested for their ability to induce EB germination. The results showed that maize cultivars can induce EB germination, and that germination rates in a cut-root experiment and a hydroponic experi- ment were consistent. Maize cvs Changcheng 799 and Zhengdan 958 induced the highest EB germination rates, while cvs Luyu 13 and Zhengyu 203 were the least effective. These four maize cultivars were further studied in a pot experiment. Rhizosphere soil, rhizosphere soil extracts, root extracts and shoot extracts from these cultivars were all able to induce EB germination, with cv. Changcheng 799 inducing the highest germination rates. Root extracts generally induced higher germination rates than shoot extracts. It is suggested that Changcheng 799 could be planted as a trap crop for control of EB.

Effect of fungal and plant metabolites on broomrapes (Orobanche and Phelipanche spp.) seed germination and radicle growth.

Orobanche and Phelipanche species (the broomrapes) are root parasitic plants, some of which cause heavy yield losses on important crops. The development of herbicides based on natural metabolites from microbial and plant origin, targeting early stages on parasitic plant development, might contribute to the reduction of broomrape seed bank in agricultural soils. Therefore, the effect of metabolites belonging to different classes of natural compounds on broomrape seed germination and radicle development was assayed in vitro. Among the metabolites tested, epi-sphaeropsidone, cyclopaldic acid, and those belonging to the sesquiterpene class induced broomrape germination in a species-specific manner. epi-Epoformin, sphaeropsidin A, and cytochalasans inhibited germination of GR24-treated broomrape seeds. The growth of broomrape radicle was strongly inhibited by sphaeropsidin A and compounds belonging to cyclohexene epoxide and cytochalasan classes. Broomrape radicles treated with epi-sphaeropsidone developed a layer of papillae while radicles treated with cytochalasans or with sphaeropsidin A turned necrotic. These findings allow new lead natural herbicides for the management of parasitic weeds to be identified.

Influence of Temperature and Plant Growth Stages of Red Clover and Wheat on Small Broomrape (Orobanche minor) Germination

Small broomrape is a holoparasitic plant that attaches to the roots of red clover as well as several other host plants. Hosts and false hosts produce stimulants that induce small broomrape germination but small broomrape does not attach to a false host. Wheat has been identified as a false host for small broomrape; therefore, studies were conducted to investigate the effect of red clover and wheat root exudates on small broomrape germination. In one study, the effect of exudates from red clover and wheat at multiple growth stages on small broomrape germination was evaluated. Red clover induced small broomrape germination at all growth stages tested but was greatest (78%) in the presence of exudates from red clover at the three-trifoliolate stage. Maximum small broomrape germination was 25% when exposed to exudates produced by one-leaf-stage wheat. In a second study, the relationship between small broomrape germination and host growth condition was evaluated using root exudates from red clover or wheat grown under several temperature conditions for either 4 or 8 wk. For the different temperatures, there were no differences in small broomrape germination when exudates of red clover grown for 4 wk were used. Small broomrape germination was reduced when exposed to exudates from red clover plants grown for 8 wk at 10 C compared with plants grown at 15, 20, and 25 C. Differences in small broomrape seed germination were observed with temperature under which wheat was grown for 4 wk, but not for 8 wk. Although wheat exudates resulted in less small broomrape seed germination than red clover exudates, growing wheat as a false host in a small broomrape-infested field could be an important component of an integrated management plan.

Non-host facilitators, a new category that unexpectedly favours parasitic weeds

In Europe, the current decrease of herbicide application increases and diversifies weed flora in crops. As a consequence, there is an increase in the number of pests that use weeds to spread. This change is a major threat to crop production. For instance broomrape—Phelipanche ramosa—is an obligate parasite of dicotyledonous species that reduces crop yield by up to 80 % of oilseed rape in France. In highly infected fields, broomrape can in turn infect various weed species and thus persist and even proliferate in the absence of host crops. Up to now, three categories of interactions between broomrape and plant species have been identified in monospecific stands: host, non-host and false host. In multispecific stands, broomrape germination and attachment have never been studied despite the use of multispecific crop associations to protect host crops with non-host species in tropical conditions. Therefore, we studied parasite germination and attachment of monospecific vs. multispecific stands, associating broomrape with host plants (oilseed rape), non-host plants (field bindweed) or a combination of both. Experiments were conducted in vitro for 6 weeks and in pots for 3 months. We measured the percentage of germinated broomrape seeds in the presence of field bindweed or oilseed rape. We analysed the attachment of the parasite on its host. Results unexpectedly show a nearly threefold increase in the infection of host oilseed rape in the presence of field bindweed. We thus proposed a new plant species category called ‘non-host facilitator’ and a new parasitic interaction process named ‘facilitation’. The underlying mechanism is still unknown though we observed that field bindweed was able to support secondary parasite attachments originating from primary attachments on a nearby oilseed rape root system. Our discovery implies the rethinking of parasite management in arable crops, particularly the rotation or association of host with non-host crops, since the non-host crops could actually be non-host facilitators.

Some soybean cultivars have ability to induce germination of sunflower broomrape.

