Salt stress and fluctuating light have separate effects on photosynthetic acclimation, but interactively affect biomass

Biogenic volatile organic compounds (BVOCs) emissions and physiological changes in Pinus densiflora and Quercus acutissima seedlings under elevated particulate matter (PM)

Urban trees effectively reduce air pollution, including particulate matter (PM), which is a major concern in East Asia. While acting as biofilters, urban trees can be affected by PM exposure, which hinders their growth and physiological functions, thereby reducing their pollution mitigation ability. Trees absorb pollutants but also emit biogenic volatile organic compounds (BVOCs), which can act as precursors to other forms of air pollution. To better understand the effects of PM on urban trees, this study examined how two tree species, Pinus densiflora and Quercus acutissima, respond to elevated PM levels under controlled conditions at a concentration of 300 μg m-3. The aim was to investigate how increased PM levels affect BVOCs emissions and physiological responses in seedlings, and how these physiological changes influence BVOCs emission pattern. The results revealed species-specific responses in BVOCs emissions under PM stress with being especially oxygenated monoterpenes more than non‑oxygenated monoterpenes. Increased PM adsorption was found to reduce photosynthetic abilities, including photosynthesis (Anet), carboxylation capacity (Vcmax), and electron transport rate (J). This reduction in photosynthetic efficiency was further evidenced by decreased chlorophyll content and light absorption, which were assessed through chlorophyll fluorescence measurements. Additionally, the study evaluated oxidative stress indicators, such as lipid peroxidation and the accumulation of reactive oxygen species (ROS), to provide a comprehensive understanding of the species' responses to elevated PM conditions. The study found that elevated PM conditions were closely linked to an increase in oxygenated monoterpenes, which was associated with both oxidative stress and impaired physiological function. These observations emphasize the need for strategic urban tree selection to enhance air quality and suggest that understanding species-specific BVOCs emissions in response to PM is crucial for optimizing urban green spaces for health and environmental benefits.

Chloride accumulation in inland rivers of China and its toxic impact on cotton

The escalation of major ion concentrations in freshwater and soil poses diverse effects on ecosystems and the environment. Excessive ions can exhibit toxicity to aquatic organisms and terrestrial plants. Currently, research on ion toxicity primarily focuses on cation toxicity. Notably, there is a noticeable research gap in understanding the impact of chloride ion (Cl-) on plant growth and development, as well as on the defense mechanisms against Cl- toxicity. In the present study, sampling was conducted on major rivers in China to measure Cl- concentrations. The results revealed that certain rivers exhibited excessive levels of Cl-, emphasizing the critical need to address Cl- toxicity issues. Subsequently, when salt-tolerant cotton seedlings were subjected to various chloride treatments, it was observed that excessive Cl- severely hindered plant growth and development. A combined analysis of transcriptomic and metabolomic data shed light on significantly enriched pathways related to galactose metabolism, arginine and proline metabolism, carotenoid metabolism, and alpha-linolenic acid metabolism under chloride stress. In summary, this research provides a scientific foundation and references for environmental management and water resource protection and offers novel insights for mitigating the adverse impacts of Cl-, thereby contributing to the preservation of ecosystem health.

Advances and mechanisms of fungal symbionts in improving the salt tolerance of crops

Soil salinization leads to reduced crop yields and waste of land resources, thereby impacting global food security. To meet the increasing demand for food and simultaneously alleviate pressure on soil resources, the development of sustainable agriculture is imperative. In contrast to physical and chemical methods, bioremediation represents an efficient and environmentally friendly approach. Fungal symbionts have been found to be associated with most plants in natural ecosystems, colonizing and residing within the internal tissues of host plants. Moreover, the potential of fungal symbionts in improving saline-alkaline soil has been widely recognized and confirmed. Numerous reports have documented the effectiveness of arbuscular mycorrhizal fungi in alleviating salt stress in plants. Meanwhile, research on other endophytic fungi for mitigating plant salt stress has emerged in recent years, which contributes to refining mechanisms for enhancing plant salt tolerance. In this review, we summarized various mechanisms by which endophytic fungi enhance plant salt tolerance. We also provided an overview of the challenges and development directions in the field of fungal symbiosis, with the aim of offering a viable strategy for the bioremediation of saline-alkali soils.

