Publications by authors named "Riichi Oguchi"

23 Publications

  • Page 1 of 1

A new method for separate evaluation of PSII with inactive oxygen evolving complex and active D1 by the pulse-amplitude modulated chlorophyll fluorometry.

Funct Plant Biol 2021 Sep 13. Epub 2021 Sep 13.

A method that separately quantifies the PSII with inactive oxygen-evolving complex (OEC) and active D1 retaining the primary quinone acceptor (QA)-reducing activity from the PSII with damaged D1 in the leaf was developed using PAM fluorometry. It is necessary to fully reduce QA to obtain Fm, the maximum fluorescence. However, QA in PSII with inactive OEC and active D1 would not be fully reduced by a saturating flash. We used the acceptor-side inhibitor DCMU to fully reduce QA. Leaves of cucumber (Cucumis sativus L.) were chilled at 4°C in dark or illuminated with UV-A to selectively inactivate OEC. After these treatments, Fv/Fm, the maximum quantum yield, in the leaves vacuum-infiltrated with DCMU were greater than those in water-infiltrated leaves. In contrast, when the leaves were illuminated by red light to photodamage D1, Fv/Fm did not differ between DCMU- and water-infiltrated leaves. These results indicate relevance of the present evaluation of the fraction of PSII with inactive OEC and active D1. Several examinations in the laboratory and glasshouse showed that PSII with inactive OEC and active D1 was only rarely observed. The present simple method would serve as a useful tool to clarify the details of the PSII photoinhibition.
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http://dx.doi.org/10.1071/FP21073DOI Listing
September 2021

Wah Soon Chow, a teacher, a friend and a colleague.

Photosynth Res 2021 Aug;149(1-2):253-258

Climate Change Cluster (C3), University of Technology Sydney, Sydney, NSW, 2007, Australia.

To finish this special issue, some friends, colleagues and students of Prof. Chow (Emeritus Professor, the Research School of Biology, the Australian National University) have written small tributes to acknowledge not only his eminent career but to describe his wonderful personality.
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http://dx.doi.org/10.1007/s11120-021-00864-wDOI Listing
August 2021

The effect of different spectral light quality on the photoinhibition of Photosystem I in intact leaves.

Photosynth Res 2021 Aug 6;149(1-2):83-92. Epub 2021 Jan 6.

Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT, 2601, Australia.

Light energy causes damage to Photosystem I (PSI) and Photosystem II (PSII). The majority of the previous photoinhibition studies have been conducted with PSII, which shows much larger photoinhibition than PSI; therefore, relatively little is known about the mechanism of PSI photoinhibition so far. A previous report showed that the photoinhibition action spectrum measured with PSI activity of isolated thylakoid is similar to the absorption spectrum of chlorophyll. However, it is known that the extent of PSI photoinhibition is much smaller in vivo compared to in vitro. It is also possible that the different extent of PSII photoinhibition, caused by different spectral light qualities, can affect the photoinhibition of PSI in vivo because PSI receives electrons from PSII. In the present research, to study the effect of light quality and the effect of the extent of PSII photoinhibition on the PSI photoinhibition in vivo, intact leaves were photoinhibited under four different light qualities. The rate coefficient of PSI photoinhibition was significantly higher in blue and red light compared to white light. The rate of PSI photoinhibition at the same photon-exposure was the largest in blue and red light and followed by white and green light. These results support the notion that light absorption by chlorophyll is responsible for the PSI photoinhibition, even in intact leaves. The variation among light colors in the relationships between the extent of photoinhibition of PSII and that of PSI indicate that PSI and PSII are independently photoinhibited with different mechanisms in the early stage of in vivo photoinhibition.
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http://dx.doi.org/10.1007/s11120-020-00805-zDOI Listing
August 2021

Functional shifts in leaves of woody invaders of deciduous forests between their home and away ranges.

Tree Physiol 2019 09;39(9):1551-1560

Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi, Japan.

Temperate forests are widely invaded by shade-tolerant shrubs and trees, including those of Eastern North America (ENA). However, it remains unknown whether these invaders are 'preadapted' for success in their new ranges due to unique aspects of their evolutionary history or whether selection due to enemy release or other postintroduction processes have driven rapid evolution in the invaded range. We sampled leaf traits of populations of woody understory invaders across light gradients in their native range in Japan and in their invaded ENA range to examine potential phenotypic shifts related to carbon gain and nitrogen use between ranges. We also measured leaf traits in three co-occurring ENA native shrub species. In their invaded range, invaders invested significantly less in leaf chlorophyll content (both per unit leaf mass and area) compared with native range populations of the same species, yet maintained similar rates of photosynthesis in low light. In addition, compared with ENA natives, ENA invaders displayed greater trait variation in response to increasing light availability (forest edges, gaps), giving them a potential advantage over ENA natives in a variety of light conditions. We conclude that, for this group of species, newly evolved phenotypes in the invaded range are more important than preadaptation for their success as shade-tolerant forest invaders.
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http://dx.doi.org/10.1093/treephys/tpz065DOI Listing
September 2019

Dependence of functional traits related to growth rates and their CO response on multiple habitat climate factors across Arabidopsis thaliana populations.

