Slow-onset regulatory processes control TPU limitation after a
period of acclimation
On the minutes timescale, TPU-limited photosynthesis is regulated by
rubisco deactivation, photosynthetic control at the cytochromeb6f complex, and qE .
Rubisco deactivation begins within minutes and persists for at least a
day (Fig. 3). Unlike photosynthetic control andqE , which are induced by acidification of the
thylakoid lumen, the mechanism of rubisco deactivation is unknown. Under
TPU-limiting conditions, ATP synthase is constricted (Kanazawa & Kramer
2002; Takizawa, Kanazawa & Kramer 2008; Kiirats et al. 2009)
probably due to low phosphate concentration, which leads to a lower
ATP/ADP ratio (Sharkey et al. 1986b; Stitt 1986; Furbank et
al. 1987) and therefore reduced rubisco activase activity. We measured
a reduction in total rubisco activity after activation with
6-phosphogluconate (Fig. 3b), which could be caused by tight binding
inhibitors (Keys, Major & Parry 1995; Paul et al. 1996; Parryet al. 1997). This can contribute to reduced rubisco activity.
Reversible deactivation of rubisco is the primary contributor to the
reduction in Vcmax measured over the course of
acclimation (Fig 1).
Over time, photoinhibition becomes responsible for dissipating more
excess energy, supplanting qE . Measured Jat 1000 µmol m-2 s-1 began
decreasing quickly and did not recover fully overnight (Fig. 1). In
addition, after acclimation, total NPQt was
higher at all levels of CO2, andNPQt did not increase at elevated
CO2. PMF (ECSt ) is overall lower
and has a reduced response to increasing CO2. This
indicates that qE is becoming less important in
energy flux compared to qi , especially in
response to TPU limitation. The NPQ must come from other sources, such
as quenching from photoinhibition or state transitions. State
transitions are somewhat limited in higher plants, with only 15-20% of
the light harvesting complex capable of relocation (Rochaix 2011), so
photoinhibition is the most likely cause. The energy dissipation due to
photoinhibition is enough to protect the photosystems, which makesqE unnecessary.
Acclimation to TPU limitation requires balancing of both carbon and
energy flux. At the end of acclimation, we found that energy flux is
balanced by photoinhibition, and that carbon flux is balanced by rubisco
deactivation. These two systems work synergistically. Rubisco
deactivation reduces the potential demand for ATP and NADPH when
CO2 fixation could exceed the potential for end-product
production. Control of electron transport by photoinhibition decreases
the potential to overload the electron transport chain from the
beginning. In this way, even though photoinhibition is rightly
considered a negative effect on the plant, it is effective in protecting
PSI; PSII is damaged, but there are effective repair mechanisms for PSII
(Ohad, Kyle & Arntzen 1984; Vass et al. 1992; Sonoike 1996).
These two effects combine to reduce pressure on inorganic phosphate
pools by reducing the potential use of phosphate from both sides.