The respiratory chain in plant mitochondria has two pathways: the ATP-coupling cytochrome pathway (CP) and ATP-uncoupling alternative oxidase (AOX). Under high-light (HL) conditions, AOX has been considered to have important roles in avoiding the over-reduction of chloroplasts. The inhibition of the respiratory chain under HL conditions accelerates photosystem II (PSII) damage and represses PSII repair in Arabidopsis thaliana leaves. However, it is not known precisely how the respiratory chain contributes to maintenance of photosynthetic electron transport activity. In this study, we investigated photosynthetic electron transport activity, protein amounts of the photosynthetic electron transport, and electron pool size in the intersystem photosynthetic electron transport and the stroma, before and after 2 h-HL treatment in the leaves of wild-type (WT) and AOX1a-knockout (aox1a) A. thaliana, in which the CP was partially inhibited by a complex-III inhibitor. The electron transport rate around PSII was decreased by CP inhibition in aox1a. After 2 h-HL treatment, CP inhibition barely changed the protein amounts of photosynthetic electron transport, but the protein amount of AOX was increased in WT. The lack of AOX further led to over-reduction of chloroplasts under CP inhibition. Under HL conditions, AOX is important in preventing over-reduction of chloroplasts as an electron sink.

Wetland plants with high aeration capacity can produce energy and maintain growth in waterlogged, anaerobic soils. In this study, we aimed to gain insight into the survival mechanisms of wetland plants in anaerobic soils by comparing the morphological characteristics and respiratory metabolism of Phragmites australis with high aeration capacity under aerobic and anaerobic hydroponic conditions. We examined growth, root aerenchyma formation, O 2 concentration in roots, and respiratory traits in shoots and roots. In low‐O 2 treatments, P. australis exhibited morphological changes, including shorter shoots, more tillers, and increased adventitious root formation, which enhanced O 2 transport in waterlogged soils. The O 2 concentration in root aerenchyma significantly decreased toward the root tip in low‐O 2 treatment. The O 2 uptake rates and maximal activities of alternative pathways were comparable between two O 2 treatments in both organs. The ratio of ATP production rate of the whole roots to that of the whole plant was low in plants grown in low‐O 2 treatment. The maximal enzyme activities in the glycolysis and the TCA cycle were also comparable between O₂ treatments. However, under low‐O 2 conditions, estimated ATP production rates and total ATP contents of whole shoots increased, but those of whole roots did not. The enhanced ATP production in shoots may support growth under low‐O 2 conditions. In conclusion, P. australis , with high aeration capacity, can adapt to long‐term rhizosphere hypoxia by modifying morphological and respiratory traits in both shoots and roots. These ecophysiological traits may have applications in ecological engineering for improving wastewater and soil quality in anaerobic rhizospheres.

Hygrophila difformis, a heterophyllous amphibious plant, shows the photosynthetic acclimation of leaves developed under not only submerged (submerged leaves) but also terrestrial conditions (terrestrial leaves) to submerged conditions. However, the underlying mechanisms of underwater photosynthesis in those leaves are unknown. In this study, the biochemical and anatomical characteristics related to carbon acquisition for underwater photosynthesis were compared between terrestrial, submerged terrestrial (ST), and submerged leaves of H. difformis. The submerged leaves had a higher underwater photosynthetic rate under low dissolved inorganic carbon (DIC) conditions than the terrestrial leaves. The ST leaves showed rates between those of the terrestrial and submerged leaves. The ST and submerged leaves exhibited higher carbonic anhydrase activity than the terrestrial leaves. Only the submerged leaves had decreased epidermal cuticle and cell wall thicknesses. The ST and submerged leaves were observed to have thinner palisade cell walls than the terrestrial leaves. Submergence acclimation in H. difformis is achieved by HCO3- utilization and anatomical change to decrease the resistance of CO2 diffusion from intercellular airspaces. The difference between the ST and submerged leaves results mainly from differing DIC flux into the leaves due to decreases in epidermal cuticle and cell wall thicknesses in the submerged leaves.

The conversion of nitrate to ammonium, i.e., nitrate reduction, is a major consumer of reductants in plants. Previous studies have reported that the mitochondrial alternative oxidase (AOX) is upregulated under limited nitrate reduction conditions, including no/low nitrate or when ammonium is the sole nitrogen (N) source. Electron transfer from ubiquinone to AOX bypasses the proton-pumping complexes III and IV, thereby consuming reductants efficiently. Thus, upregulated AOX under limited nitrate reduction may dissipate excessive reductants and thereby attenuate oxidative stress. Nevertheless, so far there is no firm evidence for this hypothesis due to the lack of experimental systems to analyze the direct relationship between nitrate reduction and AOX. We therefore developed a novel culturing system for A. thaliana that manipulates shoot activities of nitrate reduction and AOX separately without causing N starvation, ammonium toxicity, or lack of nitrate signal. Using shoots processed with this system, we examined genome-wide gene expression and growth to better understand the relationship between AOX and nitrate reduction. The results showed that, only when nitrate reduction was limited, AOX deficiency significantly upregulated genes involved in mitochondrial oxidative stress, reductant shuttles, and non-phosphorylating bypasses of the respiratory chain, and inhibited growth. Thus, we conclude that AOX alleviates mitochondrial oxidative stress and sustains plant growth under limited nitrate reduction.