J Plant Res 2018 Nov 25;131(6):987-999. Epub 2018 Jul 25.

Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi, 980-8578, Japan.

The values of many plant traits are often different even within a species as a result of local adaptation. Here, we studied how multiple climate variables influence trait values in Arabidopsis thaliana grown under common conditions. We examined 9 climate variables and 29 traits related to vegetative growth rate in 44 global A. thaliana accessions grown at ambient or elevated CO concentration ([CO]) and applied a multiple regression analysis. We found that genetic variations in the traits related to growth rates were associated with various climate variables. At ambient [CO], plant size was positively correlated with precipitation in the original habitat. This may be a result of larger biomass investment in roots at the initial stage in plants adapting to a lower precipitation. Stomatal conductance and photosynthetic nitrogen use efficiency were negatively correlated with vapor pressure deficit, probably as a result of the trade-off between photosynthetic water- and nitrogen-use efficiency. These results suggest that precipitation and air humidity influence belowground and aboveground traits, respectively. Elevated [CO] altered climate dependences in some of the studied traits. The CO response of relative growth rate was negatively correlated with altitude, indicating that plants inhabiting a higher altitude have less plasticity to changing [CO]. These results are useful not only for understanding evolutionary process but also to predict the plant species that are favored under future global change.
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http://dx.doi.org/10.1007/s10265-018-1058-1DOI Listing
November 2018

Mutant selection in the self-incompatible plant radish ( L. ) using two-step TILLING.

Breed Sci 2017 Jun 31;67(3):268-276. Epub 2017 May 31.

Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan.

Radish ( L. var. ), a widely cultivated root vegetable crop, possesses a large sink organ (the root), implying that photosynthetic activity in radish can be enhanced by altering both the source and sink capacity of the plant. However, since radish is a self-incompatible plant, improved mutation-breeding strategies are needed for this crop. TILLING (Targeting Induced Local Lesions IN Genomes) is a powerful method used for reverse genetics. In this study, we developed a new TILLING strategy involving a two-step mutant selection process for mutagenized radish plants: the first selection is performed to identify a BCM line, that is, progenies of M plants crossed with wild-type, and the second step is performed to identify BCM individuals with mutations. We focused on Rubisco as a target, since Rubisco is the most abundant plant protein and a key photosynthetic enzyme. We found that the radish genome contains six genes and one pseudogene encoding small Rubisco subunits. We screened 955 EMS-induced BCM lines using our newly developed TILLING strategy and obtained six mutant lines for the six genes, encoding proteins with four different types of amino acid substitutions. Finally, we selected a homozygous mutant and subjected it to physiological measurements.
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http://dx.doi.org/10.1270/jsbbs.16200DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5515317PMC
June 2017

The effect of interspecific variation in photosynthetic plasticity on 4-year growth rate and 8-year survival of understorey tree seedlings in response to gap formations in a cool-temperate deciduous forest.

Tree Physiol 2017 08;37(8):1113-1127

Graduate School of Life Sciences, Tohoku University, 6-3, Aramaki Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan.

Gap formation increases the light intensity in the forest understorey. The growth responses of seedlings to the increase in light availability show interspecific variation, which is considered to promote biodiversity in forests. At the leaf level, some species increase their photosynthetic capacity in response to gap formation, whereas others do not. Here we address the question of whether the interspecific difference in the photosynthetic response results in the interspecific variation in the growth response. If so, the interspecific difference in photosynthetic response would also contribute to species coexistence in forests. We also address the further relevant question of why some species do not increase their photosynthetic capacity. We assumed that some cost of photosynthetic plasticity may constrain acquisition of the plasticity in some species, and hypothesized that species with larger photosynthetic plasticity exhibit better growth after gap formation and lower survivorship in the shade understorey of a cool-temperate deciduous forest. We created gaps by felling canopy trees and studied the relationship between the photosynthetic response and the subsequent growth rate of seedlings. Naturally growing seedlings of six deciduous woody species were used and their mortality was examined for 8 years. The light-saturated rate of photosynthesis (Pmax) and the relative growth rate (RGR) of the seedlings of all study species increased at gap plots. The extent of these increases varied among the species. The stimulation of RGR over 4 years after gap formation was strongly correlated with change in photosynthetic capacity of newly expanded leaves. The increase in RGR and Pmax correlated with the 8-year mortality at control plots. These results suggest a trade-off between photosynthetic plasticity and the understorey shade tolerance. Gap-demanding species may acquire photosynthetic plasticity, sacrificing shade tolerances, whereas gap-independent species may acquire shade tolerances, sacrificing photosynthetic plasticity. This strategic difference among species would contribute to species coexistence in cool-temperate deciduous forests.
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http://dx.doi.org/10.1093/treephys/tpx042DOI Listing
August 2017

Light environment within a leaf. II. Progress in the past one-third century.

