Reversible changes in structure and function of photosynthetic apparatus of pea (Pisum sativum) leaves under drought stress

Light intensity moderates photosynthesis by optimizing photosystem mechanisms under high VPD stress

In recent decades, the global increase in vapor pressure deficit (VPD) has significantly inhibited plant growth and photosynthesis. Light intensity, a crucial environmental regulator, plays a vital role in stress response and photosynthetic adjustment. This study investigated whether increasing light intensity under high VPD conditions could optimise the photosystem and thereby enhance photosynthesis. We designed experiments using factorial combinations of two VPD levels (HVPD; high VPD, AVPD; appropriate VPD) and two irradiance gradients (L300; 300 μmol photons m-2 s-1, L600; 600 μmol photons m-2 s-1). Under high VPD, plants protect their photosystems by reducing light energy absorption and limiting photosynthetic electron flow, which results in reduced photosynthesis. However, when exposed to HVPD + L600, plants exhibited increased light energy absorption, as evidenced by elevated chlorophyll b and carotenoid levels, enhanced response to irradiance, and decreased NPQ and Y(NO). This regimen also enhanced photosynthetic electron transport by increasing the total driving force and plastoquinone pool, consequently improving the photochemical efficiency of the photosystem and ultimately boosting the net photosynthetic rate by 46.9%. This study confirmed that modulating light intensity under high VPD stress can improve photosynthesis by optimizing the photosystem. This novel approach can be utilized to enhance tomato production in arid regions.

Decoding the physicochemical basis of resurrection: the journey of lichen Flavoparmelia caperata through prolonged water scarcity to full rehydration

Desiccation tolerance is a complex phenomenon observed in the lichen Flavoparmelia ceparata. To understand the reactivation process of desiccated thalli, completely dried samples were rehydrated. The rehydration process of this lichen occurs in two phases. The first phase, characterized by rapid rehydration, involves the conversion of non-functional reaction centers (RCs) into functional PSII RCs, and the accumulation of ROS along with the increment in SOD antioxidant enzyme. These coordinated mechanisms initiate the light reaction of photosynthesis by forming active light-harvesting complexes (LHCs). This adaptation ensures efficient recovery, as evidenced by specific energy fluxes (ABS/RC, TR/RC, ET/RC, and DI/RC), phenomenological fluxes (ABS/CS, TR/CS, ET/CS, and DI/CS), quantum efficiencies (ФP0, ФE0, and ФD0), primary and secondary photochemistry, photochemical and non-photochemical quenching, and performance index, highlighting the essential role of rapid water uptake in restoring turgor pressure for cell structure and function maintenance. The interconnected network of antioxidant defenses, including catalase (CAT) and peroxidase (POD), underscores the plant's ability to cope with oxidative stress during resilience. The acid phosphomonoesterase (PME) enzymatic activity corresponds to its role in releasing phosphate for essential cellular functions and post-rehydration thallus growth. The activity of CAT, GPOD, and PME signifies the gradual reactivation of lichen F. caperata. Moreover, the investigation into chlorophyll a fluorescence emphasizes the efficient reactivation of the photosynthetic process in F. caperata. In conclusion, lichen F. caperata demonstrates significant potential for desiccation tolerance through the rapid transformation of chloroplasts, chlorophylls, and PSII RCs from their inactive to active states upon rehydration. This research not only enhances our understanding of desiccation tolerance in resurrection plants but also highlights the importance of lichens, particularly F. caperata, as valuable models for studying plant resilience in challenging environments.

