Physiology of the parasitic association between maize and witchweed (Striga hermonthica): is ABA involved?

Striga hermonthica (Del.) Benth. is an obligate hemiparasitic angiosperm which can cause severe losses of yield in cereal crops in the semi-arid tropics. The effects of this parasite on the growth and stomatal conductance of tree varieties of maize (Zea mays L.) during the first 6 weeks of the association have been studied. From 24 d after planting (DAP), infected plants were significantly shorter than uninfected controls. When the plants were harvested 45 DAP, infected plants had fewer fully expanded leaves, less leaf biomass and less pseudo-stem biomass than uninfected controls. However, the parasitized plants had more root biomass and hence a higher root:shoot ratio than uninfected controls. The stomatal conductance of infected hosts was severely inhibited by comparison with that in uninfected plants. The possibility that abscisic acid (ABA) may be involved in the regulation of the parasitic association was investigated. ABA concentrations in leaf tissue of maize (cv. Cargimontana) and S. hermonthica were determined by radioimmunoassay. While there was a difference between cultivars in the extent of the response, the concentrations of ABA were significantly higher in infected maize plants than in the uninfected controls. In S. hermonthica, leaf tissue ABA concentration was found be an order of magnitude higher than in the host leaf tissue. Detached leaves of S. hermonthica which were dehydrated at room temperature until they had lost 10-20% of their fresh weight contained three times the ABA concentration of control leaves. This suggests that leaves of S. hermonthica can synthesize or re-mobilize ABA in response to water deficit. It is not yet known whether this contributes to the higher concentration in infected hosts, but the results suggest that ABA has a role in this parasitic association.