J Plant Res 2016 May 10;129(3):353-63. Epub 2016 Mar 10.

Graduate School of Life Sciences, Tohoku University, 6-3 Aoba, Sendai, 980-8578, Japan.

Studies directly related to light environments within a leaf, conduced mainly in the past one-third century, are reviewed. In particular, studies that revealed the profiles of light absorption and photosynthetic capacity are highlighted. Progress in this research field has been accelerated by devising innovative techniques. Roles of the main photosynthetic tissues, the palisade and spongy tissues, as the light guide and diffuser, respectively, are discussed. When the leaf is illuminated with diffuse light, light is absorbed more by the chloroplasts located near the illuminated surface. The meanings of the occupation of the mesophyll surfaces facing the intercellular spaces by chloroplasts and chloroplast movement are also discussed. The discrepancy between the light absorption profile and that of photosynthetic capacity is examined most intensively.
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http://dx.doi.org/10.1007/s10265-016-0808-1DOI Listing
May 2016

Obstacles in the quantification of the cyclic electron flux around Photosystem I in leaves of C3 plants.

Photosynth Res 2016 Sep 4;129(3):239-51. Epub 2016 Feb 4.

Division of Plant Sciences, Research School of Biology, The Australian National University, 46 Sullivans Creek Road, Acton, ACT, 2601, Australia.

Sixty years ago Arnon and co-workers discovered photophosphorylation driven by a cyclic electron flux (CEF) around Photosystem I. Since then understanding the physiological roles and the regulation of CEF has progressed, mainly via genetic approaches. One basic problem remains, however: quantifying CEF in the absence of a net product. Quantification of CEF under physiological conditions is a crucial prerequisite for investigating the physiological roles of CEF. Here we summarize current progress in methods of CEF quantification in leaves and, in some cases, in isolated thylakoids, of C3 plants. Evidently, all present methods have their own shortcomings. We conclude that to quantify CEF in vivo, the best way currently is to measure the electron flux through PS I (ETR1) and that through PS II and PS I in series (ETR2) for the whole leaf tissue under identical conditions. The difference between ETR1 and ETR2 is an upper estimate of CEF, mainly consisting, in C3 plants, of a major PGR5-PGRL1-dependent CEF component and a minor chloroplast NDH-dependent component, where PGR5 stands for Proton Gradient Regulation 5 protein, PGRL1 for PGR5-like photosynthesis phenotype 1, and NDH for Chloroplast NADH dehydrogenase-like complex. These two CEF components can be separated by the use of antimycin A to inhibit the former (major) component. Membrane inlet mass spectrometry utilizing stable oxygen isotopes provides a reliable estimation of ETR2, whilst ETR1 can be estimated from a method based on the photochemical yield of PS I, Y(I). However, some issues for the recommended method remain unresolved.
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http://dx.doi.org/10.1007/s11120-016-0223-4DOI Listing
September 2016

Which plant trait explains the variations in relative growth rate and its response to elevated carbon dioxide concentration among Arabidopsis thaliana ecotypes derived from a variety of habitats?

Oecologia 2016 Mar 22;180(3):865-76. Epub 2015 Oct 22.

Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi, 980-8578, Japan.

Elevated atmospheric carbon dioxide (CO2) concentration ([CO2]) enhances plant growth, but this enhancement varies considerably. It is still uncertain which plant traits are quantitatively related to the variation in plant growth. To identify the traits responsible, we developed a growth analysis model that included primary parameters associated with morphology, nitrogen (N) use, and leaf and root activities. We analysed the vegetative growth of 44 ecotypes of Arabidopsis thaliana L. grown at ambient and elevated [CO2] (800 μmol mol(-1)). The 44 ecotypes were selected such that they were derived from various altitudes and latitudes. Relative growth rate (RGR; growth rate per unit plant mass) and its response to [CO2] varied by 1.5- and 1.7-fold among ecotypes, respectively. The variation in RGR at both [CO2]s was mainly explained by the variation in leaf N productivity (LNP; growth rate per leaf N),which was strongly related to photosynthetic N use efficiency (PNUE). The variation in the response of RGR to [CO2] was also explained by the variation in the response of LNP to [CO2]. Genomic analyses indicated that there was no phylogenetic constraint on inter-ecotype variation in the CO2 response of RGR or LNP. We conclude that the significant variation in plant growth and its response to [CO2] among ecotypes reflects the variation in N use for photosynthesis among ecotypes, and that the response of PNUE to CO2 is an important target for predicting and/or breeding plants that have high growth rates at elevated [CO2].
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http://dx.doi.org/10.1007/s00442-015-3479-zDOI Listing
March 2016