BACKGROUND AND AIMS:HCO3- can be a major carbon resource for photosynthesis in underwater environments. Here we investigate the underlying mechanism of uptake and membrane transport of HCO3- in submerged leaves of Hygrophila difformis, a heterophyllous amphibious plant. To characterize these mechanisms, we evaluated the sensitivity of underwater photosynthesis to an external carbonic anhydrase (CA) inhibitor and an anion exchanger protein inhibitor, and we attempted to identify components of the mechanism of HCO3- utilization. METHODS:We evaluated the effects of the external CA inhibitor and anion exchanger protein inhibitor on the NaHCO3 response of photosynthetic O2 evolution in submerged leaves of H. difformis. Furthermore, we performed a comparative transcriptomic analysis between terrestrial and submerged leaves. KEY RESULTS:Photosynthesis in the submerged leaves was decreased by both the external CA inhibitor and anion exchanger protein inhibitor, but no additive effect was observed. Among upregulated genes in submerged leaves, two α-CAs, Hdα-CA1 and Hdα-CA2, and one β-carbonic anhydrase, Hdβ-CA1, were detected. Based on their putative amino acid sequences, the α-CAs are predicted to be localized in the apoplastic region. Recombinant Hdα-CA1 and Hdβ-CA1 showed dominant CO2 hydration activity over HCO3- dehydration activity. CONCLUSIONS:We propose that the use of HCO3- for photosynthesis in submerged leaves of H. difformis is driven by the cooperation between an external CA, Hdα-CA1, and an unidentified HCO3- transporter.

ATP-uncoupling alternative oxidase (AOX) in the plant respiratory chain is often induced under stress conditions such as low temperature (LT). The importance of AOX in photosynthesis has been examined, and leaves having larger amounts of AOX tended to show larger decrease in photosynthetic electron transport rate (ETR) by AOX inhibition. However, the details were not clarified. Here, we used three ecotypes of Arabidopsis thaliana which differed in AOX amounts and their responses to LT, and examined whether AOX amount was related to the degree of decrease in ETR by AOX inhibition. In Tiv-0, which originates from a warmer site, grown at high temperature (HT), AOX inhibition decreased ETR, but not in the other ecotypes. LT treatment significantly increased ETR and AOX, especially in Bur-0, but AOX inhibition did not decrease ETR in LT plants of any ecotype. AOX inhibition significantly increased the non-regulated energy dissipation in photosystem II (PSII), Y(NO), and decreased the maximal quantum yield of PSII, Fv/Fm, especially in LT plants. Since AOX inhibition did not affect the parameters of PSI, AOX inhibition may directly affect the reaction center of PSII in LT plants.

Ephedra intermedia Schrenk et C.A.Meyer (Ephedraceae, Gnetales, Gymnospermae) contains ephedrine and pseudoephedrine, and has been used as Ephedra herb, a traditional medicine. Instead of degenerated leaves, photosynthetic assimilation is performed in their stems. Recently domestic cultivation of this species has been required, but relevant nutrient conditions for cultivation remain uncertain. In this study, we examined stem photosynthetic parameters in E. intermedia plants grown under different nutrient conditions in order to clarify relationships between the photosynthetic parameters and growth in their stems. Also, we examined whether the regulation of two photosystems of the photosynthetic electron transport system in E. intermedia stems is similar to that of angiosperm leaves. We cultivated E. intermedia plants under various nutrient conditions with different levels of nitrogen (N), and regularly measured CO2 assimilation rates, parameters of the photosynthetic electron transport system, and growth of E. intermedia stems. The stem growth was better under the condition with the additional ordinary chemical fertilizer compared to those with urea or magnesium lime. The seasonal changes were observed in the CO2 assimilation rates, Y(II), the quantum yield of photosystem II (PSII), and Y(ND), the non-photochemical yield due to donor side limitations in photosystem I (PSI). However, these parameters differed between nutrient conditions to small extents, and were not related to stem growth. Therefore, the measured parameters of photosynthesis could not explain the difference in stem growth between nutrient conditions. Although the photosynthetic parameters seasonally changed, the tight relationships between Y(ND), the PSI parameter, and Y(II) or Y(NPQ), the PSII parameters, were found throughout the seasons. These relationships between two photosystems in stems of gymnosperm E. intermedia were as the same as those in angiosperm leaves, although a flavodiiron protein can function as the electron sink in the stems of E. intermedia. This balance between two photosystems may be regulated by the system dependent on the thylakoid lumen pH, which may lead to the protection of two photosystems throughout the seasons.

Mangrove plants, which have evolved to inhabit tidal flats, may adjust their physiological and morphological traits to optimize their growth in saline habitats. Furthermore, the confined distribution of mangroves within warm regions suggests that warm temperature is advantageous to their growth in saline environments. We analyzed growth, morphology and respiratory responses to moderate salinity and temperature in a mangrove species, Rhizophora stylosa. The growth of R. stylosa was accelerated in moderate salinity compared with its growth in fresh water. Under warm conditions, the increased growth is accompanied by increased specific leaf area (SLA) and specific root length. Low temperature resulted in a low relative growth rate due to a low leaf area ratio and small SLA, regardless of salinity. Salinity lowered the ratio of the amounts of alternative oxidase to cytochrome c oxidase in the mitochondrial respiratory chain in leaves. Salinity enhanced the leaf respiration rate for maintenance, but under warm conditions this enhancement was compensated by a low leaf respiration rate for growth. In contrast, salinity enhanced overall leaf respiration rates at low temperature. Our results indicate that under moderate saline conditions R. stylosa leaves require warm temperatures to grow with a high rate of resource acquisition without enhancing respiratory cost.

