Analysis of elevated temperature-induced inhibition of photosystem II using chlorophyll a fluorescence induction kinetics in wheat leaves (Triticum aestivum)

Regrowth dynamics and morpho-physiological characteristics of Plantago lanceolata under different defoliation frequencies and intensities

Traditional pastures in temperate regions face limitations such as reduced growth and nutritional quality during the summer season. Plantain (P. lanceolata L.) offers advantages like increased yield and decreased nitrogen losses from grazing ruminants. Effective grazing management is essential for pasture health, and defoliation frequency and intensity play a pivotal role. This study aimed to evaluate plantain's regrowth, yield, and morpho-physiological and chemical responses under different defoliation frequencies and intensities, with the goal of enhancing its management in pastures. The study was conducted in pots within a controlled-environment growth chamber, examining the impact of three defoliation frequencies (based on extended leaf length: 15, 25 and 35 cm) and two defoliation intensities (5 and 8 cm of residual heights) with four replicates (24 pots as experimental units). The variables of interest were morphological characteristics, dry matter (DM) accumulation, herbage chemical composition, growth rate traits, and photosynthetic parameters. Defoliation frequency affected plantain's growth and nutritional composition. More frequent cuts (15 cm) resulted in lower DM yield per cut and lower stem content, while less frequent cuts (35 cm) produced higher values. Defoliation intensity influenced the proportion of leaves and stems in the total DM, with 5 cm cuts favoring leaves. Nutrient content was also affected by defoliation frequency, with less frequent cuts (35 cm) showing lower crude protein concentration and metabolizable energy content but higher neutral detergent fiber and water-soluble carbohydrate concentration. Plantain's growth rate variables were mainly influenced by defoliation frequency, with less frequent cuts promoting faster leaf appearance and growth of new leaves. The basal fluorescence variables and chlorophyll content were affected by cutting frequency, being highest when cut less frequently (35 cm), while no differences were found in the actual quantum efficiency among different defoliation frequencies and intensities. The fraction of light dedicated to non-photochemical quenching was highest when cut less frequently and more intensively. Overall, defoliation at 25 cm of extended leaf length balanced plantain forage quality and regrowth capacity.

Harmony in detoxification: Microalgae unleashing the potential of lignocellulosic pretreatment wastewater for resource utilization

Lignocellulosic biomass is a pivotal renewable resource in biorefinery process, requiring pretreatment, primarily chemical pretreatment, for effective depolymerization and subsequent transformation. This process yields solid residue for saccharification and lignocellulosic pretreatment wastewater (LPW), which comprises sugars and inhibitors such as phenols and furans. This study explored the microalgal capacity to treat LPW, focusing on two key hydrolysate inhibitors: furfural and vanillin, which impact the growth of six green microalgae. Chlorella sorokiniana exhibited higher tolerance to furfural and vanillin. However, both inhibitors hindered the growth of C. sorokiniana and disrupted algal photosynthetic system, with vanillin displaying superior inhibition. A synergistic inhibitory effect (Q < 0.85) was observed with furfural and vanillin on algal growth. Furfural transformation to low-toxic furfuryl alcohol was rapid, yet the addition of vanillin hindered this process. Vanillin stimulated carbohydrate accumulation, with 50.48 % observed in the 0.1 g/L furfural + 0.1 g/L vanillin group. Additionally, vanillin enhanced the accumulation of C16: 0 and C18: 2, reaching 21.71 % and 40.36 %, respectively, with 0.1 g/L vanillin. This study proposed a microalgae-based detoxification and resource utilization approach for LPW, enhancing the comprehensive utilization of lignocellulosic components. The observed biomass modifications also suggested potential applications for biofuel production, contributing to the evolving landscape of sustainable biorefinery processes.

Crop-Specific Responses to Cold Stress and Priming: Insights from Chlorophyll Fluorescence and Spectral Reflectance Analysis in Maize and Soybean

This study aimed to investigate the impact of cold stress and priming on photosynthesis in the early development of maize and soybean, crops with diverse photosynthetic pathways. The main objectives were to determine the effect of cold stress on chlorophyll a fluorescence parameters and spectral reflectance indices, to determine the effect of cold stress priming and possible stress memory and to determine the relationship between different parameters used in determining the stress response. Fourteen maize inbred lines and twelve soybean cultivars were subjected to control, cold stress, and priming followed by cold stress in a walk-in growth chamber. Measurements were conducted using a portable fluorometer and a handheld reflectance instrument. Cold stress induced an overall downregulation of PSII-related specific energy fluxes and efficiencies, the inactivation of RCs resulting in higher energy dissipation, and electron transport chain impairment in both crops. Spectral reflectance indices suggested cold stress resulted in pigment differences between crops. The effect of priming was more pronounced in maize than in soybean with mostly a cumulatively negative effect. However, priming stabilized the electron trapping efficiency and upregulated the electron transfer system in maize, indicating an adaptive response. Overall, this comprehensive analysis provides insights into the complex physiological responses of maize and soybean to cold stress, emphasizing the need for further genotype-specific cold stress response and priming effect research.

Improving strawberry plant resilience to salinity and alkalinity through the use of diverse spectra of supplemental lighting

BACKGROUND

This study explores the impact of various light spectra on the photosynthetic performance of strawberry plants subjected to salinity, alkalinity, and combined salinity/alkalinity stress. We employed supplemental lighting through Light-emitting Diodes (LEDs) with specific wavelengths: monochromatic blue (460 nm), monochromatic red (660 nm), dichromatic blue/red (1:3 ratio), and white/yellow (400-700 nm), all at an intensity of 200 µmol m-2 S-1. Additionally, a control group (ambient light) without LED treatment was included in the study. The tested experimental variants were: optimal growth conditions (control), alkalinity (40 mM NaHCO3), salinity (80 mM NaCl), and a combination of salinity/alkalinity.

RESULTS

The results revealed a notable decrease in photosynthetic efficiency under both salinity and alkalinity stresses, especially when these stresses were combined, in comparison to the no-stress condition. However, the application of supplemental lighting, particularly with the red and blue/red spectra, mitigated the adverse effects of stress. The imposed stress conditions had a detrimental impact on both gas exchange parameters and photosynthetic efficiency of the plants. In contrast, treatments involving blue, red, and blue/red light exhibited a beneficial effect on photosynthetic efficiency compared to other lighting conditions. Further analysis of JIP-test parameters confirmed that these specific light treatments significantly ameliorated the stress impacts.

CONCLUSIONS

In summary, the utilization of blue, red, and blue/red light spectra has the potential to enhance plant resilience in the face of salinity and alkalinity stresses. This discovery presents a promising strategy for cultivating plants in anticipation of future challenging environmental conditions.

Changes in vegetation-water response in the Sahel-Sudan during recent decades

Study region

The Africa Sahel-Sudan region, defined by annual rainfall between 150 and 1200 mm.

Study focus

Understanding the mechanism of vegetation response to water availability could help mitigate the potential adverse effects of climate change on global dryland ecosystems. In the Sahel-Sudan region, spatio-temporal changes and drivers of the vegetation-water response remain unclear. This study employs long-term satellite water and vegetation products as proxies of water availability and vegetation productivity to analyze changes in vegetation-water sensitivity and the cumulative effect duration (CED) representing a measure of the legacy effect of the impact of water constraints on vegetation. A random forest model was subsequently used to analyze potential climatic drivers of the observed vegetation response.

New hydrological insights for the region

During 1982-2016 we found a significant decrease (p < 0.05) in the sensitivity of vegetation productivity to water constraints in 26% of the Sahel-Sudan region, while 9% of the area showed a significantly increased sensitivity, mainly in the sub-humid zone. We further showed that CED significantly increased and decreased, respectively in around 9% of the study area in both cases. Our climatic driver attribution analysis suggested the existence of varying underlying mechanisms governing vegetation productivity in response to water deficit across the Sahel-Sudan dryland ecosystems. Our findings emphasize the need for diverse strategies in sustainable ecosystem management to effectively address these varying mechanisms.

