NITROGEN RELATIONS OF THE SORGHUM-STRIGA-HERMONTHICA HOST-PARASITE ASSOCIATION - GROWTH AND PHOTOSYNTHESIS

The extent to which the parasitic angiosperm Striga hermonthica reduces the growth of its sorghum host is dependent on the concentration of nitrogen (as NH4NO3 in 40% Long Ashton Solution) supplied to the plants. The biomass of 0.5, 1 and 2 mol m-3 N-grown infected plants was 22, 30 and 66%, respectively, of uninfected plants after 140 d growth. The biomass of 3 and 4 mol m-3 N-grown infected plants differed little from uninfected plants. No grain was set in 0.5 and 1 mol m-3 N-grown infected plants, grain yield reached 42 and 73% of controls in 2 and 3 mol m-3 N-grown plants, and was unaffected in 4 mol m-3 N-grown plants. Striga hermonthica also altered the allometry and architecture of the host, at all but the highest N concentration. Higher N concentration (3 and 4 mol m-3 N) reduced the growth of S. hermonthica. Foliar N concentrations in sorghum ranged from 11 mg g-1 dwt. in 0.5 mol m-3 N-grown plants, to 28 mg g-1 dwt. in 4 mol m-3 N-grown plants, and were not affected by S. hermonthica. Higher N concentrations were measured in S. hermonthica, and ranged from 18 to 45 mg g-1 dwt. in 0.5 and 3 mol m-3 N-grown plants, respectively. The relationship between photosynthesiS (CO2 flux) and N concentration differed between uninfected and infected sorghum. This was most apparent in 0.5 mol m-3 N-grown plants, with rates of 16 and 11 mumol m-2 s-1 in uninfected and infected plants, respectively (at 1500-1800 mumol m-2 s-1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 mumol m-2 s-1 at 4 mol m-3 N. Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host. At low levels of infection in 1 mol m-3 N-grown plants, the negative effect of the parasite was reversed, and photosynthesis in infected plants exceeded that in uninfected plants by 20%. Photosynthesis in S. hermonthica at 3 mol m-3 N (8 mumol m-2 s-1) was double that in 0.5 mol m-3 N-grown plants. Stable carbon isotope and gas exchange measurements data demonstrated that this higher level of autotrophic carbon fixation was accompanied by a lower dependency on heterotrophic carbon. The latter ranged from 27 to 6% in 0.5 mol m-3 and 3 mol m-3 N-grown plants, respectively.