Here, we review data of nocturnal time courses in both leaf respiratory O2-uptake and CO2-efflux to look for patterns that may provide insight into underlying mechanisms. We show that these respiratory gas fluxes are temporally correlated but not as tightly linked as is often assumed, which results in the respiratory quotient changing over the nighttime period. Temporal variation in respiration is controlled by changes in the concentration of both respiratory substrates and products, which could be directly reflected in the O2-uptake. Additionally, there may be further control of the CO2-efflux, which may also be caused by diel/circadian changes in the relative composition of substrates and products as well as variation in the use of tricarboxylic acid (TCA) intermediates. At present, net terrestrial carbon uptake, the fine balance between photosynthesis and respiration, offsets roughly one-third of total annual anthropogenic emissions (Friedlingstein et al., 2022). As a fundamental metabolic process, autotrophic respiration drives plant growth while consuming oxygen (O2) and releasing CO2. Yet, the interplay between environmental and metabolic regulation of respiration remains poorly understood, but critical to refining predictions of terrestrial carbon uptake in a warming world. Resolving underlying mechanisms governing diel changes in respiration will not only result in an increased understanding of respiration (Amthor, 2000), but also inform applications in biotechnology, such as engineering plant respiration to enhance crop yield (Garcia et al., 2023), and modelling by improving the accuracy of predictions of biosphere–atmosphere CO2-exchange (Clark et al., 2011). Recent research has highlighted the importance of non-temperature control of diel changes in leaf-to-canopy gas exchange (Resco de Dios et al., 2017a,b) in general, and in leaf respiratory CO2-efflux (RCO2) in particular (Bruhn et al., 2022; Faber et al., 2022), providing further insight into the role of photoperiod in shaping consistent temporal patterns of respiratory metabolism among diverse plants (Bruhn, 2023). Indeed, temperature alone accounts on average for only about one-half of the total observed nocturnal variation in leaf RCO2 in mature leaves (Bruhn et al., 2022). In general, leaf RCO2,To (i.e. leaf RCO2 at constant temperature) decreases during the course of the night (Fig. 1), potentially reflecting a change in substrate availability or in demand for respiratory products (Fig. 1). Recent work has documented a decrease in leaf RCO2,To through the night across species (Bruhn et al., 2022; Fig. 1). However, the literature is unclear regarding nighttime variation in leaf RO2,To (Fig. 1). Leaf RCO2 is a net result of many different (de)carboxylation processes and leaf RO2 is a net result of oxidation and reduction processes (Lambers & Oliveira, 2019). The molar ratio of CO2-efflux : O2-uptake is termed the respiratory quotient (RQ). If leaf RO2,To remains constant during the night, then a declining leaf RCO2,To would result in a decreasing RQTo during the night (Fig. 1). A declining RQTo could indicate a general trend in nocturnal net (de)carboxylations that is temporally decoupled from ATP demand (Fig. 1). On the other hand, if both leaf RO2,To and leaf RCO2,To decrease during the night, does leaf RQ remain constant during night, perhaps indicating a temporally tight coupling between the demand for ATP and net (de)carboxylations (Fig. 1)? It is already known that plant RQ may change from light to dark transitions (Le & Millar, 2023); however, a general overview of nocturnal variation of leaf RQ (Fig. 1) does not exist and relatively few measurements exist. Here, we review the literature data of the relative nocturnal variation in leaf RO2,To and compare it to that of leaf RCO2,To. In addition, we review the literature data available for potential relative nocturnal variation in leaf RQTo. Finally, we discuss perspectives with regard to a mechanistic understanding of diel variation in leaf R. This may aid future directions of research providing insight into underlying processes governing nighttime patterns of respiration and its metabolic control. Across studies, species, conditions, and length of time periods during the nighttime (i.e. from time of RTo,t-initial to time of RTo,t), the vast majority of observations of RTo,t : RTo,t-initial for RCO2,To and RO2,To are below 1.0, indicating a reduction in CO2 efflux or O2 uptake during the night (Fig. 2a). The median of RTo,t : RTo,t-initial is 0.75 for RCO2,To and 0.85 for RO2,To. Across studies, species, and conditions RTo,t : RTo,t-initial for CO2-efflux appears in general to decrease earlier or to a greater extent in the night than does RTo,t : RTo,t-initial for O2-uptake (Fig. 2b, Supporting Information Fig. S2). From 0 to 3 h into the night RTo,t : RTo,t-initial for CO2-efflux is below 1.0, whereas RTo,t : RTo,t-initial for O2-uptake within the same period appears to increase to values higher than 1.0 for some species (Figs 2b, S2). However, for several of the limited number of species/studies available for this analysis, RTo,t : RTo,t-initial for CO2-efflux and O2-uptake appears to increase late at night (Fig. S2). Furthermore, after 17 h in darkness, the RTo,t : RTo,t-initial is rather similar for both CO2-efflux and O2-uptake (Fig. 2b). The proportional decrease in leaf respiration through the night is on average c. 19% higher for leaf CO2-efflux than that of leaf O2-uptake (Fig. S3). Across the four species and different length of periods of night in different studies, leaf RQ appears to decrease in relation to its initial value during the first 9 h of darkness after which leaf RQ appears to increase as it approaches the daytime value (Fig. 2c). The phenomenon of a deceasing leaf CO2-efflux during the night (RTo,t : RTo,t-initial < 1.0) shown here and in recent work (Bruhn et al., 2022; Bruhn, 2023) appears also to be the case for respiratory O2-uptake in most species (Figs 2b,c, S2). Therefore, not only do net (de)carboxylation processes decrease during night at constant temperature, so do oxygenation processes in the leaf. This evidence of declining oxygenation in the dark indicates a nocturnal decrease in leaf mitochondrial electron transport and therefore potential demand for ATP. This further indicates nocturnal variation in the rates of processes supported by respiration and/or their metabolic costs (Azcón-Bieto et al., 1983; Amthor, 2000), irrespective of temperature changes at night. The linear regression for RTo,t : RTo,t-initial measured as O2-uptake (Fig. 2b) is a mean response across several different species (Fig. S2). As this analysis (Figs 2, S2) can only make use of the limited number of species available, it is too early to draw conclusions regarding the species-specific differences in the extent of decreasing respiratory O2-uptake during night. However, it highlights that future experimental studies of the mechanisms underlying nocturnal changes in leaf R should consider the time scale over which potential effectors alter both leaf RCO2,To, and leaf RO2,To. We encourage future studies of RTo,t : RTo,t-initial measured as O2-uptake and suggest there is much to be learned from a deeper understanding of species variation in response to environmental variation. Studies of leaf RQ over an extended period of darkness (lasting several days but measured only on a 24-h interval), indicate that leaf RQ may stay constant in darkness over these timescales (Tcherkez et al., 2003; Hanf et al., 2015). However, evidence indicates that RQ of leaves decreases during the earliest hours of the dark period (Fig. 2c), which is a result of different proportional nocturnal declines in leaf respiratory CO2-efflux and O2-uptake (Fig. 2b). This suggests that there may not only be a light–darkness transition in relative composition of respiratory substrates (Le & Millar, 2023), but also a progressive change during nighttime (Fig. 1). The regression in Fig. 2(c) includes the point at 17 h from Fig. 2(b), despite this point representing only one species. We include this late time point in the common regression since RQ in all species appears to increase later at night (Fig. S1). The RQ value varies depending on not only the respiratory substrates but also the compounds produced. For example, the increased consumption of reducing equivalents from the TCA cycle via cellular processes decreases the rate of mitochondrial O2 consumption, which increases the RQ value (Buckely & Adams, 2011; Lambers & Oliveira, 2019). By contrast, organic acid production decreases the RQ value. The relative decrease in RQ during the first few hours of the dark period (Fig. 2c) suggests that lipids or proteins may gradually be relatively more important as substrates during the beginning of the night, which may indicate what Lambers & Oliveira (2019) termed 'starvation' of leaves. Perhaps the transient decline in RQ coincides with partial degradation of certain leaf proteins (Fig. 1) that do not require maintenance during the nighttime, for example, Rubisco? Later at night, an increase in leaf RQ (Fig. 2c) may indicate a temporal change in the relative composition of respiratory substrates, approaching that of daytime/light (Graf et al., 2010). In addition to changes in respiratory substrates, changes in demands for reducing equivalents or TCA cycle intermediates may be reflected in the changes in RQ at night (Fig. 1). Furthermore, respiratory O2-uptake occurs mainly in the cytochrome and alternative pathways in the respiratory chain (Vanlerberghe et al., 2020). Since the alternative pathway is not linked to respiratory ATP production, changes in the O2-uptake rate cannot be directly interpreted as a reduced demand for ATP, without additional information regarding electron partitioning (Del-Saz et al., 2018). Therefore, changes in composition of respiratory substrates as well as demand for respiratory intermediates could affect RQ via electron partitioning between the two pathways to meet overall ATP demand (Fig. 1). Future research is needed on the nocturnal changes in composition of available