Fast onset kinetics in responses to TPU limitation are directed
by electron build-up on Qa
When plants were subjected to TPU-limiting conditions, the most
immediate effects were transient changes in the redox states of electron
transport components. It is known that while TPU-limited, increasing
CO2 levels cause a reduction inφII because, while A cannot increase, the
rate of photorespiration will go down (Stitt 1986; Sharkey et al.1988; Stitt & Grosse 1988). This, combined with the common observations
of elevated PMF and non-photochemical quenching during TPU
limitation, indicates the importance of qE in
dissipating absorbed light energy when electron transport capacity
exceeds TPU capacity. However, qE does not
activate instantaneously, with the xanthophyll cycle and PSBS
recruitment to the reaction center operating on the minutes timescale
(Li et al. 2002). Therefore, we could reasonably predict excess
accumulation of electrons on electron transport intermediates and PSI
electron acceptors. Reduction of Qa decreases the
quantum efficiency of photochemistry because PSII cannot accept any more
energy. The energy that would be going towards photochemistry is instead
shunted to nonphotochemical quenching, resulting in an increased yield
of nonphotochemical quenching. This means thatφNPQ increases even thoughNPQt changes on a slower timescale. Immediately
after entering TPU limitation, electrons build up on the electron
transport chain due to decreased electron sink strength, and the bulk of
the excess energy is most immediately handled by controls within the
electron transport chain.
Though the reduction of Qa reduces the yield of
photochemistry, the reduction of PSI following the imposition of TPU
limitation is more concerning. Acceptor-side limitation of PSI is highly
stressful due to the accumulation of ROS (Li, Wakao, Fischer & Niyogi
2009) and the inability of PSI to repair itself (Sonoike 1996, 2011).
Electron transfer to PSI from the cytochrome b6f complex is
slowed by elevated PMF due to the requirement to oxidize plastoquinol
(Kramer & Crofts 1993, 1996). We found, however, that PMF does not
build up fast enough to adjust to the limiting demand from the
Calvin-Benson cycle and regulate electron flow to PSI, and electrons do
indeed accumulate on PSI. This is not due to an accelerated rate of PSI
reduction through the cytochrome b6f complex
(ket , Fig. 4), so it must instead be due to an
acceptor side limitation of PSI. Increasing [CO2]
under TPU limitation reduces the rate of photorespiration, and ifA cannot increase due to TPU limitation the overall rate of
consumption of both ATP and NADPH decreases. The NADPH pool turnover
(half-time 0.01 s-1) is faster than that of ATP
(half-time 0.28 s-1, Arrivault et al. , 2009),
so the reduced consumption of electron transport products will affect
NADP+ availability first. Restriction of NADPH
oxidation has been suggested previously as the cause of oscillations in
TPU limitation (Furbank, Foyer & Walker 1987). The restriction of
NADP+ flux can be seen in the re-reduction of PSI
during a saturation flash at the point of greatest PSI reduction (Fig
5a). During this saturation flash, light is in excess of what is
required to oxidize PSI, and the only limitation would be the electron
carriers removing the electrons from PSI.
The accumulation of electrons on electron carriers of the electron
transport chain is resolved by slower regulation. PMF increases, causing
a decrease in ket and an increase inNPQt . As these slower control mechanisms take
hold, the transients in the other parameters slow and then stop. This is
one example of damped oscillations, commonly found associated with TPU
limitation (Ogawa 1982; Sivak & Walker 1986, 1987). The oscillations
are caused by perturbations in the electron requirements of the
Calvin-Benson cycle forcing Qa- based
control of electron transport; they are damped by the onset of PMF-based
controls of electron transport. Some, but not all measurements of
oscillations are consistent with the period and convergence rates in our
measurements of oscillations. We therefore propose that electron carrier
reduction as described here is responsible for some, but not all,
observations of oscillations in TPU limitation.
It has been repeatedly observed that TPU limitation is rare to
nonexistent in wild plants (Rogers et al. 2020), but can often be
found in experiments using high levels of light with elevated
CO2, decreased O2, and/or low
temperature. The most stressful moments will be when the plant enters
TPU limitation and electrons accumulate on PSI electron acceptors, and
therefore would be most stressful when the experimental design would
cause fluctuations in photosynthetic abilities. For example, when a
plant is held at low temperature, fluctuating light levels would
exacerbate the stressful TPU conditions. We believe this can be
stressful for plants in FACE experiments, where absolute consistency in
CO2 levels across the field and perfect mixing cannot be
reasonably expected. Allen et al. (2020) discussed the
difficulties in maintaining perfect mixing across a FACE field, and we
believe that the stress of entering and exiting TPU due to fluctuating
CO2 levels can be a drag on plant performance,
potentially reducing the expected stimulation of growth in high
CO2 FACE experiments (Long, Ainsworth, Leakey, Ort & No
2006).