Toxic effects of lanthanum(III) on photosynthetic performance of rice seedlings: Combined chlorophyll fluorescence, chloroplast structure and thylakoid membrane protein assessment

Rare earth elements (REEs) are emerging as an anticipated pollution in the environment due to their active use in many areas. However, the effects of REEs on the photosynthesis of rice have not been thoroughly explored. Therefore, this study emphasizes how high levels of La(III) affect the thylakoid membrane of rice seedlings, thereby inhibiting photosynthesis and growth. Here, we reported that rice plants treated with La(III) exhibited an increase in La accumulation in the leaves, accompanied by a decrease in chlorophyll content and photosynthetic capacity. La(III) exposure decreased Mg content in leaves, but possibly increased other nutrients including Cu, Mn, and Zn through systemic endocytosis. K-band and L-band appeared in the fluorescence OJIP transients, indicating La(III) stress destroyed the donor and receptor sides of photosystem II (PSII). Numerous reaction centers (RC/CSm) were inactivated by La(III) treatment, which resulted in a reduction in electron transport capacity (decreased ETo/RC and ETo/CSm) and an increase in the dissipation of the excess excitation energy by heat (increased DIo/RC and DIo/CSm). The BN-PAGE analysis of thylakoid membrane protein complexes showed that La(III) induced the degradation of supercomplexes, PSII core, LHCII, PSI core, LHCI, and F1-ATPase binding Cyt b6f complex. Collectively, this study revealed that La(III) causes significant degradation of thylakoid membrane proteins, thereby promoting the decomposition of photosynthetic complexes, ultimately destroying the chloroplast structure and reducing the photosynthetic performance of rice seedlings.

Chlorophyll fluorescence is a potential indicator to measure photochemical efficiency in early to late soybean maturity groups under changing day lengths and temperatures

In this study, we employed chlorophyll a fluorescence technique, to indicate plant health and status in response to changing day lengths (photoperiods) and temperatures in soybean early and late maturity groups. Chlorophyll a fluorescence study indicates changes in light reactions in photosystem II. Experiments were performed for 3-day lengths (12.5, 13.5, and 14.5 h) and five temperatures (22/14°C, 26/18°C, 30/22°C, 34/26°C, and 40/32°C), respectively. The I-P phase declined for changing day lengths. Active reaction centers decreased at long day length for maturity group III. We observed that low temperatures impacted the acceptor side of photosystem II and partially impacted electron transport toward the photosystem I end electron acceptor. Results emphasized that higher temperatures (40/32°C) triggered damage at the oxygen-evolving complex and decreased electron transport and photosynthesis. We studied specific leaf areas and aboveground mass. Aboveground parameters were consistent with the fluorescence study. Chlorophyll a fluorescence can be used as a potential technique for high-throughput phenotyping methods. The traits selected in the study proved to be possible indicators to provide information on the health status of various maturity groups under changing temperatures and day lengths. These traits can also be deciding criteria for breeding programs to develop inbreed soybean lines for stress tolerance and sensitivity based on latitudinal variations.

An efficient triose phosphate synthesis and distribution in wheat provides tolerance to higher field temperatures

High temperatures in the field hinder bread wheat high-yield production, mainly because of the adverse effects of heat over photosynthesis. The Yaqui Valley, the main wheat producer region in Mexico, is a zone prone to have temperatures over 30°C. The aim of this work was to test the flag leaf photosynthetic performance in 10 bread wheat genotypes grown under high temperatures in the field. The study took place during two seasons (2019-2020 and 2020-2021). In each season, control seeds were sown in December, while heat-stressed were sown in late January to subject wheat to heat stress (HS) during the grain-filling stage. HS reduced Grain yield from 20 to 58% in the first season. HS did not reduce chlorophyll content and light-dependent reactions were unaffected in any of the tested genotypes. Rubisco, chloroplast fructose 1,6-biphosphatase (FBPase), and sucrose phosphate synthase (SPS) activities were measured spectrophotometrically. Rubisco activity did not decrease under HS in any of the genotypes. FBPase activity was reduced by HS indicating that triose phosphate flux to starch synthesis was reduced, while SPS was not affected, and thus, sucrose synthesis was maintained. HS reduced aerial biomass in the 10 chosen genotypes. Genotypes SOKWB.1, SOKWB.3, and BORLAUG100 maintained their yield under HS, pointing to a potential success in their introduction in this region for breeding heat-tolerant bread wheat.

Redox Signaling in Plant Heat Stress Response

The increase in environmental temperature due to global warming is a critical threat to plant growth and productivity. Heat stress can cause impairment in several biochemical and physiological processes. Plants sense and respond to this adverse environmental condition by activating a plethora of defense systems. Among them, the heat stress response (HSR) involves an intricate network of heat shock factors (HSFs) and heat shock proteins (HSPs). However, a growing amount of evidence suggests that reactive oxygen species (ROS), besides potentially being responsible for cellular oxidative damage, can act as signal molecules in HSR, leading to adaptative responses. The role of ROS as toxic or signal molecules depends on the fine balance between their production and scavenging. Enzymatic and non-enzymatic antioxidants represent the first line of defense against oxidative damage and their activity is critical to maintaining an optimal redox environment. However, the HS-dependent ROS burst temporarily oxidizes the cellular environment, triggering redox-dependent signaling cascades. This review provides an overview of the redox-activated mechanisms that participate in the HSR.

Evaluation of growth and antioxidant responses of freshwater microalgae Chlorella sorokiniana and Scenedesmus dimorphus under exposure of moxifloxacin

As one of the fourth-generation fluoroquinolone antibiotics, moxifloxacin (MOX) has been frequently released to the aquatic environment, threatening local organisms. However, researches on its ecotoxicity to aquatic organisms are still limited. This study analyzed effects of MOX on the growth, photosynthesis and oxidative stress of two common types of freshwater microalgae, Chlorella sorokiniana and Scenedesmus dimorphus. The 96 h-EC₅₀ values of MOX for C. sorokiniana and S. dimorphus were 28.42 and 26.37 mg/L, respectively. Although variations of specific indicators for photosynthetic fluorescence intensity were different, photosystems of two types of microalgae were irreversibly damaged. The malondialdehyde content and superoxide dismutase of C. sorokiniana and S. dimorphus evidently increased, indicating that the exposure of MOX caused serious oxidative stress. Chlorophyll a, b and carotenoids contents of C. sorokiniana increased, probably resulting from the resistance to oxidative stress, whereas they were inhibited due to oxidation damage as for S. dimorphus. Risk quotients (RQs) of MOX for C. sorokiniana and S. dimorphus in wastewater were 7.882 and 8.495, respectively, which demonstrated that MOX had a considerable risk to aquatic environment, especially in the context of its increasing use in practice.

Enhancement of Heat and Drought Stress Tolerance in Rice by Genetic Manipulation: A Systematic Review

As a result of global warming, plants are subjected to ever-increasing abiotic stresses including heat and drought. Drought stress frequently co-occurs with heat stress as a result of water evaporation. These stressors have adverse effects on crop production, which in turn affects human food security. Rice is a major food resource grown widely in crop-producing regions throughout the world. However, increasingly common heat and drought stresses in growth regions can have negative impacts on seedling morphogenesis, reproductive organ establishment, overall yield, and quality. This review centers on responses to heat and drought stress in rice. Current knowledge of molecular regulation mechanisms is summarized. We focus on approaches to cope with heat and drought stress, both at the genetic level and from an agricultural practice perspective. This review establishes a basis for improving rice stress tolerance, grain quality, and yield for human benefit.

Photosynthetic and physiological responses of different peony cultivars to high temperature

In order to investigate the causes of the differences in heat tolerance ('Lu He Hong' and 'Zhi Hong'), we studied the physiological changes, photosynthetic properties and regulatory mechanism of the two peony cultivars at high temperature. The results showed that the physiological changed of different peony cultivars varied significantly under high temperature stress. With the extension of high temperature stress time, MDA content of 'Lu He Hong' increased,while 'Zhi Hong' rised first and then decreased, SOD activity of 'Lu He Hong' rised first and then decreased, that of 'Zhi Hong' kept rising, POD activity of 'Lu He Hong' kept decreasing, while 'Zhi Hong' rised. The photosynthetic instrument records the change of peony photosynthesis parameters at high temperature; the chlorophyll A (Chla) fluorescence transient is recorded using the plant efficiency analyzer (PEA), analyzed according to the JIP test (O-J-I-P fluorescence transient analysis), and several parameters were derived to explain the photosynthetic efficiency difference between different peony cultivars. The tested cultivars responded differently to the survey conditions, and the PCA analysis showed that the 'Zhi Hong' was more well tolerated and showed better thermal stability of the PSII. The reduced efficiency of the 'Lu He Hong' PSII antenna leads to higher heat dissipation values to increase the light energy absorbed by unit reaction center (ABS/RC), the energy captured by unit reaction center (TR₀/RC), and the energy dissipated by unit reaction center (DI₀/RC), which significantly leads to its lower total photosynthetic performance (PI_total). The light capture complex of the variety 'Zhi Hong' has high connectivity with its reaction center, less damage to OEC activity, and better stability of the PSII system. The results show that 'Zhi Hong' improves heat resistance by stabilizing the cell membrane, a strong antioxidant system, as well as a more stable photosynthetic system. The results of this study provide a theoretical basis for the screening of heat-resistant peonies suitable for cultivation in Jiangnan area and for the selection and breeding of heat-resistant cultivars.