substrates for leaf R (Le & Millar, 2023; Stitt & Zeeman, 2012), leaf biosynthesis (Penning de Vries et al., 1974), relative engagement of the electron transport pathways (Noguchi et al., 2001b), and changes in leaf N-metabolism – all factors potentially affecting RQ. Here, we have focused on revealing common trends in different proportional nocturnal declines in leaf respiratory CO2-efflux vs O2-uptake across species. The few studies available do not yet allow for a rigorous analysis of potential effects of leaf chemical composition, plant functional groups or details of growth conditions. We are at present not aware of field studies of proportional nocturnal declines in leaf respiration as both CO2-efflux vs O2-uptake within the same species, and are thus limited to a review of lab/growth cabinet studies. However, Bruhn et al. (2022) have shown that the proportional nocturnal declines in leaf respiratory CO2-efflux measured at constant temperature does not differ systematically between laboratory and field conditions, but does differ statistically significantly between species (Bruhn et al., 2022). Here, we extend this observation to O2-uptake (Fig. S2). An important implication of our analysis is that, in addition to a need for rigorous awareness of the timing of measurements in future studies of potential single controlling factors of leaf R (e.g. temperature, substrate levels, and need for ATP), we may need to study other controlling factors simultaneously, that is, substrate quality, demands for reducing equivalents or TCA cycle intermediates. This is further substantiated by the fact that the proportional decrease in leaf respiration at constant temperature through night is on average c. 19% higher for leaf CO2-efflux than that of leaf O2-uptake (Fig. S3). The phenomenon of decreasing respiration during the night appears to be common in plants, and consistent for both gas-exchange measurement techniques based on leaf CO2-efflux and O2-uptake. However, the temporal dynamics of nocturnal leaf CO2-efflux and O2-uptake also appear to differ from each other, particularly at the beginning of night, reflecting a shift in the respiratory quotient and thus the balance between oxidation and decarboxylation pathways underpinning respiratory metabolism. It may be necessary to develop an independent mechanistic understanding of the temporal control of leaf respiration in terms of O2-uptake and CO2-efflux. There may be general control originating from the cellular levels of substrates and products, which may be directly reflected in leaf energy demand as reflected by O2-uptake (and accounting for any changes in the relative engagement of the AOX). Additionally, there may be further control of the CO2-efflux, which may be indicate changes in the relative composition and use of substrates, and the specific products produced, as well as use of TCA intermediates. Future research into understanding and modelling leaf respiratory CO2-efflux would benefit from: (1) awareness of diel timescales during studies of leaf R; (2) knowledge of nocturnal variation in RQ for more species and conditions; (3) studies on the temporal variation in the relative importance of substrate composition, biosynthesis, and relative engagement of mitochondrial electron transport pathways on leaf R; and (4) disentangling the role of the level of respiratory substrates on nighttime leaf R. These endeavours will benefit from the deployment of new technologies that simultaneously resolve oxygen and carbon dioxide fluxes and isotopologues to integrate respiratory-derived fluxes to underlying substrates. When quantifying trends in nocturnal variation in leaf R, it is necessary to standardize data in a way that the relative effects are directly comparable across measured leaves, species, studies, and growing conditions. Therefore, we standardized leaf RTo at a given point in time (t) at night (RTo,t) to the first measurement of RTo,t at the beginning of the dark-period, that is RTo,t-initial. Thus, RTo,t : RTo,t-initial is simply a ratio describing the proportional decrease in leaf RTo at time t during the night when measured at constant temperature within each plant species and within each published study. Included published data were limited to species × study and species × study × treatment combinations where: (1) there was at least one initial measurement of leaf RTo at beginning of the dark period (RTo,t-initial) and at least one measurement of leaf RTo,t later in the dark period/night; and (2) the growth conditions were held constant throughout the period of measurements in the experiment, that is laboratory based studies to ensure clearly defined growth- and measurement temperature. There are currently more observations available from the literature for RCO2,To than for RO2,To in leaves fulfilling our analytical criteria (Fig. 2). For the underlying data, we did not require that both leaf RCO2,To and RO2,To were measured in a given study. Sixteen species with measured RCO2,To were: Alocasia odora (Noguchi et al., 1996, 2001a; Noguchi & Terashima, 1997); Amaranthus hypochondriacus (Bunce, 2007); Arabidopsis thaliana (Trethewey & ap Rees, 1994; Watanabe et al., 2014); bean (Gessler et al., 2017); Beta vulgaris (Fondy & Geiger, 1982); Bistorta bistortoides (McCutchan & Monson, 2001); Campanula rotundifolia (McCutchan & Monson, 2001); Flaveria linearis (Leonardos et al., 2006); Glycine max (Bunce, 2007); Gossypium spp. (Gessler et al., 2017); Halimium halimifolium (Lehmann et al., 2016); Hordeum distichum (Farrar & Farrar, 1985); Horeuam vulgare (Baysdorfer et al., 1987); Phaseolus vulgaris (Noguchi et al., 2001a); Spinacia oleracea (Noguchi et al., 1996; Noguchi & Terashima, 1997); Triticum aestivum (Azcón-Bieto & Osmond, 1983; Averill & ap Rees, 1995). Six species with measured RO2,To were: A. odora (Noguchi & Terashima, 1997); A. thaliana (Trethewey & ap Rees, 1994; Watanabe et al., 2014; O'Leary et al., 2017); Eucalyptus camaldulensis (Scafaro et al., 2017); Pisum sativum (Azcón-Bieto et al., 1983); S. oleracea (Azcón-Bieto et al., 1983; Noguchi & Terashima, 1997); T. aestivum (Averill & ap Rees, 1995; O'Leary et al., 2017). Currently, data for both RCO2,To and RO2,To measured more than once at the same times at night are available for only a few species × study combinations (Fig. 2b,c). The four species present in six studies were: A. odora (Noguchi & Terashima, 1997); A. thaliana (Trethewey & ap Rees, 1994; Watanabe et al., 2014); S. oleracea (Noguchi & Terashima, 1997); and T. aestivum (Averill & ap Rees, 1995). In the six studies included in Fig. 2(c), RQ at the onset of darkness/end of light period (RQTo,initial) was as follows: c. 1.8 in T. aestivum (open triangles; Azcón-Bieto & Osmond, 1983), c. 1.02 in T. aestivum (open triangles; Averill & ap Rees, 1995), c. 1.1 in S. oleracea (closed triangles; Noguchi & Terashima, 1997), c. 1.2 in A. odora (closed circles, Noguchi & Terashima, 1997), c. 1.2 in A. thaliana (open circles, Watanabe et al., 2014), and c. 1.2 in A. thaliana wild-type (open circles, Trethewey & ap Rees, 1994). None declared. DB did the analyses. DB, KN, KLG, and MGT all contributed to the writing. Data underlying Fig. 2 are available in Dataset S1. Dataset S1 Data underlying Fig. 2 (differential nighttime decreases in leaf respiratory CO2-efflux and O2-uptake – all data). Fig. S1 Values of leaf respiratory quotient at constant temperature (RQ,To) during nighttime in different species/study combinations. Fig. S2 Changes in RTo,t : RTo,t-initial of leaves of four species during the night. Fig. S3 Plot (w. regression through origin) of RTo,t : RTo,t-initial as O2-uptake as function RTo,t : RTo,t-initial as CO2-efflux across all available species and data. Please note: Wiley is not responsible for the content or functionality of any Supporting Information supplied by the authors. Any queries (other than missing material) should be directed to the New Phytologist Central Office. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Background and Aims Evergreen herbaceous species in the deciduous forest understorey maintain their photosystems in long-lived leaves under dynamic seasonal changes in light and temperature. However, in evergreen understorey herbs, it is unknown how photosynthetic electron transport acclimates to seasonal changes in forest understorey environments, and what photoprotection systems function in excess energy dissipation under high-light and low-temperature environments in winter. Methods Here, we used Asarum tamaense, an evergreen herbaceous species in the deciduous forest understorey with a single-flush and long-lived leaves, and measured photosynthetic CO2 assimilation and electron transport in leaves throughout the year. The contents of photosynthetic proteins, pigments and primary metabolites were determined from regularly collected leaves. Key results Both the rates of CO2 assimilation and electron transport under saturated light were kept low in summer, but increased in autumn and winter in A. tamaense leaves. Although the contents of photosynthetic proteins including Rubisco did not increase in autumn and winter, the proton motive force and Delta pH across the thylakoid membrane were high in summer and decreased from summer to winter to a great extent. These decreases alleviated the suppression by lumen acidification and increased the electron transport rate in winter. The content and composition of carotenoids changed seasonally, which may affect changes in non-photochemical quenching from summer to winter. Winter leaves accumulated proline and malate, which may support cold acclimation. Conclusions In A. tamaense leaves, the increase in photosynthetic electron transport rates in winter was not due to an increase in photosynthetic enzyme contents, but due to the activation of photosynthetic enzymes and/or release of limitation of photosynthetic electron flow. These seasonal changes in the regulation of electron transport and also the changes in several photoprotection systems should support the acclimation of photosynthetic C gain under dynamic environmental changes throughout the year.

