Allelopathic weed management in wheat (Triticum aestivum L.) through essential oil emulsions and aqueous botanical extracts-based novel bioherbicides

Phytotoxic activity of essential oil emulsions (EOEs) of aromatic plants such as Mentha piperita, Pelargonium graveolens, Matricaria chamomilla, Chrysopogon zizanioides, Pogostemon patchouli, Mentha arvensis, and aqueous extracts of Andrographis paniculata were evaluated for problematic weeds like Avena fatua and Phalaris minor, along with wheat (Triticum aestivum) as a test crop through laboratory bioassay studies during 2020–2021. The chemical composition of essential oils was analyzed through GC/GC–MS. Results of the laboratory bioassay revealed that EOEs of M. piperita, C. zizanioides, M. arvensis, and an aqueous extract of A. paniculata as pre-emergence bioherbicides strongly inhibited germination and seedling growth of the tested weed species in a dose dependent manner, with P. minor and A. fatua being selectively more sensitive than T. aestivum. Percent seed germination varied between treatments, ranging from 3.45–48.28%, 3.45–100%, and 4.55–90.91% among T. aestivum, A. fatua, and P. minor, respectively. Based on these results, these four treatments showing the highest phytotoxic activity were further evaluated through a pot culture study. The results showed that treatments with essential oil emulsions and aqueous extracts had a negative impact on seedling Vigor Index I and Vigor Index II. Comparing physiological processes in P. minor, like relative electrolyte leakages, essential oil of M. arvensis at concentration 1% + 8% (pre emergence + early post emergence) had the highest levels of 47.16%, followed by the value of 38.86% which was recorded in C. zizanioides at concentration 1% + 8% (pre emergence + early post emergence) and 22.80% reported in M. piperita at concentration 1% + 8% (pre emergence + early post emergence) treated plants. These levels were higher than those in the untreated control (10.13%). Indeed, at higher concentrations of each treatment, grain yield per plant decreased in the range of 7–29% with respect to the untreated control. The data on phytotoxicity rating showed that visible injury symptoms in T. aestivum plants were less severe as compared to the symptoms recorded in A. fatua and P. minor weeds. This is the first in-depth study to show that emulsions of essential oils from P. graveolens, C. zizanioides, and P. patchouli are phytotoxic. It is also the first time that M. arvensis and C. zizanioides have been shown to be pre-emergence bioherbicides in wheat. Hence, our study presents these EOEs as novel candidates of biological origin that can be used in sustainable weed management, especially in organic farming systems.