Phytotoxic essential oil from black turmeric (Curcuma caesia Roxb.) rhizome: Screening, efficacy, chemical basis, uptake and mode of transport

Plant-based phytotoxins play an important role in plant-plant interaction and are of great promise for the development of bioherbicide. In this study, screening of essential oils from six different Curcuma rhizomes identified black turmeric (Curcuma caesia Roxb.) oil as an efficient growth inhibitor in wheatgrass coleoptile bioassay (IC50 57.1 µg/mL). The phytotoxic efficacy of this oil was further confirmed through the dose dependent (10.0–300.0 µg/mL) inhibition of germination, coleoptile and radicle growth of wheatgrass seeds in water medium (IC50 176.7, 90.6 and 93.0 µg/mL respectively) and the pre-germinated seeds in agar medium. Activity-guided fractionation and purification of the crude oil identified curzerenone, a major furanosesquiterpene in this oil as the most active phytotoxin (IC50 13.0 µg/mL in coleoptile bioassay; 188.3, 34.7 and 36.7 µg/mL respectively in water medium). Structure-activity relationship study indicated the importance of all the functional groups for its phytotoxicity. A significant contribution by the oxygenated monoterpenes towards oil phytotoxicity was also evidenced. Though inhibitory efficacy of the oil was comparatively weak in soil, a profound activity through aerial diffusion was observed against germination and/or growth of fresh and pre-germinated wheatgrass seeds (IC50 1.4–6.2 mg/L air). The study also confirmed the uptake of active phytotoxins with no detectable transformation by the seeds in water and their persistence in the soil for at least one and half months. Further, its applicability was demonstrated through an efficient post-emergence growth inhibition (IC50 37.0–81.4 µg/mL) of bermudagrass weed in water and agar medium. [Display omitted] •Screening: black turmeric (Curcuma caesia) essential oil is a potent phytotoxin.•Major phytotoxins: curzerenone (a furanosesquiterpene) and oxygenated monoterpenes.•Modes of transport: water, agar, soil and aerial diffusion.•Uptake of active phytotoxins by the seeds with no detectable transformation.•Efficient inhibitor of post-emergence growth in bermudagrass weed.