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Plant and Cell Physiology, 1986, Vol. 27, No. 6 1169-1175
© 1986

Article

Changes in Steady-State Photosynthetic 14C-Incorporation into cellular Metabolites of Chlorellax pyrenoidosa during Transitions from High to Low Irradiance

Scott E. Taylor1, Norman Terry and James A. Bassham1

Department of Plant and Soil Biology, 108 Hilgard Hall, University of California Berkeley, CA 94720, U.S.A.

Changes in the levels of 14C-labelled metabolites were monitored in Chlorella pyrenoidosa cells during a transition from high to low irradiance, i.e., from 700 to 430 µmol quanta (400–700 nm) m–2 s–1. Chlorella cells assimilated 14CO2 photosynthetically (steady-state 14C-labelling) for 12 min at the high irradiance and then 10 min at the low irradiance. With the transition to low light, the level of 14C-labelled ribulose 1,5- bisphosphate (RuBP) did not decrease, even though the rate of total 14C-incorporation decreased by 80%. The data suggest that RuBP carboxylase deactivates rapidly (within 1 or 2 min) on exposure to low light, causing RuBP pool sizes to be maintained (or even increased) in spite of a decreased rate of RuBP regeneration. There was also evidence of light modulation of other enzymes, including some enzymes involved in sucrose synthesis. The rate of sucrose synthesis decreased with decrease in light intensity while the level of uridine 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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