Polyphasic chlorophyll a fluorescence kinetics and leaf protein analyses to track dynamics of photosynthetic performance in mulberry during progressive drought

Ultrasonic-assisted extraction of total flavonoids from Zanthoxylum bungeanum residue and their allelopathic mechanism on Microcystis aeruginosa

Water eutrophication has emerged as a pressing concern for massive algal blooms, and these harmful blooms can potentially generate harmful toxins, which can detrimentally impact the aquatic environment and human health. Consequently, it is imperative to identify a safe and efficient approach to combat algal blooms to safeguard the ecological safety of water. This study aimed to investigate the procedure for extracting total flavonoids from Z. bungeanum residue and assess its antioxidant properties. The most favorable parameters for extracting total flavonoids from Z. bungeanum residue were a liquid-solid ratio (LSR) of 20 mL/g, a solvent concentration of 60%, an extraction period of 55 min, and an ultrasonic temperature of 80 °C. Meanwhile, the photosynthetic inhibitory mechanism of Z. bungeanum residue extracts against M. aeruginosa was assessed with a particular focus on the concentration-dependent toxicity effect. Z. bungeanum residue extracts damaged the oxygen-evolving complex structure, influenced energy capture and distribution, and inhibited the electron transport of PSII in M. aeruginosa. Furthermore, the enhanced capacity for ROS detoxification enables treated cells to sustain their photosynthetic activity. The findings of this study hold considerable relevance for the ecological management community and offer potential avenues for the practical utilization of resources in controlling algal blooms.

Mixed acid treatment for removal of green macroalgae from Neopyropia aquaculture nets: Field experiment in the Subei Shoal, China

To develop an effective method to eliminate green macroalgae attached to Neopyropia aquaculture nets, we explored the influence of mixed acid solution on the photosynthetic fluorescence characteristics of Ulva spp. (green macroalgae) and Neopyropia yezoensis (red macroalgae) from Dafeng and Rudong aquaculture areas in Jiangsu Province, China. Treatment with mixed acid solution (0.0475 % hydrochloric acid:citric acid (pH 2.0) at a ratio of 4:3) for 60 s caused death of Ulva spp., but did not affect N. yezoensis. Additionally, a mixed acid solution effectively eliminated green macroalgae from Neopyropia aquaculture rafts and the marine environment remained unaffected. Hence, the application of mixed acid solution treatment has demonstrated significant efficacy in eradicating green macroalgae adhered to Neopyropia aquaculture rafts, thus presenting a promising strategy for mitigating green macroalgae proliferation in Neopyropia aquaculture areas and curbing their contribution to green tides.

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

Introduction

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

Methods

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

Results

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

Discussion

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

Elevated CO2 concentration enhances drought resistance of soybean by regulating cell structure, cuticular wax synthesis, photosynthesis, and oxidative stress response

The atmospheric [CO2] and the frequency and intensity of extreme weather events such as drought are increased, leading to uncertainty to soybean production. Elevated [CO2] (eCO2) partially mitigates the adverse effects of drought stress on crop growth and photosynthetic performance, but the mitigative mechanism is not well understood. In this study, soybean seedlings under drought stress simulated by PEG-6000 were grown in climate chambers with different [CO2] (400 μmol mol-1 and 700 μmol mol-1). The changes in anatomical structure, wax content, photosynthesis, and antioxidant enzyme were investigated by the analysis of physiology and transcriptome sequencing (RNA-seq). The results showed that eCO2 increased the thickness of mesophyll cells and decreased the thickness of epidermal cells accompanied by reduced stomatal conductance, thus reducing water loss in soybean grown under drought stress. Meanwhile, eCO2 up-regulated genes related to wax anabolism, thus producing more epidermal wax. Under drought stress, eCO2 increased net photosynthetic rate (PN), ribulose-1,5-bisphosphate carboxylase/oxygenase activity, and alerted the gene expressions in photosynthesis. The increased sucrose synthesis and decreased sucrose decomposition contributed to the progressive increase in the soluble saccharide contents under drought stress with or without eCO2. In addition, eCO2 increased the expressions of genes associated with peroxidase (POD) and proline (Pro), thus enhancing POD activity and Pro content and improving the drought resistance in soybean. Taken together, these findings deepen our understanding of the effects of eCO2 on alleviating drought stress in soybean and provide potential target genes for the genetic improvement of drought tolerance in soybean.

Natural variation in photosynthesis and water use efficiency of locally adapted Persian walnut populations under drought stress and recovery

Persian walnut is a drought-sensitive species with considerable genetic variation in the photosynthesis and water use efficiency of its populations, which is largely unexplored. Here, we aimed to elucidate changes in the efficiency of photosynthesis and water content using a diverse panel of 60 walnut families which were submitted to a progressive drought for 24 days, followed by two weeks of re-watering. Severe water-withholding reduced leaf relative water content (RWC) by 20%, net photosynthetic rate (Pn) by 50%, stomatal conductance (gs) by 60%, intercellular CO2 concentration (Ci) by 30%, and transpiration rate (Tr) by 50%, but improved water use efficiency (WUE) by 25%. Severe water-withholding also inhibited photosystem II functionality as indicated by reduced quantum yield of intersystem electron transport (φEo) and transfer of electrons per reaction center (ET0/RC), also enhanced accumulation of QA (VJ) resulted in the reduction of the photosynthetic performance (PIABS) and maximal quantum yield of PSII (FV/FM); while elevated quantum yield of energy dissipation (φDo), energy fluxes for absorption (ABS/RC) and dissipated energy flux (DI0/RC) in walnut families. Cluster analysis classified families into three main groups (tolerant, moderately tolerant, and sensitive), with the tolerant group from dry climates exhibiting lesser alterations in assessed parameters than the other groups. Multivariate analysis of phenotypic data demonstrated that RWC and biophysical parameters related to the chlorophyll fluorescence such as FV/FM, φEo, φDo, PIABS, ABS/RC, ET0/RC, and DI0/RC represent fast, robust and non-destructive biomarkers for walnut performance under drought stress. Finally, phenotype-environment association analysis showed significant correlation of some photosynthetic traits with geoclimatic factors, suggesting a key role of climate and geography in the adaptation of walnut to its habitat conditions.

