Regulation of photosynthetic carbon assimilation.

It may be concluded that the conversion of PGA to DPGA plays a key role in induction and in the regulation of cycle activity. The high concentrations of PGA in actively photosynthesizing chloroplasts reflect this role and the control exerted by adenylate ratios. Thus the cycle can operate at its maximum rate only in the presence of high PGA and low ribulose 5-phosphate concentrations. Once induction is complete, the reductive pentose phosphate pathway will continue to function at its maximum rate if sink activity within the cytoplasm makes available sufficient Pi to support rapid export of triose phosphate. If triose phosphate tends to build up in the straoma, it will favor pentose monophosphate accumulation. A relative excess of ribulose 5-phosphate would, in turn, inhibit PGA reduction (and hence its own formation) by drawing too heavily on the available ATP.