PLANT PHYSIOLOGY , Vol 105, Issue 4 1115-1123, Copyright © 1994 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Regulation of Photosynthetic Induction State by the Magnitude and Duration of Low Light Exposure
G. F. Sassenrath-Cole and R. W. Pearcy
Department of Botany, University of California, Davis, California 95616-8537
This study was undertaken to examine the dependence of the regulatory
enzymes of photosynthetic induction on photon flux density (PFD) exposure
in soybean (Glycine max L.). The induction state varies as a function of
both the magnitude and duration of the PFD levels experienced prior to an
increase in PFD. The photosynthetic induction state results from the
combined activity of separate processes that each in turn depend on prior
PFD environment in different ways. Direct measurement of enzyme activities
coupled with determination of in situ metabolite pool sizes indicated that
the fast-induction component was associated with the activation state of
stromal fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) and showed rapid
deactivation in the dark and at low PFD. The fast-induction component was
activated at low PFD levels, around 70 [mu]mol photons m-2 s-1.
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 2.7.1.19)
deactivated very slowly in the dark and required higher PFD for activation.
Both enzymes saturated at lower PFD than did photosynthesis, around 400
[mu]mol photons m-2 s-1. Ribulose-5-phosphate kinase (EC 2.7.1.19) appeared
never to be limiting to photosynthesis, and saturated at much lower PFD
than either FBPase or Rubisco. Determination of photosynthetic metabolite
pool sizes from leaves at different positions within a soybean canopy
showed a limitation to carbon uptake at the stromal FBPase and possibly the
sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37) in shade leaves upon initial
illumination at saturating PFD levels.
