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Changes in Steady-State Photosynthetic 14C-Incorporation into cellular Metabolites of Chlorellax pyrenoidosa during Transitions from High to Low Irradiance
Authors:Taylor  Scott E; Terry  Norman; Bassham  James A
Institution:Department of Plant and Soil Biology, 108 Hilgard Hall, University of California Berkeley, CA 94720, U.S.A.
Abstract:Changes in the levels of 14C-labelled metabolites were monitoredin Chlorella pyrenoidosa cells during a transition from highto low irradiance, i.e., from 700 to 430 µmol quanta (400–700nm) m–2 s–1. Chlorella cells assimilated 14CO2 photosynthetically(steady-state 14C-labelling) for 12 min at the high irradianceand then 10 min at the low irradiance. With the transition tolow light, the level of 14C-labelled ribulose 1,5- bisphosphate(RuBP) did not decrease, even though the rate of total 14C-incorporationdecreased by 80%. The data suggest that RuBP carboxylase deactivatesrapidly (within 1 or 2 min) on exposure to low light, causingRuBP pool sizes to be maintained (or even increased) in spiteof a decreased rate of RuBP regeneration. There was also evidenceof light modulation of other enzymes, including some enzymesinvolved in sucrose synthesis. The rate of sucrose synthesisdecreased with decrease in light intensity while the level ofuridine diphosphoglucose increased, but within a few minutes,both returned to their former levels. 1Present address: Chemical Biodynamics, Lawrence Berkeley Laboratory,Building 3, 1 Cyclotron Road, Berkeley, CA 94720, U.S.A. (Received March 8, 1986; Accepted June 25, 1986)
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