The parasitic angiosperm Striga hermonthica can reduce photosynthesis of 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 absence of the parasitic angiosperm Striga hermonthica, with or without a single addition of nitrogen fertilizer (150 kg N ha(-1)) using a factorial design, A progressive decline in rates of photosynthesis of Striga-infected plants were observed for the sorghum cultivar CK60 from 30 d after planting (DAP) and for maize from 40 DAP, until measurements ended 63 DAP. At this time photosynthetic rates were 46% and 31% lower in the Striga-infected sorghum and maize cultivars, respectively, compared 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 photosynthesis reflected stunting of the cereals, as determined by the height of the youngest emerged ligule, however, only the grain yield of the sorghum cultivar CK60 was significantly reduced by the presence of 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. hermonthica can reduce photosynthetic rates of field-grown sorghum and maize, and suggest that an ability to maintain high rates of photosynthesis whilst infected may be an important correlate of tolerance to the parasite.