Nicotinamide adenine dinucleotides (NAD+ and NADP+) are electron mediators involved in various metabolic pathways. NADP(H) are produced by NAD kinase (NADK) through the phosphorylation of NAD(H). The Arabidopsis NADK3 (AtNADK3) is reported to preferentially phosphorylate NADH to NADPH and is localized in the peroxisome. To elucidate the biological function of AtNADK3 in Arabidopsis, we compared metabolites of nadk1, nadk2 and nadk3 Arabidopsis T-DNA inserted mutants. Metabolome analysis revealed that glycine and serine, which are intermediate metabolites of photorespiration, both increased in the nadk3 mutants. Plants grown for 6 weeks under short-day conditions showed increased NAD(H), indicating a decrease in the phosphorylation ratio in the NAD(P)(H) equilibrium. Furthermore, high CO2 (0.15%) treatment induced a decrease in glycine and serine in nadk3 mutants. The nadk3 showed a significant decrease in post-illumination CO2 burst, suggesting that the photorespiratory flux was disrupted in the nadk3 mutant. In addition, an increase in CO2 compensation points and a decrease in CO2 assimilation rate were observed in the nadk3 mutants. These results indicate that the lack of AtNADK3 causes a disruption in the intracellular metabolism, such as in amino acid synthesis and photorespiration.

