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  • 1
    Electronic Resource
    Electronic Resource
    Diurnal changes in photochemical efficiency, the reduction state of Q, radiationless energy dissipation, and non-photochemical fluorescence quenching in cacti exposed to natural sunlight in northern Venezuela (1989)
    Springer
    Oecologia 80 (1989), S. 553-561 
    Details
    ISSN: 1432-1939
    Keywords: Cacti ; Chlorophyll fluorescence quenching ; Crassulacean acid metabolism (CAM) ; Desert ecophysiology ; Photochemistry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Diurnal measurements of chlorophyll a fluorescence from cacti (Nopalea cochenillifera, Opuntia ficus-indica, and Opuntia wentiana) growing in northern Venezuela were used to determine photochemical fluorescence quenching related to the reduction state of the primary electron acceptor of PS II as well as non-photochemical fluorescence quenching which reflects the fraction of energy going primarily into radiationless deexcitation. The cladodes used in this study were oriented such that one surface received direct sunlight in the morning and the other one during the afternoon. Both surfaces exhibited large increases in radiationless energy dissipation from the photochemical system accompanied by decreases in PS II photochemical efficiency during direct exposure to natural sunlight. During exposure to sunlight in the morning, dissipation of absorbed light energy through photosynthesis and radiationless energy dissipation was sufficient to maintain Q, the primary electron acceptor for PS II, in a low reduction state. During exposure to sunlight in the afternoon, however, the reduction state of Q rose to levels greater than 50%, presumably due to a decrease in photosynthetic electron transport as the decarboxylation of the nocturnally accumulated malic acid was completed. Exposure to direct sunlight in the afternoon also led to more sustained increases in radiationless energy dissipation. Furthermore, the increases in radiationless energy dissipation during exposure of a water-stressed cladode of O. wentiana to direct sunlight were much greater than those from other well-watered cacti, presumably due to sustained stomatal closure and decreased rates of photosynthetic electron transport. These results indicate that the radiationless dissipation of absorbed light is an important process in these CAM plants under natural conditions, and may reflect a protective mechanism against the potentially damaging effects of the accumulation of excessive energy, particularly under conditions where CO2 availability is restricted.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00380081
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Differences in the susceptibility to light stress in two lichens forming a phycosymbiodeme, one partner possessing and one lacking the xanthophyll cycle (1990)
    Springer
    Oecologia 84 (1990), S. 451-456 
    Details
    ISSN: 1432-1939
    Keywords: Carotenoids ; Chlorophyll fluorescence ; Lichens ; Light stress ; Phycosymbiodeme
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The effect of high light levels on the two partners of a Pseudocyphellaria phycosymbiodeme (Pseudocyphellaria rufovirescens, with a green phycobiont, and P. murrayi with a blue-green phycobiont), which naturally occurs in deep shade, was examined and found to differ between the partners. Green algae can rapidly accumulate zeaxanthin, which we suggest is involved in photoprotection, through the xanthophyll cycle. Blue-green algae lack this cycle, and P. murrayi did not contain or form any zeaxanthin under our experimental conditions. Upon illumination, the thallus lobes with green algae exhibited strong nonphotochemical fluorescence quenching indicative of the radiationless dissipation of excess excitation energy, whereas thallus lobes with blue-green algae did not possess this capacity. The reduction state of photosystem II was higher by approximately 30% at each PFD beyond the light-limiting range in the blue-green algal partner compared with the green algal partner. Furthermore, a 2-h exposure to high light levels resulted in large reductions in the efficiency of photosynthetic energy conversion which were rapidly reversible in the lichen with green algae, but were long-lasting in the lichen with blue-green algae. Changes in fluorescence characteristics indicated that the cause of the depression in photosynthetic energy conversion was a reversible increase in radiationless dissipation in the green algal partner and “photoinhibitory damage” in the blue-green algal partner. These findings represent further evidence that zeaxanthin is involved in the photoprotective dissipation of excessive excitation energy in photosynthetic membranes. The difference in the capacity for rapid zeaxanthin formation between the two partners of the Pseudocyphellaria phycosymbiodeme may be important in the habitat selection of the two species when living separate from one another.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00328159
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    Paper (German National Licenses)
    Fulltext
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