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

Vetches (Vicia spp.) are important forage legumes in the Middle East and Me diterranean regions. Most vetches are highly susceptible to the root holopa rasites Orobanche aegyptiaca and O. crenata, suffering severe quality and y ield losses. However, purple vetch (V. atropurpurea) has shown good resista nce to Orobanche. Microscopic studies were performed to reveal anatomical d ifferences 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 she ets in a polyethylene-bag system. Whole root observations using stereoscopi c microscopy detected necrotic lesions surrounding the attachments of Oroba nche radicles on resistant vetch roots. Hand-cut sections examined under th e light microscope revealed that in both susceptible and resistant vetch ge notypes the Orobanche seeds germinated, attached and penetrated the host ro ot epidermis and cortex. A reddish-brown secretion was observed in the apop last at the interface between the parasite haustorium and the host tissues, including the cell walls of the resistant vetch xylem vessels. Fixed and e mbedded sections observed under the light microscope showed that in the sus ceptible genotype the parasite haustorium penetrated through the endodermis into the host vascular cylinder, successfully forming a continuum with hos t vascular vessels. However, in the resistant genotype, the parasite hausto rium was blocked at the root endodermis layer. The blockage was coupled wit h secretion of unidentified material, thus preventing the parasite from est ablishing. These findings indicate that mechanical and possibly chemical ba rriers are responsible for the host defence mechanism(s) conferring resista nce of V. atropurpurea to O. aegyptiaca. (C) 2000 Annals of Botany Company.