Properties and functional activity of the photosynthetic apparatus in Vida faba L. plants grown at 10, 50, 100 and 150 W PAR were studied. Responses of the photosynthetic apparatus in high, HI (125 W m-^) and low irradiance, LI (10 W m-2) plants to a change in irradiance (/) depended on the leaf age. The chlorophyll (a + b) content per unit of the leaf area remained relatively constant during the first 3 d after a shift in /. Acclimation of the photosynthetic electron transport activity to I depended on / in the preceding period and on the phase of leaf development.