Nitrogen relations of the sorghum‐Sfriga hermonthica hostparasite association: growth and photosynthesis

Abstract

ABSTRACTThe 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 140d 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 μmol m−2 s−1 in uninfected and infected plants, respectively (at 1500–1800 μmol m−2 s−1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 μmol 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 μmol 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 hetero trophic carbon. The latter ranged from 27 to 6% in 0 5 mol m−3 and 3 mol m−3 N‐grown plants, respectively.