Initial stomatal conductance increases photosynthetic induction of trees leaves more from sunlit than from shaded environments: a meta-analysis

It has long been held that tree species/leaves from shaded environments show faster rate of photosynthetic induction than species/leaves from sunlit environments, but the evidence so far is conflicting and the underlying mechanisms are still under debate. To address the debate, we compiled a dataset for 87 tree species and compared the initial increasing slope during the first 2-min induction (SA) and stomatal and biochemical characteristics between sun and shade species from the same study, and those between sun and shade leaves within the same species. In 77% of between-species comparisons, the species with high steady-state photosynthetic rate in the high light (Af) exhibited a larger SA than the species with low Af. In 67% within-species comparisons, the sun leaves exhibited a larger SA than the shade leaves. However, in only a few instances did the sun species/leaves more rapidly achieve 50% of full induction, with an even smaller SA, than the shade species/leaves. At both the species and leaf level, SA increased with increasing initial stomatal conductance before induction (gsi). Despite exhibiting reduced intrinsic water-use efficiency in low light, a large SA proportionally enhances photosynthetic carbon gain during the first 2-min induction in the sun species and leaves. Thus, in terms of the increase in absolute rate of photosynthesis, tree species/leaves from sunlit environments display faster photosynthetic induction responses than those from shaded environments. Our results call for re-consideration of contrasting photosynthetic strategies in photosynthetic adaption/acclimation to dynamic light environments across species.

Estimation of Photosynthetic Induction Is Significantly Affected by Light Environments of Local Leaves and Whole Plants in Oryza Genus

Photosynthetic induction and stomatal kinetics are acknowledged as pivotal factors in regulating both plant growth and water use efficiency under fluctuating light conditions. However, the considerable variability in methodologies and light regimes used to assess the dynamics of photosynthesis (A) and stomatal conductance (gs) during light induction across studies poses challenges for comparison across species. Moreover, the influence of stomatal morphology on both steady-state and non-steady-state gs remains poorly understood. In this study, we show the strong impact of IRGA Chamber Illumination and Whole Plant Illumination on the photosynthetic induction of two rice species. Our findings reveal that these illuminations significantly enhance photosynthetic induction by modulating both stomatal and biochemical processes. Moreover, we observed that a higher density of smaller stomata plays a critical role in enhancing the stomatal opening and photosynthetic induction to fluctuating light conditions, although it exerts minimal influence on steady-state gs and A under constant light conditions. Therefore, future studies aiming to estimate photosynthetic induction and stomatal kinetics should consider the light environments at both the leaf and whole plant levels.

Sensitivity and responses of chloroplasts to salt stress in plants

Chloroplast, the site for photosynthesis and various biochemical reactions, is subject to many environmental stresses including salt stress, which affects chloroplast structure, photosynthetic processes, osmotic balance, ROS homeostasis, and so on. The maintenance of normal chloroplast function is essential for the survival of plants. Plants have developed different mechanisms to cope with salt-induced toxicity on chloroplasts to ensure the normal function of chloroplasts. The salt tolerance mechanism is complex and varies with plant species, so many aspects of these mechanisms are not entirely clear yet. In this review, we explore the effect of salinity on chloroplast structure and function, and discuss the adaptive mechanisms by which chloroplasts respond to salt stress. Understanding the sensitivity and responses of chloroplasts to salt stress will help us understand the important role of chloroplasts in plant salt stress adaptation and lay the foundation for enhancing plant salt tolerance.

Mesophyll conductance limits photosynthesis in fluctuating light under combined drought and heat stresses

Drought and heat stresses usually occur concomitantly in nature, with increasing frequency and intensity of both stresses expected due to climate change. The synergistic agricultural impacts of these compound climate extremes are much greater than those of the individual stresses. However, the mechanisms by which drought and heat stresses separately and concomitantly affect dynamic photosynthesis have not been thoroughly assessed. To elucidate this, we used tomato (Solanum lycopersicum) seedlings to measure dynamic photosynthesis under individual and compound stresses of drought and heat. Individual drought and heat stresses limited dynamic photosynthesis at the stages of diffusional conductance to CO2 and biochemistry, respectively. However, the primary limiting factor for photosynthesis shifted to mesophyll conductance under the compound stresses. Compared with the control, photosynthetic carbon gain in fluctuating light decreased by 38%, 73%, and 114% under the individual drought, heat, and compound stresses, respectively. Therefore, compound stresses caused a greater reduction in photosynthetic carbon gain in fluctuating light conditions than individual stress. These findings highlight the importance of mitigating the effects of compound climate extremes on crop productivity by targeting mesophyll conductance and improving dynamic photosynthesis.

