Plant Physiology 76:968-971 (1984)
© 1984 American Society of Plant Biologists
Articles
Photosynthesis and Ribulose 1,5-Bisphosphate Concentrations in Intact Leaves of Xanthium strumarium L. 1
Keith A. Mott2,
Richard G. Jensen,
James W. O'Leary and
Joseph A. Berry
Department of Biochemistry, University of Arizona, Tucson, Arizona 85721,
Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721,
Environmental Research Laboratory, Tucson International Airport, Tucson, Arizona 85706,
Carnegie Institution of Washington, 290 Panama Street, Stanford, California 94305
The interacting effects of the rate of ribulose 1,5-bisphosphate (RuBP) regeneration and the rate of RuBP utilization as influenced by the amount and activation of RuBP carboxylase on photosynthesis and RuBP concentrations were resolved in experiments which examined the kinetics of the response of photosynthesis and RuBP concentrations after step changes from a rate-saturating to a rate-limiting light intensity in Xanthium strumarium. Because RuBP carboxylase requires several minutes to deactivate in vivo, it was possible to observe the effect of reducing the rate of RuBP regeneration on the RuBP concentration at constant enzyme activation state by sampling very soon after reducing the light intensity. Samples taken over longer time periods showed the effect of changes in enzyme activation at constant RuBP regeneration rate on RuBP concentration and photosynthetic rate. Within 15 s of lowering the light intensity from 1500 to 600 microEinsteins per square meter per second the RuBP concentration in the leaves dropped below the enzyme active site concentration, indicating that RuBP regeneration rate was limiting for photosynthesis. After longer intervals of time, the RuBP concentration in the leaf increased as the RuBP carboxylase assumed a new steady state activation level. No change in the rate of photosynthesis was observed during the interval that RuBP concentration increased. It is concluded that the rate of photosynthesis at the lower light intensity was limited by the rate of RuBP regeneration and that parallel changes in the activation of RuBP carboxylase occurred such that concentrations of RuBP at steady state were not altered by changes in light intensity.
2 Current address: Carnegie Institution of Washington, Department of Plant Biology, 290 Panama St., Stanford, CA 94305.
1 Supported by The Science and Education Administration of the United States Department of Agriculture under grant 82-CRSR-1-1010, and by grants PCM 82-07687 and DEB 8110202 from the National Science Foundation. This is Arizona Agricultural Experiment Station Paper No. 3927 and CIW-DPB publication No. 865.
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