Estimation of the steady-state cyclic electron flux around PSI in spinach leaf discs in white light, CO-enriched air and other varied conditions.

Funct Plant Biol 2013 Oct;40(10):1018-1028

Division of Plant Science, Research School of Biology, College of Medicine, Biology and Environment, The Australian National University, Canberra, ACT 0200, Australia.

Cyclic electron flux (CEF) around PSI is essential for efficient photosynthesis and aids photoprotection, especially in stressful conditions, but the difficulty in quantifying CEF is non-trivial. The total electron flux through PSI (ETR1) and the linear electron flux (LEFO2) through both photosystems in spinach leaf discs were estimated from the photochemical yield of PSI and the gross oxygen evolution rate, respectively, in CO2-enriched air. ΔFlux=ETR1 - LEFO2 is an upper estimate of CEF. Infiltration of leaf discs with 150μM antimycin A did not affect LEFO2, but decreased ΔFlux 10-fold. ΔFlux was practically negligible below 350μmolphotonsm-2s-1, but increased linearly above it. The following results were obtained at 980μmolphotonsm-2s-1. ΔFlux increased 3-fold as the temperature increased from 5°C to 40°C. It did not decline at high temperature, even when LEFO2 decreased. ΔFlux increased by 80% as the relative water content of leaf discs decreased from 100 to 40%, when LEFO2 decreased 2-fold. The method of using ΔFlux as a non-intrusive upper estimate of steady-state CEF in leaf tissue appears reasonable when photorespiration is suppressed.
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http://dx.doi.org/10.1071/FP13010DOI Listing
October 2013

Cotton bracts are adapted to a microenvironment of concentrated CO2 produced by rapid fruit respiration.

Ann Bot 2013 Jul 26;112(1):31-40. Epub 2013 Apr 26.

The Key Laboratory of Oasis Eco-agriculture, Xinjiang Production and Construction Group, Shihezi University, Shihezi 832003, China.

Background And Aims: Elucidation of the mechanisms by which plants adapt to elevated CO2 is needed; however, most studies of the mechanisms investigated the response of plants adapted to current atmospheric CO2. The rapid respiration rate of cotton (Gossypium hirsutum) fruits (bolls) produces a concentrated CO2 microenvironment around the bolls and bracts. It has been observed that the intercellular CO2 concentration of a whole fruit (bract and boll) ranges from 500 to 1300 µmol mol(-1) depending on the irradiance, even in ambient air. Arguably, this CO2 microenvironment has existed for at least 1·1 million years since the appearance of tetraploid cotton. Therefore, it was hypothesized that the mechanisms by which cotton bracts have adapted to elevated CO2 will indicate how plants will adapt to future increased atmospheric CO2 concentration. Specifically, it is hypothesized that with elevated CO2 the capacity to regenerate ribulose-1,5-bisphosphate (RuBP) will increase relative to RuBP carboxylation.

Methods: To test this hypothesis, the morphological and physiological traits of bracts and leaves of cotton were measured, including stomatal density, gas exchange and protein contents.

Key Results: Compared with leaves, bracts showed significantly lower stomatal conductance which resulted in a significantly higher water use efficiency. Both gas exchange and protein content showed a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio (Jmax/Vcmax) in bracts than in leaves.

Conclusions: These results agree with the theoretical prediction that adaptation of photosynthesis to elevated CO2 requires increased RuBP regeneration. Cotton bracts are readily available material for studying adaption to elevated CO2.
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http://dx.doi.org/10.1093/aob/mct091DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3690982PMC
July 2013

The time course of photoinactivation of photosystem II in leaves revisited.

Photosynth Res 2012 Sep 27;113(1-3):157-64. Epub 2012 May 27.

College of Animal Science & Technology, North-West Agriculture and Forestry University, Yangling, 712100, Shaanxi, China.