Environmental stress - what can we learn from chlorophyll a fluorescence analysis in woody plants? A review

Chlorophyll a fluorescence (ChF) signal analysis has become a widely used and rapid, non-invasive technique to study the photosynthetic process under stress conditions. It monitors plant responses to various environmental factors affecting plants under experimental and field conditions. Thus, it enables extensive research in ecology and benefits forestry, agriculture, horticulture, and arboriculture. Woody plants, especially trees, as organisms with a considerable life span, have a different life strategy than herbaceous plants and show more complex responses to stress. The range of changes in photosynthetic efficiency of trees depends on their age, ontogeny, species-specific characteristics, and acclimation ability. This review compiles the results of the most commonly used ChF techniques at the foliar scale. We describe the results of experimental studies to identify stress factors that affect photosynthetic efficiency and analyse the experience of assessing tree vigour in natural and human-modified environments. We discuss both the circumstances under which ChF can be successfully used to assess woody plant health and the ChF parameters that can be useful in field research. Finally, we summarise the advantages and limitations of the ChF method in research on trees, shrubs, and woody vines.

Photosynthesis and growth of Amaranthus tricolor under strontium stress

Amaranthaceae are effective plants for cleaning soil contaminated by heavy metals and radionuclides. In this paper, Amaranthus tricolor was used to investigate the response of the plant photosynthesis to various concentration of strontium ions (0.2, 0.6, 3 and 6 mM), in order to determine the possibility of A. tricolor to remediate strontium contamination. The results showed that strontium ions (0.2-6 mM) had effect on light energy conversion and utilization in A. tricolor. Low level of strontium (0.2 mM) promoted the energy utilization in A. tricolor, while higher Sr concentration (3 mM or higher) increased the excess light energy in the plants. Under strontium stress of 6 mM, the acceptor side of PSII in A. tricolor leaves was more vulnerable to strontium stress than the donor side. Furthermore, strontium stress led to accumulation of Q_A^- and block in Q_B downstream of the electron transfer chain in PSII of A. tricolor leaves. The tolerance ability of A. tricolor to strontium and remediation is also reflected in its biomass and strontium content in plants. Strontium at 3 mM or below promoted the growth of A. tricolor, while higher concentration inhibited the plant growth, but without obvious wilting or curling of leaves. The maximal dry weight increased by 36.29% in shoots, and 60.14% in roots when the spiked-strontium concentration reached 0.2 mM. The maximal strontium content achieved 8.75 mg/g dry wt in shoots, and 1.71 mg/g dry wt in roots respectively, when strontium concentration was 6 mM. Transfer factors (TFs: ratio of Sr content in shoots to that in roots) of strontium in A. tricolor ranged from 2.85 to 5.93, while bio-concentration factors (BCFs: ratio of Sr content in shoots to that in solutions) ranged from 22.57 to 49.66. In summary, A. tricolor showed the excellent potential to remediate strontium contamination.

Response of Amaranthus tricolor to cesium stress in hydroponic system: Growth, photosynthesis and cesium accumulation

Remediation of the cesium-contaminated environment is of paramount importance, and phytoremediation is a cost-effective and green technique. In this paper, the response of Amaranthus tricolor to cesium ions in hydroponic solution was investigated at various cesium concentration (0, 0.05, 0.2, 0.4 and 0.6 mM), in terms of the growth weight, height and photosynthesis. The maximal Cs content in stems and leaves of A. tricolor was 13.05 mg/g dry wt under spiked Cs level of 0.4 mM in solution. The maximal transfer factor (TF) and bioconcentration factor (BCF) were 1.87 and 181.25 respectively, when the corresponding Cs content in roots and shoots was 7.04 mg/g and 13.05 mg/g dry wt respectively. TFs are higher than 1 in the conditions of normal plant growth. The growth of A. tricolor was enhanced after the treatment of Cs at low concentrations (0.05 and 0.2 mM), while it was inhibited at 0.4 and 0.6 mM. The leaf number and dry weight of stem, leaf parts and root parts were maximum at the spiked cesium level of 0.2 mM, which significantly increased by 19.19%, 47.56% and 94.56% respectively, compared with the control samples. Under 0.6 mM cesium stress, curl and withering of the leaves occurred, and the plant growth and cesium accumulation dropped to the minimum. Cs at the spiked level of 0.6 mM in solution inhibited the performance of PSII, especially in terms of blockage in electron transfer process beyond Q_A and restraint of P700 reduction. On contrast, the performance of PSII was enhanced by the spiked Cs at level of 0.2 mM, leading to the growing density of reaction centers per excited cross-section and increasing electron transfer process beyond Q_A. In summary, A. tricolor has potential for remediating the Cs-contaminated environment.

PEG-induced physiological drought for screening winter wheat genotypes sensitivity - integrated biochemical and chlorophyll a fluorescence analysis

This study aimed to screen different winter wheat genotypes at the onset of metabolic changes induced by water deficit to comprehend possible adaptive features of photosynthetic apparatus function and structure to physiological drought. The drought treatment was the most influential variable affecting plant growth and relative water content, and genotype variability determined with what intensity varieties of winter wheat seedlings responded to water deficit. PEG-induced drought, as expected, changed phenomenological energy fluxes and the efficiency with which an electron is transferred to final PSI acceptors. Based on the effect size, fluorescence parameters were grouped to represent photochemical parameters, that is, the donor and acceptor side of PSII (PC1); the thermal phase of the photosynthetic process, or the electron flow around PSI, and the chain of electrons between PSII and PSI (PC2); and phenomenological energy fluxes per cross-section (PC3). Furthermore, four distinct clusters of genotypes were discerned based on their response to imposed physiological drought, and integrated analysis enabled an explanation of their reactions' specificity. The most reliable JIP-test parameters for detecting and comparing the drought impact among tested genotypes were the variable fluorescence at K, L, I step, and PI_TOT. To conclude, developing and improving screening methods for identifying and evaluating functional relationships of relevant characteristics that are useful for acclimation, acclimatization, and adaptation to different types of drought stress can contribute to the progress in breeding research of winter wheat drought-tolerant lines.

