Abstract
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.
Highlights
Among all pests, weeds have the largest economic impact in agriculture (Pimentel et al, 2005), and among weeds, the holoparasitic broomrapes (Orobanche and Phelipanche species) are noxious, since they compete with crops for nutrients by invading their roots for nutrient extraction
The weed root extractions yielded percentages of organic extract relative to the lyophilized root weight that ranged from 0.23% for Amaranthus albus L. to a maximum of 1.85% for Fumaria officinalis L. (Table 1)
The potential was assessed for the weed root extracts to induce broomrape suicidal germination
Summary
Weeds have the largest economic impact in agriculture (Pimentel et al, 2005), and among weeds, the holoparasitic broomrapes (Orobanche and Phelipanche species) are noxious, since they compete with crops for nutrients by invading their roots for nutrient extraction. For successful crop plant infections, broomrape germination is inhibited until detection of host roots through root-derived germination stimulants (Lechat et al, 2012; Fernández-Aparicio et al, 2009). From each germinated broomrape seed a short radicle emerges that grows towards the host root and develops a multicellular haustorium. This attaches and penetrates the host root, and forms connections with the host vascular system to withdraw water and nutrients (Riopel and Timko 1995; Joel, 2013). Nutrient withdrawal through haustorial connections results in severe reductions of crop yields (Fernández-Aparicio et al, 2016). Because of synchronization of host-parasite life cycles and the intimate haustorial attachments, application of methods that kill the parasitic weeds without damaging host crop are not easy, so parasitic weeds are amongst the most difficult weeds to control (Fernández-Aparicio et al, 2020)
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