Acknowledgements
This work was supported by the National Natural Science Foundation of
China (Grant No. 41530533). We thank Prof. Cailin Wang of Jiangsu
Academy of Agricultural Sciences for providing rice seeds.
1. Introduction
Atmospheric carbon dioxide concentration ([CO2])
increased to 411.4 ± 0.1 μmol mol-1 in 2019, which was
~47% higher than pre-industrial level. Meanwhile, the
annual average growth rate of [CO2] has accelerating
in the past ten years, from 1.61 μmol mol-1 in 2009 to
2.6 μmol mol-1 in 2019
(https://www.esrl.noaa.gov/).
Photosynthesis is the process by which plants, algae, cyanobacteria and
photosynthetic bacteria convert light energy into chemical energy that
supports their activities. Rising [CO2] promoted
plant photosynthesis and thereby enhanced biomass production and grain
yield of crops (Ainsworth & Long, 2004; Ainsworth & Rogers, 2007;
Leakery et al ., 2009). Photosystem II (PSII) is the first
integral membrane protein complex in the light-dependent reactions that
executes the initial reaction of photosynthesis in plants
(Taiz & Zeiger, 2010). Within PSII,
antenna complexes absorbed photons of light to power the transfer of
electron from the reaction center to the primary quinone acceptor of
PSII, QA (Kalaji et al ., 2014; Nobel, 2020). Electron transport
through PSII was most susceptible to environment changes (Taub et
al ., 2000). As a result, changes of activities of light-harvesting,
electron transport and energy-transduction altered metabolic potential
for photosynthesis (Lawlor &
Tezara, 2009).
The efficiency of an absorbed photon trapped by PSII reactor center
could be an estimate of the maximum photochemical efficiency of PSII
(Fv/Fm)
(Baker & Oxborough, 2004). The mean value of
Fv/Fm was 0.83~0.84 for
most C3 species (Björkman & Demmig, 1987; Pfündel, 1998), with lowered
values indicating stresses on plants (Maxwell & Johnson, 2000;
Lichtenthaler et al ., 2005), which may be caused by the decrease
in the fraction of PSII active reaction centers that are capable of
photochemistry and/or the increase in non-photochemical quenching (Baker
& Oxborough, 2004). The trapped exciton moved electron into the
electron transport chain further
QA-and ultimately led to CO2 fixation (Baker & Oxborough,
2004; Kalaji et al ., 2017). Quantum yield of electron transport
(φEo) expressed the
probability that an absorbed photon led to an electron transport further
QA- (Kalajiet al ., 2011; Holland et al ., 2016). Those processes were
at the beginning of the electron flow in photosynthesis (Strasseret al ., 2004), and higher efficiency promoted electron flow from
H2O to NADP+. Performance index
(PIABS) was a
powerful and integrative expression to investigate plants’ overall
photosynthetic performance, which covered energy conservation from the
absorption of photons by PSII to the reduction of intersystem electron
acceptors (Lepedus et al .,
2012; Kalaji et al ., 2018; Faseela et al ., 2019). Those
parameters mentioned above, calculated or inferred from chlorophyll
fluorescence measurements, were considered to be non-destructive and
effective tools and were widespread used to detect the response of PSII
efficiency to various environmental stresses (Strasser et al .,
2000; Baker, 2008; Kalaji et al ., 2011; Faseela et al .,
2019), such as high temperature, drought, salt and chemical influences
(e.g. Fricke & Peters, 2002; Feng et al ., 2014; Gao et
al ., 2016; Zhou et al ., 2018; Y. Li et al ., 2019).
Elevated [CO2] directly or indirectly impacted the
PSII performance (Zong et al ., 2014), and researchers (e.g.
Naumburg et al ., 2004; Ruhil et al ., 2015) were
increasingly concerned about that. Wheat grown under elevated
[CO2] maintained higher
Fv/Fm at
tillering and booting stages
(Shanmugam et al ., 2013), and elevated [CO2]
inducted Fv/Fm reduction at 25 days
after anthesis (Martínez-Carrasco et al ., 2005), and Robredoet al . (2010) found that Fv/Fm of
barley in well-irrigated was not affect by elevated
[CO2]. However, Zhu et al . (2018) reported
that elevated [CO2] had no effect on wheat
Fv/Fm, but decreased the probability
that trapped exciton moves electron into the electron transport chain
further QA- (ψo) and
φEo. Another study suggested that elevated
[CO2] had positive effect on
Fv/Fm of aged rice leaves (Ibarakiet al ., 2005).
Those researches have well studied the response of crop photosystem
activity to short-term (one growing season) increased
[CO2] by 200~400 μmol
mol-1, and the
fact that
[CO2] stepwise increases in a long-term was not
taken into consideration. For example, the annual average increment of
[CO2] in 2019 was 2.6 μmol mol-1even if the growth rate has been accelerated
(https://www.esrl.noaa.gov/). Klironomos (2005) observed
significant difference in below-ground plant production between abrupt
and stepwise [CO2] increase treatments. Moreover,
abundant researches have evidenced that long-term elevation of
[CO2] often reduce or remove the initial stimulation
of photosynthesis resulted from short-term
increase [CO2]
(Ainworth & Long, 2004; Long et al ., 2004, Ainsworth & Rogers
2007; Albert et al ., 2011). Furthermore, long-term elevation of
[CO2] led to larger enhancement in grain yield and
more reduction in grain quality than short-term increase
[CO2] (X. Li et al ., 2019). However,
long-term effects of elevated
[CO2] over multi-generations on PSII functionality
remain largely unknown, and similarities and differences of the PSII
response to stepwise and constant increase of [CO2]
are unclear.
In addition, effects of elevated [CO2] on PSII
efficiency was observed at different times of a day. For example,
Fv/Fm was detected at noon (12:00)
(Shanmugam et al ., 2013), during the afternoon (13:00-16:00)
(Ziska & Teramura,1992), and at an hour after sunset (Ibaraki et
al ., 2005). The Fv/Fm of rice showed
obvious diurnal variation (Li et al ., 2002; Panda. 2011). It is
possible that non-photochemical quenching can be completely relaxed and
photoinhibition can be reversed after overnight dark adaptation (Loganet al ., 1999; Demmig-Adams et al ., 2006). Therefore,
predawn observation was more appropriate if researchers focus on
long-term responses of plant to treatment (Kalaji et al ., 2014).
A meta-analysis showed that a small decrease in
Fv/Fm occurred when measured during the
diurnal period than predawn (Poorter et al ., 2019). However, it
was not well understood whether the effect of elevated
[CO2] on PSII efficiency in crop was consistent from
predawn to dusk in crop.
Rice is one of the three major food
crops in the world, which feeds more than half of the world’s
population. Increased food demand in the future requires more attention
to the study of the impact of elevated [CO2] on rice
growth and production.
Therefore, we set up two long-term treatments of elevated
[CO2] from 2016 to 2019 in rice growing seasons. One
treatment was a stepwise increase of +40 µmol mol-1per season, the other was a constant increase of +200 µmol
mol-1 for all seasons. We report the difference of the
effects between short-term and multi-generations
[CO2] elevation on rice PSII efficiency.