Photosynthetic regulation in fluctuating light under combined stresses of high temperature and dehydration in three contrasting mosses

Photosynthesis regulation is fundamental for the response to environmental dynamics, especially for bryophytes during their adaptation to terrestrial life. Alternative electron flow mediated by flavodiiron proteins (FLV) and cyclic electron flow (CEF) around photosystem I (PSI) play seminal roles in the response to abiotic stresses in mosses; nevertheless, their correlation and relative contribution to photoprotection of mosses exposed to combined stresses remain unclear. In the present study, the photosynthetic performance and recovery capacity of three moss species from different growth habitats were examined during heat and dehydration with fluctuating light. Our results showed that dehydration at 22 °C for 24 h caused little photodamage, and most of the parameters recovered to their original values after rehydration. In contrast, dehydration at 38 °C caused drastic injuries, especially to PSII, which was mainly caused by the inactivation of non-photochemical quenching (NPQ). Dehydration also induced a high accumulation of O₂^- and H₂O₂. A consistently higher CEF as well as a positive correlation between CEF and FLV was observed in resistant R. japonicum, implying CEF played a more important protective role for R. japonicum. In H. plumaeforme and P. cuspidatum, the positive relationship under mild stress switched to negative when stress became severe. Therefore, FLV pathway was sensitive to environmental fluctuations and maybe less efficient than CEF thus, readily to be lost during land colonization and evolution in angiosperms. Our work provides insights into the coordination of various pathways to fine-tune photosynthetic protection and can be used as a basis for species screening and development of breeding strategies for degraded ecosystem restoration with pioneering mosses.

The interactive effects of drought and heat stress on photosynthetic efficiency and biochemical defense mechanisms of Amaranthus species

Drought and heat stress are major abiotic stress factors that limit photosynthesis and other related metabolic processes that hamper plant growth and productivity. Identifying plants that can tolerate abiotic stress conditions is essential for sustainable agriculture. Amaranthus plants can tolerate adverse weather conditions, especially drought and heat, and their leaves and grain are highly nutritious. Because of these traits, amaranth has been identified as a possible crop to be grown in marginal crop production systems. Therefore, this study investigated the photochemical and biochemical responses of Amaranthus caudatus, Amaranthus hypochondriacus, Amaranthus cruentus, and Amaranthus spinosus to drought stress, heat shock treatments, and a combination of both. After the six-leaf stage in a greenhouse, plants were subjected to drought stress, heat shock treatments, and a combination of both. Chlorophyll a fluorescence was used to evaluate the photochemical responses of photosystem II to heat shock while subjected to drought stress. It was found that heat shock and a combination of drought and heat shock damages photosystem II, but the level of damage varies considerably between the species. We concluded that A. cruentus and A. spinosus are more heat and drought-tolerant than Amaranthus caudatus and Amaranthus hypochondriacus.

The effect of supplementary light on the photosynthetic apparatus of strawberry plants under salinity and alkalinity stress

Considering the destructive effect of stresses on the photosynthetic apparatus of plants and the important role of light in photosynthesis, we investigated the effect of complementary light on the photosynthetic apparatus under salinity and alkalinity stress conditions. Light-emitting diodes (LEDs) in monochromatic blue (460 nm), monochromatic red (660 nm), dichromatic blue/red (1:3), white/yellow (400–700 nm) at 200 μmol m⁻² S⁻¹, and without LED treatment were used. The stress treatments were in three stages: Control (no stress), Alkalinity (40 mM NaHCO₃), and Salinity (80 mM NaCl). Our results showed that salinity and alkaline stress reduced CO₂ assimilation by 62.64% and 40.81%, respectively, compared to the control treatment. The blue light spectrum had the highest increase in water use efficiency (54%) compared to the treatment without supplementary light. Under salinity and alkalinity stress, L, K, and H bands increased and G bands decreased compared to the control treatment, with blue/red light causing the highest increase in L and K bands under both stress conditions. In salinity and alkalinity stress, white/yellow and blue/red spectra caused the highest increase in H bands. Complementary light spectra increased the G band compared to the treatment without complementary light. There was a significant decrease in power indices and quantum power parameters due to salt and alkalinity stress. The use of light spectra, especially blue, red, and blue/red light, increased these parameters compared with treatment without complementary light. Different light spectra have different effects on the photosynthetic apparatus of plants. It can be concluded that using red, blue spectra and their combination can increase the resistance of plants to stress conditions and be adopted as a strategy in planting plants under stress conditions.

Overexpression of DfRaf from Fragrant Woodfern (Dryopteris fragrans) Enhances High-Temperature Tolerance in Tobacco (Nicotiana tabacum)

Heat stress seriously affects medicinal herbs’ growth and yield. Rubisco accumulation factor (Raf) is a key mediator regulating the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which plays important roles in carbon assimilation and the Calvin cycle in plants. Raf has been studied in many plants, but has rarely been studied in the important medicinal plant fragrant woodfern (Dryopteris fragrans). The aim of this study was to analyze the effects of Raf on carbohydrate metabolism and the response to heat stress in medicinal plants. In this study, high temperature treatment upregulated the expression of DfRaf, which was significantly higher than that of phosphoribokinase (DfPRK), Rubisco small subunits (DfRbcS), Rubisco large subunits (DfRbcL) and Rubisco activase (DfRCA). The subcellular localization showed that the DfRaf proteins were primarily located in the nucleus; DfPRK, DfRbcS, DfRbcL and DfRCA proteins were primarily located in the chloroplast. We found that overexpression of DfRaf led to increased activity of Rubisco, RCA and PRK under high-temperature stress. The H₂O₂, O₂⁻ and MDA content of the DfRaf-OV-L2 and DfRaf-OV-L6 transgenic lines were significantly lower than those of WT and VC plants under high-temperature stress. The photosynthetic pigments, proline, soluble sugar content and ROS-scavenging ability of the DfRaf-OV-L2 and DfRaf-OV-L6 transgenic lines were higher than those of WT and VC plants under high-temperature stress. The results showed that overexpression of the DfRaf gene increased the Rubisco activity, which enhanced the high-temperature tolerance of plants.

Overcoming Reproductive Compromise Under Heat Stress in Wheat: Physiological and Genetic Regulation, and Breeding Strategy

The reproductive compromise under heat stress is a major obstacle to achieve high grain yield and quality in wheat worldwide. Securing reproductive success is the key solution to sustain wheat productivity by understanding the physiological mechanism and molecular basis in conferring heat tolerance and utilizing the candidate gene resources for breeding. In this study, we examined the performance on both carbon supply source (as leaf photosynthetic rate) and carbon sink intake (as grain yields and quality) in wheat under heat stress varying with timing, duration, and intensity, and we further surveyed physiological processes from source to sink and the associated genetic basis in regulating reproductive thermotolerance; in addition, we summarized the quantitative trait loci (QTLs) and genes identified for heat stress tolerance associated with reproductive stages. Discovery of novel genes for thermotolerance is made more efficient via the combination of transcriptomics, proteomics, metabolomics, and phenomics. Gene editing of specific genes for novel varieties governing heat tolerance is also discussed.

Phytotoxicity of atrazine combined with cadmium on photosynthetic apparatus of the emergent plant species Iris pseudacorus

The combined pollution, instead of single pollution, has become a widespread contamination phenomenon in aquatic environment. However, little information is now available about the joint effects of the combined pollution, especially co-existed pesticides and heavy metals, on aquatic plants. In the present study, using continuous excitation chlorophyll fluorescence parameters and the OJIP transient, comparisons of herbicide atrazine (ATZ) phytotoxicity on Iris pseudacorus between in the presence and absence of cadmium (Cd) were evaluated over an exposure period of three weeks under laboratory conditions. Results showed that both ATZ and Cd were toxic to I. pseudacorus. The ratio F_v/F_o, specific electron transport energy (ET₀/RC), and photochemistry efficiency (PI_abs and PI_total) of this emergent plant species at individual ATZ and Cd concentrations were significantly lower than those of the control. ATZ mainly inhibited electron transport beyond Q_A at PSII acceptor side as indicated by the sharp rise of the J-step level of fluorescence rise kinetics. A pronounced K-step and the loss of I-step due to the damage on the OEC and PSI also occurred when ATZ was at or above 1.0 mg·L^-1. In comparison to ATZ alone, ATZ combined with Cd resulted in a lower amplitude rise in J-step with apparent J-I and I-P phases; and significantly lower F_o with higher F_v/F_o, as well as greater ET₀/RC with higher values of PI_abs and PI_total. However, the adverse influences of ATZ combined with Cd on the above indicators were still significant as compared with the control. Therefore, the coexistence of Cd alleviated the individual phytotoxicities of ATZ, whereas combined pollution of ATZ and Cd still induced the decline in photosynthetic performance of I. pseudacorus, and its potential ecological impacts on the aquatic vegetation cannot be ignored. Our findings offer a better understanding of the joint effects of the pesticide and heavy metal on non-target aquatic plants, and provided valuable insights into the interaction of these pollutants in aquatic environment.