Drought is a serious abiotic stress that leads to decreased yield. In pot experiments, the reaction center chlorophyll of photosystem I (PSI) (P700) has been reported to be oxidized in response to drought stress. In the present study, we examined whether drought stress that decreases grain yield was detected by P700 oxidation in paddy-field-grown rice (Oryza sativa L.) plants. The P700 reduction and SPAD values were also evaluated. Drought stress was imposed by the prolonged mid-season drainage in 2019 and 2020. P700 oxidation was measured at saturated CO2 levels before heading. P700 oxidation was not stimulated in the 2019-drought plots. In contrast, P700 oxidation was stimulated in the 2020-drought plot. However, this phenomenon in 2020 was primarily accounted for by decreases in total leaf-N levels, as previously observed in the rice plants grown under low N availability. Rough rice yields and ratios of filled spikelets tended to decrease in the drought plots. Therefore, drought stress responsible for a decrease in grain yields could not be directly detected by P700 oxidation in paddy-field-grown rice plants, whereas the P700 oxidation responded to changes in total leaf-N levels due to the drought stress treatment. The same conclusion was drawn for the P700 reduction and SPAD value.

Summary Seasonal differences in diaspore dispersal of three mangrove species, Kandelia obovata, Bruguiera gymnorrhiza and Rhizophora stylosa, suggest that respiratory energy production and demand may differ as a result of interspecific differences in temperature dependence of growth and maintenance processes during seedling establishment. We analyzed growth, temperature dependencies of respiratory O2 consumption and amounts of respiratory chain enzymes in seedlings of these species grown at various temperatures. Respiration rates measured at the low reference temperature, RREF, were highest in leaves of 15°C‐grown K. obovata, whose dispersal occurs in the cold season, while root RREF of 15°C‐grown R. stylosa was 60% those of the other species, possibly because of warm conditions during its establishment phase. In leaves and roots of K. obovata and leaves of R. stylosa, the overall activation energy, Eo, changed with growth temperature associated with changes in the ratios of the amount of protein in the two respiratory pathways. However, Eo of seedlings of B. gymnorrhiza, which has a long dispersal phase, were constant and independent of growth temperature. The different temperature responses of seedling respiration and growth among these three species may reflect the seasonal temperature range of seedling dispersal and establishment in each species.