Effects of Environmental and Non-Environmental Factors on Dynamic Photosynthetic Carbon Assimilation in Leaves under Changing Light

Major research on photosynthesis has been carried out under steady light. However, in the natural environment, steady light is rare, and light intensity is always changing. Changing light affects (usually reduces) photosynthetic carbon assimilation and causes decreases in biomass and yield. Ecologists first observed the importance of changing light for plant growth in the understory; other researchers noticed that changing light in the crop canopy also seriously affects yield. Here, we review the effects of environmental and non-environmental factors on dynamic photosynthetic carbon assimilation under changing light in higher plants. In general, dynamic photosynthesis is more sensitive to environmental and non-environmental factors than steady photosynthesis, and dynamic photosynthesis is more diverse than steady photosynthesis. Finally, we discuss the challenges of photosynthetic research under changing light.

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.

Drought stress delays photosynthetic induction and accelerates photoinhibition under short-term fluctuating light in tomato

Fluctuating light (FL) and drought stress usually occur concomitantly. However, whether drought stress affects photosynthetic performance under FL remains unknown. Here, we measured gas exchange, chlorophyll fluorescence, and P700 redox state under FL in drought-stressed tomato (Solanum lycopersicum) seedlings. Drought stress significantly delayed the induction kinetics of stomatal and mesophyll conductances after transition from low to high light and thus delayed photosynthetic induction under FL. Therefore, drought stress exacerbated the loss of carbon gain under FL. Furthermore, restriction of CO₂ fixation under drought stress aggravated the over-reduction of photosystem I (PSI) upon transition from low to high light. The resulting stronger FL-induced PSI photoinhibition significantly suppressed linear electron flow and PSI photoprotection. These results indicated that drought stress not only caused a larger loss of carbon gain under FL but also accelerated FL-induced photoinhibition of PSI. Furthermore, drought stress enhanced relative cyclic electron flow in FL, which partially compensated for restricted CO₂ fixation and thus favored PSI photoprotection under FL. To our knowledge, we here show new insight into how drought stress affects photosynthetic performance under FL.

Transcriptome, proteome and functional characterization reveals salt stress tolerance mechanisms in upland cotton (Gossypium hirsutum L.)

Uncovering the underlying mechanism of salt tolerance is important to breed cotton varieties with improved salt tolerance. In this study, transcriptome and proteome sequencing were performed on upland cotton (Gossypium hirsutum L.) variety under salt stress, and integrated analysis was carried out to exploit salt-tolerance genes in cotton. Enrichment analysis using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) was performed on differentially expressed genes (DEGs) obtained from transcriptome and proteome sequencing. GO enrichment was carried out mainly in the cell membrane, organelle, cellular process, metabolic process, and stress response. The expression of 23,981 genes was changed in physiological and biochemical processes such as cell metabolism. The metabolic pathways obtained by KEGG enrichment included glycerolipid metabolism, sesquiterpene and triterpenoid biosynthesis, flavonoid production, and plant hormone signal transduction. Combined transcriptome and proteome analysis to screen and annotate DEGs yielded 24 candidate genes with significant differential expression. The quantitative real-time polymerase chain reaction (qRT-PCR) validation of the candidate genes showed that two genes (Gh_D11G0978 and Gh_D10G0907) responded significantly to the induction of NaCl, and these two genes were further selected as target genes for gene cloning and functional validation through virus-induced gene silencing (VIGS). The silenced plants exhibited early wilting with a greater degree of salt damage under salt treatment. Moreover, they showed higher levels of reactive oxygen species (ROS) than the control. Therefore, we can infer that these two genes have a pivotal role in the response to salt stress in upland cotton. The findings in this research will facilitate the breeding of salt tolerance cotton varieties that can be grown on saline alkaline lands.