The Dynamic Behaviors of Photosynthesis during Non-Motile Cell Germination in Haematococcus pluvialis

Haematococcus pluvialis undergoes a three-phase process during the process of germination: first, repeated mitotic events; next, cytokinesis to form the zoospore; and finally, a fast release of motile cells. Physiological properties were measured using chlorophyll a fluorescence (OJIP) transient. The most obvious increase in K-value and L-value appeared at 17 h, suggesting that oxygen-evolving complex damage and lower energetic connectivity of the photosystem II units of the mother non-motile cell occurred. Compared to phase I, the values of the maximum quantum yield of PSII photochemistry (FV/FM) and PIABS increased significantly in phases II and III, suggesting that photosynthetic photochemical activity was greatly up-regulated during cytokinesis to form zoospores and the fast release of motile cells. Moreover, the significant increase in the K-band at 17 h and 22 h indicates that the PSII donor side was the limiting factor during the initial period of germination. All these results suggest that the cellular photosynthetic activity continues to strengthen during cytokinesis to form the zoospore and the fast release of motile cells, and it was postulated to meet the demands for sporangium swelling and new organelle formation.

Physiological Response to Short-Term Heat Stress in the Leaves of Traditional and Modern Plum (Prunus domestica L.) Cultivars

The aim of this study was to evaluate physiological responses to short-term heat stress in the leaves of traditional (Bistrica) and modern (Toptaste) plum cultivars. In this study, detached plum leaves were incubated at 25 °C (control) and 40 °C (stress). After 1 h of exposure to heat (40 °C), chlorophyll a fluorescence transients were measured, and several biochemical parameters were analyzed. Elevated temperature caused heat stress in both plum cultivars, seen as a decrease in water content (WT), but in the leaves of the cultivar Bistrica, an accumulation of proline and phenols, as well as an accumulation of photosynthetic pigments, suggest the activation of a significant response to unfavorable conditions. Conversely, in the leaves of Toptaste, a significant accumulation of malondialdehyde (MDA) and an activation of guaiacol peroxidase (GPOD), all together with a decreased soluble proteins content, indicate an inadequate response to maintaining homeostasis in the leaf metabolism. The impact of an elevated temperature on photosynthesis was significant in both plum cultivars as reflected in the decrease in performance indexes (PIABS and PItotal) and the maximum quantum yield of PSII (Fv/Fm), with significantly pronounced changes found in Toptaste. Unlike the traditional plum cultivar, Bistrica, in the modern cultivar, Toptaste, short-term heat stress increased the minimal fluorescence (F0) and absorption (ABS/RC), as well as Chl b in total chlorophylls. Additionally, the inactivation of RCs (RC/ABS) suggests that excitation energy was not trapped efficiently in the electron chain transport, which resulted in stronger dissipation (DI0/RC) and the formation of ROSs. Considering all presented results, it can be presumed that the traditional cultivar Bistrica has better tolerance to heat stress than the modern cultivar Toptaste. The cultivar, Bistrica, can be used as a basis in further plum breeding programs, as a source of tolerance for high temperature stress.

Manipulation of light spectrum can improve the performance of photosynthetic apparatus of strawberry plants growing under salt and alkalinity stress

Strawberry is one of the plants sensitive to salt and alkalinity stress. Light quality affects plant growth and metabolic activities. However, there is no clear answer in the literature on how light can improve the performance of the photosynthetic apparatus of this species under salt and alkalinity stress. The aim of this work was to investigate the effects of different spectra of supplemental light on strawberry (cv. Camarosa) under salt and alkalinity stress conditions. Light spectra of blue (with peak 460 nm), red (with peak 660 nm), blue/red (1:3), white/yellow (1:1) (400–700 nm) and ambient light were used as control. There were three stress treatments: control (no stress), alkalinity (40 mM NaHCO₃), and salinity (80 mM NaCl). Under stress conditions, red and red/blue light had a positive effect on CO₂ assimilation. In addition, blue/red light increased intrinsic water use efficiency (WUEi) under both stress conditions. Salinity and alkalinity stress decreased OJIP curves compared to the control treatment. Blue light caused an increase in its in plants under salinity stress, and red and blue/red light caused an increase in its in plants under alkalinity. Both salt and alkalinity stress caused a significant reduction in photosystem II (PSII) performance indices and quantum yield parameters. Adjustment of light spectra, especially red light, increased these parameters. It can be concluded that the adverse effects of salt and alkalinity stress on photosynthesis can be partially alleviated by changing the light spectra.

Combining Heat Stress with Pre-Existing Drought Exacerbated the Effects on Chlorophyll Fluorescence Rise Kinetics in Four Contrasting Plant Species

Although drought and high temperature are two main factors affecting crop productivity and forest vegetation dynamics in many areas worldwide, little work has been done to describe the effects of heat combined with pre-existing drought on photochemical function in diverse plant species. This study investigated the biophysical status of photosystem II (PSII) and its dynamic responses under 2-day heat stress during a 2-week drought by measuring the polyphasic chlorophyll fluorescence rise (OJIP) kinetics. This study examined four contrasting species: a C3 crop/grass (wheat), a C4 crop/grass (sorghum), a temperate tree species (Fraxinus chinensis) and a tropical tree species (Radermachera sinica). Principal component analysis showed that the combination of heat and drought deviated from the effect of heat or drought alone. For all four species, a linear mixed-effects model analysis of variance of the OJIP parameters showed that the deviation arose from decreased quantum yield and increased heat dissipation of PSII. The results confirmed, in four contrasting plant species, that heat stress, when combined with pre-existing drought, exacerbated the effects on PSII photochemistry. These findings provide direction to future research and applications of chlorophyll fluorescence rise OJIP kinetics in agriculture and forestry, for facing increasingly more severe intensity and duration of both heat and drought events under climate change.

Responses of photosynthesis and antioxidants to simulated acid rain in mulberry seedlings

Acid rain, which has negative impacts on the vegetation of ecological systems, is widespread in Northern and Southern China. However, relatively little is known about the effects of acid rain on the growth and yield of economically important tree species in China. To address this issue, we studied the responses of mulberry seedlings to simulated acid rain (SAR) at different pH values. At pH 4.5, SAR induced increased antioxidant activities, total antioxidant capacity, and the accumulation of reactive oxygen species (OFR) relative to controls. However, the growth of the seedlings under SAR treatments at pH 4.5 and pH 5.6 was greater than controls. No significant differences in photosynthesis and chlorophyll a fluorescence quenching parameters were observed between the SAR treatments at pH 4.5 and pH 5.6 and controls. However, the SAR treatment at pH 3.5 resulted in altered leaf surface characteristics and changes to chloroplast ultrastructure, together with an increase in membrane electrical conductivity and an accumulation of OFR and malondialdehyde. In contrast, leaf antioxidant enzyme activities were decreased, together with electron transport parameters and photosynthesis. Taken together, these results show that the effects of acid rain on the growth and leaf physiology of mulberry seedling are dependent on pH. Moreover, mulberry seedlings had a high tolerance to acid rain at pH 4.5.

Impact of saline solution on growth and photosystem II during in vitro cultivation of Bromelia antiacantha (Bromeliaceae)

Abstract In vitro cultivation is a technique with wide application for micropropagation. However, each species has specific mineral needs for this type of cultivation. The objective was to assess the impacts of the saline solution culture medium on the performance of the photosynthetic apparatus and growth of Bromelia antiacantha during in vitro cultivation, and thus to elucidate the mitigation of the nutritional imbalance that can interfere in the electron transport in the plants. Plants were cultivated in a salt concentration gradient of MS medium (0%, 25%, 50%, 75% or 100%). The growth traits and fluorescence a chlorophyll were analyzed. Intermediate concentrations of MS medium resulted in plants with a larger number of leaves and longer root length. The OJIP curves and results of the JIP test showed that the plants grown without MS salts presented less efficient photosystem II (PSII), as indicated by the performance index [Pi(total)]. In contrast, the intermediate concentrations (MS 25% and 50%) had a positive effect on the performance of the photosynthetic apparatus. The MS 25% medium can be used for in vitro cultivation of B. antiacantha, enabling the development of plants with suitable physiological qualities for planting in the field.

