Abstract:Protoplasts from hypocotyls of etiolated mung bean (Phaseolus raditus L. ) seedlings were maintained at a constant osmotic potential at 20±2℃, and they were found to swell gradually after being pulsed with red light (R) (10.5 W · m-2, 3 min) when CaCl2 was present in the medium. The volume reached maximum during 30--60 min after R-irradiation and decreased swelling afterwards. Farred light (FR) irradiation in presence or absence of Ca2+ did not influence the protoplast volume. The R-effect was photoreversible by subse- quent FR (2.5 W · m-2, 5 min) irradiation, usually seen over two R-FR cycles. Furthermore, swelling response was in positivecorrelation with red light intensity and duration of R pulse, indicating the involvement of phytoehrome. FR became less effective in reversing the effect of R after 10 min in dark between R and FR. Protoplast swelling occurred only when Ca2+ ions (1 mmol/L) then Ca2+ ions (1 mmol/L) is added to the medium 5 rain after R. The effect of Ca2+ could not be replaced by Mg2+, Ba2+, Zn2+, or K+. The time course of water (3H20) uptake into protoplasts after R-irradiation was consistent with the trend of protoplast swelling, indicating the existence of certain relationship between the swelling and water uptake of the protoplasts.