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.