Atmospheric CO2 Concentration and N Availability Affect the Balance of the Two Photosystems in Mature Leaves of Rice Plants Grown at a Free-Air CO2 Enrichment Site.

Hiroshi Ozaki,Hidemitsu Sakai,Takeshi Tokida,Hirofumi Nakamura,Toshihiro Hasegawa,Ko Noguchi

doi:10.3389/fpls.2020.00786

Abstract

Atmospheric CO2 concentration ([CO2]) has been substantially increasing. Responses of leaf photosynthesis to elevated [CO2] have been intensively investigated because leaf photosynthesis is one of the most important determinants of crop yield. The responses of photosynthesis to elevated [CO2] can depend on nitrogen (N) availability. Here, we aimed to investigate the significance of the appropriate balance between two photosystems [photosystem I (PSI) and photosystem II (PSII)] under various [CO2] and N levels, and thus to clarify if responses of photosynthetic electron transport rates (ETRs) of the two photosystems to elevated [CO2] are altered by N availability. Thus, we examined parameters of the two photosystems in mature leaves of rice plants grown under two [CO2] levels (ambient and 200 μmol mol–1 above ambient) and three N fertilization levels at the Tsukuba free-air CO2 enrichment experimental facility in Japan. Responses of ETR of PSII (ETRII) and ETR of PSI (ETRI) to [CO2] levels differed among N levels. When moderate levels of N were applied (MN), ETRI was higher under elevated [CO2], whereas at high levels of N were applied (HN), both ETRII and ETRI were lower under elevated [CO2] compared with ambient [CO2]. Under HN, the decreases in ETRII and ETRI under elevated [CO2] were due to increases in the non-photochemical quenching of PSII [Y(NPQ)] and the donor side limitation of PSI [Y(ND)], respectively. The relationship between the effective quantum yields of PSI [Y(I)] and PSII [Y(II)] changed under elevated [CO2] and low levels of N (LN). Under both conditions, the ratio of Y(I) to Y(II) was higher than under other conditions. The elevated [CO2] and low N changed the balance of the two photosystems. This change may be important because it can induce the cyclic electron flow around PSI, leading to induction of non-photochemical quenching to avoid photoinhibition.

Highlights

Atmospheric CO2 concentration ([CO2]) has increased substantially since the Industrial Revolution
We aimed to investigate the significance of the appropriate balance between two photosystems under various [CO2] and N levels, and addressed two following hypotheses. (i) The linear electron transport rate (ETR) responds to elevated [CO2], and its response depends on N availability. (ii) The response of photosystem I (PSI) to elevated [CO2] and N availability is different from the response of photosystem II (PSII), and the balance between the two photosystems differs depending on [CO2] or N availability
The results indicated that the balance between the two photosystems changed under elevated [CO2] and low levels of N (LN)

Summary

Introduction

Atmospheric CO2 concentration ([CO2]) has increased substantially since the Industrial Revolution. Plants that acclimate to long-term elevated [CO2] conditions show lower increases in photosynthesis and yield than expected (Long et al, 2004). Plants grown under elevated [CO2] often show a larger decrease in Vcmax compared with Jmax, the limitation of photosynthetic rates under elevated [CO2] will shift from Vcmax to Jmax. The long-term responses of Vcmax and Jmax to elevated [CO2], analyzed in soybean plants at FACE site, showed a shift of the limitation from Vcmax to Jmax (Bernacchi et al, 2005). Plants grown at the CO2 spring exhibited a larger decrease in Vcmax compared to Jmax (Saban et al, 2019)

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