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^,.
Changes in the chlorophyll (Chl) and carotenoid (Car) contents and photosynthetic activity of leaves of three-year-old maple trees were studied under an extremely high irradiance (HI) (5 000 and 7 000 pmol m-2 s‘*) and subsequent low irradiance (LI) (10 pmol m-2 s‘*). Speciál attention was paid to a possible linear correlation between zeaxanthin (z) accumulation and the decrease in variable Chl fluorescence (ratio Fy/FnO during photoinhibitory treatment. Hl-induced violaxanthin (v) transformation into z was a fast one-step response proceeding within 2-5 min. Changes in the Chl a and b contents or in non-xanthophyll cycle Car were either not observed (30 min at 5 000 pmol m*2 s‘i) or a slight decline of the Chl a, lutein, 3-carotene and z contents only occurred at prolonged exposure (60 min at 7 000 pmol m'^ s'i)- Photosynthetic activity measured via the ratios (Fy/F^, Fy/F^, Rfj 690, Rfj 735) sank during this treatment to a much higher extent at 7 000 than at 5 000 pmol m'^ s'*. Ratios Fy/F<„ Rfd 690 and Rfj 735 proved to be better indicators of photoinhibition with a larger amplitudě ťhan the ratio Fy/F,„. Unlike literatuře reports, no linear correlation was found between Hl-induced decrease of fluorescence ratios and the z accumulation. Decrease of the fluorescence ratios appears to be biphasic in nátuře: fast initial fall which parallels the z formation is followed by a phase when ratios Fy/F^,, Fy/F^,, Rj^ 690 and Rf^ 735 sink further without any changes in the z pool. Regeneration of photosynthetic activity under LI also proceeded in two steps; a relatively fast increase in ratios with little or no changes in the z pool followed by a slow many hours' restoring of the former fluorescence ratios, paralleled by the z transformation into v. The back-reaction of z to v under LI was increasingly retarded with higher irradiance. Z formation and decrease in the fluorescence ratios are thus probably two Hl-induced processes which may be mutually independent.
During the vegetation period from June to October 1990 the growth, photosynthetic C02-fixation, pigment content and chlorophyll (Chl) fluorescence characteristics were compared in the needles of 5-year-old spruce dones Picea abies (L.) Karst., cultivated for two years on the soil and the sand, with or without addition of minerál fertilizers. Minerál deflciency in the spruce dones grown on the sand instead of a humus containing soil resulted in somewhat reduced lengthwise growth of shoots and needles, lower level of Chl and carotenoids (Car), as well as lower net photosynthetic rates (F^) per needle area unit and in generál also lower stomata conductance (gH2o) values. In contrast, the variable Chl fluorescence measured via the Chl fluorescence decrease ratio (Rfj = fd/fs) was only little affected, indicating an intemal íunctional photosynthetic apparatus even under minerál deflciency with a lower Chl content. In fact, Fj,} expressed on Chl basis was even signiflcantly higher in the newly developed 1990 needles of the sand cultures than in the soil ones. Application of a forestry fertilizer with basic minerals (K, Ca, Mg and some N) to the sand cultures increased growth of shoots and needles, levels of Chl and Čar, F^ and stomata conductance, and decreased the Chl fluorescence ratio F690/F735 and lability of the photosynthetic apparatus, as also seen in increased values of the ratio Chl/Car (a+b/x+c). The hasič minerál elements are thus necessaiy for the photosynthetic apparatus development, as well as stability of its pigment content and physiological function.