[Metabolism of some phosphorylated compounds and photophosphorylation in Maize leaves]

Following a period of steady state photosynthesis, in air, maize leaves were illuminated in a CO2-free atmosphere consisting of N2 or N2-O2 (80-20, v/v).Isotopic techniques have been used for the measurement in cells of pool sizes of some phosphorylated intermediates and for the study of the turn-over of these compounds.In N2 atmosphere and white light, a rapid decrease of the PGA level and an accumulation of Ru 1-5 P2 are observed, whereas DHAP and Fru 1-6 P2 levels remain high. With (32)P feeding in short-time experiments, Ru 1-5 P2, ATP and ADP are highly labelled, and there is a low but significant labelling of PGA and DHAP. Our interpretation is that basic reactions of the CALVIN cycle are occurring: phosphorylation of Ru 5 P to Ru 1-5 P2 (phosphorylating step), carboxylation of Ru 1-5 P2 to PGA (the CO2 belongs to an intracellular pool of unknown nature), reduction of PGA to trioses phosphates (reductive step), regeneration of the CO2 acceptor via the synthesis of Ru 5P (regenerative step). In N2 atmosphere the second step is the limiting one because of the low intracellular CO2 level; the consequence is an increase in the amount of the CO2 acceptor and of the compounds belonging to the regenerative step.When leaves have been treated with CMU before an illumination (white light) in N2 atmosphere, increased levels of PGA are observed as compared with the preceding experiments, whereas those of DHAP and Fru 1-6 P2 are lower; in short-time experiments, the PGA is labelled to a much greater extent than Ru 1-5 P2 and DHAP. In this case the limiting steps of the CALVIN cycle are: a) the reductive step, affecting the levels of the intermediates of the regenerative step: DHAP, Fru 1-6 P2 and Ru 5P. b) the phosphorylative step, because of the inhibition of the non-cyclic photophosphorylations: there is a decrease of the ATP cellular level. In N2 atmosphere a far-red illumination has the same effect on pool-sizes and labelling of compounds as CMU in white light, owing to the inhibition of the reducing power.In N2-O2 atmosphere (white light), the PGA level is higher than in N2 atmosphere; the effect of O2 is discussed (stimulation of the oxydative pentose phosphate cycle?).A physiological part played by the important reservoir of PGA accumulating in vivo, especially in far-red light, is suggested in "Discussion".