Photosynthetic mechanism of maize yield under fluctuating light environments in the field

The photosynthetic mechanism of crop yields in fluctuating light environments in the field remains controversial. To further elucidate this mechanism, we conducted field and simulation experiments using maize (Zea mays) plants. Increased planting density enhanced the light fluctuation frequency and reduced the duration of daily high light, as well as the light-saturated photosynthetic rate, biomass, and yield per plant. Further analysis confirmed a highly significant positive correlation between biomass and yield per plant and the duration of photosynthesis related to daily high light. The simulation experiment indicated that the light-saturated photosynthetic rate of maize leaves decreased gradually and considerably when shortening the daily duration of high light. Under an identical duration of high light exposure, increasing the fluctuation frequency decreased the light-saturated photosynthetic rate slightly. Proteomic data also demonstrated that photosynthesis was mainly affected by the duration of high light and not by the light fluctuation frequency. Consequently, the current study proposes that an appropriate duration of daily high light under fluctuating light environments is the key factor for greatly improving photosynthesis. This is a promising mechanism by which the photosynthetic productivity and yield of maize can be enhanced under complex light environments in the field.

Arbuscular mycorrhizal fungi affect the expression of PxNHX gene family, improve photosynthesis and promote Populus simonii×P. nigra growth under saline-alkali stress

INTRODUCTION

Saline-alkali stress seriously endangers the normal growth of Populus simonii×P. nigra. Arbuscular mycorrhizal (AM) fungi can enhance the saline-alkali tolerance of plants by establishing a symbiotic relationship with them.

METHODS

In this study, a pot experiment was conducted to simulate a saline-alkali environment where Populus simonii×P. nigra were inoculated with Funneliformis mosseae to explore their effects on the saline-alkali tolerance of Populus simonii×P. nigra.

RESULTS AND DISCUSSION

Our results show that a total of 8 NHX gene family members are identified in Populus simonii×P. nigra. F. mosseae regulate the distribution of Na+ by inducing the expression of PxNHXs. The pH value of poplar rhizosphere soil is reduced, result in the promote absorption of Na⁺ by poplar, that ultimately improved the soil environment. Under saline-alkali stress, F. mosseae improve the chlorophyll fluorescence and photosynthetic parameters of poplar, promote the absorption of water, K⁺ and Ca²⁺, thus increase the plant height and fresh weight of aboveground parts, and promote the growth of poplar. Our results provide a theoretical basis for further exploring the application of AM fungi to improve the saline-alkali tolerance of plants.

Polybrominated diphenyl ethers interact with the key protein involved in carbohydrate metabolism in rice

Rice exposed to organic pollutants such as polybrominated diphenyl ethers (PBDEs) usually experiences reduced biomass and increased soluble sugar content. This study showed that 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) led to increased glucose, fructose, and sucrose in rice leaves, accompanied by decreased photosynthetic rate and biomass. In order to identify the key enzyme that BDE-47 interacted with, a diazirine-alkynyl photoaffinity probe was designed, and photoaffinity labeling based chemoproteomics was conducted. Among all differentially expressed proteins, fructose-1, 6-bisphosphate aldolase (FBA) involved in carbohydrate metabolism was most likely the target protein of BDE-47. Spectral techniques and molecular docking analysis further revealed that the pollutant-protein interaction was driven by hydrophobic force. BDE-47 inhibited FBA catalytic efficiency by competing with its substrate, fructose-1, 6-diphosphate (F-1, 6-P), leading to soluble sugar accumulation, photosynthetic rate decline and biomass reduction. This study unraveled the influencing mechanism of PBDEs on rice by combining the novel photoaffinity labeling-based chemoproteomics with conventional proteomics. The improved knowledge on direct interaction between organic pollutants and proteins will help alleviate the harmful effects of soil pollution on plants.

