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 w ith Sorghum 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 germination or the attachment stages, since its roots do not support fewer Striga than does CK-60B. As Framida roots support the lowest number of young Striga stems with scale leaves, its resistance appears to occur during the establishment of a functional haustorium. Metabolite uptake by the haustorium and growth 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 material likely to be rich in phenolic compounds was observed in and around Framida conductive tissues, but not CK-60B tissues. These features indicate the involvement of at least three steps in development of resistance in Framida roots: the first is linked to a partial inhibition of development of the young haustorium; the second could play a role in the physiological events that decrease nutrient translocation towards the haustoria; and the last seems to be associated with the accumulation of a coloured phenolic-like material.