THE PARASITIC ANGIOSPERM STRIGA-HERMONTHICA CAN REDUCE PHOTOSYNTHESISOF ITS SORGHUM AND MAIZE HOSTS IN THE FIELD

Two cultivars of sorghum (CK60 and Ochuti) and one cultivar of maize ( H511) were grown in field plots in western Kenya in the presence or ab sence of the parasitic angiosperm Striga hermonthica, with or without a single addition of nitrogen fertilizer (150 kg N ha(-1)) using a fac torial design, A progressive decline in rates of photosynthesis of Str iga-infected plants were observed for the sorghum cultivar CK60 from 3 0 d after planting (DAP) and for maize from 40 DAP, until measurements ended 63 DAP. At this time photosynthetic rates were 46% and 31% lowe r in the Striga-infected sorghum and maize cultivars, respectively, co mpared to uninfected control plants. No decline in photosynthesis was observed in the second sorghum cultivar studied, Ochuti, a local land race reported to show some tolerance to the parasite. The trends in ph otosynthesis reflected stunting of the cereals, as determined by the h eight of the youngest emerged ligule, however, only the grain yield of the sorghum cultivar CK60 was significantly reduced by the presence o f the parasite. The nitrogen application influenced neither the growth nor the photosynthetic parameters measured, and possible explanations for the absence of responses are discussed. It is concluded that S. h ermonthica can reduce photosynthetic rates of field-grown sorghum and maize, and suggest that an ability to maintain high rates of photosynt hesis whilst infected may be an important correlate of tolerance to th e parasite.