REGULATION OF PHOTOSYNTHETIC INDUCTION STATE BY THE MAGNITUDE AND DURATION OF LOW-LIGHT EXPOSURE

This study was undertaken to examine the dependence of the regulatory enzymes of photosynthetic induction on photon flux density (PFD) expos ure in soybean (Glycine max L.). The induction state varies as a funct ion of both the magnitude and duration of the PFD levels experienced p rior to an increase in PFD. The photosynthetic induction state results from the combined activity of separate processes that each in turn de pend on prior PFD environment in different ways. Direct measurement of enzyme activities coupled with determination of in situ metabolite po ol sizes indicated that the fast-induction component was associated wi th 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 PF D. The fast-induction component was activated at low PFD levels, aroun d 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 lowe r 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 limitin g to photosynthesis, and saturated at much lower PFD than either FBPas e or Rubisco. Determination of photosynthetic metabolite pool sizes fr om leaves at different positions within a soybean canopy showed a limi tation to carbon uptake at the stromal FBPase and possibly the sedohep tulose-1,7-bisphosphatase (EC 3.1.3.37) in shade leaves upon initial i llumination at saturating PFD levels.