High-resolution dissection of photosystem II electron transport reveals differential response to water deficit and heat stress in isolation and combination in pearl millet [Pennisetum glaucum (L.) R. Br.]

Heat and Water Deficit Stress (WDS) tend to impede and restrict the efficiency of photosynthesis, chlorophyll fluorescence, and maximum photochemical quantum yield in plants based on their characteristic ability to interfere with the electron transport system in photosystem II. Dissection of the electron transport pathway in Photosystem II (PSII) under water deficit and Heat Stress (HS) can be insightful in gaining knowledge on the various attributes of the photosynthetic performance of a plant. We attempt a high-resolution dissection of electron transport in PSII with studies on chlorophyll a fast fluorescence kinetics and non-photochemical quenching (NPQ) as a response to and recovery from these stresses in pearl millet [Pennisetum glaucum (L.) R. Br.] in isolation and combination. In this study, we bring out the mechanisms by which both heat and water stress, in isolation and in combination, affect the photosynthetic electron transport in Photosystem II. Our results indicate that oxygen evolution complex (OEC) damage is the primary effect of heat stress and is not seen with the same intensity in the water-stressed plants. Low exciton absorption flux in heat stress and combined stress was seen due to OEC damage, and this caused an electron transport traffic jam in the donor side of PS II. Both the specific energy flux model and the phenomenological flux model developed from the derived values in our study show that water deficit stress in combination with heat stress has a much stronger effect than the stresses in isolation on the overall electron transport pathway of the PS II in pearl millet plants.

Assessment of the Photosynthetic Apparatus Functions by Chlorophyll Fluorescence and P700 Absorbance in C3 and C4 Plants under Physiological Conditions and under Salt Stress

Functions of the photosynthetic apparatus of C3 (Pisum sativum L.) and C4 (Zea mays L.) plants under physiological conditions and after treatment with different NaCl concentrations (0-200 mM) were investigated using chlorophyll a fluorescence (pulse-amplitude-modulated (PAM) and JIP test) and P700 photooxidation measurement. Data revealed lower density of the photosynthetic structures (RC/CSo), larger relative size of the plastoquinone (PQ) pool (N) and higher electron transport capacity and photosynthetic rate (parameter RFd) in C4 than in C3 plants. Furthermore, the differences were observed between the two studied species in the parameters characterizing the possibility of reduction in the photosystem (PSI) end acceptors (REo/RC, REo/CSo and δRo). Data revealed that NaCl treatment caused a decrease in the density of the photosynthetic structures and relative size of the PQ pool as well as decrease in the electron transport to the PSI end electron acceptors and the probability of their reduction as well as an increase in the thermal dissipation. The effects were stronger in pea than in maize. The enhanced energy losses after high salt treatment in maize were mainly from the increase in the regulated energy losses (ΦNPQ), while in pea from the increase in non-regulated energy losses (ΦNO). The reduction in the electron transport from QA to the PSI end electron acceptors influenced PSI activity. Analysis of the P700 photooxidation and its decay kinetics revealed an influence of two PSI populations in pea after treatment with 150 mM and 200 mM NaCl, while in maize the negligible changes were registered only at 200 mM NaCl. The experimental results clearly show less salt tolerance of pea than maize.

Low Light Increases the Abundance of Light Reaction Proteins: Proteomics Analysis of Maize (Zea mays L.) Grown at High Planting Density

Maize (Zea mays L.) is usually planted at high density, so most of its leaves grow in low light. Certain morphological and physiological traits improve leaf photosynthetic capacity under low light, but how light absorption, transmission, and transport respond at the proteomic level remains unclear. Here, we used tandem mass tag (TMT) quantitative proteomics to investigate maize photosynthesis-related proteins under low light due to dense planting, finding increased levels of proteins related to photosystem II (PSII), PSI, and cytochrome b₆f. These increases likely promote intersystem electron transport and increased PSI end electron acceptor abundance. OJIP transient curves revealed increases in some fluorescence parameters under low light: quantum yield for electron transport (φE_o), probability that an electron moves beyond the primary acceptor Q_A^- (ψ_o), efficiency/probability of electron transfer from intersystem electron carriers to reduction end electron acceptors at the PSI acceptor side (δR_o), quantum yield for reduction of end electron acceptors at the PSI acceptor side (φR_o), and overall performance up to the PSI end electron acceptors (PI_total). Thus, densely planted maize shows elevated light utilization through increased electron transport efficiency, which promotes coordination between PSII and PSI, as reflected by higher apparent quantum efficiency (AQE), lower light compensation point (LCP), and lower dark respiration rate (R_d).

Physiological, Biochemical, and Root Proteome Networks Revealed New Insights Into Salt Tolerance Mechanisms in Pongamia pinnata (L.) Pierre

Cultivation of potential biofuel tree species such as Pongamia pinnata would rehabilitate saline marginal lands toward economic gains. We carried out a physiological, biochemical, and proteomic analysis to identify key regulatory responses which are associated with salt tolerance mechanisms at the shoot and root levels. Pongamia seedlings were grown at 300 and 500 mM NaCl (∼3% NaCl; sea saline equivalent) concentrations for 15 and 30 days, gas exchange measurements including leaf net photosynthetic rate (A _sat ), stomatal conductance (g _s ), and transpiration rate (E), and varying chlorophyll a fluorescence kinetics were recorded. The whole root proteome was quantified using the free-labeled nanoLC-MS/MS technique to investigate crucial proteins involved in signaling pathways associated with salt tolerance. Pongamia showed no visible salt-induced morphological symptoms. However, Pongamia showed about 50% decline in gas exchange parameters including A _sat , E, and g _s 15 and 30 days after salt treatment (DAS). The maximum potential quantum efficiency of photosystem (PS) II (Fv/Fm) was maintained at approximately 0.8 in salt-treated plants. The thermal component of PSII (DIo) was increased by 1.6-fold in the salt-treated plants. A total of 1,062 protein species were identified with 130 commonly abundant protein species. Our results also elucidate high abundance of protein species related to flavonoid biosynthesis, seed storage protein species, and carbohydrate metabolism under salt stress. Overall, these analyses suggest that Pongamia exhibited sustained leaf morphology by lowering net photosynthetic rates and emitting most of its light energy as heat. Our root proteomic results indicated that these protein species were most likely recruited from secondary and anaerobic metabolism, which could provide defense for roots against Na⁺ toxicity under salt stress conditions.