Since photosystem II (PS II) performs the demanding function of water oxidation using light energy, it is susceptible to photoinactivation during photosynthesis. The time course of photoinactivation of PS II yields useful information about the process. Depending on how PS II function is assayed, however, the time course seems to differ. Here, we revisit this problem by using two additional assays: (1) the quantum yield of oxygen evolution in limiting, continuous light and (2) the flash-induced cumulative delivery of PS II electrons to the oxidized primary donor (P700(+)) in PS I measured as a 'P700 kinetics area'. The P700 kinetics area is based on the fact that the two photosystems function in series: when P700 is completely photo-oxidized by a flash added to continuous far-red light, electrons delivered from PS II to PS I by the flash tend to re-reduce P700(+) transiently to an extent depending on the PS II functionality, while the far-red light photo-oxidizes P700 back to the steady-state concentration. The quantum yield of oxygen evolution in limiting, continuous light indeed decreased in a way that deviated from a single-negative exponential. However, measurement of the quantum yield of oxygen in limiting light may be complicated by changes in mitochondrial respiration between darkness and limiting light. Similarly, an assay based on chlorophyll fluorescence may be complicated by the varying depth in leaf tissue from which the signal is detected after progressive photoinactivation of PS II. On the other hand, the P700 kinetics area appears to be a reasonable assay, which is a measure of functional PS II in the whole leaf tissue and independent of changes in mitochondrial respiration. The P700 kinetics area decreased in a single-negative exponential fashion during progressive photoinactivation of PS II in a number of plant species, at least at functional PS II contents ≥6 % of the initial value, in agreement with the conclusion of Sarvikas et al. (Photosynth Res 103:7-17, 2010). That is, the single-negative-exponential time course does not provide evidence for photoprotection of functional PS II complexes by photoinactivated, connected neighbours.
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http://dx.doi.org/10.1007/s11120-012-9743-8DOI Listing
September 2012

Quantifying and monitoring functional photosystem II and the stoichiometry of the two photosystems in leaf segments: approaches and approximations.

Photosynth Res 2012 Sep 26;113(1-3):63-74. Epub 2012 May 26.

Research School of Biology, College of Medicine, Biology and Environment, The Australian National University, Canberra, ACT, 0200, Australia.

Given its unique function in light-induced water oxidation and its susceptibility to photoinactivation during photosynthesis, photosystem II (PS II) is often the focus of studies of photosynthetic structure and function, particularly in environmental stress conditions. Here we review four approaches for quantifying or monitoring PS II functionality or the stoichiometry of the two photosystems in leaf segments, scrutinizing the approximations in each approach. (1) Chlorophyll fluorescence parameters are convenient to derive, but the information-rich signal suffers from the localized nature of its detection in leaf tissue. (2) The gross O(2) yield per single-turnover flash in CO(2)-enriched air is a more direct measurement of the functional content, assuming that each functional PS II evolves one O(2) molecule after four flashes. However, the gross O(2) yield per single-turnover flash (multiplied by four) could over-estimate the content of functional PS II if mitochondrial respiration is lower in flash illumination than in darkness. (3) The cumulative delivery of electrons from PS II to P700(+) (oxidized primary donor in PS I) after a flash is added to steady background far-red light is a whole-tissue measurement, such that a single linear correlation with functional PS II applies to leaves of all plant species investigated so far. However, the magnitude obtained in a simple analysis (with the signal normalized to the maximum photo-oxidizable P700 signal), which should equal the ratio of PS II to PS I centers, was too small to match the independently-obtained photosystem stoichiometry. Further, an under-estimation of functional PS II content could occur if some electrons were intercepted before reaching PS I. (4) The electrochromic signal from leaf segments appears to reliably quantify the photosystem stoichiometry, either by progressively photoinactivating PS II or suppressing PS I via photo-oxidation of a known fraction of the P700 with steady far-red light. Together, these approaches have the potential for quantitatively probing PS II in vivo in leaf segments, with prospects for application of the latter two approaches in the field.
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http://dx.doi.org/10.1007/s11120-012-9740-yDOI Listing
September 2012

Important photosynthetic contribution from the non-foliar green organs in cotton at the late growth stage.

Planta 2012 Feb 9;235(2):325-36. Epub 2011 Sep 9.

The Key Laboratory of Oasis Eco-agriculture, Xinjiang Production and Construction Group, Shihezi University, Shihezi 832003, People's Republic of China.

