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

摘要

The nature of the lack of oxygen inhibition of C3-photosynthesis at low temperature was investigated in white clover (Trifolium repens L.). Detached leaves were brought to steady-state photosynthesis in air (34 Pa p(CO2), 21 kPa p(O2), balance N2) at temperatures of 20°C and 8°C, respectively. Net photosynthesis, ribulose 1,5-bisphosphate (RuBP) and ATP contents, and ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activities were followed before and after changing to 2·0 kPa p(O2). At 20°C, lowering p(O2) increased net photosynthesis by 37%. This increase corresponded closely with the increase expected from the effect on the kinetic properties of RuBPCO. Conversely, at 8°C net photosynthesis rapidly decreased following a decrease in p(O2) and then increased again reaching a steady-state level which was only 7% higher than at 21 kPa p(O2). The steady-state rates of RuBP and associated ATP consumption were both estimated to have decreased. ATP and RuBP contents decreased by 18% and 33% respectively, immediately after the change in p(O2) suggesting that RuBP regeneration was reduced at low p(O2) due to reduced photophosphorylation. Subsequently, RuBP content increased again. Steady-state RuBP content at 2·0 kPa p(O2) was 24% higher than at 21 kPa p(O2). RuBPCO activity decreased by 22%, indicating control of steady-state RuBP consumption by RuBPCO activity. It is suggested that lack of oxygen inhibition of photosynthesis at low temperature is due to decreased photophosphorylation at low temperature and low p(O2). This may be due to assimilate accumulation within the chloroplasts. Decreased photophosphorylation seems to decrease RuBP synthesis and RuBPCO activity, possibly due to an acidification of the chloroplast stroma