Resistance of Sorghum to Witchweed

Abstract

STRAINS of Sorghum vulgare resistant to root parasite witchweed (Striga lutea) have been bred in South Africa over a number of years.¹ One of these (var. ‘Framida’) is characterized by its inability to germinate the parasite seed, which requires a stimulatory substance normally excreted from the host plant roots. A comparison of the resistant variety ‘Framida’, and a susceptible variety ‘Light Mori’ (from Nigeria) illustrates this point very clearly. Seedlings of the two varieties were grown in coarse washed sand in 3-in. plastic pots (50 seedlings per pot) for a period of nine days at ambient temperature (23–31°C.). After this the root solutions were eluted with 20 ml. of distilled water. The eluted solutions were tested for their ability to germinate the parasite seed in Petri-dish culture at full strength (as eluted) and at dilutions of 1:10 and 1:100. Typical results are shown in Fig. 1. It can be seen that the concentration of germination stimulent from the susceptible variety (‘Light Mori’) is supra-optimal and that a dilution of 1:10 does not reduce germination of the parasite seeds significantly. In the resistant variety, however, a similar dilution reduces germination almost completely; while even at the highest concentration germination is significantly lower. Since it is unlikely that a concentration of root growth stimulant approaching that of the experimental conditions will be encountered in Nature, this seems to be at least one mechanism by which resistance is achieved.