PLANT PHYSIOLOGY , Vol 106, Issue 2 679-687, Copyright © 1994 by American Society of Plant Biologists
|
METABOLISM AND ENZYMOLOGY |
CO2 Uptake and Electron Transport Rates in Wild-Type and a Starchless Mutant of Nicotiana sylvestris (The Role and Regulation of Starch Synthesis at Saturating CO2 Concentrations)
H. Eichelmann and A. Laisk
Institute of Molecular and Cell Biology, Tartu University, 181 Riia Street, Tartu, Estonia EE2400
CO2 uptake rate, chlorophyll fluorescence, and 830-nm absorbance were
measured in wild-type (wt) Nicotiana sylvestris (Speg. et Comes) and
starchless 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. O2 enhancement
of photosynthesis at CO2 and light saturation was relatively much greater
in the mutant than in the wt. In 21% O2, the electron transport rate (ETR)
calculated from fluorescence peaked near the beginning of the CO2
saturation of photosynthesis. With the further increase of CO2
concentration ETR remained nearly constant or declined a little in the wt
but drastically declined in the mutant. Absorbance measurements at 830 nm
indicated photosystem I acceptor side reduction in both plants at
saturating CO2 and light. Assimilatory charge (postillumination CO2 uptake)
measurements indicated trapping of chloroplast inorganic phosphate,
supposedly in hexose phosphates, in the mutant. It is concluded that starch
synthesis gradually substitutes for photorespiration as electron acceptor
with increasing CO2 concentration in the wt but not in the mutant. It is
suggested that starch synthesis is co-controlled by the activity of the
chloroplast fructose bisphosphatase.