Non-foliar green organs are recognized as important carbon sources after leaves. However, the contribution of each organ to total yield has not been comprehensively studied in relation to the time-course of changes in surface area and photosynthetic activity of different organs at different growth stages. We studied the contribution of leaves, main stem, bracts and capsule wall in cotton by measuring their time-course of surface area development, O(2) evolution capacity and photosynthetic enzyme activity. Because of the early senescence of leaves, non-foliar organs increased their surface area up to 38.2% of total at late growth stage. Bracts and capsule wall showed less ontogenetic decrease in O(2) evolution capacity per area and photosynthetic enzyme activity than leaves at the late growth stage. The total capacity for O(2) evolution of stalks and bolls (bracts plus capsule wall) was 12.7 and 23.7% (total ca. 36.4%), respectively, as estimated by multiplying their surface area by their O(2) evolution capacity per area. We also kept the bolls (from 15 days after anthesis) or main stem (at the early full bolling stage) in darkness for comparison with non-darkened controls. Darkening the bolls and main stem reduced the boll weight by 24.1 and 9%, respectively, and the seed weight by 35.9 and 16.3%, respectively. We conclude that non-foliar organs significantly contribute to the yield at the late growth stage.
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http://dx.doi.org/10.1007/s00425-011-1511-zDOI Listing
February 2012

Intra-leaf gradients of photoinhibition induced by different color lights: implications for the dual mechanisms of photoinhibition and for the application of conventional chlorophyll fluorometers.

New Phytol 2011 Jul 18;191(1):146-159. Epub 2011 Mar 18.

Plant Sciences, Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan.

• We studied how different color lights cause gradients of photoinhibition within a leaf, to attempt to resolve the controversy of whether photon absorption by chlorophyll or by manganese (Mn) is the primary cause of photoinhibition, as suggested by the excess-energy hypothesis or the two-step hypothesis, respectively. • Lincomycin-treated leaf discs were photoinhibited by white, blue, green or red light. Combining a microfiber fluorometer, a fiber-thinning technique and a micro-manipulator enabled us to measure the chlorophyll fluorescence signals within a leaf. Photoinhibition gradients were also compared with results from various conventional fluorometers to estimate their depth of signal detection. • The severity of photoinhibition was in the descending order of blue, red and green light near the adaxial surface, and in the descending order of blue, green and red light in the deeper tissue, which correlated with the chlorophyll and the Mn absorption spectrums, respectively. These results cannot be explained by either hypothesis alone. • These data strongly suggest that both the excess-energy and the two-step mechanisms occur in photoinhibition, and fluorometers with red or blue measuring light give overestimated or underestimated F(v)/F(m) values of photoinhibited leaves compared with the whole tissue average, respectively; that is, they measured deeper or shallower leaf tissue, respectively.
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http://dx.doi.org/10.1111/j.1469-8137.2011.03669.xDOI Listing
July 2011

Operation of dual mechanisms that both lead to photoinactivation of Photosystem II in leaves by visible light.

Physiol Plant 2011 May 4;142(1):47-55. Epub 2011 Mar 4.

Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan.

Photosystem II (PS II) is photoinactivated during photosynthesis, requiring repair to maintain full function during the day. What is the mechanism(s) of the initial events that lead to photoinactivation of PS II? Two hypotheses have been put forward. The 'excess-energy hypothesis' states that excess energy absorbed by chlorophyll (Chl), neither utilized in photosynthesis nor dissipated harmlessly in non-photochemical quenching, leads to PS II photoinactivation; the 'Mn hypothesis' (also termed the two-step hypothesis) states that light absorption by the Mn cluster in PS II is the primary effect that leads to dissociation of Mn, followed by damage to the reaction centre by light absorption by Chl. Observations from various studies support one or the other hypothesis, but each hypothesis alone cannot explain all the observations. We propose that both mechanisms operate in the leaf, with the relative contribution from each mechanism depending on growth conditions or plant species. Indeed, in a single system, namely, the interior of a leaf, we could observe one or the other mechanism at work, depending on the location within the tissue. There is no reason to expect the two mechanisms to be mutually exclusive.
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http://dx.doi.org/10.1111/j.1399-3054.2011.01452.xDOI Listing
May 2011

The involvement of dual mechanisms of photoinactivation of photosystem II in Capsicum annuum L. Plants.

Plant Cell Physiol 2009 Oct 7;50(10):1815-25. Epub 2009 Sep 7.

Plant Sciences, Department of Biological Sciences, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan.

For plants, light is an indispensable resource. However, it also causes a loss of photosynthetic activity associated with photoinactivation of photosystem II (PSII). In studies of the mechanism of this photoinactivation, there are two conflicting hypotheses at present. One is that excess energy received by leaves, being neither utilized by photosynthesis nor dissipated safely in non-photochemical quenching, causes the photoinactivation. The other involves a two-step mechanism in which excitation of Mn by photons is the primary cause. In the former hypothesis, photoinactivation of PSII should not occur in low light that provides little excess energy, but in the latter hypothesis it should. Therefore, we tested these two hypotheses in different irradiances. We used a system that can measure the fraction of functional PSII complexes under natural conditions and over a long period in intact leaves, which were attached to a plant treated with lincomycin taken up via the roots. The leaves were photoinactivated in low, medium or high light (30, 60 or 950 micromol m(-2) s(-1)) with white, blue, green or red light-emitting diode arrays. Our results showed that the extent of photoinactivation per photon exposure was higher in high light than in low light, consistent with the abundance of excess energy. However, photoinactivation did occur in low light with little excess energy, and blue light caused the greatest extent of photoinactivation followed by white, green and red light in this order, an order that can be predicted from the Mn absorbance spectrum. These results suggest that both mechanisms occur in the photoinactivation process.
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http://dx.doi.org/10.1093/pcp/pcp123DOI Listing
October 2009