Protection of PSI and PSII complexes of wheat from toxic effect of anthracene by Bacillus subtilis (NCIM 5594)

Contamination of polycyclic aromatic hydrocarbons (PAHs) in environment indicates a serious problem to the present era. These are carcinogenic and mutagenic compounds and pose a potential risk to photosynthetic organisms. The present study illustrates the protection of Photosystem I and Photosystem II complexes of wheat plant by Bacillus subtilis (NCIM 5594) from toxic effects of anthracene (ANT). Initially, Chl a fluorescence induction curve measurement revealed declined J-I and I-P phase in ANT-treated plants. Efficiency of light absorption, trapping, and electron transport was reduced in ANT-treated plants, while in ANT + Bacillus subtilis (NCIM 5594)-treated plants value of these parameters was restored. Effect of ANT and ANT + Bacillus subtilis (NCIM 5594) on energy conversion of Photosystem I and Photosystem II was measured. Quantum yield of Photosystem I (YI) and Photosystem II (YII) was decreased in the presence of ANT, while these values were recovered in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Reduction in Y(II) was associated with an increase in non-regulated energy dissipation NO. Likewise the reduction of Y(I) was induced due to donor-side and acceptor-side limitation of Photosystem I caused by toxic effect of ANT. Toxic effects of ANT on electron transport rate (ETR_I and ETR_II) were found to be reduced in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Activation of Cyclic electron flow around Photosystem I in ANT-treated plants was recovered by bacteria. It was concluded that toxic effect of ANT on Photosystem I and Photosystem II complexes was recovered by Bacillus subtilis (NCIM 5594) strain, and thus it is useful strain for crop improvement in ANT-polluted soil.

Comparison of CAM expression, photochemistry and antioxidant responses in Sedum album and Portulaca oleracea under combined stress

Previous studies of crassulacean acid metabolism (CAM) pathway during stress have been directed at individual drought and salinity stress, here, we studied the effects of a combination of drought and salt on CAM expression, chlorophyll fluorescence and antioxidant parameters in the C₃ -CAM facultative Sedum album and C₄ -CAM facultative Portulaca oleracea plants. While salinity alone was not able to induce functional CAM expression in P. oleracea leaves, we showed that salinity induced low level of nocturnal acid accumulation in S. album species. After 20 d of exposure to the combination of simultaneous salt and drought stress, P. oleracea plants exhibited more resistance to photoinhibition as compared to S. album plants. The decrease of maximum quantum yield (F_v /F_m ) in S. album leaves under combined stress was in parallel with the largest suppression of CAM expression of >50%, probably displaying the withdrawal of functional CAM back to C₃ pathway. However, under drought treatment alone, S. album plants exhibited higher photosynthetic flexibility, which was associated with the up-regulation of antioxidant enzymes activities and maintenance of glutathione (GSH) pool, and consequently higher photochemical functioning. The levels of nitric oxide (NO) correlated well with CAM expression, which was observed only in S. album, suggesting that NO acts in a different way in C₃ and C₄ species during CAM induction. Additionally, in both species, over the course of CAM induction, the changes in CAM expression parameters exhibited a similar pattern to that of antioxidant capacity and photochemical functioning parameters.

The oxygen-evolving complex: a super catalyst for life on earth, in response to abiotic stresses

The oxygen-evolving complex is integrated into photosystem (PSII). An essential part of oxygenic photosynthetic apparatus, embedded in the thylakoid membrane of chloroplasts. The OEC is a super catalyst to split water into molecular oxygen in the presence of light. The OEC consist of four Mn atoms, one Ca atom and five oxygen atoms (CaMn₄O₅) and this cluster is maintained by its surrounding proteins viz., PsbQ, PsbP, PsbO, PsbR. The function of this super catalyst with a high turnover frequency of 500 s^-1 in standard condition. Chlorophyll a fluorescence (OJIP transients) are used to understand structural and functional cohesion of photosynthetic apparatus. A further K-peak in OJIP curve reflects damage at the OEC donor site in response to salinity, drought, and high temperature. The decline in performance indices (PI, SFI) also revealed structural damage of photosynthetic apparatus that leads to disruption of electron transport rate under abiotic conditions. This review discusses the structural and function cohesion of the OEC in plant against variable abiotic conditions.

Spatial and Temporal Bioactive Compound Contents and Chlorophyll Fluorescence of Kale (Brassica oleracea L.) Under UV-B Exposure Near Harvest Time in Controlled Environments

UV-B irradiation has been used to enhance the secondary metabolite content in plants, but its spatial effect on plants has not been considered. The objective of this study was to compare spatial photosynthetic traits and bioactive compound accumulation in kale (Brassica oleracea L. var Acephala) according to the distribution and length of UV-B exposure near harvest. Plants were exposed to UV-B of 0-3, 3-6 and 6-9 W m^-2 for 4 h per day at 5 days (Exp. 1) and 4.2 W m^-2 at 5, 4, 3, 2 or 1 days (Exp. 2) before harvest. In spatial distribution, the higher the UV-B intensity, the lower the mean F_v /F_m (maximal photochemical efficiency of PSII) and the higher the concentration of total flavonoid compound (TFC). With UV-B stress, F_v /F_m and fluorescence transient parameters decreased except for DI₀ /CS (dissipated energy flux per cross section) and PI_abs (performance index of PSII). When exposed to UV-B radiation for 2 days before harvest, the total phenolic compounds and TFC per plant were highest, not always proportional to the local F_v /F_m but affected by dry weight. Short-term UV-B stress near harvest would be more efficient for the accumulation of bioactive compounds by minimizing the loss of plant weight.

Characterization of Photosynthetic Phenotypes and Chloroplast Ultrastructural Changes of Soybean (Glycine max) in Response to Elevated Air Temperatures

Heat stress negatively affects photosynthesis in crop plants. Chlorophyll fluorescence provides information about the efficiency of the light-dependent reactions of photosynthesis and can be measured non-destructively and rapidly. Four soybean (Glycine max) genotypes were grown in controlled environments at 28/20°C (control), followed by imposition of control, 38/28°C, and 45/28°C day/night temperature regimes for 7 days. Coordinated chlorophyll fluorescence, gas exchange, and chloroplast ultrastructure measurements were conducted over the course of the 7-day temperature treatments and revealed contrasting responses among the different genotypes. Although generally similar, the extent of the impact of elevated temperatures on net photosynthesis differed among genotypes. Despite dramatic effects on photosynthetic light reactions, net photosynthetic rates were not reduced by exposure to 45°C on the 1^st day of treatment imposition. Temporal dynamics of light reaction characteristics over the course of the 7-day heat-wave simulation revealed distinct responses among the genotypes. Similarly, chloroplast ultrastructure examination identified contrasting responses of DT97-4290 and PI603166, particularly with respect to starch characteristics. These changes were positively associated with differences in the percent area of chloroplasts that were occupied by starch grains. Elevated temperature increased number and size of starch grains on the 1^st day of DT97-4290 which was coordinated with increased minimum chlorophyll fluorescence (F₀) and reduced leaf net CO₂ assimilation (A). Whereas on the 7^th day the elevated temperature treatment showed reduced numbers and sizes of starch grains in chloroplasts and was coordinated with similar levels of F₀ and A to the control treatment. Unlike starch dynamics of PI603166 which elevated temperature had little effect on. The genotypic differences in photosynthetic and chloroplast ultrastructure responses to elevated temperatures identified here are of interest for the development of more tolerant soybean cultivars and to facilitate the dissection of molecular mechanisms underpinning heat stress tolerance of soybean photosynthesis.

