Photosynthetic response to fluctuating environments and photoprotective strategies under abiotic stress

Endophytic Bacillus pumilus G5 Interacting with Silicon to Improve Drought Stress Resilience in Glycyrrhiza uralensis Fisch. by Modulating Nitrogen Absorption, Assimilation, and Metabolism Pathways

Drought stress has become the primary severe threat to global agriculture production, including medicinal plants. Plant growth-promoting bacteria (PGPB) and environmentally friendly element silicon (Si) have emerged as effective methods in alleviating drought stress in various plants. Here, the effects of the plant endophytic G5 interaction with Si on regulating nitrogen absorption, assimilation, and metabolism pathways were investigated in the morphophysiological and gene attributes of Glycyrrhiza uralensis exposed to drought. Results showed that G5+Si application improved nitrogen absorption and assimilation by increasing the available nitrogen content in the soil, further improving the nitrogen utilization efficiency. Then, G5+Si triggered the accumulation of the major adjustment substances proline, γ-aminobutyric acid, putrescine, and chlorophyll, which played an important role in contributing to maintaining balance and energy supply in G. uralensis exposed to drought. These findings will provide new ideas for the combined application of PGPR and Si on both soil and plant systems in a drought habitat.

Natural genetic variation in dynamic photosynthesis is correlated with stomatal anatomical traits in diverse tomato species across geographical habitats

Plants grown under field conditions experience fluctuating light. Understanding the natural genetic variations for a similarly dynamic photosynthetic response among untapped germplasm resources, as well as the underlying mechanisms, may offer breeding strategies to improve production using molecular approaches. Here, we measured gas exchange under fluctuating light, along with stomatal density and size, in eight wild tomato species and two tomato cultivars. The photosynthetic induction response showed significant diversity, with some wild species having faster induction rates than the two cultivars. Species with faster photosynthetic induction rates had higher daily integrated photosynthesis, but lower average water use efficiency because of high stomatal conductance under natural fluctuating light. The variation in photosynthetic induction was closely associated with the speed of stomatal responses, highlighting its critical role in maximizing photosynthesis under fluctuating light conditions. Moreover, stomatal size was negatively correlated with stomatal density within a species, and plants with smaller stomata at a higher density had a quicker photosynthetic response than those with larger stomata at lower density. Our findings show that the response of stomatal conductance plays a pivotal role in photosynthetic induction, with smaller stomata at higher density proving advantageous for photosynthesis under fluctuating light in tomato species. The interspecific variation in the rate of stomatal responses could offer an untapped resource for optimizing dynamic photosynthetic responses under field conditions.

Roles of ApcD and orange carotenoid protein in photoinduction of electron transport upon dark-light transition in the Synechocystis PCC 6803 mutant deficient in flavodiiron protein Flv1

Flavodiiron proteins Flv1/Flv3 accept electrons from photosystem (PS) I. In this work we investigated light adaptation mechanisms of Flv1-deficient mutant of Synechocystis PCC 6803, incapable to form the Flv1/Flv3 heterodimer. First seconds of dark-light transition were studied by parallel measurements of light-induced changes in chlorophyll fluorescence, P700 redox transformations, fluorescence emission at 77 K, and OCP-dependent fluorescence quenching. During the period of Calvin cycle activation upon dark-light transition, the linear electron transport (LET) in wild type is supported by the Flv1/Flv3 heterodimer, whereas in Δflv1 mutant activation of LET upon illumination is preceded by cyclic electron flow that maintains State 2. The State 2-State 1 transition and Orange Carotenoid Protein (OCP)-dependent non-photochemical quenching occur independently of each other, begin in about 10 s after the illumination of the cells and are accompanied by a short-term re-reduction of the PSI reaction center (P700+). ApcD is important for the State 2-State 1 transition in the Δflv1 mutant, but S-M rise in chlorophyll fluorescence was not completely inhibited in Δflv1/ΔapcD mutant. LET in Δflv1 mutant starts earlier than the S-M rise in chlorophyll fluorescence, and the oxidation of plastoquinol (PQH2) pool promotes the activation of PSII, transient re-reduction of P700+ and transition to State 1. An attempt to induce state transition in the wild type under high intensity light using methyl viologen, highly oxidizing P700 and PQH2, was unsuccessful, showing that oxidation of intersystem electron-transport carriers might be insufficient for the induction of State 2-State 1 transition in wild type of Synechocystis under high light.

NADH dehydrogenase-like complex L subunit improves salt tolerance by enhancing photosynthetic electron transport

Cyclic electron transport (CET) around photosystem I (PSI) mediated by the NADH dehydrogenase-like (NDH) complex is closely related to plant salt tolerance. However, whether overexpression of a core subunit of the NDH complex affects the photosynthetic electron transport under salt stress is currently unclear. Here, we expressed the NDH complex L subunit (Ndhl) genes ZmNdhl1 and ZmNdhl2 from C4 plant maize (Zea mays) or OsNdhl from C3 plant rice (Oryza sativa) using a constitutive promoter in rice. Transgenic rice lines expressing ZmNdhl1, ZmNdhl2, or OsNdhl displayed enhanced salt tolerance, as indicated by greater plant height, dry weight, and leaf relative water content, as well as lower malondialdehyde content compared to wild-type plants under salt stress. Fluorescence parameters such as post-illumination rise (PIR), the prompt chlorophyll a fluorescence transient (OJIP), modulated 820-nm reflection (MR), and delayed chlorophyll a fluorescence (DF) remained relatively normal in transgenic plants during salt stress. These results indicate that expression of ZmNdhl1, ZmNdhl2, or OsNdhl increases cyclic electron transport activity, slows down damage to linear electron transport, alleviates oxidative damage to the PSI reaction center and plastocyanin, and reduces damage to electron transport on the receptor side of PSI in rice leaves under salt stress. Thus, expression of Ndhl genes from maize or rice improves salt tolerance by enhancing photosynthetic electron transport in rice. Maize and rice Ndhl genes played a similar role in enhancing salinity tolerance and avoiding photosynthetic damage.

Drought responsiveness in six wheat genotypes: identification of stress resistance indicators

Introduction

Wheat (Triticum aestivum L.) is among the world's most important staple food crops. In the current climate change scenario, a better understanding of wheat response mechanisms to water stress could help to enhance its productivity in arid ecosystems.

Methods

In this study, water relations, gas exchange, membrane integrity, agronomic traits and molecular analysis were evaluated in six wheat genotypes (D117, Syndiouk, Tunisian durum7 (Td7), Utique, Mahmoudi AG3 and BT) subjected to drought-stress.

Results and discussion

For all the studied genotypes, drought stress altered leaf area, chlorophyll content, stomatal density, photosynthetic rate and water-use efficiency, while the relative water content at turgor loss point (RWC0) remained stable. Changes in osmotic potential at turgor loss point (Ψπ0), bulk modulus of elasticity (Ɛmax) and stomatal regulation, differed greatly among the studied genotypes. For the drought-sensitive genotypes AG3 and BT, no significant changes were observed in Ψπ0, whereas the stomatal conductance (gs) and transpiration rate (E) decreased under stress conditions. These two varieties avoided turgor loss during drought treatment through an accurate stomatal control, resulting in a significant reduction in yield components. On the contrary, for Syndiouk, D117, Td7 and Utique genotypes, a solute accumulation and an increase in cell wall rigidity were the main mechanisms developed during drought stress. These mechanisms were efficient in enhancing soil water uptake, limiting leaf water loss and protecting cells membranes against leakage induced by oxidative damages. Furthermore, leaf soluble sugars accumulation was the major component of osmotic adjustment in drought-stressed wheat plants. The transcriptional analysis of genes involved in the final step of the ABA biosynthesis (AAO) and in the synthesis of an aquaporin (PIP2:1) revealed distinct responses to drought stress among the selected genotypes. In the resistant genotypes, PIP2:1 was significantly upregulated whereas in the sensitive ones, its expression showed only a slight induction. Conversely, the sensitive genotypes exhibited higher levels of AAO gene expression compared to the resistant genotypes. Our results suggest that drought tolerance in wheat is regulated by the interaction between the dynamics of leaf water status and stomatal behavior. Based on our findings, Syndiouk, D117, Utique and Td7, could be used in breeding programs for developing high-yielding and drought-tolerant wheat varieties.

