The regulation of the water potential gradient in the host and parasite relationship between sorghum bicolor and striga hermonthica

Abstract

A glasshouse experiment was carried out to investigate the factors controlling the abstraction of xylem fluid from its host by the parasite Striga hermonthica (Scrophulariaceae). Striga had a mean daily transpiration rate far exceeding that of its host sorghum (Sorghum bicolor), with infestation by Striga also shown to lower the transpiration rate of the host. Stopping the host's transpiration was shown to decrease the transpiration rate of the parasite. Stopping the parasite's transpiration only gave an initial increase in the host's transpiration rate which was not sustained. The parasite had a lower water potential than its host, values being -0.42 MPa and -0.23 MPa, respectively, and an accompanying higher osmotic pressure of 0.68 MPa against 0.51 MPa for sorghum. Modifying the water potential gradient by bagging both partners together showed that the differential in osmotic pressure and water potential was largely maintained by the parasite's higher rate of transpiration. A favourable water potential gradient towards the parasite still existed following the cessation of transpiration, this being generated by the haustorial resistance to hydraulic conductivity which was found to be some 1.5-4.5 times greater than that offered by the parasite shoot. Both the high rate of transpiration and the increased resistance across the haustoria would appear to be necessary means to facilitate the diversion of host resources to the parasite.