Short-term warming does not affect intrinsic thermotolerance but induces strong sustaining photoprotection in tropical evergreen citrus genotypes

Consequences of warming and postwarming events on photosynthetic thermotolerance (P_T ) and photoprotective responses in tropical evergreen species remain elusive. We chose Citrus to answer some of the emerging questions related to tropical evergreen species' P_T behaviour including (i) how wide is the genotypic variation in P_T ? (ii) how does P_T respond to short-term warming and (iii) how do photosynthesis and photoprotective functions respond over short-term warming and postwarming events? A study on 21 genotypes revealed significant genotypic differences in P_T , though these were not large. We selected five genotypes with divergent P_T and simulated warming events: T_max 26/20°C (day-time highest maximum/night-time lowest maximum) (Week 1) < T_max 33/30°C (Week 2) < T_max 36/32°C (Week 3) followed by T_max 26/16°C (Week 4, recovery). The P_T of all genotypes remained unaltered despite strong leaf megathermy (leaf temperature > air temperature) during warming events. Though moderate warming showed genotype-specific stimulation in photosynthesis, higher warming unequivocally led to severe loss in net photosynthesis and induced higher nonphotochemical quenching. Even after a week of postwarming, photoprotective mechanisms strongly persisted. Our study points towards a conservative P_T in evergreen citrus genotypes and their need for sustaining higher photoprotection during warming as well as postwarming recovery conditions.

Photosynthetic acclamatory response of Panicum antidotale Retz. populations to root zone desiccation stress

Growth of plants is severely reduced due to water stress by affecting photosynthesis including photosystem II (PSII) activity and electron transport. This study emphasised on comparative and priority targeted changes in PSII activity due to progressive drought in seven populations of Panicum antidotale (P. antidotale) collected from Cholistan Desert and non-Cholistan regions. Tillers of equal growth of seven populations of P. antidotale grown in plastic pots filled with soil were subjected progressive drought by withholding water irrigation for three weeks. Progressive drought reduced the soil moisture content, leaf relative water content, photosynthetic pigments and fresh and dry biomass of shoots in all seven populations. Populations from Dingarh Fort, Dingarh Grassland and Haiderwali had higher growth than those of other populations. Cholistani populations especially in Dingarh Grassland and Haiderwali had greater ability of osmotic adjustment as reflected by osmotic potential and greater accumulation of total soluble proteins. Maximum H2O2 under water stress was observed in populations from Muzaffargarh and Khanewal but these were intermediate in MDA content. Under water stress, populations from Muzaffargarh and Dingarh Fort had greater K+ accumulation in their leaves. During progressive drought, non-Cholistani populations showed complete leaf rolling after 23 days of drought, and these populations could not withstand with more water stress condition while Cholistani populations tolerated more water stress condition for 31 days. Moreover, progressive drought caused PSII damages after 19 days and it became severe after 23 days in non-Cholistani populations of P. antidotale than in Cholistani populations.

Amelioration of plant responses to drought under elevated CO2 by rejuvenating photosynthesis and nitrogen use efficiency: implications for future climate-resilient crops

The contemporary global agriculture is beset with serious threats from diverse eco-environmental conditions causing decreases in crop yields by ~ 15%. These yield losses might increase further due to climate change scenarios leading to increased food prices triggering social unrest and famines. Urbanization and industrialization are often associated with rapid increases in greenhouse gases (GHGs) especially atmospheric CO₂ concentration [(CO₂)]. Increase in atmospheric [CO₂] significantly improved crop photosynthesis and productivity initially which vary with plant species, genotype, [CO₂] exposure time and biotic as well as abiotic stress factors. Numerous attempts have been made using different plant species to unravel the physiological, cellular and molecular effects of elevated [CO₂] as well as drought. This review focuses on plant responses to elevated [CO₂] and drought individually as well as in combination with special reference to physiology of photosynthesis including its acclimation. Furthermore, the functional role of nitrogen use efficiency (NUE) and its relation to photosynthetic acclimation and crop productivity under elevated [CO₂] and drought are reviewed. In addition, we also discussed different strategies to ameliorate the limitations of ribulose-1,5-bisphosphate (RuBP) carboxylation and RuBP regeneration. Further, improved stomatal and mesophyll conductance and NUE for enhanced crop productivity under fast changing global climate conditions through biotechnological approaches are also discussed here. We conclude that multiple gene editing approaches for key events in photosynthetic processes would serve as the best strategy to generate resilient crop plants with improved productivity under fast changing climate.

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.

Photosynthetic performance of quinoa (Chenopodium quinoa Willd.) after exposure to a gradual drought stress followed by a recovery period

Drought is an abiotic scourge, one of the major environmental stress factors that adversely affect plant growth and photosynthesis machinery through a disruption of cell organelles, arrangement thylakoid membranes and the electron transport chain. Herein, we probed the effect of drought stress on photosynthetic performance of Chenopodium quinoa Willd. Beforehand, plants were subjected to water deficit (as 15% Field Capacity, FC) for one (D-1W) or two weeks (D-2W), and were then re-watered at 95% FC for 2 weeks. Light and electron microscopy analysis of leaves showed no apparent changes in mesophyll cell organization and chloroplast ultrastructure after one week of drought stress, while a swelling of thylakoids and starch accumulation were observed after the prolonged drought (D-2W). The latter induced a decrease in both PSI and PSII quantum yields which was accompanied by an increase in F₀ (minimum fluorescence) and a decline in F_m (maximum fluorescence). Drought stress influenced the fluorescence transients, where the major changes at the OJIP prompt FI level were detected in the OJ and IP phases. Prolonged drought induced a decrease in chl a fluorescence at IP phase which was readjusted and established back after re-watering and even more an increase was observed after 2 weeks of recovery. The maximum quantum yield of primary photochemistry (φ_Po) was unaffected by the different drought stress regimes. Drought induced an increase in the ABS/RC and DI₀/RC ratios which was concurrent to a stable φ_Po (maximum quantum yield of PSII primary photochemistry). A substantial decrease in PI_(ABS) was detected especially, during severe drought stress (D-2W) suggesting a drop in the PSII efficiency and the level of electron transport through the plastoquinone pool (PQ pool) towards oxidized PSI RCs (P₇₀₀⁺). The immunoblot analysis of the main PSII proteins revealed considerable changes in the D1, D2, CP47, OEC, PsbQ and LHCII proteins under drought. These changes depend on the stress duration and recovery period. The main message of this investigation is the elevated recovery capacities of PSII and PSI photochemical activities after re-watering.

