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^,.
Differences between photosynthetic characteristics of chlorophyll (Chl) 6-deficient aurea mutant (Su/su) and the green (su/su) variety of Nicotiana tabacum were tested. Fully mature leaves of 3-5-month-old plants grown in potted soil were investigated. Main differences were found as follows: Respective Chl a and b contents were 3 and 5 times lower in aurea plants, while xanthophylls and carotenes contents per unit of Chl were higher. The content of light-harvesting complex (LHC) was lower and LHC composition differed in aurea mutant. PS 1 activity calculated per unit of Chl content was higher in aurea mutant. The green variety showed the lower photosynthetic rates (Pn) at saturating irradiances whether calculated on the Chl or leaf area basis. At excessive light declined with green plants. Carboxylation efficiency (CE) (CO2 response slope of Pjj at low CO2 concentrations) was higher for aurea mutant. Time- integrated intercellular CO2 concentrations derived from ^^C discrimination were higher for aurea mutant (304 cm^ m'^) than for green plants (283 cm^ m'^), which together with higher mean stomatal conductance in aurea matched CE differences.