When exposed to Y-radiation (12, 8 and 3.5 kGy), the growth of beán seedlings {Phaseolus vulgaris L.) was stopped and after some hours or days the plants began to wilt in a dose-dependent manner, starting from the leaf rim. The rate of the dark respiration {R) of leaves increased and that of net photosynthesis {P^ was strongly reduced. The regulation of stomata opening and closure was lost and the stomatal conductance (g^) of the y-ray exposed plants was strongly reduced. The reduced was only partly due to either the partial or almost Ml stomata closure. Chlorophyll (Chl) fluorescence measurements with a two-wavelength fluorometer and a PAM fluorometer showed an increasingly reduced variable fluorescence Fy, lower values of Rfj, of ground fluorescence Fq, and of the fluorescence ratios Fy/F,n and Fy/F^. This indicated a damage to the photosynthetic apparatus. The increasing loss of photosynthetic pigments in the 350 krad exposed plants was also detected via an increase in the fluorescence ratio F690/F730. The performance of the light driven xanthophyll cycle (violaxanthin/zeaxanthin transformation) proceeded in the y-ray treated plants only at reduced rates. The y-ray damage of plants can best be detected by measurements of stomatal conductance, and various Chl fluorescence ratios such as Rf(j, Fy/Fj, and Fy/F^,.