Negative effects of long-term moderate salinity and short-term drought stress on the photosynthetic performance of Hybrid Pennisetum

Plants are always suffering periods of soil water deficit and sustained soil salinity during their life cycle. Unraveling the mechanisms underpinning the responses of plants, especially the photosynthesis, to drought, salinity, and co-occurring stresses is critical for both the protection of natural vegetation and the stabilization of crop production. To better understand the downregulation of photosynthetic capability induced by soil salinity and drought, gas exchange parameters, leaf pigment contents, and chlorophyll (Chl) a fluorescence transients were analyzed in leaves of Hybrid Pennisetum. Our results showed that long-term moderate salinity, short-term drought, and the combination of these stressors decreased leaf pigment content by 11.4-31.5% and net photosynthetic rate (P_n) by 14.6-67.6% compared to those in untreated plants. The reduction of P_n in Hybrid Pennisetum under long-term salinity stress mainly occurred by stomatal limitation, whereas non-stomatal limitation played a dominant role under short-term drought stress. The changes in Chl a fluorescence kinetics (especially the appearance of the L-band and K-band) in both stress treatments showed that salinity and drought stress damaged the structural stability of photosystem II (PSII) and disturbed the equilibrium between the electrons at the acceptor and donor sides of PSII. Furthermore, although the negative effect of drought stress on leaf photosynthesis was much greater than that of salinity stress, moderate salt stress alleviated the negative effect of drought stress on the photosynthetic performance of Hybrid Pennisetum after long acclimation times.

Proteomic analyses unraveling water stress response in two Eucalyptus species originating from contrasting environments for aridity

Eucalyptus are widely cultivated in several regions of the world due to their adaptability to different climatic conditions and amenable to tree breeding programs. With changes in environmental conditions pointing to an increase in aridity in many areas of the globe, the demand for genetic materials that adapt to this situation is required. Therefore, the aim of this work was to identify contrasting differences between two Eucalyptus species under water stress through the identification of differentially abundant proteins. For this, total protein extraction was proceeded from leaves of both species maintained at 40 and 80% of field capacity (FC). The 80% FC water regime was considered as the control and the 40% FC, severe water stress. The proteins were separated by 2-DE with subsequent identification of those differentially abundant by liquid nanocromatography coupled to high resolution MS (Q-Exactive). Comparative proteomics allowed to identify four proteins (ATP synthase gamma and alpha, glutamine synthetase and a vacuolar protein) that were more abundant in drought-tolerant species and simultaneously less abundant or unchanged in the drought- sensitive species, an uncharacterized protein found exclusively in plants under drought stress and also 10 proteins (plastid-lipid, ruBisCO activase, ruBisCO, protease ClpA, transketolase, isoflavone reductase, ferredoxin-NADP reductase, malate dehydrogenase, aminobutyrate transaminase and sedoheptulose-1-bisphosphatase) induced exclusively in the drought-tolerant species in response to water stress. These results suggest that such proteins may play a crucial role as potential markers of water stress tolerance through the identification of species-specific proteins, and future targets for genetic engineering.

Physiological Response of Miscanthus x giganteus to Plant Growth Regulators in Nutritionally Poor Soil

Miscanthus x giganteus (Mxg) is a promising second-generation biofuel crop with high production of energetic biomass. Our aim was to determine the level of plant stress of Mxg grown in poor quality soils using non-invasive physiological parameters and to test whether the stress could be reduced by application of plant growth regulators (PGRs). Plant fitness was quantified by measuring of leaf fluorescence using 24 indexes to select the most suitable fluorescence indicators for quantification of this type of abiotic stress. Simultaneously, visible stress signs were observed on stems and leaves and differences in variants were revealed also by microscopy of leaf sections. Leaf fluorescence analysis, visual observation and changes of leaf anatomy revealed significant stress in all studied subjects compared to those cultivated in good quality soil. Besides commonly used F_v/F_m (potential photosynthetic efficiency) and P.I. (performance index), which showed very low sensitivity, we suggest other fluorescence parameters (like dissipation, DIo/RC) for revealing finer differences. We can conclude that measurement of leaf fluorescence is a suitable method for revealing stress affecting Mxg in poor soils. However, none of investigated parameters proved significant positive effect of PGRs on stress reduction. Therefore, direct improvement of soil quality by fertilization should be considered for stress reduction and improving the biomass quality in this type of soils.

Response of photosynthetic capacity and antioxidative system of chloroplast in two wucai (Brassica campestris L.) genotypes against chilling stress

Chilling stress during the growing season could cause a series of changes in wucai (Brassica campestris L.). WS-1 (chilling-tolerant genotype) and Ta2 (chilling-sensitive genotype) were sampled in present study to explore the chilling tolerance mechanisms. Our results indicated that photosynthetic parameters exhibited lower level in Ta2 than in WS-1 under chilling stress. The rapid chlorophyll fluorescence dynamics curve showed that chilling resulted in a greater inactivation of photosystem II reaction center in Ta2. Reactive oxygen species and malondialdehyde content of chloroplast in Ta2 were higher than WS-1. The ascorbate-glutathione cycle in chloroplast of WS-1 played a more crucial role than Ta2, which was confirmed by higher activities of antioxidant enzymes including Ascorbate peroxidase, Glutathione reductase, Monodehydroascorbate reductase and Dehydroascorbate reductase and higher content of AsA and GSH. In addition, the ultrastructure of chloroplasts in Ta2 was more severely damaged. After low temperature stress, the shape of starch granules in Ta2 changed from elliptical to round and the volume became larger than that of WS-1. The thylakoid structure of Ta2 also became dispersed from the original tight arrangement. Combined with our previous study under heat stress, WS-1 can tolerant both chilling stress and heat stress, which was partly due to a stable photosynthetic system and the higher active antioxidant system in plants, in comparison to Ta2.