Sink-source imbalance triggers delayed photosynthetic induction: Transcriptomic and physiological evidence

Sink-source imbalance causes accumulation of nonstructural carbohydrates (NSCs) and photosynthetic downregulation. However, despite numerous studies, it remains unclear whether NSC accumulation or N deficiency more directly decreases steady-state maximum photosynthesis and photosynthetic induction, as well as underlying gene expression profiles. We evaluated the relationship between photosynthetic capacity and NSC accumulation induced by cold girdling, sucrose feeding, and low nitrogen treatment in Glycine max and Phaseolus vulgaris. In G. max, changes in transcriptome profiles were further investigated, focusing on the physiological processes of photosynthesis and NSC accumulation. NSC accumulation decreased the maximum photosynthetic capacity and delayed photosynthetic induction in both species. In G. max, such photosynthetic downregulation was explained by coordinated downregulation of photosynthetic genes involved in the Calvin cycle, Rubisco activase, photochemical reactions, and stomatal opening. Furthermore, sink-source imbalance may have triggered a change in the balance of sugar-phosphate translocators in chloroplast membranes, which may have promoted starch accumulation in chloroplasts. Our findings provide an overall picture of photosynthetic downregulation and NSC accumulation in G. max, demonstrating that photosynthetic downregulation is triggered by NSC accumulation and cannot be explained solely by N deficiency.

High light-induced changes in whole-cell proteomic profile and its correlation with the organization of thylakoid super-complex in cyclic electron transport mutants of Chlamydomonas reinhardtii

Light and nutrients are essential components of photosynthesis. Activating the signaling cascades is critical in starting adaptive processes in response to high light. In this study, we have used wild-type (WT), cyclic electron transport (CET) mutants like Proton Gradient Regulation (PGR) (PGRL1), and PGR5 to elucidate the actual role in regulation and assembly of photosynthetic pigment-protein complexes under high light. Here, we have correlated the biophysical, biochemical, and proteomic approaches to understand the targeted proteins and the organization of thylakoid pigment-protein complexes in the photoacclimation. The proteomic analysis showed that 320 proteins were significantly affected under high light compared to the control and are mainly involved in the photosynthetic electron transport chain, protein synthesis, metabolic process, glycolysis, and proteins involved in cytoskeleton assembly. Additionally, we observed that the cytochrome (Cyt) b₆ expression is increased in the pgr5 mutant to regulate proton motive force and ATPase across the thylakoid membrane. The increased Cyt b₆ function in pgr5 could be due to the compromised function of chloroplast (cp) ATP synthase subunits for energy generation and photoprotection under high light. Moreover, our proteome data show that the photosystem subunit II (PSBS) protein isoforms (PSBS1 and PSBS2) expressed more than the Light-Harvesting Complex Stress-Related (LHCSR) protein in pgr5 compared to WT and pgrl1 under high light. The immunoblot data shows the photosystem II proteins D1 and D2 accumulated more in pgrl1 and pgr5 than WT under high light. In high light, CP43 and CP47 showed a reduced amount in pgr5 under high light due to changes in chlorophyll and carotenoid content around the PSII protein, which coordinates as a cofactor for efficient energy transfer from the light-harvesting antenna to the photosystem core. BN-PAGE and circular dichroism studies indicate changes in macromolecular assembly and thylakoid super-complexes destacking in pgrl1 and pgr5 due to changes in the pigment-protein complexes under high light. Based on this study, we emphasize that this is an excellent aid in understanding the role of CET mutants in thylakoid protein abundances and super-complex organization under high light.

CO2 mesophyll conductance regulated by light: a review

MAIN CONCLUSION

Light quality and intensity regulate plant mesophyll conductance, which has played an essential role in photosynthesis by controlling leaf structural and biochemical properties. Mesophyll conductance (g_m), a crucial physiological factor influencing the photosynthetic rate of leaves, is used to describe the resistance of CO₂ from the sub-stomatal cavity into the chloroplast up to the carboxylation site. Leaf structural and biochemical components, as well as external environmental factors such as light, temperature, and water, all impact g_m. As an essential factor of plant photosynthesis, light affects plant growth and development and plays a vital role in regulating g_m as well as determining photosynthesis and yield. This review aimed to summarize the mechanisms of g_m response to light. Both structural and biochemical perspectives were combined to reveal the effects of light quality and intensity on the g_m, providing a guide for selecting the optimal conditions for intensifying photosynthesis in plants.

Constrained disorder principle-based variability is fundamental for biological processes: Beyond biological relativity and physiological regulatory networks

The constrained disorder principle (CDP) defines systems based on their degree of disorder bounded by dynamic boundaries. The principle explains stochasticity in living and non-living systems. Denis Noble described the importance of stochasticity in biology, emphasizing stochastic processes at molecular, cellular, and higher levels in organisms as having a role beyond simple noise. The CDP and Noble's theories (NT) claim that biological systems use stochasticity. This paper presents the CDP and NT, discussing common notions and differences between the two theories. The paper presents the CDP-based concept of taking the disorder beyond its role in nature to correct malfunctions of systems and improve the efficiency of biological systems. The use of CDP-based algorithms embedded in second-generation artificial intelligence platforms is described. In summary, noise is inherent to complex systems and has a functional role. The CDP provides the option of using noise to improve functionality.

Photosynthetic acclimation to changing environments

Plants are exposed to environments that fluctuate of timescales varying from seconds to months. Leaves that develop in one set of conditions optimise their metabolism to the conditions experienced, in a process called developmental acclimation. However, when plants experience a sustained change in conditions, existing leaves will also acclimate dynamically to the new conditions. Typically this process takes several days. In this review, we discuss this dynamic acclimation process, focussing on the responses of the photosynthetic apparatus to light and temperature. We briefly discuss the principal changes occurring in the chloroplast, before examining what is known, and not known, about the sensing and signalling processes that underlie acclimation, identifying likely regulators of acclimation.

LED omics in Rocket Salad (Diplotaxis tenuifolia): Comparative Analysis in Different Light-Emitting Diode (LED) Spectrum and Energy Consumption

By applying three different LED light treatments, designated as blue (B), red (R)/blue (B), red (R) and white (W) light, as well as the control, the effect on Diplotaxis tenuifolia phenotype (yield and quality), and physiological, biochemical, and molecular status, as well as growing system resource use efficiency, was examined. We observed that basic leaf characteristics, such as leaf area, leaf number, relative chlorophyll content, as well as root characteristics, such as total root length and root architecture, remained unaffected by different LEDs. Yield expressed in fresh weight was slightly lower in LED lights than in the control (1113 g m^-2), with R light producing the least (679 g m^-2). However, total soluble solids were significantly affected (highest, 5.5° Brix, in R light) and FRAP was improved in all LED lights (highest, 191.8 μg/g FW, in B) in comparison to the control, while the nitrate content was less (lowest, 949.2 μg/g FW, in R). Differential gene expression showed that B LED light affected more genes in comparison to R and R/B lights. Although total phenolic content was improved under all LED lights (highest, 1.05 mg/g FW, in R/B), we did not detect a significant amount of DEGs in the phenylpropanoid pathway. R light positively impacts the expression of the genes encoding for photosynthesis components. On the other hand, the positive impact of R light on SSC was possibly due to the expression of key genes being induced, such as SUS1. In summary, this research is an integrative and innovative study, where the exploration of the effect of different LED lights on rocket growing under protected cultivation, in a closed chamber cultivation system, was performed at multiple levels.

Effect of gibberellic acid on photosynthesis and oxidative stress response in maize under weak light conditions

Gibberellin (GA) is an important endogenous hormone involved in plant responses to abiotic stresses. Experiments were conducted at the Research and Education Center of Agronomy, Shenyang Agricultural University (Shenyang, China) in 2021.We used a pair of near-isogenic inbred maize lines comprising, SN98A (light-sensitive inbred line) and SN98B (light-insensitive inbred line) to study the effects of exogenous gibberellin A3 (GA₃) application on different light-sensitive inbred lines under weak light conditions. The concentration of GA₃ was selected as 20, 40 and 60 mg L^-1. After shade treatment, the photosynthetic physiological indexes of SN98A were always lower than SN98B, and the net photosynthetic rate of SN98A was 10.12% lower than SN98B on the 20th day after shade treatment. GA₃ treatments significantly reduced the barren stalk ratios in SN98A and improved its seed setting rates by increasing the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), photosynthetic pigment contents, photochemical efficiency of photosystem II (PS II) (Fv/Fm), photochemical quenching coefficient (qP), effective quantum yield of PSII photochemistry (Φ_PSII), and antioxidant enzyme activities, where the most effective treatment was 60 mg L^-1GA₃. Compared with CK group, the seed setting rate increased by 33.87%. GA₃ treatment also regulated the metabolism of reactive oxygen species (ROS) and reduced the superoxide anion ( O 2 - ) production rate, H₂O₂ content, and malondialdehyde content. The superoxide anion ( O 2 - ) production rate, H₂O₂ content and malondialdehyde content of SN98A sprayed with 60 mg L^-1 GA₃ decreased by 17.32%,10.44% and 50.33% compared with CK group, respectively. Compared with the control, GA₃ treatment significantly (P < 0.05) increased the expression levels of APX and GR in SN98A, and APX, Fe-SOD, and GR in SN98B. Weak light stress decreased the expression of GA20ox2, which was related to gibberellin synthesis, and the endogenous gibberellin synthesis of SN98A. Weak light stress accelerated leaf senescence, and exogenous GA₃ application inhibited the ROS levels in the leaves and maintained normal physiological functions in the leaves. These results indicate that exogenous GA₃ enhances the adaptability of plants to low light stress by regulating photosynthesis, ROS metabolism and protection mechanisms, as well as the expression of key genes, which may be an economical and environmentally friendly method to solve the low light stress problem in maize production.

