Egyptian Broomrape (Orobanche aegyptiaca Pers.) and Small Broomrape (Orobanche minor Sm.) Parasitism of Red Clover (Trifolium pratense L.) in Vitro

Broomrapes, Orobanche spp., are holoparasites that affect the growth of a variety of broadleaf crops. One of the distinct characteristics of the family Orobanchaceae is the lack of chlorophyll, and hence inability to synthesize their own food. Broomrapes subsist on the roots of the host plant from which they derive the carbon, water, and nutrients needed for further growth. Parasitism as such leads to yield reductions, and in case of heavy infestations, complete crop failure. Among other plants parasitized by broomrapes are several legumes, some of which are also the world's most economically important crops. As part of their unique biology, legumes provide an ecological niche for diazotropic soil bacteria, which belong to the family Rhizobiaceae. In return, the host plant receives fixed nitrogen from the nodules, specialized structures produced on the roots of most legume plants upon inoculation with bacteria. Orobanche spp. germination depends on the presence of chemical stimulant in host root exudates. It has been reported that inoculation of some legumes resulted in greater infestation by parasitic weeds. In addition, bacterial nodules were assumed to provide a place for broomrape invasion of host legume. Furthermore, infestations were observed to be more intense in aerobic conditions when rhizobia are most active. It is possible that production of the stimulant could be correlated with the infection of roots with Rhizobium, and that the nodules formed on the roots could play a role in broomrape parasitism; however, others have not confirmed this. Studies were conducted to investigate the possibility of interaction between Orobanche spp. attack and Rhizobium nodulation in legumes. Seed germination, number of parasitic attachments, as well as the morphology of two broomrape species, small broomrape (Orobanche minor Sm.) and Egyptian broomrape (Orobanche aegyptiaca Pers.), were studied. O. minor showed a greater percent seed germination, and formed a greater number of attachments on red clover (Trifolium pratense L.) inoculated with Rhizobium leguminosarum bv. trifolii in comparison with non-inoculated plants. However, the addition of the inoculum did not appear to enhance O. aegyptiaca seed germination or the number of its attachments on the host roots compared with the controls. Morphological observations of O. minor attachments on red clover suggest that parasitic attachments were not situated over the bacterial nodules, but perhaps involve parasite-induced enzymatic degradation followed by mechanical protrusion of host plant root cortex, possibly utilizing host plant-rhizobacteria interactions as well.