Comparative Proteomics Reveals Cold Acclimation Machinery Through Enhanced Carbohydrate and Amino Acid Metabolism in Wucai (Brassica Campestris L.)

Limited information is available on the cold acclimation of non-heading Chinese cabbage (NHCC) under low temperatures. In this study, the isobaric tags for relative and absolute quantification (iTRAQ) were used to illustrate the molecular machinery of cold acclimation. Compared to the control (Cont), altogether, 89 differentially expressed proteins (DEPs) were identified in wucai leaves responding to low temperatures (LT). Among these proteins, 35 proteins were up-regulated ((and 54 were down-regulated). These differentially expressed proteins were categorized as having roles in carbohydrate metabolism, photosynthesis and energy metabolism, oxidative defense, amino acid metabolism, metabolic progress, cold regulation, methylation progress, and signal transduction. The fructose, glucose, and sucrose were dramatically increased in response to cold acclimation. It was firstly reported that aspartate, serine, glutamate, proline, and threonine were significantly accumulated under low temperatures. Results of quantitative real-time PCR analysis of nine DEPs displayed that the transcriptional expression patterns of six genes were consistent with their protein expression abundance. Our results demonstrated that wucai acclimated to low temperatures through regulating the expression of several crucial proteins. Additionally, carbohydrate and amino acid conversion played indispensable and vital roles in improving cold assimilation in wucai.

Performance Index and PSII Connectivity Under Drought and Contrasting Light Regimes in the CAM Orchid Phalaenopsis

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis characterized by improved water use efficiency mediated by major nocturnal CO2 fixation. Due to its inherent metabolic plasticity CAM represents a successful physiological strategy for plant adaptation to abiotic stress. The present study reports on the impact of drought stress and different light intensities (PPFD 50 and 200 μmol m-2 s-1) on the photosynthetic performance of the obligate CAM orchid Phalaenopsis "Edessa" by integrating diel gas exchange patterns with assessments of the light reactions by analyzing fast chlorophyll a fluorescence induction. Parameters such as PIabs (performance index), different energy fluxes per active reaction centre (RC) reflecting the electron flow from photosystem II to photosystem I and the energetic communication between PSII complexes defined as connectivity were considered for the first time in a CAM plant. A higher PS II connectivity for plants grown under low light (p ∼ 0.51) compared to plants grown under high light (p ∼ 0.31) brought about similar specific energy fluxes of light absorbance, dissipation and processing through the electron transport chain, irrespective of the light treatment. With a 25% higher maximum quantum yield and comparable biomass formation, low light grown plants indeed proved to process light energy more efficiently compared to high light grown plants. The performance index was identified as a very reliable and sensitive parameter to indicate the onset and progress of drought stress. Under restricted CO2 availability (due to closed stomata) leaves showed higher energy dissipation and partial inactivation of PSII reaction centres to reduce the energy input to the electron transport chain and as such aid in avoiding overexcitation and photodamage. Especially during CAM idling there is a discrepancy between continuous input of light energy but severely reduced availability of both water and CO2, which represents the ultimate electron acceptor. Taken together, our results show a unique flexibility of CAM plants to optimize the light reactions under different environmental conditions in a dual way by either attenuating or increasing energy flux.

Defense manifestations of enzymatic and non-enzymatic antioxidants in Ricinus communis L. exposed to lead in hydroponics

Lead (Pb) is a major inorganic pollutant with no biological significance and has been a global concern. Phytotoxicity of lead induces toxic effects by generating reactive oxygen species (ROS), which inhibits most of the cellular processes in plants. Hydro-ponic experiments were performed with Ricinus communis to investigate the toxicity and antioxidant responses by exposing to different concentrations of lead (0, 200 and 400 µM) for 10 days. Pb stress caused a significant increase in electrolyte leakage, non-enzymatic antioxidants (phenols and flavonoids) and a decrease in the elemental profile of the plant. Histochemical visualization clearly indicates the significant increase of H2O2 production in dose-dependent manner under Pb stress. Likewise, an increase in catalase, guaiacol peroxidase and superoxide dismutase activity was also evident. Ascorbate peroxidase and MDAR, on the other hand, responded biphasically to Pb treatments showing a decrease in concentration. The decline in redox ratio GSH/GSSG was imposed by the indirect oxidative stress of Pb. Hence these findings showed the ameliorative potential of R. communis to sustain Pb toxicity under oxidative stress.

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.

Enhanced arsenic tolerance and secondary metabolism by modulation of gene expression and proteome profile in Artemisia annua L. after application of exogenous salicylic acid

This study was designed to investigate the effect of exogenous application of salicylic acid (SA) on proteins pattern and secondary metabolites in arsenic (As) stressed Artemisia annua. A. annua was treated by As 100 μM, SA 100 μM and combined treatment of SA 100 μM + As 100 μM upto 3 days. Significant accumulation of As was observed in roots than shoots at As 100 μM treatment. Under As treatment, oxidative stress was induced as indicated by increased TBARS content. Biomass, carotenoid, flavonoids were enhanced whereas total chlorophyll pigment was reduced under As treatment. Combined treatment of SA 100 μM + As 100 μM was more effective for increment of biomass, total chlorophyll content, and flavonoids as compared to As 100 μM treatment. Protein profiling revealed 20 differentially abundant proteins by 2-DE PAGE and MALDI-TOF-MS analysis. Identified proteins were related to photosynthesis, energy metabolism, transcriptional regulators, secondary metabolism, lipid metabolism, transport proteins and unknown/hypothetical proteins. All identified proteins were significantly increased in abundance under combined treatments of SA 100 μM + As 100 μM. The expression analysis of key genes involved in biosynthesis of lipid metabolism, signal molecule, transcriptional regulators, artemisinin biosynthetic genes, isoprenoids pathway, terpenes and flavonoids pathway were significantly upregulated under combined treatments of SA 100 μM + As 100 μM, suggesting a fine linkage in regulation of primary and secondary metabolism to modulate tolerance capacity and to improve phytoremediation property of A. annua against arsenic toxicity.