Long-Term Alleviation of the Functional Phenotype in Chlorophyll-Deficient Wheat and Impact on Productivity: A Semi-Field Phenotyping Experiment

Wheat mutants with a reduced chlorophyll synthesis are affected by a defective control of the photosynthetic electron flow, but tend to recover a wild-type phenotype. The sensitivity of some mutants to light fluctuations suggested that cultivation outdoors could significantly impact productivity. Six mutant lines of Triticum durum or Triticum aestivum with their respective wild-type cultivars were cultivated with a regular seasonal cycle (October-May) in a semi-field experiment. Leaf chlorophyll content and fluorescence parameters were analysed at the early (November) and late (May) developmental stages, and checked for correlation with morphometric and grain-production parameters. The alleviation of the phenotype severity concerned primarily the recovery of the photosynthetic-membrane functionality, but not the leaf chlorophyll content. Photosystem II (PSII) was less photoprotected in the mutants, but a moderate PSII photoinhibition could help control the electron flow into the chain. The accumulation of interchain electron carriers was a primary acclimative response towards the naturally fluctuating environment, maximally exploited by the mature durum-wheat mutants. The mutation itself and/or the energy-consuming compensatory mechanisms markedly influenced the plant morphogenesis, leading especially to reduced tillering, which in turn resulted in lower grain production per plant. Consistently with the interrelation between early photosynthetic phenotype and grain-yield per plant, chlorophyll-fluorescence indexes related to the level of photoprotective thermal dissipation (pNPQ), photosystem II antenna size (ABS/RC), and pool of electron carriers (Sm) are proposed as good candidates for the in-field phenotyping of chlorophyll-deficient wheat.

Adjustments in photosynthetic pigments, PS II photochemistry and photoprotection in a tropical C4 forage plant exposed to warming and elevated [CO2]

Global climate change will impact crops and grasslands, affecting growth and yield. However, is not clear how the combination of warming and increased atmospheric carbon dioxide concentrations ([CO₂]) will affect the photosystem II (PSII) photochemistry and the photosynthetic tissue photoinhibition and photoprotection on tropical forages. Here, we evaluated the effects of elevated [CO₂] (∼600 μmol mol^-1) and warming (+2 °C increase temperature) on the photochemistry of photosystem II and the photoprotection strategies of a tropical C4 forage Panicum maximum Jacq. grown in a Trop-T-FACE facility under well-watered conditions without nutrient limitation. Analysis of the maximum photochemical efficiency of PSII (F_v/F_m), the effective PSII quantum yield Y(II), the quantum yield of regulated energy dissipation Y(NPQ), the quantum yield of non-regulated energy dissipation Y(NO), and the malondialdehyde (MDA) contents in leaves revealed that the photosynthetic apparatus of plants did not suffer photoinhibitory damage, and plants did not increase lipid peroxidation in response to warming and [CO₂] enrichment. Plants under warming treatment showed a 12% higher chlorophyll contents and a 58% decrease in α-tocopherol contents. In contrast, carotenoid composition (zeaxanthin and β-carotene) and ascorbate levels were not altered by elevated [CO₂] and warming. The elevated temperature increased both net photosynthesis rate and aboveground biomass but elevated [CO₂] increased only net photosynthesis. Adjustments in chlorophyll, de-epoxidation state of the xanthophylls cycle, and tocopherol contents suggest leaves of P. maximum can acclimate to 2 °C warmer temperature and elevated [CO₂] when plants are grown with enough water and nutrients during tropical autumn-winter season.

Temporal heterogeneity in photosystem II photochemistry in Artemisia ordosica under a fluctuating desert environment

Acclimation strategies in xerophytic plants to stressed environmental conditions vary with temporal scales. Our understanding of environmentally-induced variation in photosystem II (PSII) processes as a function of temporal scales is limited, as most studies have thus far been based on short-term, laboratory-controlled experiments. In a study of PSII processes, we acquired near-continuous, field-based measurements of PSII-energy partitioning in a dominant desert-shrub species, namely Artemisia ordosica, over a six-year period from 2012-2017. Continuous-wavelet transformation (CWT) and wavelet coherence analyses (WTC) were employed to examine the role of environmental variables in controlling the variation in the three main PSII-energy allocation pathways, i.e., photochemical efficiency and regulated and non-regulated thermal dissipation, i.e., Φ _PSII, Φ _NPQ, and Φ _NO, respectively, across a time-frequency domain from hours to years. Convergent cross mapping (CCM) was subsequently used to isolate cause-and-effect interactions in PSII-energy partitioning response. The CWT method revealed that the three PSII-energy allocation pathways all had distinct daily periodicities, oscillating abruptly at intermediate timescales from days to weeks. On a diurnal scale, WTC revealed that all three pathways were influenced by photosynthetically active radiation (PAR), air temperature (T _a), and vapor pressure deficit (VPD). By comparing associated time lags for the three forms of energy partitioning at diurnal scales, revealed that the sensitivity of response was more acutely influenced by PAR, declining thereafter with the other environmental variables, such that the order of influence was greatest for T _a, followed by VPD, and then soil water content (SWC). PSII-energy partitioning on a seasonal scale, in contrast, displayed greater variability among the different environmental variables, e.g., Φ _PSII and Φ _NO being more predisposed to changes in T _a, and Φ _NPQ to changes in VPD. CCM confirmed the causal relationship between pairings of PSII-energy allocation pathways, according to shrub phenology. A. ordosica is shown to have an innate ability to (i) repair damaged PSII-photochemical apparatus (maximum quantum yield of PSII photochemistry, with F _v/F _m > 0.78), and (ii) acclimatize to excessive PAR, dry-air conditions, and prolonged drought. A. ordosica is relatively sensitive to extreme temperature and exhibits photoinhibition.

Anisohydric characteristics of a rice genotype 'ARC 11094' contribute to increased photosynthetic carbon fixation in response to high light

Photosynthetic induction, which is the response of the CO₂ assimilation rate to a stepwise increase in light intensity, potentially affects plant carbon gain and crop productivity in field environments. Although natural variations in photosynthetic induction are determined by CO₂ supply and its fixation, detailed factors, especially CO₂ supply, are unclear. This study investigated photosynthesis at steady and non-steady states in three rice (Oryza sativa L.) genotypes: ARC 11094, Takanari and Koshihikari. Stomatal traits and water relations in the plants were evaluated to characterise CO₂ supply. Photosynthetic induction in ARC 11094 and Takanari was superior to that in Koshihikari owing to an efficient CO₂ supply. The CO₂ supply in Takanari is attributed to its high stomatal density, small guard cell length and extensive root mass, whereas that in ARC 11094 is attributed to its high stomatal conductance per stoma and stomatal opening in leaves with insufficient water (i.e., anisohydric stomatal behaviour). Our results suggest that there are various mechanisms for realising an efficient CO₂ supply during the induction response. These characteristics can be useful for improving photosynthetic induction and, thus, crop productivity in field environments in future breeding programmes.

Insights on the regulation of photosynthesis in pea leaves exposed to oscillating light

Plants growing in nature often experience fluctuating irradiance. However, in the laboratory, the dynamics of photosynthesis are usually explored by instantaneously exposing dark-adapted plants to constant light and examining the dark-to-light transition, which is a poor approximation of natural phenomena. With the aim creating a better approximation, we exposed leaves of pea (Pisum sativum) to oscillating light and measured changes in the functioning of PSI and PSII, and of the proton motive force at the thylakoid membrane. We found that the dynamics depended on the oscillation period, revealing information about the underlying regulatory networks. As demonstrated for a selected oscillation period of 60 s, the regulation tries to keep the reaction centers of PSI and PSII open. We present an evaluation of the data obtained, and discuss the involvement of particular processes in the regulation of photosynthesis. The forced oscillations provided an information-rich fingerprint of complex regulatory networks. We expect future progress in understanding these networks from experiments involving chemical interventions and plant mutants, and by using mathematical modeling and systems identification and control tools.

