Diurnal and Developmental Changes in Energy Allocation of Absorbed Light at PSII in Field-Grown Rice Satoshi Ishida1, Nozomu Uebayashi1, Youshi Tazoe1, Masahiro Ikeuchi1, Koki Homma2, Fumihiko Sato1 and Tsuyoshi Endo1,* Division of Integrated Life Sciences, Graduate School of Biostudies, Kyoto University, Kyoto, 606-8502 Japan Laboratory of Crop Science, Graduate School of Agriculture. Kyoto University, Kyoto, 606-8502 Japan *Corresponding author: E-mail, [email protected]; Fax, +81-75-753-6398. (Received May 21, 2013; Accepted November 12, 2013)
2
Keywords: Chl fluorescence Diurnal changes Energy allocation in PSII Non-photochemical quenching Photoinhibition Rice (Oryza sativa). Abbreviations: Fm (Fm0 , Fm00 ), maximum fluorescence obtained by a saturating light pulse at pre-dawn (or during daytime under light, or during daytime after a brief dark treatment); Fo (Fo0 ), minimum fluorescence obtained under the measuring light in the dark (or in the light); Fs, steadystate fluorescence under light; fast (Jfast), quantum yield (or energy flux) of dissipation associated with NPQ that relaxed rapidly in the dark; f,D (Jf,D), quantum yield (or energy flux) of basic dissipation in PSII; NPQ (JNPQ), quantum yield (or energy flux) of dissipation associated with NPQ; PSII (JPSII), quantum yield (or flow rate) of electron transport in PSII;
slow (Jslow), the quantum yield (or energy flux) of dissipation associated with NPQ that relaxed slowly in the dark; NPQ, non-photochemical quenching of Chl fluorescence; PAR, photosynthetically active radiation; qP, fluorescence parameters that indicate the fraction of open PSII centers.
Introduction In plants, only some of the absorbed light energy is used for photosynthetic electron transport in PSII, and a large portion of the energy is lost through regulatory thermal dissipation, which can be visualized as non-photochemical quenching (NPQ) based on a quenching analysis of Chl fluorescence (Schreiber et al. 1986). NPQ-associated thermal dissipation has been shown to be an essential photoprotective mechanism of PSII. However, it has not been clear how large a portion of light energy absorbed in PSII is dissipated through this mechanism, since the original parameters for non-photochemical quenching, such as qN = 1 – (Fm0 – Fo0 )/(Fm – Fo) and NPQ = (Fm – Fm0 )/Fm0 , are not based on the quantum yield, and thus they cannot be compared quantitatively with the quantum yield of electron transport expressed as PSII = (Fm0 – Fs)/Fm0 (Genty et al. 1989). Therefore, attempts have been made to simulate the dissipation associated with NPQ on the basis of quantum yield. The first of such attempts was reported by DemmigAdams et al. (1996) based on the puddle model of energy transfer, in which the light energy absorbed in antennae Chl is always transferred to the same reaction centers. Later, new simulations based on the lake model of energy transfer, in which the excitation energy of Chls can be exchanged among reaction centers, were proposed independently by Kramer et al. (2004) and Hendrickson et al. (2004). NPQ-associated dissipation can be further divided into subcategories because NPQ has been shown to be induced by at least three independent mechanisms: qE (energy quenching), qT (NPQ associated with a state transition) and qI (NPQ associated with photoinhibition) (Quick and Stitt 1989). The most
Plant Cell Physiol. 55(1): 171–182 (2014) doi:10.1093/pcp/pct169, available online at www.pcp.oxfordjournals.org ! The Author 2013. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: [email protected]
Plant Cell Physiol. 55(1): 171–182 (2014) doi:10.1093/pcp/pct169 ! The Author 2013.
171
Downloaded from http://pcp.oxfordjournals.org/ at University of Texas at Austin on June 5, 2014
The allocation of absorbed light energy in PSII to electron transport and heat dissipation processes in rice grown under waterlogged conditions was estimated with the lake model of energy transfer. With regard to diurnal changes in energy allocation, the peak of the energy flux to electron transport, JPSII, occurred in the morning and the peak of the energy flux to heat dissipation associated with non-photochemical quenching of Chl fluorescence, JNPQ, occurred in the afternoon. With regard to seasonal changes in energy allocation, JPSII in the rapidly growing phase was greater than that in the ripening phase, even though the leaves of rice receive less light in the growing phase than in the ripening period in Japan. This seasonal decrease in JPSII was accompanied by an increase in JNPQ. One of the reasons for the lower JPSII in the ripening phase might be a more sever afternoon suppression of JPSII. To estimate energy dissipation due to photoinhibition of PSII, JNPQ was divided into Jfast, which is associated with fast-recovering NPQ mainly due to qE, and Jslow, which is mainly due to photoinhibition. The integrated daily energy loss by photoinhibiton was calculated to be about 3–8% of light energy absorption in PSII. Strategies for the utilization of light energy adopted by rice are discussed. For example, very efficient photosynthesis under non-saturating light in the rapidly growing phase is proposed.