Proteomic analysis of Camellia sinensis (L.) reveals a synergistic network in the response to drought stress and recovery

Drought is a crucial limiting factor for tea yield and quality. To systematically characterize the molecular response of tea plants to drought stress and its capacity to recover, we used iTRAQ-based comparative proteomic approach to investigate the effects of drought on protein expression profiles in tea seedlings subjected to different drought treatments. A total of 3274 proteins were identified, of which 2169 and 2300 showed differential expressions during drought and recovery, respectively. Functional annotation showed that multiple biological processes were regulated, suggesting that tea plants probably employed multiple and synergistic resistance mechanisms in dealing with drought stress. Hierarchical clustering showed that chlorophyll a/b-binding proteins were up-regulated in DB and RE, suggesting that tea plants might regulate expression of chlorophyll a/b-binding proteins to maintain the photosystem II function during drought stress. Abundant proteins involved in sulfur-containing metabolite pathways, such as glutathione, taurine, hypotaurine, methionine, and cysteine, changed significantly during drought stress. Among them, TL29 interacted with LHCb6 to connect S-containing metabolites with chlorophyll a/b-binding proteins. This suggests that sulfur-containing compounds play important roles in the response to drought stress in tea plants. In addition, the expression of PAL was up-regulated in DA and down-regulated in DB. Cinnamyl alcohol dehydrogenase, caffeic acid O-methyltransferase, and 4-coumarate-CoA ligase also showed significant changes in expression levels, which regulated the biosynthesis of polyphenols. The results indicate that slight drought stress might promote polyphenol biosynthesis, while serious drought stress leads to inhibition. The expression of lipoxygenase and short-chain dehydrogenase increased during slight drought stress and some volatile metabolite pathways were enriched, indicating that drought stress might affect the tea aroma. The study provides valuable information that will lay the foundation for studies investigating the functions of drought response genes in tea leaves.

Fe deficiency induced changes in rice (Oryza sativa L.) thylakoids

Iron deficiency is an important abiotic stress that limits productivity of crops all over the world. We selected a hybrid rice (Oryza sativa L.), LYPJ, which is super high-yield and widely cultured in China, to investigate changes in the components and structure of thylakoid membranes and photosynthetic performance in response to iron deficiency. Our results demonstrated that photosystem I (PSI) is the primary target for iron deficiency, while the changes in photosystem II (PSII) are important for rebuilding a balance in disrupted energy utilization and dissipation caused by differential degradation of photosynthetic components. The result of immunoblot analysis suggested that the core subunit PsaA declined drastically, while PsbA remained relatively stable. Furthermore, several organizational changes of the photosynthetic apparatus were found by BN-PAGE, including a marked decrease in the PSI core complexes, the Cytb ₆ /f complex, and the trimeric form of the LHCII antenna, consistent with the observed unstacking grana. The fluorescence induction analysis indicated a descending PSII activity with energy dissipation enhanced markedly. In addition, we proposed that the crippled CO₂ assimilation could be compensated by the enhanced of phosphoenolpyruvate carboxylase (PEPC), which is suggested by the decreased ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and photosynthetic efficiency.

Influence of sub-lethal crude oil concentration on growth, water relations and photosynthetic capacity of maize (Zea mays L.) plants

Maize tolerance potential to oil pollution was assessed by growing Zea mays in soil contaminated with varying levels of crude oil (0, 2.5 and 5.0 % v/w basis). Crude oil contamination reduced soil microflora which may be beneficial to plant growth. It was observed that oil pollution caused a remarkable decrease in biomass, leaf water potential, turgor potential, photosynthetic pigments, quantum yield of photosystem II (PSII) (Fv/Fm), net CO2 assimilation rate, leaf nitrogen and total free amino acids. Gas exchange characteristics suggested that reduction in photosynthetic rate was mainly due to metabolic limitations. Fast chlorophyll a kinetic analysis suggested that crude oil damaged PSII donor and acceptor sides and downregulated electron transport as well as PSI end electron acceptors thereby resulting in lower PSII efficiency in converting harvested light energy into biochemical energy. However, maize plants tried to acclimate to moderate level of oil pollution by increasing root diameter and root length relative to its shoot biomass, to uptake more water and mineral nutrients.

Rubisco Activase Is Also a Multiple Responder to Abiotic Stresses in Rice

Ribulose-1,5-bisphosphate carboxylase/oxygenase activase (RCA) is a nuclear gene that encodes a chloroplast protein that plays an important role in photosynthesis. Some reports have indicated that it may play a role in acclimation to different abiotic stresses. In this paper, we analyzed the stress-responsive elements in the 2.0 kb 5’-upstream regions of the RCA gene promoter and the primary, secondary and tertiary structure of the protein. We identified some cis-elements of multiple stress-related components in the RCA promoter. Amino acid and evolution analyses showed that the RCA protein had conserved regions between different species; however, the size and type varied. The secondary structures, binding sites and tertiary structures of the RCA proteins were also different. This might reflect the differences in the transcription and translation levels of the two RCA isoforms during adaptation to different abiotic stresses. Although both the transcription and translation levels of RCA isoforms in the rice leaves increased under various stresses, the large isoform was increased more significantly in the chloroplast stroma and thylakoid. It can be concluded that RCA, especially RCA_L, is also a multiple responder to abiotic stresses in rice, which provides new insights into RCA functions.

Operative photo assimilation associated proteome modulations are critical for iron-dependent cadmium tolerance in Oryza sativa L

Iron-dependent Cd tolerance in Oryza sativa L. cv 7029 had been explored. Photo assimilatory process such as photosynthesis and nitrogen fixation found to be functional in the presence of excess Fe during Cd stress. Cd-inducible Fe deficiency demonstrated with upregulation of iron uptake gene families such as OsIRT and OsYSL was attributed as foremost reason for retardation of photo assimilation in the course of Cd treatment. Upholding of photo assimilation during Fe supplement was associated with proteome modulations. Monitoring of proteome responses in leaf and root revealed proteins imperative for alleviation of Cd stress. Specifically, proteins that take part in photosynthesis, glucose metabolism, nitrogen fixation, and abiotic stress tolerance played key role in Fe-dependent Cd detoxification. It is concluded that Fe supply help to operate photo assimilation which enable favorable proteome responses of Cd stress tolerance.

