INFLUENCE OF NITROGEN ON GROWTH AND PHOTOSYNTHESIS OF A C-3 CEREAL, ORYZA-SATIVA, INFECTED WITH THE ROOT HEMIPARASITE STRIGA-HERMONTHICA

Striga hermonthica is a root hemiparasitic angiosperm native to the African semi-arid tropics. It is a major weed of C-4 cereals but locally it is also an important weed of the C-3 plant, rice (Oryza sativa). Infected rice plants produced 17% and 42% of the total biomass of uninfected plants when grown at two different ammonium nitrate concentrations, 1 and 3 mol m(-3), respectively. S. hermonthica prevented grain production at both concentrations of nitrogen. At the lower concentration no heads were produced. At the higher concentration head weight was only 6% of uninfected controls. S. hermonthica also altered the partitioning of dry matter between plant parts, such that shoot growth was reduced to a greater extent than root growth. As a consequence the root-to-shoot ratio of infected plants was approximately five times greater than that of uninfected control plants. Light saturated rates of photosynthesis in infected plants were 56% and 70% of those of uninfected controls, at low and high nitrogen, respectively. Infection also led to lower values of stomatal conductance although the substomatal CO2 concentration was unaffected. Analysis of the response of photosynthesis to substomatal CO2 concentration (A/C-i curves) demonstrated that lower rates of photosynthesis could not be solely attributed to lower stomatal conductances. Lower initial slopes and asymptotic rates suggest that both carboxylation and processes controlling regeneration of ribulose-1,5-bisphosphate are reduced by infection. The data are discussed with respect to the influence of S. hermonthica on the growth and photosynthesis of C-4 hosts, where in contrast to the situation with rice, nitrogen feeding results in a marked alleviation of the effects of the parasite on the host.