Regular Paper
1
S. Ishida et al.
Results Dark recovery of NPQ in field-grown rice To estimate the degree of photoinhibition during the daytime, rice plants under full sunlight were dark adapted, then Fv/Fm was measured (Fig. 1). To distinguish this from pre-dawn Fv/Fm, the maximum quantum yield thus measured during daytime was expressed as Fv00 /Fm00 . Recovery of NPQ, i.e. an increase in
172
Fig. 1 Recovery of Fv00 /Fm00 during dark treatment in rice leaves. Rice plants were cultivated in pots for 6 weeks in a greenhouse. At noon on a sunny day in July, plants were moved to a dark room and further exposed to 1,100 mmol m2 s1 white light, which corresponded to the light intensity of full sunlight in this season (see Figs. 2, 10), for 1 h to induce uniform photoinhibition among individual leaves (n = 8) used for the measurements. The plants were then dark treated, and Fv00 /Fm00 values were measured repeatedly.
Fv00 /Fm00 , during the dark adaptation, appeared biphasic, as found in other plants. The first phase is generally regarded as the recovery process of energy quenching (qE) and the slow phase as that of photoinhibition (qI) (Quick and Stitt 1989). From the recovery curve of NPQ, a part of NPQ, which was found after 20 min dark adaptation, can represent slowly reversible NPQ. Therefore, in this study, we define photoinhibion as light-induced inactivation of PSII, which can be visualized as the remaining part of NPQ after 20 min dark adaptation. ‘Photoinhibition’ thus defined was an indicator of the balance between light-induced inactivation of PSII centers and reconstruction of the damaged PSII complex.
Diurnal changes in photoinhibition To estimate photoinhibition in field conditions, diurnal changes in Fv00 /Fm00 in sunlit upright (vertical) leaves of rice were examined in waterlogged field on July 22, 23 and 31, and August 5 (rapidly growing phase before heading) and September 11 and 18 (ripening phase) in 2010. The youngest fully expanded leaves were chosen for the measurements. Heading occurred on August 16. We chose south-facing upright leaves, which represented the major portion of actively photosynthesizing leaves, for the measurements. Typical diurnal changes in Fv00 /Fm00 are shown in Fig. 2, and the data for other days are shown in Supplementary Fig. S1. Values of Fv00 /Fm00 decreased by 6–10% at around noon. On most days, the peak of photoinhibition corresponded well with the peak of photosynthetically active radiation (PAR). Significant photoinhibition occurred even when the maximum light intensity at noon was
2
Keywords: Chl fluorescence Diurnal changes Energy allocation in PSII Non-photochemical quenching Photoinhibition Rice (Oryza sativa). Abbreviations: Fm (Fm0 , Fm00 ), maximum fluorescence obtained by a saturating light pulse at pre-dawn (or during daytime under light, or during daytime after a brief dark treatment); Fo (Fo0 ), minimum fluorescence obtained under the measuring light in the dark (or in the light); Fs, steadystate fluorescence under light; fast (Jfast), quantum yield (or energy flux) of dissipation associated with NPQ that relaxed rapidly in the dark; f,D (Jf,D), quantum yield (or energy flux) of basic dissipation in PSII; NPQ (JNPQ), quantum yield (or energy flux) of dissipation associated with NPQ; PSII (JPSII), quantum yield (or flow rate) of electron transport in PSII;
slow (Jslow), the quantum yield (or energy flux) of dissipation associated with NPQ that relaxed slowly in the dark; NPQ, non-photochemical quenching of Chl fluorescence; PAR, photosynthetically active radiation; qP, fluorescence parameters that indicate the fraction of open PSII centers.
