Responsiveness to Soil Phosphorus and Arbuscular Mycorrhizal Fungi by Striga-Susceptible and Striga-Resistant Sorghum Genotypes.

Striga, a genus of obligate parasitic weeds, has been identified as the most important biological factor limiting agricultural productivity in sub-Saharan Africa. Germination of Striga seeds is triggered by strigolactone root exudates from host plants. Strigolactones also induce hyphal branching in arbuscular mycorrhizal (AM) fungi, which are important for plant uptake of phosphorus in low phosphorus soils. Mechanisms of Striga resistance based on reduced strigolactone production may also convey resistance to AM fungi which would require higher inputs of phosphorus fertilizer to attain optimal crop growth. There is evidence for genetic differences in mycorrhizal responsiveness in other grain crops; therefore it is beneficial for breeders to be aware of these differences when developing Striga-resistant sorghum cultivars. This project aims to determine phosphorus and mycorrhizal responsiveness of sorghum genotypes important for or developed by breeders working on Striga resistance. Phosphorus response curves were determined for twelve genotypes using pasteurized low phosphorus soil amended to achieve four different phosphorus levels. Simple linear regression was performed on root and shoot dry weight. Results indicate variability in phosphorus responsiveness within Striga resistant and susceptible genotypes. Seven of these genotypes were selected for continued research, which analyzed differences in mycorrhizal responsiveness in relation to Striga resistance and responsiveness to phosphorus. Treatments included three levels of phosphorus amendment and the addition of Glomus mosseae inoculum. Preliminary data suggests all genotypes responded positively to increasing P levels and the presence of AM fungi.