Response of photosynthetic induction to irradiance transition in mulberry leaf

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.