The leaf anatomy of a broad-leaved evergreen allows an increase in leaf nitrogen content in winter.

Physiol Plant 2009 Jul 19;136(3):299-309. Epub 2009 Feb 19.

Graduate school of Life Sciences, Tohoku University, Aoba Sendai 980-8578, Japan.

In temperate regions, evergreen species are exposed to large seasonal changes in air temperature and irradiance. They change photosynthetic characteristics of leaves responding to such environmental changes. Recent studies have suggested that photosynthetic acclimation is strongly constrained by leaf anatomy such as leaf thickness, mesophyll and chloroplast surface facing the intercellular space, and the chloroplast volume. We studied how these parameters of leaf anatomy are related with photosynthetic seasonal acclimation. We evaluated differential effects of winter and summer irradiance on leaf anatomy and photosynthesis. Using a broad-leaved evergreen Aucuba japonica, we performed a transfer experiment in which irradiance regimes were changed at the beginning of autumn and of spring. We found that a vacant space on mesophyll surface in summer enabled chloroplast volume to increase in winter. The leaf nitrogen and Rubisco content were higher in winter than in summer. They were correlated significantly with chloroplast volume and with chloroplast surface area facing the intercellular space. Thus, summer leaves were thicker than needed to accommodate mesophyll surface chloroplasts at this time of year but this allowed for increases in mesophyll surface chloroplasts in the winter. It appears that summer leaf anatomical characteristics help facilitate photosynthetic acclimation to winter conditions. Photosynthetic capacity and photosynthetic nitrogen use efficiency were lower in winter than in summer but it appears that these reductions were partially compensated by higher Rubisco contents and mesophyll surface chloroplast area in winter foliage.
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http://dx.doi.org/10.1111/j.1399-3054.2009.01224.xDOI Listing
July 2009

Green light drives leaf photosynthesis more efficiently than red light in strong white light: revisiting the enigmatic question of why leaves are green.

Plant Cell Physiol 2009 Apr 25;50(4):684-97. Epub 2009 Feb 25.

Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.

The literature and our present examinations indicate that the intra-leaf light absorption profile is in most cases steeper than the photosynthetic capacity profile. In strong white light, therefore, the quantum yield of photosynthesis would be lower in the upper chloroplasts, located near the illuminated surface, than that in the lower chloroplasts. Because green light can penetrate further into the leaf than red or blue light, in strong white light, any additional green light absorbed by the lower chloroplasts would increase leaf photosynthesis to a greater extent than would additional red or blue light. Based on the assessment of effects of the additional monochromatic light on leaf photosynthesis, we developed the differential quantum yield method that quantifies efficiency of any monochromatic light in white light. Application of this method to sunflower leaves clearly showed that, in moderate to strong white light, green light drove photosynthesis more effectively than red light. The green leaf should have a considerable volume of chloroplasts to accommodate the inefficient carboxylation enzyme, Rubisco, and deliver appropriate light to all the chloroplasts. By using chlorophylls that absorb green light weakly, modifying mesophyll structure and adjusting the Rubisco/chlorophyll ratio, the leaf appears to satisfy two somewhat conflicting requirements: to increase the absorptance of photosynthetically active radiation, and to drive photosynthesis efficiently in all the chloroplasts. We also discuss some serious problems that are caused by neglecting these intra-leaf profiles when estimating whole leaf electron transport rates and assessing photoinhibition by fluorescence techniques.
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http://dx.doi.org/10.1093/pcp/pcp034DOI Listing
April 2009

Recovery of photoinactivated photosystem II in leaves: retardation due to restricted mobility of photosystem II in the thylakoid membrane.

Photosynth Res 2008 Oct-Dec;98(1-3):621-9. Epub 2008 Sep 20.

Research School of Biological Sciences, The Australian National University, Canberra, ACT 0200, Australia.