Exploring the Genetic Basis of Drought Tolerance in Alhagi camelorum: A Comprehensive Transcriptome Study of Osmotic Stress Adaptations

Alhagi camelorum, a desert shrub known for its impressive drought tolerance, exhibits notable resilience under arid conditions. However, the underlying mechanisms driving its drought resistance remain largely unexplored. This study aims to investigate these mechanisms by exposing A. camelorum to osmotic stress using varying polyethylene glycol (PEG) concentrations (1%, 5%, 10%) in a controlled laboratory setting. Growth analysis revealed significant inhibition and phenotypic changes with increasing PEG levels. Transcriptomic analysis, including differentially expressed gene identification, GO enrichment analysis, and hierarchical cluster analysis of genes in roots and shoots, identified key pathways associated with drought adaptation, such as ABA-activated signaling, cell wall biogenesis, photosynthesis, and secondary metabolite biosynthesis. Notably, some genes involved in these pathways exhibited tissue-specific expression patterns and showed PEG concentration-dependent regulation. Key findings include the dose-dependent (R2 > 0.8) upregulation of a proline-rich protein (Asp01G030840) and a BURP domain-containing protein (Asp02G039780), as well as critical genes involved in cell wall biogenesis (encoding Pectinesterase inhibitor domain-containing and Fasciclin-like arabinogalactan protein), and secondary metabolite biosynthesis (encoding enzymes for terpenoid and flavonoid biosynthesis). The regulation of these genes is likely influenced by phytohormones such as ABA and other stress-related hormones, along with significant transcription factors like ABI4, TALE, MYB61, GRAS, and ERF. These insights lay the groundwork for further research into the functional roles of these genes, their regulatory networks, and their potential applications in enhancing drought resistance in desert plants and agricultural crops.

Mild osmotic stress offers photoprotection in Chlamydomonas reinhardtii under high light

The exposure of autotrophs to high light intensities significantly impacts their photosynthetic performance. When combined with unpredictable climate changes, the lethality of these effects is exacerbated and, often surpassing the organisms' threshold for tolerance. In this regard, our study centres on examining the mitigating effects of mild osmotic stress induced by 2% Polyethylene Glycol (PEG) in conjunction with high-light conditions, using Chlamydomonas reinhardtii as a model system. Cells were cultivated under low PEG-induced osmotic stress at various light intensities, and their responses were analyzed through biochemical and biophysical approaches. Remarkably, cells grown under lower PEG concentrations exhibited superior growth, increased biomass, and enhanced photosynthetic efficiency under high light compared to non-PEG-treated cells. Surprisingly, their non-photochemical quenching (NPQ) levels were lower, indicating the operation of a distinct photoprotective mechanism in PEG-grown samples. The PEG-grown cells demonstrated higher chlorophyll content but lower carotenoid content, supporting the NPQ data. Circular dichroism analysis suggested that the macro-organization of super-complexes was minimally disrupted in PEG-grown samples, even under high light. This was further supported by Blue native PAGE, which showed greater stability of the super-complexes in PEG-grown cells, implying heightened stability in pigment-protein interactions. Immunoblot analysis revealed minimal differences in core reaction center proteins between PEG-grown and non-PEG cells. Notably, this protective mechanism was absent in the cell wall-deficient mutant CC503. We propose that the partial photoprotection observed is attributed to the PEG shielding the cell wall. This result holds promise for enhancing algal biomass production under natural environmental conditions influenced by fluctuating light intensity.

Drought Tolerance in Plants: Physiological and Molecular Responses

Drought, a significant environmental challenge, presents a substantial risk to worldwide agriculture and the security of food supplies. In response, plants can perceive stimuli from their environment and activate defense pathways via various modulating networks to cope with stress. Drought tolerance, a multifaceted attribute, can be dissected into distinct contributing mechanisms and factors. Osmotic stress, dehydration stress, dysfunction of plasma and endosome membranes, loss of cellular turgidity, inhibition of metabolite synthesis, cellular energy depletion, impaired chloroplast function, and oxidative stress are among the most critical consequences of drought on plant cells. Understanding the intricate interplay of these physiological and molecular responses provides insights into the adaptive strategies plants employ to navigate through drought stress. Plant cells express various mechanisms to withstand and reverse the cellular effects of drought stress. These mechanisms include osmotic adjustment to preserve cellular turgor, synthesis of protective proteins like dehydrins, and triggering antioxidant systems to counterbalance oxidative stress. A better understanding of drought tolerance is crucial for devising specific methods to improve crop resilience and promote sustainable agricultural practices in environments with limited water resources. This review explores the physiological and molecular responses employed by plants to address the challenges of drought stress.

