NITROGEN RELATIONS OF THE SORGHUM STRIGA-HERMONTHICA HOST-PARASITE ASSOCIATION - GERMINATION, ATTACHMENT AND EARLY GROWTH

The probability of the parasitic angiosperm Striga hermonthica achieving a successful union with its sorghum host is in part dependent on the concentration of ammonium nitrate in the growth medium. Germination rates were higher with 1 mm than with 3 mm ammonium nitrate. Ammonium nitrate did not influence either the stability of active components of host root exudate, or their perception by the parasite's seed. High concentrations of ammonium nitrate did not significantly reduce the leakage of potassium ions from sorghum roots (although patterns of exudation may differ between groups of compounds). Thus, it appears that ammonium nitrate reduced either the production of stimulatory compounds, or their specific leakage from host roots. Subsequent attachment of the parasite radicle and early growth of the plant were also lower at higher concentration of ammonium nitrate, and possible reasons for these observations are discussed.