Characterization of the Formation and Distribution of Photosynthetic Products by Sedum praealtum Chloroplasts

Photoassimilation of ¹⁴CO₂ by intact chloroplasts from the Crassulacean acid metabolism plant Sedum praealtum was investigated. The main water-soluble, photosynthetic products were dihydroxyacetone phosphate (DHAP), glycerate 3-phosphate (PGA), and a neutral saccharide fraction. Only a minor amount of glycolate was produced. A portion of neutral saccharide synthesis was shown to result from extrachloroplastic contamination, and the nature of this contamination was investigated with light and electron microscopy. The amount of photoassimilated carbon partitioned into starch increased at both very low and high concentrations of orthophosphate. High concentrations of exogenous PGA also stimulated starch synthesis.

DHAP and PGA were the preferred forms of carbon exported to the medium, although indirect evidence suported hexose monophosphate export. The export of PGA and DHAP to the medium was stimulated by high exogenous orthophosphate, but depletion of chloroplastic reductive pentose phosphate intermediates did not occur. As a result only a relatively small inhibition in the rate of CO₂ assimilation occurred.

The rate of photoassimilation was stimulated by exogenous PGA, ribose 5-phosphate, fructose 1,6-bisphosphate, fructose 6-phosphate, and glucose 6-phosphate. Inhibition occurred with phosphoenolpyruvate and high concentrations of PGA and ribose 5-phosphate. PGA inhibition did not result from depletion of chloroplastic orthophosphate or from inhibition of ribulose 1,5-bisphosphate carboxylase. Exogenous PGA and phosphoenolpyruvate were shown to interact with the orthophosphate translocator.