Golden 2-like Transcription Factors Regulate Photosynthesis under UV-B Stress by Regulating the Calvin Cycle

UV-B stress can affect plant growth at different levels, and although there is a multitude of evidence confirming the effects of UV-B radiation on plant photosynthesis, there are fewer studies using physiological assays in combination with multi-omics to investigate photosynthesis in alpine plants under stressful environments. Golden 2-like (G2-like/GLK) transcription factors (TFs) are highly conserved during evolution and may be associated with abiotic stress. In this paper, we used Handy-PEA and Imaging-PAM Maxi to detect chlorophyll fluorescence in leaves of Rhododendron chrysanthum Pall. (R. chrysanthum) after UV-B stress, and we also investigated the effect of abscisic acid (ABA) on photosynthesis in plants under stress environments. We used a combination of proteomics, widely targeted metabolomics, and transcriptomics to study the changes of photosynthesis-related substances after UV-B stress. The results showed that UV-B stress was able to impair the donor side of photosystem II (PSII), inhibit electron transfer and weaken photosynthesis, and abscisic acid was able to alleviate the damage caused by UV-B stress to the photosynthetic apparatus. Significant changes in G2-like transcription factors occurred in R. chrysanthum after UV-B stress, and differentially expressed genes localized in the Calvin cycle were strongly correlated with members of the G2-like TF family. Multi-omics assays and physiological measurements together revealed that G2-like TFs can influence photosynthesis in R. chrysanthum under UV-B stress by regulating the Calvin cycle. This paper provides insights into the study of photosynthesis in plants under stress, and is conducive to the adoption of measures to improve photosynthesis in plants under stress to increase yield.

Integrating leaf spectral and water status information to effectively track chlorophyll a fluorescence parameters during dehydration

Monitoring changes in chlorophyll a (ChlFa) fluorescence during dehydration can provide insights into plant photosynthetic responses to climate change challenges, which are predicted to increase drought frequency. However, the limited knowledge of how ChlFa parameters respond to water deficit hinders the exploration of the photochemical mechanism of the photosynthetic process and the simulation of photosynthetic fluorescence models. Furthermore, how to track such responses of ChlFa parameters, especially at large scales, remains a challenge. In this study, we attempted to use spectral information reflected from leaves to follow the dynamic response patterns of ChlFa parameters of seven species under prolonged dehydration. The results showed that the investigated ChlFa parameters exhibited significant changes as dehydration progressed, with considerable variability among the different species as well as under different water conditions. This study also demonstrated that the integration of both spectral and water content information can provide an effective method for tracking ChlFa parameters during dehydration, explaining over 90% of the total variance in the measured ChlFa parameters. Collectively, these results should serve as a valuable reference for predicting the response of ChlFa parameters to dehydration and offer a potential method for estimating ChlFa parameters under drought conditions.

Nitrate nitrogen enhances the efficiency of photoprotection in Leymus chinensis under drought stress

Introduction

Global climate change exerts a significant impact on the nitrogen supply and photosynthesis ability in land-based plants. The photosynthetic capacity of dominant grassland species is important if we are to understand carbon cycling under climate change. Drought stress is one of the major factors limiting plant photosynthesis, and nitrogen (N) is an essential nutrient involved in the photosynthetic activity of leaves. The regulatory mechanisms responsible for the effects of ammonium (NH4 +) and nitrate (NO3 -) on the drought-induced photoinhibition of photosystem II (PSII) in plants have yet to be fully elucidated. Therefore, there is a significant need to gain a better understanding of the role of electron transport in the photoinhibition of PSII.

Methods

In the present study, we conducted experiments with normal watering (LD), severe drought (MD), and extreme drought (HD) treatments, along with no nitrogen (N0), ammonium (NH4), nitrate (NO3), and mixed nitrogen (NH4NO3) treatments. We analyzed pigment accumulation, reactive oxygen species (ROS) accumulation, photosynthetic enzyme activity, photosystem activity, electron transport, and O-J-I-P kinetics.

