Prolonged Use of Insecticide Dimethoate Inhibits Growth and Photosynthetic Activity of Wheat Seedlings: A Study by Laser-Induced Chlorophyll Fluorescence Spectroscopy.

This paper is an extension of the work published in Journal of Fluorescence (2011) 21: 785-791. In the previous work, we studied the effect of dimethoate (50, 100 and 200 ppm) on growth and photosynthetic activity of wheat seedlings after 10 days of dimethoate treatment. In the present study, new measurement conditions (dimethoate concentration: 25 ppm, treatment period: 20 days and 30 days) were used in addition to those used in the past work. Various plant growth parameters, photosynthetic pigment content, laser-induced chlorophyll fluorescence (LICF) spectra and fluorescence induction kinetics (FIK) curves were recorded after 10, 20 and 30 days of dimethoate treatments. LICF spectra were recorded in the region of 650-780 nm using violet diode laser (405 nm). FIK curves were recorded at 685 nm using red diode laser (635 nm). Fluorescence intensity ratio (FIR) of two fluorescence peaks around 685 and 730 nm, and variable chlorophyll fluorescence decrease ratio (Rfd) were determined from LICF spectra and FIK curves respectively. Curve-fitted parameters of LICF spectra were used for determination of FIR (F685/F730). The effect of treatment of the insecticide dimethoate on growth and photosynthetic activity of wheat seedlings was examined by using these parameters as well as the past work. In 10-days treatment, 25 and 50 ppm dimethoate showed stimulatory effect with better stimulation being observed at 25 ppm. All studied concentrations higher than 50 ppm exhibited inhibitory effect on wheat seedlings. In case of dimethoate treatment studied for longer durations (more than 10 days), all concentrations showed inhibitory effect. Lower doses which showed some positive response for short time duration become toxic with the extension of treatment periods. Thus, this study clearly confirms the toxic effect of dimethoate on wheat plants.