Carbon dependency of the hemiparasite Striga hermonthica on Sorghum bicolor determined by carbon isotopic and gas exchange analyses

The development of autotrophy in Striga hermonthica (Del.) Benth. parasitic on Sorghum bicolor L. was quantitatively determined by C-13/C-12 isotope ratio analysis and related to the photosynthetic capacity of the parasite. Net carbon assimilation by the parasite was achieved within 4-5 days after emergence and thereafter the photosynthetic capacity remained 3- to 5-fold in excess of the respiratory rate. Nearly 40% of the dry matter of the aerial parts of the parasite was derived from autotrophic assimilation after only 7 days. While the development of the aerial parts continued to be favoured, 65% of aerial dry matter being of autotrophic origin at 42 days, there was a shift towards the use of autotrophic photosynthate for root development around the onset of flowering. When host aerial parts were cut off 50 days after emergence, the parasite exhibited reduced further growth, but completed its life cycle autotrophically. As the delta(13)C values in these plants were lower than in plants that remain in association, we deduce that S. hermonthica continues to benefit from heterotrophic assimilation to the end of its life cycle.