Regeneration of Ribulose 1,5-bisphosphate and Ribulose 1,5-bisphosphate carboxylase/oxygenase Activity Associated with Lack of Oxygen Inhibition of Photosynthesis at Low Temperature

The nature of the lack of oxygen inhibition of C₃-photosynthesisat low temperature was investigated in white clover (Trifoliumrepens L.). Detached leaves were brought to steady-state photosynthesisin air (34 Pa p(CO²), 21 kPa p(O₂), balance N₂) at temperaturesof 20°C and 8°C, respectively. Net photosynthesis, ribulose1,5-bisphosphate (RuBP) and ATP contents, and ribulose 1,5-bisphosphatecarboxylase/oxygenase (RuBPCO) activities were followed beforeand after changing to 2·0 kPa p(O₂). At 20°C, lowering p(O₂) increased net photosynthesis by37%. This increase corresponded closely with the increase expectedfrom the effect on the kinetic properties of RuBPCO. Conversely,at 8°C net photosynthesis rapidly decreased following adecrease in p(O₂) and then increased again reaching a steady-statelevel which was only 7% higher than at 21 kPa p(O₂). The steady-staterates of RuBP and associated ATP consumption were both estimatedto have decreased. ATP and RuBP contents decreased by 18% and33% respectively, immediately after the change in p(O₂) suggestingthat RuBP regeneration was reduced at low p(O₂) due to reducedphotophosphorylation. Subsequently, RuBP content increased again.Steady-state RuBP content at 2·0 kPa p(O₂) was 24% higherthan at 21 kPa p(O₂). RuBPCO activity decreased by 22%, indicatingcontrol of steady-state RuBP consumption by RuBPCO activity. It is suggested that lack of oxygen inhibition of photosynthesisat low temperature is due to decreased photophosphorylationat low temperature and low p(O₂). This may be due to assimilateaccumulation within the chloroplasts. Decreased photophosphorylationseems to decrease RuBP synthesis and RuBPCO activity, possiblydue to an acidification of the chloroplast stroma. Key words: Oxygen inhibition, photosynthesis, ribulose bisphosphate carboxylase/oxygenase