Transcriptomic and metabolic signatures of diatom plasticity to light fluctuations

Unlike in terrestrial and freshwater ecosystems, light fields in oceans fluctuate due to both horizontal current and vertical mixing. Diatoms thrive and dominate the phytoplankton community in these fluctuating light fields. However, the molecular mechanisms that regulate diatom acclimation and adaptation to light fluctuations are poorly understood. Here, we performed transcriptome sequencing, metabolome profiling, and 13C-tracer labeling on the model diatom Phaeodactylum tricornutum. The diatom acclimated to constant light conditions was transferred to six different light conditions, including constant light (CL5d), short-term (1 h) high light (sHL1h), and short-term (1 h) and long-term (5 days) mild or severe light fluctuation conditions (mFL1h, sFL1h, mFL5d, and sFL5d) that mimicked land and ocean light levels. We identified 2,673 transcripts (25% of the total expressed genes) expressed differentially under different fluctuating light regimes. We also identified 497 transcription factors, 228 not reported previously, which exhibited higher expression under light fluctuations, including 7 with a light-sensitive PAS domain (Per-period circadian protein, Arnt-aryl hydrocarbon receptor nuclear translocator protein, Sim-single-minded protein) and 10 predicted to regulate genes related to light-harvesting complex proteins. Our data showed that prolonged preconditioning in severe light fluctuation enhanced photosynthesis in P. tricornutum under this condition, as evidenced by increased oxygen evolution accompanied by the upregulation of Rubisco and light-harvesting proteins. Furthermore, severe light fluctuation diverted the metabolic flux of assimilated carbon preferentially toward fatty acid storage over sugar and protein. Our results suggest that P. tricornutum use a series of complex and different responsive schemes in photosynthesis and carbon metabolism to optimize their growth under mild and severe light fluctuations. These insights underscore the importance of using more intense conditions when investigating the resilience of phytoplankton to light fluctuations.

Non-structural carbohydrate dynamics and growth in tomato plants grown at fluctuating light and temperature

Fluctuations in light intensity and temperature lead to periods of asynchrony between carbon (C) supply by photosynthesis and C demand by the plant organs. Storage and remobilization of non-structural carbohydrates (NSC) are important processes that allow plants to buffer these fluctuations. We aimed to test the hypothesis that C storage and remobilization can buffer the effects of temperature and light fluctuations on growth of tomato plants. Tomato plants were grown at temperature amplitudes of 3 or 10°C (deviation around the mean of 22°C) combined with integration periods (IP) of 2 or 10 days. Temperature and light were applied in Phase (high temperature simultaneously with high light intensity, (400 μmol m^-2 s^-1), low temperature simultaneously with low light intensity (200 μmol m^-2 s^-1) or in Antiphase (high temperature with low light intensity, low temperature with high light intensity). A control treatment with constant temperature (22°C) and a constant light intensity (300 μmol m^-2 s^-1) was also applied. After 20 days all treatments had received the same temperature and light integral. Differences in final structural dry weight were relatively small, while NSC concentrations were highly dynamic and followed changes of light and temperature (a positive correlation with decreasing temperature and increasing light intensity). High temperature and low light intensity lead to depletion of the NSC pool, but NSC level never dropped below 8% of the plant weight and this fraction was not mobilizable. Our results suggest that growing plants under fluctuating conditions do not necessarily have detrimental effects on plant growth and may improve biomass production in plants. These findings highlight the importance in the NSC pool dynamics to buffer fluctuations of light and temperature on plant structural growth.

Elevated ozone decreases the activity of Rubisco in poplar but not its activation under fluctuating light

Increasing tropospheric ozone (O3) is well-known to decrease leaf photosynthesis under steady-state light through reductions in biochemical capacity. However, the effects of O3 on photosynthetic induction and its biochemical limitations in response to fluctuating light remain unclear, despite the rapid fluctuations of light intensity occurring under field conditions. In this study, two hybrid poplar clones with different O3 sensitivities were exposed to elevated O3. Dynamic photosynthetic CO2 response measurements were conducted to quantify the impact of elevated O3 and exposure duration on biochemical limitations during photosynthetic induction. We found that elevated O3 significantly reduced the steady-state light-saturated photosynthetic rate, the maximum rate of carboxylation (Vcmax) and Rubisco content. In addition, elevated O3 significantly decreased the time constants for slow phases and weighting of the fast phase of the Vcmax induction in poplar clone '546' but not in clone '107'. However, elevated O3 did not affect the time, it took to reach a given percentage of full Vcmax activation or photosynthetic induction in either clone. Overall, photosynthetic induction was primarily limited by the activity of Rubisco rather than the regeneration of ribulose-1,5-biphosphate regardless of O3 concentration and exposure duration. The lack of O3-induced effects on the activation of Rubisco observed here would simplify the simulation of impacts of O3 on nonsteady-state photosynthesis in dynamic photosynthetic models.