The functionality of photosystem II (PS II) following high-light pre-treatment of leaf segments at a chilling temperature was monitored as F(v)/F(m), the ratio of variable to maximum chlorophyll fluorescence in the dark-adapted state and a measure of the optimal photochemical efficiency in PS II. Recovery of PS II functionality in low light (LL) and at a favourable temperature was retarded by (1) water stress and (2) growth in LL, in both spinach and Alocasia macrorrhiza L. In spinach leaf segments, water stress per se affected neither F(v)/F(m) nor the ability of the adenosine triphosphate (ATP) synthase to be activated by far-red light for ATP synthesis, but it induced chloroplast shrinkage as observed in frozen and fractured samples by scanning electron microscopy. A common feature of water stress and growth of plants in LL is the enhanced anchoring of PS II complexes, either across the shrunken lumen in water-stress conditions or across the partition gap in larger grana due to growth in LL. We suggest that such enhanced anchoring restricts the mobility of PS II complexes in the thylakoid membrane system, and hence hinders the lateral migration of photoinactivated PS II reaction centres to the stroma-located ribosomes for repair.
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http://dx.doi.org/10.1007/s11120-008-9363-5DOI Listing
March 2009

Differential effects of severe water stress on linear and cyclic electron fluxes through Photosystem I in spinach leaf discs in CO(2)-enriched air.

Planta 2008 Oct 18;228(5):803-12. Epub 2008 Jul 18.

Photobioenergetics Group, Research School of Biological Sciences, The Australian National University, Canberra, ACT 0200, Australia.

Linear and cyclic electron fluxes through Photosystem I in 1% CO(2) were quantified in spinach leaf tissue under severe water stress. Using actinic light with a peak at 697 nm for preferential light absorption by Photosystem I while also stimulating Photosystem II to improve redox poising, the cyclic electron flux after 60 s of illumination was a substantial proportion (33-44%) of the total electron flux through PSI at irradiances up to ~1,070 micromol photons m(-2) s(-1). At the maximum irradiance, the cyclic electron flux changed little with the progressive water loss from leaf tissue up to ~60%; by contrast, the linear electron flux was approximately halved. A reason for this differential effect of water stress on the capacity for cyclic and linear electron flow could be the increased crowding of soluble proteins in the stroma due to chloroplast shrinkage. Indeed the confinement of soluble proteins to a smaller chloroplast volume was indicated by cryo-scanning electron microscopy. It is known that the diffusion coefficient of large proteins is decreased when the background concentration of small proteins is raised; by contrast, the diffusion coefficient of small proteins is not affected by increasing the concentration of a large protein (Muramatsu and Minton in Proc Natl Acad Sci USA 85:2984-2988, 1988). Therefore, we suggest that linear electron flow, being coupled to the Calvin-Benson cycle, is limited by the diffusion of large macromolecules, especially the ribulose 1, 5-bisphosphate carboxylase/oxygenase complex. By contrast, cyclic electron flow, involving relatively small macromolecules such as ferredoxin, is less susceptible to inhibition by crowding in the stroma.
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http://dx.doi.org/10.1007/s00425-008-0783-4DOI Listing
October 2008

A rapid, whole-tissue determination of the functional fraction of PSII after photoinhibition of leaves based on flash-induced P700 redox kinetics.

Physiol Plant 2008 Jan;132(1):23-32

Photobioenergetics Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 0200, Australia.

Assaying the number of functional PSII complexes by the oxygen yield from leaf tissue per saturating, single-turnover flash, assuming that each functional PSII evolves one oxygen molecule after four flashes, is one of the most direct methods but time-consuming. The ratio of variable to maximum Chl fluorescence yield (F(v)/F(m)) in leaves can be correlated with the oxygen yield per flash during a progressive loss of PSII activity associated with high-light stress and is rapid and non-intrusive, but suffers from being representative of chloroplasts near the measured leaf surface; consequently, the exact correlation depends on the internal leaf structure and on which leaf surface is being measured. Our results show that the average F(v)/F(m) of the adaxial and abaxial surfaces has a reasonable linear correlation with the oxygen yield per flash after varied extents of photoinactivation of PSII. However, we obtained an even better linear correlation between (1) the integrated, transient electron flow (Sigma) to P700+, the dimeric Chl cation in PSI, after superimposing a single-turnover flash on steady background far-red light and (2) the relative oxygen yield per flash. Leaves of C3 and C4 plants, woody and herbaceous species, wild-type and a Chl-b-less mutant, and monocot and dicot plants gave a single straight line, which seems to be a universal relation for predicting the relative oxygen yield per flash from Sigma. Measurement of Sigma is non-intrusive, representative of the whole leaf tissue, rapid and applicable to attached leaves; it may even be applicable in the field.
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http://dx.doi.org/10.1111/j.1399-3054.2007.01000.xDOI Listing
January 2008
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