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-o) 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.