Melatonin Positively Influences the Photosynthetic Machinery and Antioxidant System of Avena sativa during Salinity Stress

Recent studies have demonstrated melatonin protects various crops against abiotic stresses. However, the effects of melatonin on the photosynthetic apparatus of stressed plants is poorly characterized. We investigated the effects of melatonin pretreatment on photosynthesis and tolerance to salinity stress in Avena sativa (oat) plants. Oat plants were exposed to four treatments (three replicate pots per treatment): well-watered (WW; control); watered with 300 mM salt solution for 10 days (NaCl); pretreated with 100 µM melatonin solution for 7 days then watered normally for 10 days (Mel+W); or pretreated with 100 µM melatonin for 7 days then 300 mM salt for 10 days (Mel+NaCl). Considerable differences in growth parameters, chlorophyll content, stomatal conductance, proline accumulation, lipid peroxidation, electrolyte leakage, and growth parameters were observed between groups. Genes encoding three major antioxidant enzymes were upregulated in the Mel+NaCl group compared to the other groups. Chlorophyll-a fluorescence kinetic analyses revealed that almost all photosynthetic parameters were improved in Mel+NaCl plants compared to the other treatments. Analysis of genes encoding the major extrinsic proteins of photosystem II (PSII) revealed that PsbA, PsbB, PsbC, and PsbD (but not PsbO) were highly upregulated in Mel+NaCl plants compared to the other groups, indicating melatonin positively influenced photosynthesis under control conditions and salt stress. In addition, melatonin upregulated stress-responsive NAC transcription factor genes in plants exposed to salt stress. These findings suggest melatonin pretreatment improves photosynthesis and enhances salt tolerance in oat plants.

CO2 uptake and chlorophyll a fluorescence of Suaeda fruticosa grown under diurnal rhythm and after transfer to continuous dark

Although only 2-4% of absorbed light is emitted as chlorophyll (Chl) a fluorescence, its measurement provides valuable information on photosynthesis of the plant, particularly of Photosystem II (PSII) and Photosystem I (PSI). In this paper, we have examined photosynthetic parameters of Suaeda fruticosa L. (family: Amaranthaceae), surviving under extreme xerohalophytic conditions, as influenced by diurnal rhythm or continuous dark condition. We report here CO₂ gas exchange and the kinetics of Chl a fluorescence of S. fruticosa, made every 3 hours (hrs) for 3 days, using a portable infra-red gas analyzer and a Handy PEA fluorimeter. Our measurements on CO₂ gas exchange show the maximum rate of photosynthesis to be at 08:00 hrs under diurnal condition and at 05:00 hrs under continuous dark. From the OJIP phase of Chl a fluorescence transient, we have inferred that the maximum quantum yield of PSII photochemistry must have increased during the night under diurnal rhythm, and between 11:00 and 17:00 hrs under constant dark. Overall, our study has revealed novel insights into how photosynthetic reactions are affected by the photoperiodic cycles in S. fruticosa under high salinity. This study has further revealed a unique strategy operating in this xero-halophyte where the repair mechanism for damaged PSII operates during the dark, which, we suggest, contributes to its ecological adaptation and ability to survive and reproduce under extreme saline, high light, and drought conditions. We expect these investigations to help in identifying key genes and pathways for raising crops for saline and dry areas.

UV-B Physiological Changes Under Conditions of Distress and Eustress in Sweet Basil

UV-B radiation has been previously reported to induce protective or deleterious effects on plants depending on the UV-B irradiation doses. To elucidate how these contrasting events are physiologically coordinated, we exposed sweet basil plants to two UV-B doses: low (8.5 kJ m-2 day-1, 30 min exposure) and high (68 kJ m-2 day-1, 4 h exposure), with the plants given both doses once continuously in a single day. Physiological tests during and after both UV-B exposures were performed by comparing the stress-induced damage and adverse effects on photosynthetic activity, the concentration and composition of photosynthetic and non-photosynthetic pigments, and stress-related hormones biosynthesis in basil plants. Our results showed that upon receiving a high UV-B dose, a severe inactivation of oxygen evolving complex (OEC) activity at the PSII donor side and irreversible PSII photodamage caused primarily by limitation of the acceptor side occurred, which overloaded protective mechanisms and finally led to the death of the plants. In contrast, low UV-B levels did not induce any signs of UV-B stress injuries. The OEC partial limitation and the inactivation of the electron transport chain allowed the activation of photoprotective mechanisms, avoiding irreversible damage to PSII. Overall results indicate the importance of a specific response mechanisms regulating photoprotection vs irreversible photoinhibition in basil that were modulated depending on the UV-B doses.

Transcriptomic and metabolomic profiling of Camellia sinensis L. cv. 'Suchazao' exposed to temperature stresses reveals modification in protein synthesis and photosynthetic and anthocyanin biosynthetic pathways

To determine the mechanisms in tea plants responding to temperature stresses (heat and cold), we examined the global transcriptomic and metabolomic profiles of the tea plant cultivar 'Suchazao' under moderately low temperature stress (ML), severely low temperature stress (SL), moderately high temperature stress (MH) and severely high temperature stress (SH) using RNA-seq and high performance liquid chromatography tandem mass spectrometry/mass spectrometry (HPLC-MS/MS), respectively. The identified differentially expressed genes indicated that the synthesis of stress-resistance protein might be redirected to cope with the temperature stresses. We found that heat shock protein genes Hsp90 and Hsp70 played more critical roles in tea plants in adapting to thermal stress than cold, while late embryogenesis abundant protein genes (LEA) played a greater role under cold than heat stress, more types of zinc finger genes were induced under cold stress as well. In addition, energy metabolisms were inhibited by SH, SL and ML. Furthermore, the mechanisms of anthocyanin synthesis were different under the cold and heat stresses. Indeed, the CsUGT75C1 gene, encoding UDP-glucose:anthocyanin 5-O-glucosyl transferase, was up-regulated in the SL-treated leaves but down-regulated in SH. Metabolomics analysis also showed that anthocyanin monomer levels increased under SL. These results indicate that the tea plants share certain foundational mechanisms to adjust to both cold and heat stresses. They also developed some specific mechanisms for surviving the cold or heat stresses. Our study provides effective information about the different mechanisms tea plants employ in surviving cold and heat stresses, as well as the different mechanisms of anthocyanin synthesis, which could speed up the genetic breeding of heat- and cold-tolerant tea varieties.

Chlorophyll fluorescence and carbohydrate concentration as field selection traits for heat tolerant chickpea genotypes

Chickpea (Cicer arietinum L.), a cool season crop is severely affected by heat stress, predicted to increase due to warming climates. Research for identifying heat tolerance markers for potential chickpea genotype selection is imperative. The study assessed the response of four chickpea genotypes to a natural temperature gradient in the field using chlorophyll fluorescence, non-structural carbohydrate, chlorophyll concentrations, gas exchange and grain yield. Field experiments were carried out in two winter seasons at three locations with known differences in temperature in NE South Africa. Results showed two genotypes were tolerant to heat stress with an F_v/F_m of 0.83-0.85 at the warmer site, while the two sensitive genotypes showed lower F_v/F_m of 0.78-0.80. Both dark-adapted F_v/F_m and F_q'/F_m' (where F_q' = F_m' -F) measured at comparable high light levels correlated positively with grain yield. The two tolerant genotypes also showed higher photosynthetic rates, starch, sucrose and grain yield than the sensitive genotypes at the warmer site. However, these parameters were consistently higher at the cooler sites than at the warmer. These results were further validated by a climate chamber experiment, where higher F_v/F_m decline in the sensitive compared to tolerant genotypes was observed when they were exposed to short-term heat treatments of 30/25 °C and 35/30 °C. Tolerant genotypes had higher F_v/F_m (0.78-0.81) and grain yield plant^-1(1.12-2.37g) compared to sensitive genotypes (0.74-0.75) and (0.32-0.89g plant^-1) respectively in the 35/30 °C. It is concluded that chlorophyll fluorescence and leaf carbohydrates are suitable tools for selection of heat tolerant chickpea genotypes under field conditions, while the coolest site showed favourable conditions for chickpea production.

Intrinsic kinetic model of photoautotrophic microalgae based on chlorophyll fluorescence analysis

As photoautotrophic microorganisms, microalgae feature complex mechanisms of photosynthesis and light energy transfer and as such studying their intrinsic growth kinetics is fairly difficult. In this article, the quantum yield of photochemical reaction was introduced in a study of microalgal kinetics to establish an intrinsic kinetic model of photoautotrophic microalgal growth. The blue-green algae Synechococcus sp. PCC7942 was used to verify the kinetic model developed using chlorophyll fluorescence analysis and growth kinetics determination. Results indicate that the kinetic model can realistically reflect the light energy utilization efficiency of microalgae as well as their intrinsic growth kinetic characteristics. The model and method proposed in this article may be utilized in intrinsic kinetics studies of photoautotrophic microorganisms.

