Physiology and histology of resistance to Striga hermonthica in Sorghum bicolor var. Framida

Germination, attachment to host root and growth of Striga hermonthica (Del. ) Benth. seeds and seedlings were studied in in vitro co-culture with Sorgh um bicolor (L.) Moench and in pot experiments. Two varieties, the resistant Framida and the susceptible CK-60B, were used. Histological, morphological and physiological studies revealed the key stages of resistance mechanisms involved. Resistance of Framida to Striga does not occur at the germinatio n or the attachment stages, since its roots do not support fewer Striga tha n does CK-60B. As Framida roots support the lowest number of young Striga s tems with scale leaves, its resistance appears to occur during the establis hment of a functional haustorium. Metabolite uptake by the haustorium and g rowth rate of the young parasite were lower on Framida roots than on CK-60B roots, even when similarly developed haustoria were compared. Furthermore, at a later stage of infestation, significant accumulation of a coloured ma terial likely to be rich in phenolic compounds was observed in and around F ramida conductive tissues, but not CK-60B tissues. These features indicate the involvement of at least three steps in development of resistance in Fra mida roots: the first is linked to a partial inhibition of development of t he young haustorium; the second could play a role in the physiological even ts that decrease nutrient translocation towards the haustoria; and the last seems to be associated with the accumulation of a coloured phenolic-like m aterial.