Abstract: To investigate the mechanisms underlying photosynthetic damage in leaves of alfalfa (Medicago sativa) induced by rhizospheric residues of imidacloprid (IMI)-a neonicotinoid insecticide, alfalfa roots were exposed to Hoagland nutrient solutions containing different concentrations of IMI. Based on the responses of plant growth and physiological variables to IMI stress, an IMI stress concentration of 2 mM was determined. The inhibitory effects of high-dose IMI stress on photosynthesis of alfalfa were then analyzed using chlorophyll fluorescence kinetics. Rhizosphere IMI stress caused leaf necrosis and oxidative stress in alfalfa, leading to photosynthetic inhibition, as evidenced by chloroplast structural damage, a decline in photosynthetic potential, and block of the fluorescence rise in the J-P phase of the chlorophyll fluorescence induction curve (OJIP). Further JIP testing indicated that the photosynthetic inhibition induced by high-dose IMI stress might be associated with impaired electron transport on both the donor and acceptor sides of photosystem Ⅱ (PSⅡ). IMI stress reduced energy available for electron transport (ET₀/CS₀, ET₀/RC) and increased energy dissipation (DI₀/CS₀, DI₀/RC). Fluorescence quenching analysis confirmed that high-dose IMI inhibited photochemical efficiency, triggered nonphotochemical quenching, and caused photodamage. These findings revealed the mechanism by which high-dose IMI in the rhizosphere significantly inhibits photosynthesis of alfalfa by impairing electron transport in PSⅡ, providing new insights into the environmental risks of neonicotinoid insecticides.

Key words: imidacloprid, photosystem Ⅱ, photosynthesis, chlorophyll fluorescence, Medicago sativa