Abstract:In studying the mechanism of increase in alfalfa (Medicago sativa L. ) photosynthesis under elevated atmospheric CO2, it was found that the capacity of chloroplasts for light absorption was greater, the potential activity and efficiency of primary conversion of light enlergy of PS Ⅱ , quantum yield of PS Ⅱ electron transport, and activation capacity of PS Ⅰ were stimulated, photochemical quenching coefficient was increased and non-photochemical quenching coefficient was decreased under elevated atmospheric CO2.