Growth and photosynthetic responses of maize and sorghum to the parasitic weed Striga hermonthica

Striga hermonthica (Del.) Benth. is an angiosperm root hemiparasite which is an important weed of principally C4 cereals in the semi-arid tropics; maize, sorghum and millet are the most important hosts. S. hermonthica has its greatest impact in low- input subsistence farming systems. This thesis examines the influence of S. hermonthica on sorghum and maize cultivars grown under both laboratory and field conditions and the influence of nitrogen (in the form of ammonium nitrate) on the association. S. hermonthica had a marked effect on its host. Infected plants showed lower total biomass accumulation and allocated less biomass to the shoot and grain in favour of the root, compared with uninfected plants. Altered biomass partitioning resulted in higher root: shoot ratios in infected plants compared with uninfected plants. Less intemode extension was observed in infected plants resulting in changes in plant architecture. Cereals infected with S. hermonthica had lower rates of photosynthesis compared with uninfected plants under both laboratory and field conditions. In the early stages of the S. hermonthica-cereal association lower rates of photosynthesis were mainly attributed to lower stomatal conductance although the influence of the parasite on other mechanisms, such as the activity of photosystem II, could not be discounted. The S. hermonthica-sorghum association was influenced by the supply of nitrogen both before and after attachment of S. hermonthica in laboratory grown plants. High nitrogen supply inhibited the germination and attachment of S. hermonthica and the effects of the parasite on growth and photosynthesis were ameliorated compared with plants grown at low nitrogen. High concentrations of nitrogen supplied after the attachment of S. hermonthica lowered the biomass of the parasite and alleviated the effect of S. hermonthica on host photosynthesis. In the field high applications of nitrogen fertiliser at experimental sites (150-180 kg N ha'1) had little effect on the level of S. hermonthica infection or on the growth and photosynthesis of infected and uninfected cereals. Lower doses of fertiliser (40 kg N ha'1) on a farmer’s field, however, did influence the association, with infected plants having higher rates of photosynthesis, growth and grain yield compared with infected plants on plots which received no added fertiliser. Possible explanations for the lack of response to nitrogen at experimental sites are discussed. Cereal species and cultivars differed in their sensitivity to S. hermonthica infection. Typically S. hermonthica lowered host productivity and field grown plants had grain yields 21-55% lower than uninfected plants, depending on the species/cultivar used. However, the land race sorghum cultivar Ochuti, exhibited a degree of tolerance to S. hermonthica. Photosynthesis and grain yield were little affected by S. hermonthica compared with other infected cereal varieties, suggesting that an ability to maintain high rates of photosynthesis may be an important correlate of tolerance to the parasite.