Photosynthetic Apparatus of Hydrocharis morsus-ranae in Different Solar Lighting

Hydrocharis morsus-ranae is a free-floating species growing in lakes and slow-flowing rivers near the shore in Europe and Western Asia, and as an invasive plant in the USA and Canada. Light-requiring plants of this species can also grow in the shade, up to about 30% of full sunlight. In this paper we present the data about the photosynthetic apparatus of sunny and shady H. morsus-ranae plants grown in the sun and in the shade in nature. Methods of light and transmission electron microscopy, biochemistry, chlorophyll fluorescence induction as well as the principal component analysis were used. It was found that leaves of plants growing in shade differed from those in the sun with such traits as thickness of a blade, palisade and spongy parenchyma, ultrastructure of chloroplasts, and quantum efficiency of photosynthetic electron transport, the content of chlorophylls and carotenoids, anthocyanins and phenilpropanoids. By these traits, H. morsus-ranae shady plants are similar with shade-bearing plants that indicates their adaptation to light intensity lowering. The ordination plots (PCA) suggested a clear structural and functional shift of plants growing in different lighting showing relationship to light changes in the natural environment. Thus, our results displayed the high phenotypic plasticity of the H. morsus-ranae photosynthetic apparatus, which ensures its acclimation to changing light environment and wide distribution of this species.

Novel Approaches for Sustainable Horticultural Crop Production: Advances and Prospects

Reduction of plant growth, yield and quality due to diverse environmental constrains along with climate change significantly limit the sustainable production of horticultural crops. In this review, we highlight the prospective impacts that are positive challenges for the application of beneficial microbial endophytes, nanomaterials (NMs), exogenous phytohormones strigolactones (SLs) and new breeding techniques (CRISPR), as well as controlled environment horticulture (CEH) using artificial light in sustainable production of horticultural crops. The benefits of such applications are often evaluated by measuring their impact on the metabolic, morphological and biochemical parameters of a variety of cultures, which typically results in higher yields with efficient use of resources when applied in greenhouse or field conditions. Endophytic microbes that promote plant growth play a key role in the adapting of plants to habitat, thereby improving their yield and prolonging their protection from biotic and abiotic stresses. Focusing on quality control, we considered the effects of the applications of microbial endophytes, a novel class of phytohormones SLs, as well as NMs and CEH using artificial light on horticultural commodities. In addition, the genomic editing of plants using CRISPR, including its role in modulating gene expression/transcription factors in improving crop production and tolerance, was also reviewed.

Fast chlorophyll a fluorescence induction (OJIP) phenotyping of chlorophyll-deficient wheat suggests that an enlarged acceptor pool size of Photosystem I helps compensate for a deregulated photosynthetic electron flow

The wheat lines affected by a decrease in the leaf chlorophyll content typically experience a biomass loss. A known major problem of the chlorophyll-deficient wheat mutants is their limited prevention of Photosystem I (PSI) over-reduction brought about by an insufficient cyclic electron flow, potentially exposing them to a higher sensitivity to light fluctuations. However, the resistance of some mutant lines against fluctuating light suggests the occurrence of regulatory processes compensating for the defect in cyclic electron flow. In this study, a phenotyping approach based on fast chlorophyll a fluorescence induction (OJIP transient), corroborated by P700 redox kinetics, was applied to a collection of chlorophyll-deficient wheat lines, grown under continuous or fluctuating light. Quantitative parameters calculated from the OJIP transient are considered informative about Photosystem II (PSII) functional antenna size and photochemistry, as well as the functioning of the entire photosynthetic electron transport chain. The mutants tended to recover a wild-type-like chlorophyll content, and mature plants could hardly be distinguished based on their effective PSII antenna size. Nevertheless, specific OJIP-derived parameters were strongly correlated with the phenotype severity, in particular the amplitude of the I-P phase and the I-P/J-P amplitude ratio, which are indicative of a more capacitive pool of PSI final electron acceptors (ferredoxin and ferredoxin-NADP⁺ oxidoreductase, FNR). We propose that the enlargement of such pool of electron carriers is a compensatory response operating at the acceptor side of PSI to alleviate potentially harmful over-reduced states of PSI. Our results also suggest that, in chlorophyll-deficient mutants, higher F_V /F_M cannot prove a superior PSII photochemistry and wider I-P phase is not indicative of a higher relative content of PSI.

Fillable and unfillable gaps in plant transcriptome under field and controlled environments

The differences between plants grown in field and in controlled environments have long been recognized. However, few studies have addressed the underlying molecular mechanisms. To evaluate plant responses to fluctuating environments using laboratory equipment, we developed SmartGC, a high-performance growth chamber that reproduces the fluctuating irradiance, temperature and humidity of field environments. We analysed massive transcriptome data of rice plants grown under field and SmartGC conditions to clarify the differences in plant responses to field and controlled environments. Rice transcriptome dynamics in SmartGC mimicked those in the field, particularly during the morning and evening but those in conventional growth chamber conditions did not. Further analysis revealed that fluctuation of irradiance affects transcriptome dynamics in the morning and evening, while fluctuation of temperature affects transcriptome dynamics only in the morning. We found upregulation of genes related to biotic and abiotic stress, and their expression was affected by environmental factors that cannot be mimicked by SmartGC. Our results reveal fillable and unfillable gaps in the transcriptomes of rice grown in field and controlled environments and can accelerate the understanding of plant responses to field environments for both basic biology and agricultural applications.

Black-Box Mathematical Model for Net Photosynthesis Estimation and Its Digital IoT Implementation Based on Non-Invasive Techniques: Capsicum annuum L. Study Case

Photosynthesis is a vital process for the planet. Its estimation involves the measurement of different variables and its processing through a mathematical model. This article presents a black-box mathematical model to estimate the net photosynthesis and its digital implementation. The model uses variables such as: leaf temperature, relative leaf humidity, and incident radiation. The model was elaborated with obtained data from Capsicum annuum L. plants and calibrated using genetic algorithms. The model was validated with Capsicum annuum L. and Capsicum chinense Jacq. plants, achieving average errors of 3% in Capsicum annuum L. and 18.4% in Capsicum chinense Jacq. The error in Capsicum chinense Jacq. was due to the different experimental conditions. According to evaluation, all correlation coefficients (Rho) are greater than 0.98, resulting from the comparison with the LI-COR Li-6800 equipment. The digital implementation consists of an FPGA for data acquisition and processing, as well as a Raspberry Pi for IoT and in situ interfaces; thus, generating a useful net photosynthesis device with non-invasive sensors. This proposal presents an innovative, portable, and low-scale way to estimate the photosynthetic process in vivo, in situ, and in vitro, using non-invasive techniques.

Variation in biochemical, physiological and ecophysiological traits among the teak (Tectona grandis Linn. f) seed sources of India

Teak being an iconic timber species the studies on its physiological and biochemical traits are very limited in India and worldwide. As a result, the current study aimed to assess biochemical parameters such as chlorophyll a, chlorophyll b, total chlorophyll, carotenoids, chlorophyll ab ratio, proline content, and peroxidase activity, along with physiological parameters such as Chlorophyll stability index, relative water content, and leaf area, as well as ecophysiological traits such as net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO₂ concentration (Ci), transpiration rate (Tr), Leaf temperature, intrinsic water-use efficiency (iWUE), instantaneous water use efficiency and intrinsic carboxylation efficiency of thirty teak seed sources collected from different states of India. FCRITK 19, FCRITK 21, FCRITK 25, FCRITK 29, and FCRITK 05 were reported to have a greater photosynthetic rate (> 17 µmol m⁻² s⁻¹) coupled with a relative water content of more than 50% and a chlorophyll stability index of more than 60%, which could be used in a future genetic improvement programme. Correlation analysis indicated that water use efficiency was found to be strongly but negatively correlated with transpiration rate (−0.601) and stomatal conductance (−0.910). The proline content had a substantial positive correlation with the chlorophyll stability index (0.890), signifying that they are associated with abiotic stress conditions. Cluster analysis was attempted to discriminate the sources based on biochemical, physiological and ecophysiological traits. Eleven sources (FCRITK 25, FCRITK 27, FCRITK 29, FCRITK 14, FCRITK 30, FCRITK 16, FCRITK 05, FCRITK 13, FCRITK 02, FCRITK 17 and FCRITK 15) exhibited superior performance compared to rest of the sources.

