Do maize cultivars demonstrate tolerance to the parasitic weed Striga asiatica ?

Three cultivars of maize (TMV-1, Katumani and Staha) were grown in the absence and presence of the root hemiparasite Striga asiatica in the field in eastern Tanzania. Infested Katumani and TMV-1 plants yielded 22% and 25% less grain than uninfested plants; however, the grain yield of Staha was similar in both infested and uninfested plants. Measurements of gas exchange and the quantum yield of photosystem II (PhiPSII) revealed that photosynthesis and stomatal conductance were lower in infested plants of TMV-1 and Katumani than in control plants, but were not as severely affected in S. asiatica -infested plants of the cultivar Staha. Chlorophyll fluorescence measurements, particularly the F (v) /F (m) ratio, showed that S. asiatica predisposed TMV-1 to greater photoinhibition than control plants after exposure to irradiances of 2000 mumol quanta m(-2) s(-1) in the field during the day. The lowering of the F (v) /F (m) ratio resulted from a decrease in F (m) rather than in F (o) . In contrast to TMV-1, infested Staha plants were not more photoinhibited than control plants by mid-afternoon. We suggest that the ability of Staha to yield well in the presence of S. asiatica may result, in part, from the ability of this cultivar to maintain high rates of photosynthesis in the field, thus limiting the extent of photoinhibition.