Abstract: With dwarfing interstock Fuji apples as the test materials and water treatment as the control (CK), we examined the fruit thinning effect and its influences on leaves’ photosynthesis by spraying 200, 300, and 400 mg·L^-1 metamitron during the young fruit period to solve artificial fruit thinning problems (time-consuming, much labor, and low efficiency). The results showed that metamitron application could significantly reduce the inflorescence and flowers’ fruit-setting rate by 16.5%-22.8% and 50.9%-53.9%, respectively. The treatment of 300 mg·L^-1 metamitron had the strongest fruit thinning effect, with a single fruit rate of 46.6% and a double fruit rate of 18.3%. As a photosynthesis inhibitor, metamitron application reduced the chlorophyll content of leaves and strongly affected photosynthesis. The inhibitory effect on chlorophyll content disappeared after 15 days of the treatment, while that on the net photosynthetic rate disappeared gradually after 11 days of the treatment. The application of metamitron significantly reduced the maximum quantum yield of PSⅡ reaction center (F_v/F_m), actual photochemical efficiency (Φ_PSⅡ), photochemical quenching coefficient (q_P) and non-photochemical quenching coefficient (NPQ), with such inhibitory effect having been lasted for 15 days. OJIP analysis showed that metamitron caused damage to the apple leaves’ oxygen-evolving complex, especially limiting the transfer of electrons in the PSⅡ reaction center from Q_A to Q_B. Metamitron treatment increased W_k, and significantly decreased ψ_o, RC/CS_m, and PI_abs. Besides, 300 mg·L^-1 metamitron had the most significant effect. Our results showed that metamitron destroyed the structure of the PSⅡ reaction center of apple leaves and hindered the transfer of electrons from the donor to the receptor of PSⅡ. Consequently, the photosynthetic rate was affected, and the young fruits fell off due to the lack of accumulation of photosynthetic products.

Key words: metamitron, apple, photosynthesis, fluorescence