Light-induced displacement of PLASTID MOVEMENT IMPAIRED1 precedes light-dependent chloroplast movements

Light-dependent chloroplast movements are an actin-dependent cellular response to changes in the light environment that help plants maximize photosynthetic potential and reduce photodamage. Over a dozen proteins are known to be required for normal chloroplast movements, but the molecular mechanisms regulating the transformation of light perception into chloroplast motility are not fully understood. Here, we show that in Arabidopsis (Arabidopsis thaliana) the actin-bundling plasma membrane-associated proteins THRUMIN1, PLASTID MOVEMENT IMPAIRED1 (PMI1), and KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT1 (KAC1) interact through the 14-3-3 proteins KAPPA and OMEGA. We also show that the interaction of PMI1 with 14-3-3 KAPPA and OMEGA is regulated by blue light activation of the Phototropin2 photoreceptor. Live-cell confocal microscopy revealed light-induced dynamic changes in the cellular localizations of PMI1 and KAC1. In particular, PMI1 was relocated away from irradiated areas of the plasma membrane in less than a minute after blue light exposure, consistent with PMI1 playing a critical role in initiating light-dependent chloroplast movements. We present a modified conceptual model for high light-dependent chloroplast movements in which PMI1 acts as the mobile signal that initiates a coordinated sequence of changes in protein-protein and protein-plasma membrane interactions that initiate the chloroplast movement response and determine where in the cell chloroplasts are able to anchor to the plasma membrane.

Towards improved dynamic photosynthesis in C3 crops by utilizing natural genetic variation

Under field environments, fluctuating light conditions induce dynamic photosynthesis, which affects carbon gain by crop plants. Elucidating the natural genetic variations among untapped germplasm resources and their underlying mechanisms can provide an effective strategy to improve dynamic photosynthesis and, ultimately, improve crop yields through molecular breeding approaches. In this review, we first overview two processes affecting dynamic photosynthesis, namely (i) biochemical processes associated with CO2 fixation and photoprotection and (ii) gas diffusion processes from the atmosphere to the chloroplast stroma. Next, we review the intra- and interspecific variations in dynamic photosynthesis in relation to each of these two processes. It is suggested that plant adaptations to different hydrological environments underlie natural genetic variation explained by gas diffusion through stomata. This emphasizes the importance of the coordination of photosynthetic and stomatal dynamics to optimize the balance between carbon gain and water use efficiency under field environments. Finally, we discuss future challenges in improving dynamic photosynthesis by utilizing natural genetic variation. The forward genetic approach supported by high-throughput phenotyping should be introduced to evaluate the effects of genetic and environmental factors and their interactions on the natural variation in dynamic photosynthesis.

Chickpea plant responses to polyphosphate fertiliser forms and drip fertigation frequencies: effect on photosynthetic performance and phenotypic traits

Photosynthesis is the main biophysiological process that governs plant growth and development. Under nutrient deficiency in crops and soils, many photosynthetic reactions can be disturbed. We compared two polyphosphates (Poly-A and Poly-B) and an orthophosphate fertiliser (Ortho-P) to an unfertilised treatment under three drip fertigation frequencies. Results showed that the electron transport chain between PSII and PSI was significantly enhanced in fertigated chickpea plants compared with the control treatment. The polyphosphate fertiliser (Poly-A) enhanced the number of electron acceptors of the photosynthetic linear electron transport chain compared with the other fertiliser forms. Furthermore, the time for reaching the maximum intensity F m was shortened in the fertilised chickpea plant indicating that the rate of light trapping and electron transport was enhanced under phosphorus drip fertigation. Also, the energy needed to close all reaction centres was decreased with P fertigated treatments, as revealed by the electron acceptor pool size of PSII (Sm/tFmax ). However, no significant effects of fertiliser forms or fertigation frequencies were observed on the energetic demand for reaction centres closure. Plants grown under polyphosphate fertigation absorbed significantly more phosphorus. Positive correlations between phosphorus uptake, photosynthetic yield, chickpea podding dynamic, and grain yield showed the beneficial effects of adequate phosphorus nutrition on chickpea growth and productivity.

Does Fluctuating Light Affect Crop Yield? A Focus on the Dynamic Photosynthesis of Two Soybean Varieties

In natural environments, plants are exposed to variable light conditions, but photosynthesis has been mainly studied at steady state and this might overestimate carbon (C) uptake at the canopy scale. To better elucidate the role of light fluctuations on canopy photosynthesis, we investigated how the chlorophyll content, and therefore the different absorbance of light, would affect the quantum yield in fluctuating light conditions. For this purpose, we grew a commercial variety (Eiko) and a chlorophyll deficient mutant (MinnGold) either in fluctuating (F) or non-fluctuating (NF) light conditions with sinusoidal changes in irradiance. Two different light treatments were also applied: a low light treatment (LL; max 650 μmol m^-2 s^-1) and a high light treatment (HL; max 1,000 μmol m^-2 s^-1). Canopy gas exchanges were continuously measured throughout the experiment. We found no differences in C uptake in LL treatment, either under F or NF. Light fluctuations were instead detrimental for the chlorophyll deficient mutant in HL conditions only, while the green variety seemed to be well-adapted to them. Varieties adapted to fluctuating light might be identified to target the molecular mechanisms responsible for such adaptations.

Molecular Mechanism of Organic Pollutant-Induced Reduction of Carbon Fixation and Biomass Yield in Oryza sativa L

Photosynthetic carbon fixation is fundamental for plant growth and is a key process driving the global carbon cycle. This study explored the mechanism of disturbed carbon fixation in Oryza sativa L. by organic pollutants 2,3,4,5-tetrachlorobiphenyl (CB 61), 4'-hydroxy-2,3,4,5-tetrachlorobiphenyl (4'-OH-CB 61), 2,2',4,4'-tetrabromo diphenyl ether (BDE 47), tricyclazole (TRI), and pyrene. The biomass of rice exposed to 4'-OH-CB 61, TRI, and BDE 47 was on average 80.63% of that of the control (p < 0.05), and the inhibition of net photosynthetic rate was 59.15% by 4'-OH-CB 61. Proteomics confirmed that 4'-OH-CB 61 significantly downregulated the enzymes in the photosynthetic carbon fixation pathway, which was attributed to the decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the rate-limiting enzyme in the Calvin cycle. In detail, decreased Rubisco activity (6.96-33.44%) and downregulated OsRBCS2-5 encoding small Rubisco subunits (-6.80 < log₂FC < -2.13) by 4'-OH-CB 61, TRI, and BDE 47 were in line with biomass yield reduction. Molecular docking and dynamic simulation suggested that the three pollutants potentially competed with CO₂ for binding to the active sites in Rubisco, leading to reduced CO₂ capture efficiency. These results revealed the molecular mechanism of organic pollution-induced rice yield reduction, contributing to improving the understanding of crop growth and carbon sequestration capacity of organics-contaminated soils globally.

Photosynthesis under fluctuating light in the CAM plant Vanilla planifolia

Photosynthetic induction after a sudden increase in illumination affects carbon gain. Photosynthetic dynamics under fluctuating light (FL) have been widely investigated in C3 and C4 plants but are little known in CAM plants. In our present study, the chlorophyll fluorescence, P700 redox state and electrochromic shift signals were measured to examine photosynthetic characteristics under FL in the CAM orchid Vanilla planifolia. The light use efficiency was maximized in the morning but was restricted in the afternoon, indicating that the pool of malic acid dried down in the afternoon. During photosynthetic induction in the morning, electron flow through photosystem I rapidly reached the 95% of the maximum value in 4-6 min, indicating that V. planifolia showed a fast photosynthetic induction when compared with C3 and C4 plants reported previously. Upon a sudden transition from dark to actinic light, a rapid re-oxidation of P700 was observed in V. planifolia, indicating the fast outflow of electrons from PSI to alternative electron acceptors, which was attributed to the O₂ photo-reduction mediated by water-water cycle. The functioning of water-water cycle prevented photosystem I over-reduction after transitioning from low to high light and thus protected PSI under FL. In the afternoon, cyclic electron flow was stimulated under FL to fine-tune photosynthetic apparatus when photosynthetic CO₂ was restricted. Therefore, water-water cycle cooperates with cyclic electron flow to regulate the photosynthesis under FL in the CAM orchid V. planifolia.