Sensing Plant Physiology and Environmental Stress by Automatically Tracking Fj and Fi Features in PSII Chlorophyll Fluorescence Induction

Following a step excitation, chlorophyll fluorescence (ChlF) from photosystem II (PSII) of a dark-adapted photosynthetic organism exhibits the well-known OJIP pattern. The OJIP induction has been widely used in plant science and agriculture engineering. While the J and I phases are related to transitions of photochemical reaction redox states, characteristic fluorescence intensities at the two phases (F_j and F_i ) are often treated at fixed time points in routine measurement and thus do not account for variations in plant and experimental conditions, this (1) neglects the differences in the time of appearance of these phases, which is potentially useful information for characterizing plant status and environmental factors, and (2) leads to errors in measured F_j and F_i values in the many publications. In this work, an alternative method for consistent measurement of F_j and F_i was presented. The proposed method measures the curvatures in the OJIP curve and automatically tracks the characteristic transition points under variable sample and experimental conditions. Experiments were carried out to demonstrate the concept and classification capabilities of the method. This research has established a new framework to analyze ChlF and has enhanced the application capability of ChlF. It is expect useful in analysis ChlF from PSII.

Estimation of Chlorophyll Fluorescence at Different Scales: A Review

Measuring chlorophyll fluorescence is a direct and non-destructive way to monitor vegetation. In this paper, the fluorescence retrieval methods from multiple scales, ranging from near the ground to the use of space-borne sensors, are analyzed and summarized in detail. At the leaf-scale, the chlorophyll fluorescence is measured using active and passive technology. Active remote sensing technology uses a fluorimeter to measure the chlorophyll fluorescence, and passive remote sensing technology mainly depends on the sun-induced chlorophyll fluorescence filling in the Fraunhofer lines or oxygen absorptions bands. Based on these retrieval principles, many retrieval methods have been developed, including the radiance-based methods and the reflectance-based methods near the ground, as well as physically and statistically-based methods that make use of satellite data. The advantages and disadvantages of different approaches for sun-induced chlorophyll fluorescence retrieval are compared and the key issues of the current sun-induced chlorophyll fluorescence retrieval algorithms are discussed. Finally, conclusions and key problems are proposed for the future research.

Detection of Cadmium Risk to the Photosynthetic Performance of Hybrid Pennisetum

Photosynthesis plays an essential role in plant growth and crop yield, and the mechanisms of the effects of cadmium (Cd) on photosynthetic performance require more attention. The acute toxicity of Cd in soil to the photosynthetic capacity of Hybrid Pennisetum was evaluated using gas exchange parameters, A/Ci curves, light response curves, and chlorophyll a fluorescence transients after exposure to elevated Cd concentrations (0, 10, 20, 50, 70, and 100 mg kg-1) for a 3-month period. The results indicated that leaf Cd concentration in Hybrid Pennisetum increased with the strength of soil Cd stress and ranged from 4.9 to 15.8 μg g-1 DW. The accumulation of leaf Cd severely restricted photosynthesis and its non-stomatal limitation in regulating the photosynthetic performance of Hybrid Pennisetum. The leaf chloroplasts at 10 and 20 mg kg-1 Cd concentrations showed no noticeable change, but the chlorophyll content significantly decreased by 9.0-20.4% at 50-100 mg kg-1 Cd concentrations. The Cd treatments also decreased plant ribulose-1,5-bisphosphate (RuBP) activity (Vcmax ) and regeneration capacity (Jmax ), triose phosphate utilization (TPU), light-saturated photosynthesis (Amax ), apparent quantum yield (AQY), light saturation point (LSP), and dark respiration (Rday ), but Cd treatment increased the light compensation point (LCP). The shape of chlorophyll a fluorescence transients in leaves was altered under different Cd treatments. The increased OJ phase and the decreased IP phase in fluorescence induction curves suggested that Cd toxicity inhibited both light use efficiency and photodamage avoidance ability. These results suggested that the decrease in photosynthesis through exposure to Cd may be a result of the decrease in leaf chlorophyll content, Rubisco activity, and RuBP regeneration, inhibition of triose phosphate utilization, reduction of the ability to use light and provide energy, and restrictions on electron transport in PSII.

Tissue specific disruption of photosynthetic electron transport rate in pigeonpea (Cajanus cajan L.) under elevated temperature

Nutritional rich pigeonpea (Cajanus cajan [L.]), a perennial shrub member of family Fabaceae is the sixth most important grain legume of the world. Continuous rise of temperature and current global climate scenario limits plant growth and performance but photosynthetic machineries are adversely affected. The aim of this study was the analysis of tissue specific photosynthetic pigments and photosynthetic electron transport rate under elevated temperature. Two different stages of both leaf (young and mature), and pod (young and mature) were chosen, and photosynthetic pigment and J-I-P tests were measured. Leaves and pods were detached and incubated in normal temperature (25°C) for 24 h in two different condition one light irradiance (100 µmol m-2 s-1) and another darkness as control, and treated with high temperature 45°C for 24 h and repeated previous measurements were taken. Tissue specific photosynthetic pigments variation were observed; J-I-P parameters clearly revealed that elevated temperature has greater influence on photosystem II (PSII) electron transport than photosystem I (PSI), and significant changes were observed in pods than leaves. Young tissues were adversely affected by elevated temperature. In addition, the J-I-P tests and energy pipeline model indicated that PSI electron transport rate of leaves and pods appeared to be more thermo-tolerance than those in PSII. Only a minor drop in pigments pool and photosynthetic performance was observed after 24 h of darkness. O-J-I-P transients can be used as a sensitive, nondestructive method for measuring heat stress damage and a special tool for investigating action sites of high temperature stress. Findings of this study will contribute to basic understanding of photosynthetic performance, and to screen potential thermo-tolerant genotypes of pigeonpea to sustain in either current scenario of climate change or/and erratic future climatic conditions.

Influence of nitrogen source on photochemistry and antenna size of the photosystems in marine green macroalgae, Ulva lactuca

Ulva lactuca is regarded as a prospective energy crop for biorefinery owing to its affluent biochemical composition and high growth rate. In fast-growing macroalgae, biomass development strictly depends on external nitrogen pools. Additionally, nitrogen uptake rates and photosynthetic pigment content vary with type of nitrogen source and light conditions. However, the combined influence of nitrogen source and light intensity on photosynthesis is not widely studied. In present study, pale green phenotype of U. lactuca was obtained under high light (HL) condition when inorganic nitrogen (nitrate) in the media was substituted with organic nitrogen (urea). Further, pale green phenotype survived the saturating light intensities in contrast to the normal pigmented control which bleached in HL. Detailed analysis of biochemical composition and photosynthesis was performed to understand functional antenna size and photoprotection in pale green phenotype. Under HL, urea-grown cultures exhibited increased growth rate, carbohydrate and lipid content while substantial reduction in protein, chlorophyll content and PSII antenna size was observed. Further, in vivo slow and polyphasic chlorophyll a (Chl a) fluorescence studies revealed reduction in excitation pressure on PSII along with low non-photochemical quenching thus, transmitting most of the absorbed energy into photochemistry. The results obtained could be correlated to previous report on cultivation of U. lactuca through saturating summer intensities (1000 µmole photons m^-2 s^-1) in urea based: poultry litter extract (PLE). Having proved critical role of urea in conforming photoprotection, the application PLE was authenticated for futuristic, sustainable and year-round biomass cultivation.

Arbuscular Mycorrhizal fungi (AMF) protects photosynthetic apparatus of wheat under drought stress

Drought stress (DS) is amongst one of the abiotic factors affecting plant growth by limiting productivity of crops by inhibiting photosynthesis. Damage due to DS and its protection by Arbuscular Mycorrhizal fungi (AMF) was studied on photosynthetic apparatus of wheat (Triticum aestivum) plants in pot experiments. DS was maintained by limiting irrigation to the drought stressed (DS) and AMF + DS plants. Relative Water content (RWC) was measured for leaf as well as soil to ensure drought conditions. DS plants had minimum RWC for both leaf and soil. AMF plants showed increased RWC both for leaf and soil indicating that AMF hyphae penetrated deep into the soil and provided moisture to the plants. In Chl a fluorescence induction curve (OJIP), a declined J-I and I-P phase was observed in DS plants. Efficacy of primary photochemistry declined in DS plants as result of DS, while AMF plants showed maximum photochemistry. DS leads to declined quantum efficiency of PSI and PSII in DS plants while it was restored in AMF + DS plants. Electron transport (ETRI and ETRII) decreased while quantum yield of non-photochemical quenching Y(NPQ) increased as a result of drought stress. CEF around PSI increased in DS-stressed plants. Efficient PSI complexes decreased in DS plants while in case of AMF plants PSI complexes were able to perform PSI photochemistry significantly. Thus, it is concluded that drought stress-induced damage to the structure and function of PSII and PSI was alleviated by AMF colonization.