Exogenous melatonin strongly affects dynamic photosynthesis and enhances water-water cycle in tobacco

Melatonin (MT), an important phytohormone synthesized naturally, was recently used to improve plant resistance against abiotic and biotic stresses. However, the effects of exogenous melatonin on photosynthetic performances have not yet been well clarified. We found that spraying of exogenous melatonin (100 μM) to leaves slightly affected the steady state values of CO₂ assimilation rate (A _N ), stomatal conductance (g _s ) and mesophyll conductance (g _m ) under high light in tobacco leaves. However, this exogenous melatonin strongly delayed the induction kinetics of g _s and g _m , leading to the slower induction speed of A _N . During photosynthetic induction, A _N is mainly limited by biochemistry in the absence of exogenous melatonin, but by CO₂ diffusion conductance in the presence of exogenous melatonin. Therefore, exogenous melatonin can aggravate photosynthetic carbon loss during photosynthetic induction and should be used with care for crop plants grown under natural fluctuating light. Within the first 10 min after transition from low to high light, photosynthetic electron transport rates (ETR) for A _N and photorespiration were suppressed in the presence of exogenous melatonin. Meanwhile, an important alternative electron sink, namely water-water cycle, was enhanced to dissipate excess light energy. These results indicate that exogenous melatonin upregulates water-water cycle to facilitate photoprotection. Taking together, this study is the first to demonstrate that exogenous melatonin inhibits dynamic photosynthesis and improves photoprotection in higher plants.

NaCl affects photosynthetic and stomatal dynamics by osmotic effects and reduces photosynthetic capacity by ionic effects in tomato

NaCl stress affects stomatal behavior and photosynthesis by a combination of osmotic and ionic components, but it is unknown how these components affect stomatal and photosynthetic dynamics. Tomato (Solanum lycopersicum) plants were grown in a reference nutrient solution [control; electrical conductivity (EC)=2.3 dS m-1], a solution containing additional macronutrients (osmotic effect; EC=12.6 dS m-1), or a solution with additional 100 mM NaCl (osmotic and ionic effects; EC=12.8 dS m-1). Steady-state and dynamic photosynthesis, and leaf biochemistry, were characterized throughout leaf development. The osmotic effect decreased steady-state stomatal conductance while speeding up stomatal responses to light intensity shifts. After 19 d of treatment, photosynthetic induction was reduced by the osmotic effect, which was attributable to lower initial stomatal conductance due to faster stomatal closing under low light. Ionic effects of NaCl were barely observed in dynamic stomatal and photosynthetic behavior, but led to a reduction in leaf photosynthetic capacity, CO2 carboxylation rate, and stomatal conductance in old leaves after 26 d of treatment. With increasing leaf age, rates of light-triggered stomatal movement and photosynthetic induction decreased across treatments. We conclude that NaCl impacts dynamic stomatal and photosynthetic kinetics by osmotic effects and reduces photosynthetic capacity by ionic effects.

Water Stress Inhibits Germination While Maintaining Embryo Viability of Subtropical Wetland Seeds: A Functional Approach With Phylogenetic Contrasts

Wetland species commonly exhibit a range of strategies to cope with water stress, either through drought tolerance or through avoidance of the period of limited water availability. Natural populations provide a genetic resource for ecological remediation and may also have direct economic value. We investigated the effects of drought stress on the seed germination of wetland species. Nineteen species were germinated in four concentrations of polyethylene glycol 6000 (PEG) and were evaluated daily (12-h light photoperiod) or after 35 days (continuous darkness) to determine seed germination under water stress. Germination percentage decreased with an increase in polyethylene glycol 6000 (PEG) concentration, but species' germination response to PEG concentration varied significantly. Seeds recovered their germinability after the alleviation of water stress, but the extent of recovery was species-dependent.

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.