Results

Analysis showed that increased nitrate application significantly increased the leaf chlorophyll content per unit area (Chlarea) and nitrogen content per unit area (Narea) (p< 0.05). Under HD treatment, ROS levels were lower in NO3-treated plants than in N0 plants, and there was no significant difference in photosynthetic enzyme activity between plants treated with NO3 and NH4NO3. Under drought stress, the maximum photochemical efficiency of PSII (Fv/Fm), PSII electron transport rate (ETR), and effective quantum yield of PSII (φPSII) were significant higher in NO3-treated plants (p< 0.05). Importantly, the K-band and G-band were higher in NO3-treated plants.

Discussion

These results suggest that drought stress hindered the formation of NADPH and ATP in N0 and NH4-treated L. chinensis plants, thus damaging the donor side of the PSII oxygen-evolving complex (OEC). After applying nitrate, higher photosynthetic enzyme and antioxidant enzyme activity not only protected PSII from photodamage under drought stress but also reduced the rate of damage in PSII during the growth of L. chinensis growth under drought stress.

Drought affects both photosystems in Arabidopsis thaliana

Drought is a major abiotic stress that impairs plant growth and development. Despite this, a comprehensive understanding of drought effects on the photosynthetic apparatus is lacking. In this work, we studied the consequences of 14-d drought treatment on Arabidopsis thaliana. We used biochemical and spectroscopic methods to examine photosynthetic membrane composition and functionality. Drought led to the disassembly of PSII supercomplexes and the degradation of PSII core. The light-harvesting complexes (LHCII) instead remain in the membrane but cannot act as an antenna for active PSII, thus representing a potential source of photodamage. This effect can also be observed during nonphotochemical quenching (NPQ) induction when even short pulses of saturating light can lead to photoinhibition. At a later stage, under severe drought stress, the PSI antenna size is also reduced and the PSI-LHCI supercomplexes disassemble. Surprisingly, although we did not observe changes in the PSI core protein content, the functionality of PSI is severely affected, suggesting the accumulation of nonfunctional PSI complexes. We conclude that drought affects both photosystems, although at a different stage, and that the operative quantum efficiency of PSII (ΦPSII ) is very sensitive to drought and can thus be used as a parameter for early detection of drought stress.

Molecular Mechanisms of Plant Defense against Abiotic Stress

The climatic changes and anthropogenic factors in recent decades (global warming, drought, salinity, extreme temperature, environmental pollution) have led to an increase in the negative impact of environmental factors on plants. Abiotic stress strongly influences the important processes of plants and thus affects their growth and development. The effects of stressors on the plants depend on the intensity, frequency, and duration of stress, plant species as well as a combination of various stressors. Plants have developed different mechanisms to limit adverse environmental conditions. In the publications in this Special Issue, Molecular Mechanisms of Plant Defense against Abiotic Stress, new information on plant defense mechanisms against abiotic and biotic stress is presented. The studies help us better understand plants' protection mechanisms again global climate change.

Tomato Biodiversity and Drought Tolerance: A Multilevel Review

Ongoing global climate change suggests that crops will be exposed to environmental stresses that may affect their productivity, leading to possible global food shortages. Among these stresses, drought is the most important contributor to yield loss in global agriculture. Drought stress negatively affects various physiological, genetic, biochemical, and morphological characteristics of plants. Drought also causes pollen sterility and affects flower development, resulting in reduced seed production and fruit quality. Tomato (Solanum lycopersicum L.) is one of the most economically important crops in different parts of the world, including the Mediterranean region, and it is known that drought limits crop productivity, with economic consequences. Many different tomato cultivars are currently cultivated, and they differ in terms of genetic, biochemical, and physiological traits; as such, they represent a reservoir of potential candidates for coping with drought stress. This review aims to summarize the contribution of specific physio-molecular traits to drought tolerance and how they vary among tomato cultivars. At the genetic and proteomic level, genes encoding osmotins, dehydrins, aquaporins, and MAP kinases seem to improve the drought tolerance of tomato varieties. Genes encoding ROS-scavenging enzymes and chaperone proteins are also critical. In addition, proteins involved in sucrose and CO₂ metabolism may increase tolerance. At the physiological level, plants improve drought tolerance by adjusting photosynthesis, modulating ABA, and pigment levels, and altering sugar metabolism. As a result, we underline that drought tolerance depends on the interaction of several mechanisms operating at different levels. Therefore, the selection of drought-tolerant cultivars must consider all these characteristics. In addition, we underline that cultivars may exhibit distinct, albeit overlapping, multilevel responses that allow differentiation of individual cultivars. Consequently, this review highlights the importance of tomato biodiversity for an efficient response to drought and for preserving fruit quality levels.

