The time course of acclimation to the stress of triose phosphate use limitation

Triose-phosphate utilization (TPU) limits the maximum rate at which plants
can photosynthesize. However, TPU is almost never found to be limiting
photosynthesis under ambient conditions for plants. This, along with
previous results showing adaptability of TPU at low temperature, suggest
that TPU capacity is regulated to be just above the photosynthetic rate
achievable under the prevailing conditions. A set of experiments were
performed to study the adaptability of TPU capacity when plants are
acclimated to elevated CO2 concentrations. Plants held at 1500 ppm CO2
were initially TPU-limited. After 30 hours, they no longer exhibited TPU
limitations, but they did not elevate their TPU capacity. Instead, the
maximum rates of carboxylation and electron transport declined. A time
course of regulatory responses was established. A step increase of CO2
first caused PSI to be oxidized, but after 40 s, both PSI and PSII had
excess electrons as a result of acceptor-side limitations. Electron flow
to PSI slowed, and the proton motive force increased. Eventually,
non-photochemical quenching reduced electron flow sufficiently to balance
the TPU limitation. Over several minutes rubisco deactivated contributing
to regulation of metabolism to overcome the TPU limitation.