Trehalose Alleviated Salt Stress in Tomato by Regulating ROS Metabolism, Photosynthesis, Osmolyte Synthesis, and Trehalose Metabolic Pathways

Trehalose plays a critical role in plant response to salinity but the involved regulatory mechanisms remain obscure. Here, this study explored the mechanism of exogenous trehalose-induced salt tolerance in tomato plants by the hydroponic test method. Our results indicated that 10 mM trehalose displayed remarkable plant biomass by improving growth physiology, which were supported by the results of chlorophyll fluorescence and rapid light-response curve. In the salinity environment, trehalose + NaCl treatment could greatly inhibit the decrease of malondialdehyde level, and it increases the contents of other osmotic substances, carbohydrates, K+, and K+/Na+ ratio. Meanwhile, trehalose still had similar effects after recovery from salt stress. Furthermore, trehalose pretreatment promoted trehalose metabolism; significantly increased the enzymatic activity of the trehalose metabolic pathway, including trehalose-6-phosphate synthase (TPS), trehalose-6-phosphate phosphatase (TPP), and trehalase (TRE); and upregulated the expression of SlTPS1, SlTPS5, SlTPS7, SlTPPJ, SlTPPH, and SlTRE under saline conditions. However, the transcriptional levels of SlTPS1, SlTPS5, and SlTPS7 genes and the activity of TPS enzyme were reversed after recovery. In addition, we found that hydrogen peroxide (H2O2) and superoxide anion (O2 -) were accumulated in tomato leaves because of salt stress, but these parameters were all recovered by foliar-applied trehalose, and its visualization degree was correspondingly reduced. Antioxidant enzyme activities (SOD, POD, and CAT) and related gene expression (SlCu/Zn-SOD, SlFe-SOD, SlMn-SOD, SlPOD, and SlCAT) in salt-stressed tomato leaves were also elevated by trehalose to counteract salt stress. Collectively, exogenous trehalose appeared to be the effective treatment in counteracting the negative effects of salt stress.

Photosynthetic Induction Under Fluctuating Light Is Affected by Leaf Nitrogen Content in Tomato

The response of photosynthetic CO₂ assimilation to changes of illumination affects plant growth and crop productivity under natural fluctuating light conditions. However, the effects of nitrogen (N) supply on photosynthetic physiology after transition from low to high light are seldom studied. To elucidate this, we measured gas exchange and chlorophyll fluorescence under fluctuating light in tomato (Solanum lycopersicum) seedlings grown with different N conditions. After transition from low to high light, the induction speeds of net CO₂ assimilation (A _N ), stomatal conductance (g _s ), and mesophyll conductance (g _m ) delayed with the decline in leaf N content. The time to reach 90% of maximum A _N , g _s and g _m was negatively correlated with leaf N content. This delayed photosynthetic induction in plants grown under low N concentration was mainly caused by the slow induction response of g _m rather than that of g _s . Furthermore, the photosynthetic induction upon transfer from low to high light was hardly limited by photosynthetic electron flow. These results indicate that decreased leaf N content declines carbon gain under fluctuating light in tomato. Increasing the induction kinetics of g _m has the potential to enhance the carbon gain of field crops grown in infertile soil.

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.

Protection of Photosynthesis by Halotolerant Staphylococcus sciuri ET101 in Tomato (Lycoperiscon esculentum) and Rice (Oryza sativa) Plants During Salinity Stress: Possible Interplay Between Carboxylation and Oxygenation in Stress Mitigation

Tomato (Lycoperiscon esculentum) and rice (Oryza sativa) are the two most important agricultural crops whose productivity is severely impacted by salinity stress. Soil salinity causes an irreversible damage to the photosynthetic apparatus in plants at all developmental stages leading to significant reduction in agricultural productivity. Reduction in photosynthesis is the primary response that is observed in all glycophytic plants during salt stress. Employment of salt-tolerant plant growth-promoting bacteria (PGPB) is an economical and viable approach for the remediation of saline soils and improvement of plant growth. The current study is aimed towards investigating the growth patterns and photosynthetic responses of rice and tomato plants upon inoculation with halotolerant PGPB Staphylococcus sciuri ET101 under salt stress conditions. Tomato and rice plants inoculated with PGPB showed increased growth rate and stimulated root growth, along with higher transpiration rates (E), stomatal conductance (g s ), and intracellular CO2 accumulation (Ci). Additionally, correlation of relative water content (RWC) to electrolyte leakage (EL) in tomato and rice plants showed decreased EL in inoculated plants during salt stress conditions, along with higher proline and glycine betaine content. Energy dissipation by non-photochemical quenching (NPQ) and increased photorespiration of 179.47% in tomato and 264.14% in rice plants were observed in uninoculated plants subjected to salinity stress. Furthermore, reduced photorespiration with improved salinity tolerance is observed in inoculated plants. The higher rates of photosynthesis in inoculated plants during salt stress were accompanied by increased quantum efficiency (ΦPSII) and maximum quantum yield (F v /F m ) of photosystem II. Furthermore, inoculated plants showed increased carboxylation efficiency of RuBisCO, along with higher photosynthetic electron transport rate (ETR) (J) during salinity stress. Although the total cellular ATP levels are drastically affected by salt stress in tomato and rice plants along with increased reactive oxygen species (ROS) accumulation, the restoration of cellular ATP levels in leaves of inoculated plants along with decreased ROS accumulation suggests the protective role of PGPB. Our results reveal the beneficial role of S. sciuri ET101 in protection of photosynthesis and amelioration of salinity stress responses in rice and tomato plants.