Abstract Background and Aims Mangrove plants are mostly found in tropical and sub-tropical tidal flats, and their limited distribution may be related to their responses to growth temperatures. However, the mechanisms underlying these responses have not been clarified. Here, we measured the dependencies of the growth parameters and respiration rates of leaves and roots on growth temperatures in typical mangrove species. Methods We grew two typical species of Indo-Pacific mangroves, Bruguiera gymnorrhiza and Rhizophora stylosa, at four different temperatures (15, 20, 25 and 30 °C) by irrigating with fresh water containing nutrients, and we measured growth parameters, chemical composition, and leaf and root O2 respiration rates. We then estimated the construction costs of leaves and roots and the respiration rates required for maintenance and growth. Key Results The relative growth rates of both species increased with growth temperature due to changes in physiological parameters such as net assimilation rate and respiration rate rather than to changes in structural parameters such as leaf area ratio. Both species required a threshold temperature for growth (12.2 °C in B. gymnorrhiza and 18.1 °C in R. stylosa). At the low growth temperature, root nitrogen uptake rate was lower in R. stylosa than in B. gymnorrhiza, leading to a slower growth rate in R. stylosa. This indicates that R. stylosa is more sensitive than B. gymnorrhiza to low temperature. Conclusions Our results suggest that the mangrove species require a certain warm temperature to ensure respiration rates sufficient for maintenance and growth, particularly in roots. The underground temperature probably limits their growth under the low-temperature condition. The lower sensitivity of B. gymnorrhiza to low temperature shows its potential to adapt to a wider habitat temperature range than R. stylosa. These growth and respiratory features may explain the distribution patterns of the two mangrove species.

Mangrove plants are found in tropical and sub-tropical tidal flats, and their limited distribution may be related to their responses to growth temperatures. However, the mechanisms underlying these responses have not been clarified. Typical species of Indo–Pacific mangroves, Bruguiera gymnorrhiza and Rhizophora stylosa, differ in their distribution ranges. Here, we grew these species at different temperatures and analyzed growth and respiration parameters. Relative growth rate of both species increased with growth temperature due to changes in physiological parameters such as net assimilation rate and respiration rate rather than to changes in structural parameters. At low growth temperature, root respiration rate and nitrogen uptake rate were lower in R. stylosa than in B. gymnorrhiza roots, leading to slower growth and higher sensitivity to low temperature of R. stylosa. Biomass allocation to leaves and roots was also lower in R. stylosa at any temperature. These characteristics of R. stylosa would result in lower relative growth rate of whole plant and to different distribution patterns of the two species.

Leaf nitrogen (N) level affects not only photosynthetic CO2 assimilation, but also two photosystems of the photosynthetic electron transport. The quantum yield of photosystem II [Y(II)] and the non-photochemical yield due to the donor side limitation of photosystem I [Y(ND)], which denotes the fraction of oxidized P700 (P700(+)) to total P700, oppositely change depending on leaf N level, and the negative correlation between these two parameters has been reported in leaves of plants cultivated at various N levels in growth chambers. Here, we aimed to clarify whether this correlation is maintained after short-term changes in leaf N level, and what parameters are the most responsive to the changes in leaf N level under field conditions. We cultivated rice varieties at two N fertilization levels in paddy fields, treated additional N fertilization to plants grown at low N, and measured parameters of two photosystems of mature leaves. In rice leaves under low N condition, the Y(ND) increased and the photosynthetic linear electron flow was suppressed. In this situation, the accumulation of P700(+) can function as excess energy dissipation. After the N addition, both Y(ND) and Y(II) changed, and the negative correlation between them was maintained. We used a newly-developed device to assess the photosystems. This device detected the similar changes in Y(ND) after the N addition, and the negative correlation between Y(ND) and photosynthetic O-2 evolution rates was observed in plants under various N conditions. This study has provided strong field evidence that the Y(ND) largely changes depending on leaf N level, and that the Y(II) and Y(ND) are negatively correlated with each other irrespective of leaf N level, varieties and annual variation. The Y(ND) can stably monitor the leaf N status and the linear electron flow under field conditions.

