Prohexadione calcium is herbicidal to the sunflower root parasite Orobanche cumana

The obligatory sunflower root parasite Orobanche cumana Wallr. deprives its host of essential nutrients, resulting in a dramatic reduction in 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 acetohydroxy acid synthase and consequently, growth of the parasite is inhibited, although 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 reduced emergence of O. cumana compared with the sole application of the herbicide. The aim of this study was to investigate whether prohexadione is herbicidal to O. cumana.

Prohexadione was rapidly distributed within the sunflower, reaching the roots, the site of O. cumana attack, as early as 6 h after application (HAA) on sunflower leaves. A direct impact of prohexadione on O. cumana germination was investigated and a half-maximal inhibitory concentration (IC₅₀ ) of 84 μm prohexadione was found. In addition, the inhibition of germination 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.

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 is excreted in sufficient amounts from the sunflower roots, therefore having a direct impact on O. cumana germination. © 2020 Society of Chemical Industry.