Artifleck: The Study of Artifactual Responses to Light Flecks with Inappropriate Leaves

Methods in sunfleck research commonly employ the use of experimental leaves which were constructed in homogeneous light. These experimental organs may behave unnaturally when they are challenged with fluctuating light. Photosynthetic responses to heterogeneous light and leaf macronutrient relations were determined for Cycas micronesica, Glycine max, and Zea mays leaves that were grown in homogeneous shade, heterogeneous shade, or full sun. The speed of priming where one light fleck increased the photosynthesis during a subsequent light fleck was greatest for the leaves grown in heterogeneous shade. The rate of induction and the ultimate steady-state photosynthesis were greater for the leaves that were grown in heterogeneous shade versus the leaves grown in homogeneous shade. The leaf mass per area, macronutrient concentration, and macronutrient stoichiometry were also influenced by the shade treatments. The amplitude and direction in which the three developmental light treatments influenced the response variables were not universal among the three model species. The results indicate that the historical practice of using experimental leaves which were constructed under homogeneous light to study leaf responses to fluctuating light may produce artifacts that generate dubious interpretations.

High Stomatal Conductance in the Tomato Flacca Mutant Allows for Faster Photosynthetic Induction

Due to their slow movement and closure upon shade, partially closed stomata can be a substantial limitation to photosynthesis in variable light intensities. The abscisic acid deficient flacca mutant in tomato (Solanum lycopersicum) displays very high stomatal conductance (g_s ). We aimed to determine to what extent this substantially increased g_s affects the rate of photosynthetic induction. Steady-state and dynamic photosynthesis characteristics were measured in flacca and wildtype leaves, by the use of simultaneous gas exchange and chlorophyll fluorometry. The steady-state response of photosynthesis to CO₂, maximum quantum efficiency of photosystem II photochemistry (F_v/F_m ), as well as mesophyll conductance to CO₂ diffusion were not significantly different between genotypes, suggesting similar photosynthetic biochemistry, photoprotective capacity, and internal CO₂ permeability. When leaves adapted to shade (50 µmol m^-2 s^-1) at 400 µbar CO₂ partial pressure and high humidity (7 mbar leaf-to-air vapour pressure deficit, VPD) were exposed to high irradiance (1500 µmol m^-2 s^-1), photosynthetic induction was faster in flacca compared to wildtype leaves, and this was attributable to high initial g_s in flacca (~0.6 mol m^-2 s^-1): in flacca, the times to reach 50 (t₅₀ ) and 90% (t₉₀ ) of full photosynthetic induction were 91 and 46% of wildtype values, respectively. Low humidity (15 mbar VPD) reduced g_s and slowed down photosynthetic induction in the wildtype, while no change was observed in flacca; under low humidity, t₅₀ was 63% and t₉₀ was 36% of wildtype levels in flacca. Photosynthetic induction in low CO₂ partial pressure (200 µbar) increased g_s in the wildtype (but not in flacca), and revealed no differences in the rate of photosynthetic induction between genotypes. Effects of higher g_s in flacca were also visible in transients of photosystem II operating efficiency and non-photochemical quenching. Our results show that at ambient CO₂ partial pressure, wildtype g_s is a substantial limitation to the rate of photosynthetic induction, which flacca overcomes by keeping its stomata open at all times, and it does so at the cost of reduced water use efficiency.