Effects of cadmium on photosynthesis of Schima superba young plant detected by chlorophyll fluorescence

Contamination by heavy metals has become a serious environmental pollution issue today due to its potential threat to plant, wildlife, and human health. Photosynthesis, a process in which light energy is used to produce sugar and other organic compounds, is sensitive to heavy metals. In the present study, the response of photosynthetic process and carbon assimilation of Schima superba was investigated under cadmium (Cd) stress. Three Cd concentrations (0, 300, and 600 mg kg^-1) were used designated as control (CK), low Cd (L₁), and high Cd treatment (L₂) of plants. Results showed that photosystem II (PSII) acceptor and donor side electron transport were more easily blocked in treatment compared to control, and L₂ have more significant changes than L₁. A substantial decrease of 820 nm reflection curve absorption was observed both in L₁ and L₂ treatments. Special energy fluxes showed significant difference between the control group and the treated group, which indicated that low concentration Cd stress can cause decrease in quantum yield of PSII in plants studied. Non-stomatal factors resulted in a decrease in net photosynthetic rate and a decrease in photosystem activity. Our results suggested that Cd can damage structure and function of the photosynthesis of S. superba young plants.

Decrease in the Photosynthetic Performance of Temperate Grassland Species Does Not Lead to a Decline in the Gross Primary Production of the Ecosystem

Plants, under stressful conditions, can proceed to photosynthetic adjustments in order to acclimatize and alleviate the detrimental impacts on the photosynthetic apparatus. However, it is currently unclear how adjustment of photosynthetic processes under environmental constraints by plants influences CO2 gas exchange at the ecosystem-scale. Over a 2-year period, photosynthetic performance of a temperate grassland ecosystem was characterized by conducting frequent chlorophyll fluorescence (ChlF) measurements on three primary grassland species (Lolium perenne L., Taraxacum sp., and Trifolium repens L.). Ecosystem photosynthetic performance was estimated from measurements performed on the three dominant grassland species weighed based on their relative abundance. In addition, monitoring CO2 fluxes was performed by eddy covariance. The highest decrease in photosynthetic performance was detected in summer, when environmental constraints were combined. Dicot species (Taraxacum sp. and T. repens) presented the strongest capacity to up-regulate PSI and exhibited the highest electron transport efficiency under stressful environmental conditions compared with L. perenne. The decline in ecosystem photosynthetic performance did not lead to a reduction in gross primary productivity, likely because increased light energy was available under these conditions. The carbon amounts fixed at light saturation were not influenced by alterations in photosynthetic processes, suggesting photosynthesis was not impaired. Decreased photosynthetic performance was associated with high respiration flux, but both were influenced by temperature. Our study revealed variation in photosynthetic performance of a grassland ecosystem responded to environmental constraints, but alterations in photosynthetic processes appeared to exhibit a negligible influence on ecosystem CO2 fluxes.

Long-term measurements of chlorophyll a fluorescence using the JIP-test show that combined abiotic stresses influence the photosynthetic performance of the perennial ryegrass (Lolium perenne) in a managed temperate grassland

Several experiments have highlighted the complexity of stress interactions involved in plant response. The impact in field conditions of combined environmental constraints on the mechanisms involved in plant photosynthetic response, however, remains understudied. In a long-term field study performed in a managed grassland, we investigated the photosynthetic apparatus response of the perennial ryegrass (Lolium perenne L.) to environmental constraints and its ability to recover and acclimatize. Frequent field measurements of chlorophyll a fluorescence (ChlF) were made in order to determine the photosynthetic performance response of a population of L. perenne. Strong midday declines in the maximum quantum yield of primary photochemistry (F_V F_M ) were observed in summer, when a combination of heat and high light intensity increased photosynthetic inhibition. During this period, increase in photosystem I (PSI) activity efficiency was also recorded, suggesting an increase in the photochemical pathway for de-excitation in summer. Strong climatic events (e.g. heat waves) were shown to reduce electron transport between photosystem II (PSII) and PSI. This reduction might have preserved the PSI from photo-oxidation. Periods of low soil moisture and high levels of sun irradiance increased PSII sensitivity to heat stress, suggesting increased susceptibility to combined environmental constraints. Despite the multiple inhibitions of photosynthetic functionality in summer, the L. perenne population showed increased PSII tolerance to environmental stresses in August. This might have been a response to earlier environmental constraints. It could also be linked to the selection and/or emergence of well-adapted individuals.

PSI becomes more tolerant to fluoranthene through the initiation of cyclic electron flow

Environmental pollution by organic compounds such as polycyclic aromatic hydrocarbons (PAHs) poses a potential ecological risk to photosynthetic organisms. In the present study, the toxic effects of fluoranthene (FLT) on the energy conversion of PSI and PSII in wheat (Triticum aestivum L.) plants were studied. By evaluating the performance of both PSI and PSII, which act as an internal environmental sensor, it was revealed that activity of both photosystems was negatively affected by FLT treatment. However, the quantum yield of PSII, Y(II), was reduced at 5µM FLT, whereas the quantum yield of PSI, Y(I), significantly decreased at 25µM FLT. The decline in Y(II) was accompanied by an increase in nonregulated energy dissipation, Y(NO). The decrease in Y(I) induced by FLT was caused by donor-side, and acceptor side limitation of PSI. Cyclic electron flow (CEF) was activated only at higher concentrations and was associated with the inhibition of linear electron flow (LEF) after exposure to a higher concentration of FLT. The inhibition of LEF and induction of CEF seems to be essential for the tolerance of PSI to FLT toxicity.

QTLs and Potential Candidate Genes for Heat Stress Tolerance Identified from the Mapping Populations Specifically Segregating for Fv/Fm in Wheat

Despite the fact that Fv/Fm (maximum quantum efficiency of photosystem II) is the most widely used parameter for a rapid non-destructive measure of stress detection in plants, there are barely any studies on the genetic understanding of this trait under heat stress. Our aim was to identify quantitative trait locus (QTL) and the potential candidate genes linked to Fv/Fm for improved photosynthesis under heat stress in wheat (Triticum aestivum L.). Three bi-parental F2 mapping populations were generated by crossing three heat tolerant male parents (origin: Afghanistan and Pakistan) selected for high Fv/Fm with a common heat susceptible female parent (origin: Germany) selected for lowest Fv/Fm out of a pool of 1274 wheat cultivars of diverse geographic origin. Parents together with 140 F2 individuals in each population were phenotyped by Fv/Fm under heat stress (40°C for 3 days) around anthesis. The Fv/Fm decreased by 6.3% in the susceptible parent, 1-2.5% in the tolerant parents and intermediately 4-6% in the mapping populations indicating a clear segregation for the trait. The three populations were genotyped with 34,955 DArTseq and 27 simple sequence repeat markers, out of which ca. 1800 polymorphic markers mapped to 27 linkage groups covering all the 21 chromosomes with a total genome length of about 5000 cM. Inclusive composite interval mapping resulted in the identification of one significant and heat-stress driven QTL in each population on day 3 of the heat treatment, two of which were located on chromosome 3B and one on chromosome 1D. These QTLs explained about 13-35% of the phenotypic variation for Fv/Fm with an additive effect of 0.002-0.003 with the positive allele for Fv/Fm originating from the heat tolerant parents. Approximate physical localization of these three QTLs revealed the presence of 12 potential candidate genes having a direct role in photosynthesis and/or heat tolerance. Besides providing an insight into the genetic control of Fv/Fm in the present study, the identified QTLs would be useful in breeding for heat tolerance in wheat.

A New Microfluidic Device for Classification of Microalgae Cells Based on Simultaneous Analysis of Chlorophyll Fluorescence, Side Light Scattering, Resistance Pulse Sensing

Fast on-site monitoring of foreign microalgae species carried by ship ballast water has drawn more and more attention. In this paper, we presented a new method and a compact device of classification of microalgae cells by simultaneous detection of three kinds of signals of single microalgae cells in a disposable microfluidic chip. The microfluidic classification device has advantages of fast detection, low cost, and portability. The species of a single microalgae cell can be identified by simultaneous detection of three signals of chlorophyll fluorescence (CF), side light scattering (SLS), and resistance pulse sensing (RPS) of the microalgae cell. These three signals represent the different characteristics of a microalgae cell. A compact device was designed to detect these three signals of a microalgae cell simultaneously. In order to demonstrate the performance of the developed system, the comparison experiments of the mixed samples of three different species of microalgae cells between the developed system and a commercial flow cytometer were conducted. The results show that three kinds of microalgae cells can be distinguished clearly by our developed system and the commercial flow cytometer and both results have good agreement.