Introduction In plants, only some of the absorbed light energy is used for photosynthetic electron transport in PSII, and a large portion of the energy is lost through regulatory thermal dissipation, which can be visualized as non-photochemical quenching (NPQ) based on a quenching analysis of Chl fluorescence (Schreiber et al. 1986). NPQ-associated thermal dissipation has been shown to be an essential photoprotective mechanism of PSII. However, it has not been clear how large a portion of light energy absorbed in PSII is dissipated through this mechanism, since the original parameters for non-photochemical quenching, such as qN = 1 – (Fm0 – Fo0 )/(Fm – Fo) and NPQ = (Fm – Fm0 )/Fm0 , are not based on the quantum yield, and thus they cannot be compared quantitatively with the quantum yield of electron transport expressed as PSII = (Fm0 – Fs)/Fm0 (Genty et al. 1989). Therefore, attempts have been made to simulate the dissipation associated with NPQ on the basis of quantum yield. The first of such attempts was reported by DemmigAdams et al. (1996) based on the puddle model of energy transfer, in which the light energy absorbed in antennae Chl is always transferred to the same reaction centers. Later, new simulations based on the lake model of energy transfer, in which the excitation energy of Chls can be exchanged among reaction centers, were proposed independently by Kramer et al. (2004) and Hendrickson et al. (2004). NPQ-associated dissipation can be further divided into subcategories because NPQ has been shown to be induced by at least three independent mechanisms: qE (energy quenching), qT (NPQ associated with a state transition) and qI (NPQ associated with photoinhibition) (Quick and Stitt 1989). The most
Plant Cell Physiol. 55(1): 171–182 (2014) doi:10.1093/pcp/pct169, available online at www.pcp.oxfordjournals.org ! The Author 2013. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: [email protected]
Plant Cell Physiol. 55(1): 171–182 (2014) doi:10.1093/pcp/pct169 ! The Author 2013.
171
Downloaded from http://pcp.oxfordjournals.org/ at University of Texas at Austin on June 5, 2014
The allocation of absorbed light energy in PSII to electron transport and heat dissipation processes in rice grown under waterlogged conditions was estimated with the lake model of energy transfer. With regard to diurnal changes in energy allocation, the peak of the energy flux to electron transport, JPSII, occurred in the morning and the peak of the energy flux to heat dissipation associated with non-photochemical quenching of Chl fluorescence, JNPQ, occurred in the afternoon. With regard to seasonal changes in energy allocation, JPSII in the rapidly growing phase was greater than that in the ripening phase, even though the leaves of rice receive less light in the growing phase than in the ripening period in Japan. This seasonal decrease in JPSII was accompanied by an increase in JNPQ. One of the reasons for the lower JPSII in the ripening phase might be a more sever afternoon suppression of JPSII. To estimate energy dissipation due to photoinhibition of PSII, JNPQ was divided into Jfast, which is associated with fast-recovering NPQ mainly due to qE, and Jslow, which is mainly due to photoinhibition. The integrated daily energy loss by photoinhibiton was calculated to be about 3–8% of light energy absorption in PSII. Strategies for the utilization of light energy adopted by rice are discussed. For example, very efficient photosynthesis under non-saturating light in the rapidly growing phase is proposed.
Regular Paper
1
S. Ishida et al.
Results Dark recovery of NPQ in field-grown rice To estimate the degree of photoinhibition during the daytime, rice plants under full sunlight were dark adapted, then Fv/Fm was measured (Fig. 1). To distinguish this from pre-dawn Fv/Fm, the maximum quantum yield thus measured during daytime was expressed as Fv00 /Fm00 . Recovery of NPQ, i.e. an increase in
172
Fig. 1 Recovery of Fv00 /Fm00 during dark treatment in rice leaves. Rice plants were cultivated in pots for 6 weeks in a greenhouse. At noon on a sunny day in July, plants were moved to a dark room and further exposed to 1,100 mmol m2 s1 white light, which corresponded to the light intensity of full sunlight in this season (see Figs. 2, 10), for 1 h to induce uniform photoinhibition among individual leaves (n = 8) used for the measurements. The plants were then dark treated, and Fv00 /Fm00 values were measured repeatedly.
Fv00 /Fm00 , during the dark adaptation, appeared biphasic, as found in other plants. The first phase is generally regarded as the recovery process of energy quenching (qE) and the slow phase as that of photoinhibition (qI) (Quick and Stitt 1989). From the recovery curve of NPQ, a part of NPQ, which was found after 20 min dark adaptation, can represent slowly reversible NPQ. Therefore, in this study, we define photoinhibion as light-induced inactivation of PSII, which can be visualized as the remaining part of NPQ after 20 min dark adaptation. ‘Photoinhibition’ thus defined was an indicator of the balance between light-induced inactivation of PSII centers and reconstruction of the damaged PSII complex.
Diurnal changes in photoinhibition To estimate photoinhibition in field conditions, diurnal changes in Fv00 /Fm00 in sunlit upright (vertical) leaves of rice were examined in waterlogged field on July 22, 23 and 31, and August 5 (rapidly growing phase before heading) and September 11 and 18 (ripening phase) in 2010. The youngest fully expanded leaves were chosen for the measurements. Heading occurred on August 16. We chose south-facing upright leaves, which represented the major portion of actively photosynthesizing leaves, for the measurements. Typical diurnal changes in Fv00 /Fm00 are shown in Fig. 2, and the data for other days are shown in Supplementary Fig. S1. Values of Fv00 /Fm00 decreased by 6–10% at around noon. On most days, the peak of photoinhibition corresponded well with the peak of photosynthetically active radiation (PAR). Significant photoinhibition occurred even when the maximum light intensity at noon was
