Phytotoxicity of selected herbicides to mung bean (Phaseolus aureus Roxb.)

Mung bean (Phaseolus aureus Roxb.) is grown after harvest of wheat during the fallow period. Herbicides such as metsulfuron, atrazine and isoxaflutole are recommended to control weeds in wheat–rice cropping system including weeds of fallow crop. The effects of three herbicides with different modes of action—atrazine, photosystem II inhibitor; metsulfuron, acetolactate synthase inhibitor; and isoxaflutole, 4-hydroxyphenylpyruvatedioxygenase inhibitor—on shoot height, chlorophyll concentrations and cellular damage in herbicide-treated mung bean were studied. While isoxaflutole inhibited shoot growth and chlorophyll concentration of mung bean, atrazine and metsulfuron did not cause reduction in the shoot growth of mung bean. Metsulfuron (226, 452, 1356 and 2260 μg/kg soil) and isoxaflutole (452, 1356 and 2260 μg/kg soil) in soil reduced the concentration of leaf chlorophyll of mung bean compared to the control. Atrazine in soil did not affect the total chlorophyll concentration of mung bean leaves. Electron micrographs showed that untreated mung bean had elongated chloroplasts, thylakoids organized as intact grana, distinct starch grains and a small number of plastoglubuli. Mesophyll cells of atrazine-treated mung bean leaves had swollen chloroplasts and thylakoids with disorganized grana. Leaves of metsulfuron-treated mung bean had swollen chloroplasts with a large number of starch grains. Starch grains were not observed in leaves of mung bean treated with either atrazine or isoxaflutole. Complete disruption of thylakoids was observed in isoxaflutole-treated mung bean leaves. Leaves of atrazine-treated mung bean showed detached microfibrils along with distorted and degenerated secondary walls. Metsulfuron-treated mung bean leaves showed aggregated microfibrils with completely dissolved secondary walls, while isoxaflutole-treated leaves had completely degenerated secondary walls with complete loss of microfibrils. We conclude that isoxaflutole at higher doses, influence mung bean at the morphological, physiological and cellular levels.