Early physiological response of broccoli leaf to foliar application of clove oil and its main constituents

Clove oil and its principal constituent, eugenol, display herbicidal contact action on annual broadleaf weeds. In the present study, we examined the early effects of clove oil (CL) and its main constituents: eugenol (E), eugenol + humulene (E + H) and eugenol + caryophyllene (E + C) on the physiological state of broccoli leaves during the first 60 min after application. We also addressed the hypothesis that synergism exists between the main components of clove oil and is responsible for its phytotoxic effect. The response of plant tissues was recorded using isothermal calorimetry (TAM III) during the first 60 min and chlorophyll a fluorescence (FluorCam) imaging at three time points: 20, 40 and 60 min after application. The changes in three fluorescence parameters were visually analyzed: QYmax-maximum photochemical efficiency of photosystem II, NPQ-non-photochemical quenching, and Rfd-fluorescence decrease ratio. All of the treatments caused a decrease of both heat emission and chlorophyll a fluorescence. The visible physiological changes in broccoli leaves were observed as early as 20 min after treatments. Based on a multiple regression analysis, we demonstrated a strong correlation (R2 = 0.81-0.96) between heat emission and QYmax, as well as heat emission and Rfd. The strongest phytotoxic effect was observed for CL. We did not observe the existence of synergism between eugenol and any of the other main constituents, a phenomenon believed to increase phytotoxic effects.