The role of photosynthesis related pigments in light harvesting, photoprotection and enhancement of photosynthetic yield in planta

Photosynthetic pigments are an integral and vital part of all photosynthetic machinery and are present in different types and abundances throughout the photosynthetic apparatus. Chlorophyll, carotenoids and phycobilins are the prime photosynthetic pigments which facilitate efficient light absorption in plants, algae, and cyanobacteria. The chlorophyll family plays a vital role in light harvesting by absorbing light at different wavelengths and allowing photosynthetic organisms to adapt to different environments, either in the long-term or during transient changes in light. Carotenoids play diverse roles in photosynthesis, including light capture and as crucial antioxidants to reduce photodamage and photoinhibition. In the marine habitat, phycobilins capture a wide spectrum of light and have allowed cyanobacteria and red algae to colonise deep waters where other frequencies of light are attenuated by the water column. In this review, we discuss the potential strategies that photosynthetic pigments provide, coupled with development of molecular biological techniques, to improve crop yields through enhanced light harvesting, increased photoprotection and improved photosynthetic efficiency.

Fluctuation at High Temperature Combined with Nutrients Alters the Thermal Dependence of Phytoplankton

The Metabolic Theory of Ecology (MTE) predicts that the temperature increases exert a common effect on organisms stimulating metabolic rates, this being stronger for a heterotrophic than for an autotrophic metabolism. However, no available studies within the MTE framework have focused on organisms' response under fluctuation at high temperature interacting with factors such as nutrient availability, or how this interaction could affect the coexistence between mixotrophic and strict autotrophic phytoplankton. Hence, we assess how the phytoplankton metabolism and species composition are affected under scenarios of high temperature and fluctuation at high temperature, and how nutrients alter the direction and magnitude of such impact. For that, we use a mixed culture composed of two phytoplankton species: a strict autotrophic species and a mixotrophic species. Our results indicate that, in agreement with the MTE, only fluctuation at high temperature treatment registered a greater activation energy (E_a) value for respiration than for primary production and stimulated mixotrophic over strict autotrophic species abundance compared to control treatment. Remarkably, fluctuation at high temperature had a strong negative impact on the total abundance of the mixed-culture. The interaction between nutrient enrichment and fluctuation at high temperature increased abundance of the strict autotrophic species and overall species abundance, and led to Ea values that were higher in primary production than in respiration. Changes in community composition, enhanced by nutrient enrichment, could be behind this response, which can have implications in ecosystem functioning in a changing world.

Metabolic adjustment and regulation of gene expression are essential for increased resistance to severe water deficit and resilience post-stress in soybean

Background

Soybean is the main oilseed crop grown in the world; however, drought stress affects its growth and physiology, reducing its yield. The objective of this study was to characterize the physiological, metabolic, and genetic aspects that determine differential resistance to water deficit in soybean genotypes.

Methods

Three soybean genotypes were used in this study, two lineages (L11644 and L13241), and one cultivar (EMBRAPA 48-C48). Plants were grown in pots containing 8 kg of a mixture of soil and sand (2:1) in a greenhouse under sunlight. Soil moisture in the pots was maintained at field capacity until the plants reached the stage of development V4 (third fully expanded leaf). At this time, plants were subjected to three water treatments: Well-Watered (WW) (plants kept under daily irrigation); Water Deficit (WD) (withholding irrigation until plants reached the leaf water potential at predawn of -1.5 ± 0.2 MPa); Rewatered (RW) (plants rehydrated for three days after reached the water deficit). The WW and WD water treatments were evaluated on the eighth day for genotypes L11644 and C48, and on the tenth day for L13241, after interruption of irrigation. For the three genotypes, the treatment RW was evaluated after three days of resumption of irrigation. Physiological, metabolic and gene expression analyses were performed.

Results

Water deficit inhibited growth and gas exchange in all genotypes. The accumulation of osmolytes and the concentrations of chlorophylls and abscisic acid (ABA) were higher in L13241 under stress. The metabolic adjustment of lineages in response to WD occurred in order to accumulate amino acids, carbohydrates, and polyamines in leaves. The expression of genes involved in drought resistance responses was more strongly induced in L13241. In general, rehydration provided recovery of plants to similar conditions of control treatment. Although the C48 and L11644 genotypes have shown some tolerance and resilience responses to severe water deficit, greater efficiency was observed in the L13241 genotype through adjustments in morphological, physiological, genetic and metabolic characteristics that are combined in the same plant. This study contributes to the advancement in the knowledge about the resistance to drought in cultivated plants and provides bases for the genetic improvement of the soybean culture.

A System Dynamics Approach to Model Photosynthesis at Leaf Level Under Fluctuating Light

Photosynthesis has been mainly studied under steady-state conditions even though this assumption results inadequate for assessing the biochemical responses to rapid variations occurring in natural environments. The combination of mathematical models with available data may enhance the understanding of the dynamic responses of plants to fluctuating environments and can be used to make predictions on how photosynthesis would respond to non-steady-state conditions. In this study, we present a leaf level System Dynamics photosynthesis model based and validated on an experiment performed on two soybean varieties, namely, the wild type Eiko and the chlorophyll-deficient mutant MinnGold, grown in constant and fluctuating light conditions. This mutant is known to have similar steady-state photosynthesis compared to the green wild type, but it is found to have less biomass at harvest. It has been hypothesized that this might be due to an unoptimized response to non-steady-state conditions; therefore, this mutant seems appropriate to investigate dynamic photosynthesis. The model explained well the photosynthetic responses of these two varieties to fluctuating and constant light conditions and allowed to make relevant conclusions on the different dynamic responses of the two varieties. Deviations between data and model simulations are mostly evident in the non-photochemical quenching (NPQ) dynamics due to the oversimplified combination of PsbS- and zeaxanthin-dependent kinetics, failing in finely capturing the NPQ responses at different timescales. Nevertheless, due to its simplicity, the model can provide the basis of an upscaled dynamic model at a plant level.

Drought stress reduces crop carbon gain due to delayed photosynthetic induction under fluctuating light conditions

Drought stress is a major limiting factor for crop growth and yield. Water availability in the field can cyclically change between drought and rewatering conditions, depending on precipitation patterns. Concurrently, light intensity under field conditions can fluctuate, inducing dynamic photosynthesis and transpiration during the crop growth period. The present study aimed to characterize carbon gain and water use in fluctuating light under drought and rewatering conditions in two major crops, namely rice and soybean. We conducted gas exchange measurements under fluctuating light conditions with rice and soybean plants exposed to drought treatment (9-13 days) imposed by withholding water and subsequent rewatering treatment (8-9 days). Drought stress significantly reduced the maximum CO₂ assimilation rate (A) in soybean but not in rice. Under drought conditions, A increased after a step increase in light and then gradually decreased in both crops, resulting in the significant reduction of steady-state A in rice and soybean. Moreover, drought stress delayed photosynthetic induction in both crops even when it had relatively small impact on maximum A. These results suggest that the drought effects on photosynthesis should be evaluated based on induction, maximum, and steady states. The delayed photosynthetic induction under drought owing to the reduced gas diffusional conductance via stomata resulted in a substantial loss of leaf carbon gain under fluctuating light conditions. Meanwhile, rewatering, after drought, completely recovered photosynthesis under fluctuating light in both crops. Therefore, the stability of photosynthetic induction can be a promising target to improve drought tolerance during crop breeding in the future.

Lack of plastid-encoded Ycf10, a homolog of the nuclear-encoded DLDG1 and the cyanobacterial PxcA, enhances the induction of non-photochemical quenching in tobacco

pH homeostasis in the chloroplast is crucial for the control of photosynthesis and other metabolic processes in plants. Recently, nuclear-encoded Day-Length-dependent Delayed Greening1 (DLDG1) and Fluctuating-Light Acclimation Protein1 (FLAP1) that are required for the light-inducible optimization of plastidial pH in Arabidopsis thaliana were identified. DLDG1 and FLAP1 homologs are specifically conserved in oxygenic phototrophs, and a DLDG1 homolog, Ycf10, is encoded in the chloroplast genome in plant cells. However, the function of Ycf10 and its physiological significance are unknown. To address this, we constructed ycf10 tobacco Nicotiana tabacum mutants and characterized their phenotypes. The ycf10 tobacco mutants grown under continuous-light conditions showed a pale-green phenotype only in developing leaves, and it was suppressed in short-day conditions. The ycf10 mutants also induced excessive non-photochemical quenching (NPQ) compared with those in the wild-type at the induction stage of photosynthesis. These phenotypes resemble those of Arabidopsis dldg1 mutants, suggesting that they have similar functions. However, there are distinct differences between the two mutant phenotypes: The highly induced NPQ in tobacco ycf10 and the Arabidopsis dldg1 mutants are diminished and enhanced, respectively, with increasing duration of the fluctuating actinic-light illumination. Ycf10 and DLDG1 were previously shown to localize in chloroplast envelope-membranes, suggesting that Ycf10 and DLDG1 differentially control H+ exchange across these membranes in a light-dependent manner to control photosynthesis.

