Abstract: Using “Menggusang” mulberry (Morus alba L.) seedling grown under greenhouse conditions in Heilongjiang Province, we analyzed photosynthetic induction responses to irradiance transitions via gas-exchange and chlorophyll fluorescence techniques. Results show that 10 min photosynthetic induction is needed for the photosynthetic rate ( Pn ) of mulberry leaf to reach stable state under the sudden irradiance transition from dark/low irradiance (LI) to high irradiance (HI). A biphasic photosynthetic response is evident — rapid phase (1~2 min) and slow phase (8~10 min). When irradiance is transited from LI to HI, induction of photosynthetic quantum conversion efficiency falls behind the photosynthetic CO₂ assimilation. When irradiance is changed from high to low/dark, Pn faster approaches steady-state compared to the irradiance transfer from low/dark to high. This indicates that induction efficiency of quantum conversion is limited by photosynthetic CO₂ assimilation following increase in irradiance. Induction efficiency of photosynthetic quantum conversion may therefore be regulated by the proportion of open PSⅡ reaction centers.