The susceptibility of photosynthesis to photoinhibition and its recovery were studied on cultures of the cyanobacterium Anacystis nidulans. Oxygen evolution and low temperature fluorescence kinetics were measured. Upon exposure to high light A. nidulans showed a rapid decrease in oxygen evolution followed by a quasi steady state rate of photosynthesis. This quasi steady state rate decreased with increasing photon flux density of the photoinhibitory light. Reactivation of photosynthesis in dim light after the photoinhibitory treatment was rapid: 85 to 95% recovery occurred within 2 hours. In the presence of the translation inhibitor, streptomycin (250 micrograms per milliliter), no reactivation occurred. We also found that the damage increased dramatically if the high light treatment was done with streptomycin added. A transcription inhibitor, rifampicin, did not inhibit the reactivation process. Based on these data we conclude that the photoinhibitory damage observed is the net result of a balance between the photoinhibitory process and the operation of the repairing mechanism(s). (9) imply that the repairing system of photodamages in photosynthesis includes synthesis of specific proteins. We hypothesize that a repairing system operating under photoinhibitory conditions may be significant in determining the susceptibility to photoinhibition in photosynthesis; when the damages exceed the capacity of the repairing system, photoinhibition is likely to occur.In a long-term effort to obtain a better understanding of both the molecular mechanisms of photoinhibition ofphotosynthesis, and ofthe significance and the mechanisms ofa repairing system for avoiding photoinhibitions ofphotosynthesis, this initial communication describes the time courses of photoinhibition and reactivation of photosynthesis in the cyanobacterium Anacystis nidulans. A. nidulans has been chosen for this study because it shows the typical responses in photosynthesis upon photoinhibition and we believe that its procaryotic gene organization makes it a suitable object for using molecular genetic tools to elucidate the mechanisms of both photoinhibition and reactivation of photosynthesis.MATERIALS AND METHODS Culture Conditions. Anacystis nidulans 625 (Synechococcus 6301) was grown in batch cultures in an inorganic medium (22) at 38°C. PAR was 50 ,umol m-2 s-', as measured with a Li-Cor quantum radiometer (Lambda Instruments Lincoln, NE) and the light sources were incandescent lamps (Philips PAR 38 150 inches/220 V). The cells were grown in 0.5 L of the medium in rectangular flasks (1.5 L), gently shaken, with light coming in from above, using equipment described before (15). Air was flushed over the culture, which gave cells adapted to low inorganic carbon.Photoinhibitory Treatment and Reactivation. The cells were harvested by centrifugation at 2000g for 5 min and washed once in medium at 38°C, after which the algal pellet was resuspended in 10 ml ofthe medium, buffered with 1 mm Tris (pH 7.2), made up by C02-free water. From this stock an ...
This paper presents a trade model with firm-level productivity differences and R&D-driven growth. Trade liberalization causes the least productive firms to exit but also slows the development of new products. The overall effect on productivity growth depends on the size of intertemporal knowledge spillovers in R&D. When these spillovers are relatively weak, then trade liberalization promotes productivity growth in the short run and makes consumers better off in the long run. However, when these spillovers are relatively strong, then trade liberalization retards productivity growth in the short run and makes consumers worse off in the long run. Copyright � 2010 Blackwell Publishing Ltd.
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