Characterization of Nannochloropsis oceanica Rose Bengal Mutants Sheds Light on Acclimation Mechanisms to High Light When Grown in Low Temperature

A barrier to realizing Nannochloropsis oceanica's potential for omega-3 eicosapentaenoic acid (EPA) production is the disparity between conditions that are optimal for growth and those that are optimal for EPA biomass content. A case in point is temperature: higher content of polyunsaturated fatty acid, and especially EPA, is observed in low-temperature (LT) environments, where growth rates are often inhibited. We hypothesized that mutant strains of N. oceanica resistant to the singlet-oxygen photosensitizer Rose Bengal (RB) would withstand the oxidative stress conditions that prevail in the combined stressful environment of high light (HL; 250 μmol photons m-2 s-1) and LT (18°C). This growth environment caused the wild-type (WT) strain to experience a spike in lipid peroxidation and an inability to proliferate, whereas growth and homeostatic reactive oxygen species levels were observed in the mutant strains. We suggest that the mutant strains' success in this environment can be attributed to their truncated photosystem II antennas and their increased ability to diffuse energy in those antennas as heat (non-photosynthetic quenching). As a result, the mutant strains produced upward of four times more EPA than the WT strain in this HL-LT environment. The major plastidial lipid monogalactosyldiacylglycerol was a likely target for oxidative damage, contributing to the photosynthetic inhibition of the WT strain. A mutation in the NO10G01010.1 gene, causing a subunit of the 2-oxoisovalerate dehydrogenase E1 protein to become non-functional, was determined to be the likely source of tolerance in the RB113 mutant strain.

Different responses of two Chinese cabbage (Brassica rapa L. ssp. pekinensis) cultivars in photosynthetic characteristics and chloroplast ultrastructure to salt and alkali stress

Salt and alkali stress affected the photosynthetic characteristics of Chinese cabbages. A salt-tolerant cultivar maintained its tolerance by ensuring the high ability of photosynthesis. The synthesis of organic acids and carbohydrates in leaves played important roles in improving the photosynthetic capacity of alkali-tolerant plants. Soil salinization has become an increasingly serious ecological problem, which limits the quality and yield of crops. As an important economic vegetable in winter, however, little is known about the response of Chinese cabbage to salt, alkali and salt-alkali stress in photosynthetic characteristics and chloroplast ultrastructure. Thus, two Chinese cabbage cultivars, 'Qinghua' (salt-tolerant-alkali-sensitive) and 'Biyu' (salt-sensitive-alkali-tolerant) were investigated under stresses to clarify the similarities and differences between salt tolerance and alkali tolerance pathways in Chinese cabbage. We found that the root of Qinghua, the leaf ultrastructure and net photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE), maximum photochemical quantum yield of PSII (Fv/Fm) and nonphotochemical quenching (NPQ) were not affected by salt stress. However, Biyu was seriously affected under salt stress. Its growth indexes decreased by between 60 and 30% compared with the control and the photosynthetic indexes were also seriously affected under salt stress. This indicated that the salt-tolerant cultivar Qinghua improved the photosynthetic fluorescence ability to promote the synthesis of organic matter resulting in salt tolerance. In contrast, under alkali treatment, the root of Biyu was affected by alkali stress, but could still maintain good growth, and root and leaf structure were not seriously affected and could maintain the normal operations. Biyu improved its tolerance by improving the water use efficiency, regulating the synthesis of organic acids and carbohydrates, ensuring the synthesis of organic matter and ensured the normal growth of the plant.

Photosynthesis dynamics and regulation sensed in the frequency domain

Foundations of photosynthesis research have been established mainly by studying the response of plants to changing light, typically to sudden exposure to a constant light intensity after dark acclimation or light flashes. This approach remains valid and powerful, but can be limited by requiring dark acclimation before time-domain measurements and often assumes that rate constants determining the photosynthetic response do not change between dark and light acclimation. We show that these limits can be overcome by measuring plant responses to sinusoidally modulated light of varying frequency. By its nature, such frequency-domain characterization is performed in light-acclimated plants with no need for prior dark acclimation. Amplitudes, phase shifts, and upper harmonic modulation extracted from the data for a wide range of frequencies can target different kinetic domains and regulatory feedbacks. The occurrence of upper harmonic modulation reflects nonlinear phenomena, including photosynthetic regulation. To support these claims, we measured chlorophyll fluorescence emission of the green alga Chlorella sorokiniana in light that was sinusoidally modulated in the frequency range 1000-0.001 Hz. Based on these experimental data and numerical as well as analytical mathematical models, we propose that frequency-domain measurements can become a versatile tool in plant sensing.

Photosystem I in low light-grown leaves of Alocasia odora, a shade-tolerant plant, is resistant to fluctuating light-induced photoinhibition

When intact green leaves are exposed to the fluctuating light, in which high light (HL) and low light (LL) alternate, photosystem I (PSI) is readily damaged. This PSI inhibition is mostly alleviated by the addition of far-red (FR) light. Here, we grew Alocasia odora, a shade-tolerant species, at several light levels and examined their photosynthetic traits in relation to the fluctuating light-induced PSI inhibition. We found that, even in the absence of FR, PSI in LL-grown leaves was resistant to the fluctuating light. LL leaves showed higher chlorophyll (Chl) contents on leaf area basis, lower Chl a/b ratios, lower cytochrome f/P700 ratios, and lower PSII/PSI excitation ratios assessed by the 77 K fluorescence. Also, P700 in the HL phase of the fluctuating light was more oxidized. The results of the regression analyses of the PSI photoinhibition to these traits indicate that the lower electron flow rate to P700 and more excitation energy transfer to PSI protect PSI in LL-grown leaves. Both of these contribute oxidization of P700 to the efficient quencher form P700⁺. These features may be common in LL-grown shade-tolerant species, which are often exposed to strong sunflecks in their natural habitats.

The Optical Properties of Leaf Structural Elements and Their Contribution to Photosynthetic Performance and Photoprotection

Leaves have evolved to effectively harvest light, and, in parallel, to balance photosynthetic CO2 assimilation with water losses. At times, leaves must operate under light limiting conditions while at other instances (temporally distant or even within seconds), the same leaves must modulate light capture to avoid photoinhibition and achieve a uniform internal light gradient. The light-harvesting capacity and the photosynthetic performance of a given leaf are both determined by the organization and the properties of its structural elements, with some of these having evolved as adaptations to stressful environments. In this respect, the present review focuses on the optical roles of particular leaf structural elements (the light capture module) while integrating their involvement in other important functional modules. Superficial leaf tissues (epidermis including cuticle) and structures (epidermal appendages such as trichomes) play a crucial role against light interception. The epidermis, together with the cuticle, behaves as a reflector, as a selective UV filter and, in some cases, each epidermal cell acts as a lens focusing light to the interior. Non glandular trichomes reflect a considerable part of the solar radiation and absorb mainly in the UV spectral band. Mesophyll photosynthetic tissues and biominerals are involved in the efficient propagation of light within the mesophyll. Bundle sheath extensions and sclereids transfer light to internal layers of the mesophyll, particularly important in thick and compact leaves or in leaves with a flutter habit. All of the aforementioned structural elements have been typically optimized during evolution for multiple functions, thus offering adaptive advantages in challenging environments. Hence, each particular leaf design incorporates suitable optical traits advantageously and cost-effectively with the other fundamental functions of the leaf.

Arabidopsis CHLOROPHYLLASE 1 protects young leaves from long-term photodamage by facilitating FtsH-mediated D1 degradation in photosystem II repair

The proteolytic degradation of the photodamaged D1 core subunit during the photosystem II (PSII) repair cycle is well understood, but chlorophyll turnover during D1 degradation remains unclear. Here, we report that Arabidopsis thaliana CHLOROPHYLLASE 1 (CLH1) plays important roles in the PSII repair process. The abundance of CLH1 and CLH2 peaks in young leaves and is induced by high-light exposure. Seedlings of clh1 single and clh1-1/2-2 double mutants display increased photoinhibition after long-term high-light exposure, whereas seedlings overexpressing CLH1 have enhanced light tolerance compared with the wild type. CLH1 is localized in the developing chloroplasts of young leaves and associates with the PSII-dismantling complexes RCC1 and RC47, with a preference for the latter upon exposure to high light. Furthermore, degradation of damaged D1 protein is retarded in young clh1-1/2-2 leaves after 18-h high-light exposure but is rescued by the addition of recombinant CLH1 in vitro. Moreover, overexpression of CLH1 in a variegated mutant (var2-2) that lacks thylakoid protease FtsH2, with which CLH1 interacts, suppresses the variegation and restores D1 degradation. A var2-2 clh1-1/2-2 triple mutant shows more severe variegation and seedling death. Taken together, these results establish CLH1 as a long-sought chlorophyll dephytylation enzyme that is involved in PSII repair and functions in long-term adaptation of young leaves to high-light exposure by facilitating FtsH-mediated D1 degradation.

