3.4 Electrons flow to molecular oxygen under salt in both
C4 species
The electron generated by H2O splitting can be used by
alternative sinks, in addition to the common sink to support NADPH
generation. The most commonly known sinks are the reactions involving
oxygen, including photorespiration and Mehler reaction (Chen et al.,
2004; Shirao et al., 2013). To assess the relevance of these pathways,
the oxygen dependence of electron flow was performed. SV andSA were subjected to different Actinic Light at a range of
irradiance levels (AL, 0 to 1806 μmol m-2 s-1) at the presence of
saturating CO2 (2000 μL L-1) and
either 21% or 2% of O2. Regardless the degree of NaCl
treatments, the ERTII in both species reached its
maximum at around 400 μmol m-2 s-1(Figure 5A-B). Exposure of control plants to low oxygen (2%) resulted
in a decrease of PSII ETR at saturating irradiances.
Measurements of the redox state of the primary electron donor of PSI
(P700) revealed slight effects of oxygen concentration
in control plants. With increasing irradiance, P700became progressively more oxidized in both species (Figure 5C-D).
Despite the proportion of oxidized P700(P700+) was insensitive to the oxygen
concentration in control untreated plants, the conductance of the ETC
(gETC; Figure 5E-F) declined and the ETRI follows the
same trend and decreased by the same amount (Figure 5G-H).
SV exposed to 100 mM NaCl showed lower ETRII at
high CO2 than the control plants (Figure 5A). As in the
control, electron transport through PSII decreased slightly under low
O2 (Figure 5A). In contrast, exposure to NaCl resulted
in an increase in the ETRII in SA compared to
untreated plants; this increase in electron transfer through PSII was
completely revoked under low O2 concentration (Figure
5B).
The proportion of oxidized P700(P700+) in salt-treated SV was
significantly higher under low O2 (Figure 5C). This is
accompanied by to a negligible decline in the g ETC, resulted in a
slight increase in the electron flow through PSI potentially via
cyclization across FRQ (Figure 5G). Conversely, in SA , PSI ETR
(ETRI) in the absence or presence of salt (250 mM)
decreased at low O2 (Figure 5H). This caused by an
enhancement in P700 oxidation and a fall in
gETC (Figure 5D-F).