光合产氧：新方法带来的遗忘通量（原文请自行下载或者来电索取，以下为部分内容截取）

Photosynthetic Oxygen Production New Method Brings to Light Forgotten flux

Oxygen (O2) is evolved during photosynthetic electron
transport when water is split by the oxygenevolving
complex to provide protons and electrons to
the chloroplastic electron chain, thereby generating
ATP and NADPH—the energy source and reducing
power for plant metabolism. The majority of this
chemical energy is used to drive photosynthetic carbon
metabolism, which consists of ribulose-1,5-bisphosphate
carboxylation (photosynthetic carbon reduction
cycle) and oxygenation (photosynthetic carbon oxidation
cycle); with a combined electron requirement = JA.
Four electrons are required for every O2 evolved so that
gross O2 production (GOP) is related to linear electron
transport (J) according to J/4. When linear electron
transport is used only to drive CO2
fixation, the consumption
of O2 and the release of CO2 by photosynthetic
carbon oxidation and mitochondrial respiration is
such that net O2 production (NOP) is equal to net CO2
assimilation (Anet; provided the respiratory quotient is
1, but see Tcherkez et al., 2017).
Additionally, electrons can be used for alternative
noncyclic electron transport (ANCET), including, for
example, the photoreduction of O2 itself forming reactive
oxygen species (Mehler-peroxidase reactions or
“water-water cycle”; Asada, 1999), chloroplastic anabolism
(e.g. lipids; Stumpf et al., 1963), the reduction of
oxaloacetate to malate (which is exported to the mitochondria;
Scheibe, 2004), and nitrogen assimilation
(Bloom et al., 1989). ANCET has been hypothesized
both as a way to regulate ATP/NADPH ratio to meet
the changing energy demands of cellular metabolism
and as a mechanism to prevent photodamage through
utilizing excess reductant when the photon flux density
exceeds the energy requirement of CO2
fixation (e.g.
under high irradiance, cold temperatures, water stress
closing stomata; e.g. Badger, 1985; Ort and Baker, 2002;
Robinson, 1988). Importantly, there is no formal evidence
for how electron flows interact, particularly under
fluctuating light conditions (Morales et al., 2018).
As ANCET allows for greater rates of linear electron
transport to be sustained, total electron transport (Jt)
will be greater than JA. Conversely, the effect on O2
uptake will be dependent on the metabolic pathway
involved. For example, in the Mehler-peroxidase reactions,
there is no net change in O2 so that NOP will remain
equal to Anet. But in the reduction of nitrate,
the ratio between N-linked O2 production and O2 consumption
is highly dependent on the amino acid synthesized
(Noctor and Foyer, 1998). In this case, NOPwill
not always equal Anet because O2 and CO2 may not be
balanced in metabolism(Skillman, 2008). Consequently,
concomitant measurements of CO2 and O2
fluxes are
important to the understanding of how plants regulate
the use of light energy, with different fates having very
different metabolic outcomes.
The earliest measurements of O2 evolution were unable
to distinguish GOP from uptake of O2 (Hill, 1937).
The mass spectrometry method established by Mehler
and Brown (1952) solved this problem by employing
O2 isotope tracers to independently monitor fluxes
of 16O2 and 18O2. In this method, pure 18O2 was supplied
to the gas headspace of a closed chamber, and the
decline in 18O2 was attributed to O2 uptake. O2 evolved
carries the same isotopic composition as the water from
which it is generated; in this case, the dominant isotope
in the water was 16O (Fig. 1). The 18O-labeling approach
was further applied to leaf disks (e.g. Tourneux and
Peltier, 1995), whole excised leaves (e.g. Volk and Jackson,
1972), and entire plants (Gerbaud and André, 1980),
illuminating the fate of O2 in vivo.
The limitation of closed gas exchange systems is that
measurements can only be undertaken for short periods
of time (seconds to minutes) before the CO2 concentration
is depleted. Consequently, CO2:O2 is not constant,
which changes the relative rates of carboxylation
and oxygenation so that estimates of GOP and O2 uptake
will be inaccurate. This limitation was overcome in
the mass spectrometry approach by replacing CO2
consumed through periodic influx of CO2 into the
chamber, allowing for steady-state quantification and
extending the ability to measure O2
fluxes under a
range of conditions and physiological states (Canvin
et al., 1980). At the same time, advances were being
made in the use of chlorophyll fluorescence, which
provides information on PSII quantum yield (Baker,
2008). Genty et al. (1989) provided the empirical link
between fluorescence and electron transport rate,
replacing the need to directly measure O2 evolution.
Chlorophyll fluorescence is now one of the most popular
techniques in plant physiology because of its ease
of use and relatively low cost. This has been aided by
the capacity to multiplex fluorescence measurements
with H2O and CO2 gas exchange in portable, commercially
available instruments, opening up the possibility
of measuring plant function outside of the laboratory.
Consequently, in vivo measurements of O2
fluxes have
substantially declined over the last 20 years.
In this issue of Plant Physiology, Gauthier et al. (2018)
remind us why it is so important to return our attention
to O2, providing us with a new, elegant open-path system
to measure O2
fluxes. Their method is a “reverse”
isotopic approach, involving 18O-labeling of leaf water
rather than the air so that the isotopic composition of O2
that is evolved during water splitting has a signature