Sensitivity of the Photosynthetic Apparatus in Maize and Sorghum under Different Drought Levels

Drought is one of the main environmental stress factors affecting plant growth and yield. The impact of different PEG concentrations on the photosynthetic performance of maize (Zea mays L. Mayflower) and sorghum (Sorghum bicolor L. Foehn) was investigated. The activity of the photosynthetic apparatus was assessed using chlorophyll fluorescence (PAM and JIP test) and photooxidation of P₇₀₀. The data revealed that water deficiency decreased the photochemical quenching (qP), the ratio of photochemical to nonphotochemical processes (Fv/Fo), the effective quantum yield of the photochemical energy conversion in PSII (Φ_PSII), the rate of the electron transport (ETR), and the performance indexes PI_total and PI_ABS, as the impact was stronger in sorghum than in maize and depended on drought level. The PSI photochemistry (P₇₀₀ photooxidation) in sorghum was inhibited after the application of all studied drought levels, while in maize, it was registered only after treatment with higher PEG concentrations (30% and 40%). Enhanced regulated energy losses (Φ_NPQ) and activation of the state transition under drought were also observed in maize, while in sorghum, an increase mainly in nonregulated energy losses (Φ_NO). A decrease in pigment content and relative water content and an increase in membrane damage were also registered after PEG treatment. The experimental results showed better drought tolerance of maize than sorghum. This study provides new information about the role of regulated energy losses and state transition for the protection of the photosynthetic apparatus under drought and might be a practical approach to the determination of the drought tolerance of plants.

Mechanistic Insights on Salicylic Acid Mediated Enhancement of Photosystem II Function in Oregano Seedlings Subjected to Moderate Drought Stress

Dramatic climate change has led to an increase in the intensity and frequency of drought episodes and, together with the high light conditions of the Mediterranean area, detrimentally influences crop production. Salicylic acid (SA) has been shown to supress phototoxicity, offering photosystem II (PSII) photoprotection. In the current study, we attempted to reveal the mechanism by which SA is improving PSII efficiency in oregano seedlings under moderate drought stress (MoDS). Foliar application of SA decreased chlorophyll content under normal growth conditions, but under MoDS increased chlorophyll content, compared to H₂O-sprayed oregano seedlings. SA improved the PSII efficiency of oregano seedlings under normal growth conditions at high light (HL), and under MoDS, at both low light (LL) and HL. The mechanism by which, under normal growth conditions and HL, SA sprayed oregano seedlings compared to H₂O-sprayed exhibited a more efficient PSII photochemistry, was the increased (17%) fraction of open PSII reaction centers (qp), and the increased (7%) efficiency of these open reaction centers (Fv'/Fm'), which resulted in an enhanced (24%) electron transport rate (ETR). SA application under MoDS, by modulating chlorophyll content, resulted in optimized antenna size and enhanced effective quantum yield of PSII photochemistry (Φ_PSII) under both LL (7%) and HL (25%), compared to non-SA-sprayed oregano seedlings. This increased effective quantum yield of PSII photochemistry (Φ_PSII) was due to the enhanced efficiency of the oxygen evolving complex (OEC), and the increased fraction of open PSII reaction centers (qp), which resulted in an increased electron transport rate (ETR) and a lower amount of singlet oxygen (¹O₂) production with less excess excitation energy (EXC).