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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. [ABSTRACT FROM AUTHOR]

Enhancing crop yield response to elevated CO2 concentrations (E-[CO2]) is an important adaptation measure to climate change. A high-yielding indica rice cultivar "Takanari" has recently been identified as a potential candidate for high productivity in E-[CO2] resulting from its large sink and source capacities. To fully utilize these traits, nitrogen should play a major role, but it is unknown how N levels influence the yield response of Takanari to E-[CO2]. We therefore compared grain yield and quality of Takanari with those of Koshihikari, a standard japonica cultivar, in response to Free-Air CO2 enrichment (FACE, +200 μmol mol−1) under three N levels (0, 8, and 12 g m−2) over three seasons. The biomass of both cultivars increased under E-[CO2] at all N levels; however, the harvest index decreased under E-[CO2] in the N-limited treatment for Koshihikari but not for Takanari. The decreased harvest index of Koshihikari resulted from limited enhancement of spikelet number under N-limitation. In contrast, spikelet number increased in E-[CO2] in Takanari even without N application, resulting in significant yield enhancement, averaging 18% over 3 years, whereas Koshihikari exhibited virtually no increase in yield in E-[CO2] under the N-limited condition. Grain appearance quality of Koshihikari was severely reduced by E-[CO2], most notably in N-limited and hot conditions, by a substantial increase in chalky grain, but chalky grain % did not increase in E-[CO2] even without N fertilizer. These results indicated that Takanari could retain its high yield advantage over Koshihikari with limited increase in chalkiness even under limited N conditions and that it could be a useful genetic resource for improving N use efficiency under E-[CO2]. [ABSTRACT FROM AUTHOR]

Atmospheric CO 2 concentration ([CO 2 ]) has been substantially increasing. Responses of leaf photosynthesis to elevated [CO 2 ] have been intensively investigated because leaf photosynthesis is one of the most important determinants of crop yield. The responses of photosynthesis to elevated [CO 2 ] 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 [CO 2 ] and N levels, and thus to clarify if responses of photosynthetic electron transport rates (ETRs) of the two photosystems to elevated [CO 2 ] are altered by N availability. Thus, we examined parameters of the two photosystems in mature leaves of rice plants grown under two [CO 2 ] levels (ambient and 200 μmol mol -1 above ambient) and three N fertilization levels at the Tsukuba free-air CO 2 enrichment experimental facility in Japan. Responses of ETR of PSII (ETRII) and ETR of PSI (ETRI) to [CO 2 ] levels differed among N levels. When moderate levels of N were applied (MN), ETRI was higher under elevated [CO 2 ], whereas at high levels of N were applied (HN), both ETRII and ETRI were lower under elevated [CO 2 ] compared with ambient [CO 2 ]. Under HN, the decreases in ETRII and ETRI under elevated [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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., (Copyright © 2020 Ozaki, Tokida, Nakamura, Sakai, Hasegawa and Noguchi.)

Enhancing crop yield response to elevated CO 2 concentrations (E-[CO 2 ]) is an important adaptation measure to climate change. A high-yielding indica rice cultivar "Takanari" has recently been identified as a potential candidate for high productivity in E-[CO 2 ] resulting from its large sink and source capacities. To fully utilize these traits, nitrogen should play a major role, but it is unknown how N levels influence the yield response of Takanari to E-[CO 2 ]. We therefore compared grain yield and quality of Takanari with those of Koshihikari, a standard japonica cultivar, in response to Free-Air CO 2 enrichment (FACE, +200 μmol mol -1 ) under three N levels (0, 8, and 12 g m -2 ) over three seasons. The biomass of both cultivars increased under E-[CO 2 ] at all N levels; however, the harvest index decreased under E-[CO 2 ] in the N-limited treatment for Koshihikari but not for Takanari. The decreased harvest index of Koshihikari resulted from limited enhancement of spikelet number under N-limitation. In contrast, spikelet number increased in E-[CO 2 ] in Takanari even without N application, resulting in significant yield enhancement, averaging 18% over 3 years, whereas Koshihikari exhibited virtually no increase in yield in E-[CO 2 ] under the N-limited condition. Grain appearance quality of Koshihikari was severely reduced by E-[CO 2 ], most notably in N-limited and hot conditions, by a substantial increase in chalky grain, but chalky grain % did not increase in E-[CO 2 ] even without N fertilizer. These results indicated that Takanari could retain its high yield advantage over Koshihikari with limited increase in chalkiness even under limited N conditions and that it could be a useful genetic resource for improving N use efficiency under E-[CO 2 ].