Effects of Artificially Reproduced Fluctuations in Sunlight Spectral Distribution on the Net Photosynthetic Rate of Cucumber Leaves

The effects of photosynthetic photon flux density (PPFD) fluctuations in sunlight have already been investigated; however, the spectral photon flux density distribution (SPD) has hardly been considered. Here, sunlight SPD fluctuations recorded for 200 min in October in Tokyo, Japan were artificially reproduced using an LED-artificial sunlight source system. The net photosynthetic rate (P n) of cucumber leaves under reproduced sunlight was measured and compared with the P n estimated from a steady-state PPFD-P n curve for the same leaves. The measured and estimated P n agreed except when the PPFD was low, where the measured P n was lower than the estimated P n. The ratio of measured P n to estimated P n was 0.94-0.95 for PPFD ranges of 300-700 μmol m-2 s-1, while the value was 0.98-0.99 for 900-1,300 μmol m-2 s-1, and the overall ratio was 0.97. This 3% reduction in the measured P n compared with the P n estimated from a steady-state PPFD-P n curve was significantly smaller than the approximately 20-30% reduction reported in previous experimental and simulation studies. This result suggests that the loss of integral net photosynthetic gain under fluctuating sunlight can vary among days with different fluctuation patterns or may be non-significant when fluctuations in both PPFD and relative SPD of sunlight are taken into consideration.

A Holistic Approach to Study Photosynthetic Acclimation Responses of Plants to Fluctuating Light

Plants in natural environments receive light through sunflecks, the duration and distribution of these being highly variable across the day. Consequently, plants need to adjust their photosynthetic processes to avoid photoinhibition and maximize yield. Changes in the composition of the photosynthetic apparatus in response to sustained changes in the environment are referred to as photosynthetic acclimation, a process that involves changes in protein content and composition. Considering this definition, acclimation differs from regulation, which involves processes that alter the activity of individual proteins over short-time periods, without changing the abundance of those proteins. The interconnection and overlapping of the short- and long-term photosynthetic responses, which can occur simultaneously or/and sequentially over time, make the study of long-term acclimation to fluctuating light in plants challenging. In this review we identify short-term responses of plants to fluctuating light that could act as sensors and signals for acclimation responses, with the aim of understanding how plants integrate environmental fluctuations over time and tailor their responses accordingly. Mathematical modeling has the potential to integrate physiological processes over different timescales and to help disentangle short-term regulatory responses from long-term acclimation responses. We review existing mathematical modeling techniques for studying photosynthetic responses to fluctuating light and propose new methods for addressing the topic from a holistic point of view.

Maintaining higher leaf photosynthesis after heading stage could promote biomass accumulation in rice

Leaf photosynthetic rate changes across the growing season as crop plants age. Most studies of leaf photosynthesis focus on a specific growth stage, leaving the question of which pattern of photosynthetic dynamics maximizes crop productivity unanswered. Here we obtained high-frequency data of canopy leaf CO2 assimilation rate (A) of two elite rice (Oryza sativa) cultivars and 76 inbred lines across the whole growing season. The integrated A value after heading was positively associated with crop growth rate (CGR) from heading to harvest, but that before heading was not. A curve-smoothing analysis of A after heading showed that accumulated A at > 80% of its maximum (A80) was positively correlated with CGR in analyses of all lines mixed and of lines grouped by genetic background, while the maximum A and accumulated A at ≤ 80% were less strongly correlated with CGR. We also found a genomic region (~ 12.2 Mb) that may enhance both A80 and aboveground biomass at harvest. We propose that maintaining a high A after heading, rather than having high maximum A, is a potential target for enhancing rice biomass accumulation.

Minimizing VPD Fluctuations Maintains Higher Stomatal Conductance and Photosynthesis, Resulting in Improvement of Plant Growth in Lettuce

Vapor pressure deficit (VPD) is considered to be one of the major environmental factors influencing stomatal functions and photosynthesis, as well as plant growth in crop and horticultural plants. In the greenhouse cultivation, air temperature and relative air humidity are regulated by switching on/off the evaporative systems and opening/closing the roof windows, which causes VPD fluctuation. However, it remains unclear how VPD fluctuation affects photosynthetic and growth performance in plants. Here, we examined the effects of the VPD fluctuation on the photosynthetic and growth characteristics in lettuce (Lactuca sativa L.). The parameters for gas exchange and chlorophyll fluorescence and biomass production were evaluated under the conditions of drastic (1.63 kPa for 6 min and 0.63 for 3 min) or moderate (1.32 kPa for 7 min and 0.86 kPa for 3 min) VPD fluctuation. The drastic VPD fluctuation induced gradual decrease in stomatal conductance and thus CO₂ assimilation rate during the measurements, while moderate VPD fluctuation caused no reduction of these parameters. Furthermore, data showed moderate VPD fluctuation maintained leaf expansion and the efficiency of CO₂ diffusion across leaf surface, resulting in enhanced plant growth compared with drastic VPD fluctuation. Taken together, fine regulation of VPD can be crucial for better plant growth by maintaining the photosynthetic performance in lettuce. The present work demonstrates the importance of VPD control during plant cultivation in plant factories and greenhouses.

Leaf photosynthetic and anatomical insights into mechanisms of acclimation in rice in response to long-term fluctuating light

Long-term fluctuating light (FL) conditions are very common in natural environments. The physiological and biochemical mechanisms for acclimation to FL differ between species. However, most of the current conclusions regarding acclimation to FL were made based on studies in algae or Arabidopsis thaliana. It is still unclear how rice (Oryza sativa L.) integrate multiple physiological changes to acclimate to long-term FL. In this study, we found that rice growth was repressed under long-term FL. By systematically measuring phenotypes and physiological parameters, we revealed that: (a) under short-term FL, photosystem I (PSI) was inhibited, while after 1-7 days of long-term FL, both PSI and PSII were inhibited. Higher acceptor-side limitation in electron transport and higher overall nonphotochemical quenching (NPQ) explained the lower efficiencies of PSI and PSII, respectively. (b) An increase in pH differences across the thylakoid membrane and a decrease in thylakoid proton conductivity revealed a reduction of ATP synthase activity. (c) Using electron microscopy, we showed a decrease in membrane stacking and stomatal opening after 7 days of FL treatment. Taken together, our results show that electron flow, ATP synthase activity and NPQ regulation are the major processes determining the growth performance of rice under long-term FL conditions.

Stomatal, mesophyll conductance, and biochemical limitations to photosynthesis during induction

The dynamics of leaf photosynthesis in fluctuating light affects carbon gain by plants. Mesophyll conductance (gm) limits CO2 assimilation rate (A) under the steady state, but the extent of this limitation under non-steady-state conditions is unknown. In the present study, we aimed to characterize the dynamics of gm and the limitations to A imposed by gas diffusional and biochemical processes under fluctuating light. The induction responses of A, stomatal conductance (gs), gm, and the maximum rate of RuBP carboxylation (Vcmax) or electron transport (J) were investigated in Arabidopsis (Arabidopsis thaliana (L.)) and tobacco (Nicotiana tabacum L.). We first characterized gm induction after a change from darkness to light. Each limitation to A imposed by gm, gs and Vcmax or J was significant during induction, indicating that gas diffusional and biochemical processes limit photosynthesis. Initially, gs imposed the greatest limitation to A, showing the slowest response under high light after long and short periods of darkness, assuming RuBP-carboxylation limitation. However, if RuBP-regeneration limitation was assumed, then J imposed the greatest limitation. gm did not vary much following short interruptions to light. The limitation to A imposed by gm was the smallest of all the limitations for most of the induction phase. This suggests that altering induction kinetics of mesophyll conductance would have little impact on A following a change in light. To enhance the carbon gain by plants under naturally dynamic light environments, attention should therefore be focused on faster stomatal opening or activation of electron transport.