CO2 UPTAKE AND ELECTRON-TRANSPORT RATES IN WILD-TYPE AND A STARCHLESSMUTANT OF NICOTIANA-SYLVESTRIS - THE ROLE AND REGULATION OF STARCH SYNTHESIS AT SATURATING CO2 CONCENTRATIONS

CO2 uptake rate, chlorophyll fluorescence, and 830-nm absorbance were measured in wild-type (wt) Nicotiana sylvestris (Speg. et Comes) and s tarchless mutant NS 458 leaves at different light intensities and CO2 concentrations. Initial slopes of the relationships between CO2 uptake and light and CO2 were similar, but the maximum rate at CO2 and light saturation was only 30% in the mutant compared with the wt. O-2 enhan cement of photosynthesis at CO2 and tight saturation was relatively mu ch greater in the mutant than in the wt. In 21% O-2, the electron tran sport rate (ETR) calculated from fluorescence peaked near the beginnin g of the CO2 saturation of photosynthesis. With the further increase o f CO2 concentration ETR remained nearly constant or declined a little in the wt but drastically declined in the mutant. Absorbance measureme nts at 830 nm indicated photosystem I acceptor side reduction in both plants at saturating CO2 and light. Assimilatory charge (postilluminat ion CO2 uptake) measurements indicated trapping of chloroplast inorgan ic phosphate, supposedly in hexose phosphates, in the mutant. It is co ncluded that starch synthesis gradually substitutes for photorespirati on as electron acceptor with increasing CO2 concentration in the wt bu t not in the mutant. It is suggested that starch synthesis is co-contr olled by the activity of the chloroplast fructose bisphosphatase.