Sunflower broomrape is a noxious parasitic weed which has caused severe damage to crop ecosystems. Trap crops can release a mixture of allelochemicals to induce the germination of sunflower broomrape. We studied the allelopathic effects of soybean on sunflower broomrape. Fourteen common soybean cultivars were grown in pots. Samples were collected from soybean plants and rhizosphere soil at five growth stages (V1, V3, V5, R2, and R4). The allelopathic effects of soybean reached highest at the V3 stage. Methanolic extracts of soybean roots induced higher broomrape germination than methanolic extracts of stems or leaves. The germination rates induced by root extracts (10-fold dilution) were positively correlated with germination rates induced by stem (10-fold dilution) and leaf extracts (10-fold dilution). The broomrape germination rates induced by root extracts were also positively correlated with soybean nodule diameter and dry weight. The results indicated that soybeans could induce sunflower broomrape germination. We conclude that soybean has the potential to be used as a trap crop for sunflower broomrape.

The Potential of Cell-free Cultures of Rhizobium leguminosarum, Azotobacter chroococcum and Compost Tea as Biocontrol Agents for Faba Bean Broomrape (Orobanche crenata Forsk.)

In the present study, cell-free cultures of four isolates of Rhizobium leguminosarum, an isolate of Azotobacter chroococcum and compost tea were investigated for their biocontrol potential against the root parasitic weed Orobanche crenata. Individual cell-free cultures of Azotobacter chroococcum or Rhizobium sp., dual and mixture of cell-free cultures of Rhizobium spp. or compost tea were applied to infested pots in greenhouse conditions. The treatments showed variable effects on many developmental parameters of both faba bean and broomrape. Significant decrease in the number of broomrape attachments, dry weight of the attached tubercles on faba bean roots and the reduction in percentage of broomrape seed germination were recorded. Compost tea, individual and mixture of R. leguminosarum isolates were more reducing on broomrape germination and growth than A. chroococcum alone did; being the former treatment is the best. The reduction in broomrape incidence by compost tea was due to certain phenotypic mechanisms, which acted alone or in combination. These mechanisms included negative effect of natural stimulant broomrape on seed germination, prevention of radical penetration inside the host roots, parasite yield reduction, and thus increasing the growth and vitality of faba bean. In vitro experiment indicated that seed germination percentage of broomrape was also negatively affected by the combination of root-exudates and compost tea. Radical apexes of the germinated seeds were distorted. These distortions may prevent the radicals to follow up the infestation. In conclusion, the study presents the potential of R. leguminosarum isolates and compost tea in biocontrol of broomrape. More investigations should be carried out with viable bacterial cells on the parasite plant before use in sustainable agricultural systems.

Structure-Activity Relationship Studies of Strigolactone-Related Molecules for Branching Inhibition in Garden Pea: Molecule Design for Shoot Branching

Initially known for their role in the rhizosphere in stimulating the seed germination of parasitic weeds such as the Striga and Orobanche species, and later as host recognition signals for arbuscular mycorrhizal fungi, strigolactones (SLs) were recently rediscovered as a new class of plant hormones involved in the control of shoot branching in plants. Herein, we report the synthesis of new SL analogs and, to our knowledge, the first study of SL structure-activity relationships for their hormonal activity in garden pea (Pisum sativum). Comparisons with their action for the germination of broomrape (Phelipanche ramosa) are also presented. The pea rms1 SL-deficient mutant was used in a SL bioassay based on axillary bud length after direct SL application on the bud. This assay was compared with an assay where SLs were fed via the roots using hydroponics and with a molecular assay in which transcript levels of BRANCHED1, the pea homolog of the maize TEOSINTE BRANCHED1 gene were quantified in axillary buds only 6 h after application of SLs. We have demonstrated that the presence of a Michael acceptor and a methylbutenolide or dimethylbutenolide motif in the same molecule is essential. It was established that the more active analog 23 with a dimethylbutenolide as the D-ring could be used to control the plant architecture without strongly favoring the germination of P. ramosa seeds. Bold numerals refer to numbers of compounds.

Screening of Some Cotton Varieties for Allelopathic Potential on Clover Broomrape Germination

Research on the chemical basis of allelopathy has often been hindered by the complexity of plant and soil matrices, making it difficult to track active compounds. Well‐designed bioassays under controlled environments can be useful in evaluating the allelopathic potential of plants. This paper describes two experiments about the allelopathic potential of cotton (Gossypium hirsutum L.). In the first experiment, root exudates were collected from six common cotton (G. hirsutum L. and G. barbadense L.) varieties grown hydroponically. The cotton (donor plant) root exudates were tested on clover broomrape (Orobanche minor Smith), to determine which variety had the highest allelopathic potential. In the second experiment, the allelopathic potential of 22 soon‐to‐ be‐released cotton varieties was evaluated for clover broomrape using rhizosphere soil and extracts from different plant parts. The results indicated that methanol was a more suitable reagent than distilled water for extracting germination stimulants from cotton plants. There was a significant positive correlation between the rates of clover broomrape germination in the rhizosphere soil treatments and those in the stem extract (r = 0.6018, P < 0.05) and the leaf extract treatments (r = 0.5540, P < 0.05). There was also a significant positive correlation between germination rates in the root extract treatments and the stem extract treatments (r = 0.7237, P < 0.05). In conclusion, the allelopathic potential of cotton can be assayed by observing the germination of clover broomrape seeds treated with cotton stem extracts.