Background Plants invest photosynthates in construction and maintenance of their structures and functions. Such investments are considered costs. These costs are recovered by the CO2 assimilation rate (A) in the leaves, and thus A is regarded as the immediate, short-term benefit. In photosynthesizing leaves, CO2 diffusion from the air to the carboxylation site is hindered by several structural and biochemical barriers. CO2 diffusion from the intercellular air space to the chloroplast stroma is obstructed by the mesophyll resistance. The inverses is the mesophyll conductance (g(m)). Whether various plants realize an optimal g(m), and how much investment is needed for a relevant g(m), remain unsolved. Scope This review examines relationships among leaf construction costs (CC), leaf maintenance costs (MC) and g(m) in various plants under diverse growth conditions. Through a literature survey, we demonstrate a strong linear relationship between leaf mass per area (LMA) and leaf CC. The overall correlation of CC vs. g(m) across plant phylogenetic groups is weak, but significant trends are evident within specific groups and/or environments. Investment in CC is necessary for an increase in LMA and mesophyll cell surface area (S-mes). This allows the leaf to accommodate more chloroplasts, thus increasing A. However, increases in LMA and/or S-mes often accompany other changes, such as cell wall thickening, which diminishes g(m). Such factors that make the correlations of CC and g(m) elusive are identified. Conclusions For evaluation of the contribution of g(m) to recover CC, leaf life span is the key factor. The estimation of MC in relation to g(m), especially in terms of costs required to regulate aquaporins, could be essential for efficient control of g(m) over the short term. Over the long term, costs are mainly reflected in CC, while benefits also include ultimate fitness attributes in terms of integrated carbon gain over the life of a leaf, plant survival and reproductive output.

The recovery from photoinhibition is much slower in photosystem (PS) I than in PSII; therefore, the susceptibility of PSI to photoinhibition is important with respect to photosynthetic production under special physiological conditions. Previous studies have shown that repetitive short-pulse (rSP) illumination selectively induces PSI photoinhibition. Depending on the growth light intensity or the variety/species of the plant, PSI photoinhibition is different, but the underlying mechanisms remain unknown. Here, we aimed to clarify whether the differences in the susceptibility of PSI to photoinhibition depend on environmental factors or on rice varieties and which physiological properties of the plant are related to this susceptibility. We exposed mature leaves of rice plants to rSP illumination. We examined the effects of elevated CO2 concentration and low N during growth on the susceptibility of PSI to photoinhibition and compared it in 12 different varieties. We fitted the decrease in the quantum yield of PSI during rSP illumination and estimated a parameter indicating susceptibility. Low N level increased susceptibility, whereas elevated CO2 concentration did not. The susceptibility differed among different rice varieties, and many indica varieties showed higher susceptibility than the temperate japonica varieties. Susceptibility was negatively correlated with the total chlorophyll content and N content. However, the decrease in Pm ' value, an indicator of damaged PSI, was positively correlated with chlorophyll content. This suggests that in leaves with a larger electron transport capacity, the overall PSI activity may be less susceptible to photoinhibition, but more damaged PSI may accumulate during rSP illumination.

Although N levels affect leaf photosynthetic capacity, the effects of N levels on the photochemistry of photosystems II and I (PSII and PSI, respectively) are not well-understood. In the present study, we examined this aspect in rice (Oryza sativa L. 'Hitomebore') plants grown under three different N levels at normal or high temperatures that can occur during rice culture and do not severely suppress photosynthesis. At both growth temperatures, the quantum efficiency of PSII [Y(II)] and the fraction of the primary quinone electron acceptor in its oxidized state were positively correlated with the amount of total leaf-N, whereas the quantum yields of non-photochemical quenching and donor-side limitation of PSI [Y(ND)] were negatively correlated with the amount of total leaf-N. These changes in PSII and PSI parameters were strongly correlated with each other. Growth temperatures scarcely affected these relationships. These results suggest that the photochemistry of PSII and PSI is coordinately regulated primarily depending on the amount of total leaf-N. When excess light energy occurs in low N-acclimated plants, oxidation of the reaction center chlorophyll of PSI is thought to be stimulated to protect PSI from excess light energy. It is also suggested that PSII and PSI normally operate at high temperature used in the present study. In addition, as the relationships between Y(II) and Y(ND) were found to be almost identical to those observed in osmotically stressed rice plants, common regulation is thought to be operative when excess light energy occurs due to different causes.

The temperature dependence of respiration rates and their acclimation to growth temperature vary among species/ecotypes, but the details remain unclear. Here, we compared the temperature dependence of shoot O2 consumption rates among Arabidopsis thaliana ecotypes to clarify how the temperature dependence and their acclimation to temperature differ among ecotypes, and how these differences relate to shoot growth. We examined growth analysis, temperature dependence of O2 consumption rates, and protein amounts of the respiratory chain components in shoots of twelve ecotypes of A. thaliana grown at three different temperatures. The temperature dependence of the O2 consumption rates were fitted to the modified Arrhenius model. The dynamic response of activation energy to measurement temperature was different among growth temperatures, suggesting that the plasticity of respiratory flux to temperatures differs among growth temperatures. The similar values of activation energy at growth temperature among ecotypes suggest that a similar process may determine the O2 consumption rates at the growth temperature in any ecotype. These results suggest that the growth temperature affects not only the absolute rate of O2 consumption but also the plasticity of respiratory flux in response to temperature, supporting the acclimation of shoot growth to various temperatures.