Effect of orthophosphoric acid (OPA) on germination of seeds of parthenium (Parthenium hysterophorus L.) using wheat (Triticum aestivum L.) as a reference was investigated. The test seed wheat, could germinate at a concentration of about 60 mM, but failed to form a normal seedling. At higher concentrations (about 75 mM) the OPA completely inhibited germination. Whereas parthenium seed germination was completely inhibited at 15 mM OPA. The concentration of OPA causing 50% loss in seed germination in parthenium and wheat were 0.19 and 16.7 mM, respectively. The concentration of OPA causing 50% loss of dehydrogenase activity in the seeds of parthenium and wheat were 0.15 and 1.9 mM, respectively. Spraying of OPA on parthenium stand at 15–60 mM caused wilting followed by desiccation in about 2–4 hours and death subsequently depending on the concentration of the chemical. At 45 mM concentration the OPA killed parthenium stand irrecoverably.
