Prohexadione calcium is herbicidal to the sunflower root parasite Orobanche cumana.

BACKGROUND The obligatory sunflower root parasite Orobanche cumana Wallr. deprives essential nutrients from its host resulting in dramatic reduction of yield and biomass. A post-emergence application with an imidazolinone herbicide on an imidazolinone-tolerant sunflower is highly effective against O. cumana. The herbicide inhibits the enzyme acetohydroxyacid synthase and consequently, growth of the parasite is inhibited while the sunflower survives the treatment through mutations in the target enzyme. Interestingly, field studies have shown that a combined application of an imidazolinone herbicide with prohexadione resulted in a reduced emergence of O. cumana compared to the sole application of the herbicide. The aim of this study was to investigate if prohexadione is herbicidal to Orobanche cumana. RESULTS Prohexadione was rapidly distributed within the sunflower, reaching the roots, the site of O. cumana attack, as early as six hours after application on sunflower leaves. A direct impact of prohexadione on O. cumana germination was investigated and an IC50 of 84 μM prohexadione was found. In addition, the germination inhibition by prohexadione was terminal, meaning that O. cumana seeds died after prohexadione contact as soon as they were primed for germination. Additionally, excretion studies showed that a small proportion of the applied prohexadione was excreted by sunflower roots. CONCLUSION We show that prohexadione is an inhibitor of O. cumana germination and that the growth regulator is found in sunflower roots shortly after application. We hypothesize that prohexadione gets excreted in sufficient amounts from the sunflower roots therefore having a direct impact on O. cumana germination. This article is protected by copyright. All rights reserved.