Parasitic weeds and allelopathy:From the hypothesis to the proofs

Abstract

Parasitic weeds represent a main component of weed problems facing agriculture and negatively impact agroecosystem and environment. They belong to different plant families and are either hemi- or holo- root or shoot parasites. They attack plant species of different botanical families and growth forms and distributed in different geographical regions in the world, cause great yield losses and under sever infestation they lead to complete crop loss. Parasitic weeds represent a highly ecological, biological and physiological tolerant species to both internal and external environmental changes. They adapt themselves to disperse by different common agents and to survive and tolerate severe environmental condition under which long-term ecophysiological dormancy has been very well demonstrated and developed in many of the parasitic species in absence of suitable conditions for germination mainly the availability of host plants. The relationship between these parasites and their hosts is highly complicated, both sometimes work jointly for successful attachment and complete parasitism, while in contrast, antagonistic interactions are most common between both at which different mechanisms may be involved in the resistance of host to parasite attachment, germination, penetration and development which is termed as tolerant/resistance mechanism or sometimes described as a defense mechanism. The parasite may be also very well prepared for positive response to host signals and showed ability to attach itself to the host cell surface and to penetrate with aid of enzymes or other chemical secretions that allow dissolvent and separation of host cells. In the central of these complicated chemical reactions between parasite and host plants allelochemicals are involved. Both partners can use these chemicals to negate the effect of the other either by stimulation of parasite and host growth, and thus in some cases facilitate this association or each may use them as harmful natural chemicals to achieve their final goal. The nature of parasite/host relationships, the allelochemicals role in parasitism process, their chemical nature either as stimulants or inhibitors and the development and utilization of these secondary metabolites as new tools for parasitic weed control and their possible practical application are reviewed and discussed. The most important parasitic species were considered and their relations with host plants from the allelopathy point of view and the role of this mechanism in parasitism process were thoroughly reviewed.