The differential susceptibility of vetch (Vicia spp.) to Orobanche aegyptiaca:: Anatomical studies

Vetches (Vicia spp.) are important forage legumes in the Middle East and Mediterranean regions. Most vetches are highly susceptible to the root holoparasites Orobanche aegyptiaca and O. crenata, suffering severe quality and yield losses. However, purple vetch (V. atropurpurea) has shown good resistance to Orobanche. Microscopic studies were performed to reveal anatomical differences of host-parasite interactions between susceptible and resistant vetch. Susceptible V. sativa 'Yovel' and resistant V. atropupurea 'Popany' were grown in association with O. aegyptiaca seeds, on glass microfibre sheets in a polyethylene-bag system. Whole root observations using stereoscopic microscopy detected necrotic lesions surrounding the attachments of Orobanche radicles on resistant vetch roots. Hand-cut sections examined under the light microscope revealed that in both susceptible and resistant vetch genotypes the Orobanche seeds germinated, attached and penetrated the host root epidermis and cortex. A reddish-brown secretion was observed in the apoplast at the interface between the parasite haustorium and the host tissues, including the cell walls of the resistant vetch xylem vessels. Fixed and embedded sections observed under the light microscope showed that in the susceptible genotype the parasite haustorium penetrated through the endodermis into the host vascular cylinder, successfully forming a continuum with host vascular vessels. However, in the resistant genotype, the parasite haustorium was blocked at the root endodermis layer. The blockage was coupled with secretion of unidentified material, thus preventing the parasite from establishing. These findings indicate that mechanical and possibly chemical barriers are responsible for the host defence mechanism(s) conferring resistance of V. atropurpurea to O. aegyptiaca. (C) 2000 Annals of Botany Company.