Iron- and manganese-assisted cadmium tolerance in Oryza sativa L.: lowering of rhizotoxicity next to functional photosynthesis

Cadmium toxicity is alleviated by iron and manganese supplements because of reduction in cadmium accumulation and upholding of redox regulation that prevent cadmium-inducible damage to root growth and photosynthesis. Cadmium toxicity in Oryza sativa L. MTU 7029 was investigated in the presence of different concentrations of the micronutrients Fe and Mn. It had been observed that these micronutrients reduce Cd uptake and minimize Cd-inducible rhizotoxicity. The photosynthetic electron transport chain, which is the hub of Fe containing metalloproteins, was severely affected by Cd and resulted in reduced bioproductivity under Cd stress. However, exogenous Fe restored the photosynthetic electron transport. Thus, due to the maintenance of the photosynthetic electron transport, the Cd tolerance was improved during Fe supplement. Both antioxidant enzymes and non-enzymatic antioxidant metabolites were found to play important roles in the alleviation of Cd stress under Fe or Mn supplement. It is concluded that the presence of excess Fe and Mn protects rice plants from Cd stress.

Comparison of thylakoid structure and organization in sun and shade Haberlea rhodopensis populations under desiccation and rehydration

The resurrection plant, Haberlea rhodopensis can survive nearly total desiccation only in its usual low irradiation environment. However, populations with similar capacity to recover were discovered recently in several sunny habitats. To reveal what kind of morphological, structural and thylakoid-level alterations play a role in the acclimation of this low-light adapted species to high-light environment and how do they contribute to the desiccation tolerance mechanisms, the structure of the photosynthetic apparatus, the most sensitive component of the chlorophyll-retaining resurrection plants, was analyzed by transmission electron microscopy, steady state low-temperature fluorescence and two-dimensional Blue-Native/SDS PAGE under desiccation and rehydration. In contrast to the great differences in the morphology of plants, the ultrastructure and the organization of thylakoids were surprisingly similar in well-hydrated shade and sun populations. A high ratio of photosystem (PS)I binding light harvesting complex (LHC)II, important in low- and fluctuating light environment, was characteristic to both shade and sun plant, and the ratios of the main chlorophyll-protein complexes were also similar. The intensive protective mechanisms, such as shading by steep leaf angle and accumulation of protective substances, probably reduced the light intensity at the chloroplast level. The significantly increased ratio of monomer to oligomer antennae in well-hydrated sun plants may be connected with the temporary high light exposure of chloroplasts. During desiccation, LHCII was removed from PSI and part of PSII supercomplexes disassembled with some loss of PSII core and LHCII. The different reorganization of antennae, possibly connected with different quenching mechanisms, involved an increased amount of monomers in shade plants but unchanged proportion of oligomers in sun plants. Desiccation-induced responses were more pronounced in sun plants which also had a greater capacity to recover due to their stress-acclimated attitude.

Photosynthetic changes of flag leaves during senescence stage in super high-yield hybrid rice LYPJ grown in field condition

Photosynthetic activities and thylakoid membrane protein patterns as well as the ultrastructure of chloroplasts in flag leaves were investigated during the senescence processes in high-yield hybrid rice LYPJ under field condition. The earlier decrease of PS I activity than PS II in LYPJ was primarily due to the significant degradation of PS I chlorophyll-protein complex. The degradation rate for each chlorophyll-protein complex was different and the order for the stability of thylakoid membrane complexes during flag leaf senescence in rice LYPJ was: LHCII > OEC > PSII core antenna > PSII core > PSI core > LHCI, which was partly supported by the BN-PAGE gel combined with immunoblot analysis. A decrease in the chlorophyll a/b ratio at the senescence stage was observed to coincide with stability of the LHCII subunits. Ultrastructural investigations revealed that the chloroplasts have large loosen stacking grana without interconnecting stroma thylakoids during the senescence processes. It was hypothesized that the stability of grana thylakoids harboring the major LHCII under high radiation condition in summer might played a key role in the dissipation of excess light energy. This alternative strategy would protect photosynthetic apparatus from photodamage and might be causally related to the high yield of this rice cultivar.

Analysis of the Citrullus colocynthis transcriptome during water deficit stress

Citrullus colocynthis is a very drought tolerant species, closely related to watermelon (C. lanatus var. lanatus), an economically important cucurbit crop. Drought is a threat to plant growth and development, and the discovery of drought inducible genes with various functions is of great importance. We used high throughput mRNA Illumina sequencing technology and bioinformatic strategies to analyze the C. colocynthis leaf transcriptome under drought treatment. Leaf samples at four different time points (0, 24, 36, or 48 hours of withholding water) were used for RNA extraction and Illumina sequencing. qRT-PCR of several drought responsive genes was performed to confirm the accuracy of RNA sequencing. Leaf transcriptome analysis provided the first glimpse of the drought responsive transcriptome of this unique cucurbit species. A total of 5038 full-length cDNAs were detected, with 2545 genes showing significant changes during drought stress. Principle component analysis indicated that drought was the major contributing factor regulating transcriptome changes. Up regulation of many transcription factors, stress signaling factors, detoxification genes, and genes involved in phytohormone signaling and citrulline metabolism occurred under the water deficit conditions. The C. colocynthis transcriptome data highlight the activation of a large set of drought related genes in this species, thus providing a valuable resource for future functional analysis of candidate genes in defense of drought stress.

Chlorophyll a fluorescence in evaluation of the effect of heavy metal soil contamination on perennial grasses

Chlorophyll a fluorescence gives information about the plant physiological status due to its coupling to the photosynthetic electron transfer chain and to the further biochemical processes. Environmental stresses, which acts synergistically, disturbs the photosynthesis. The OJIP test, elaborated by Strasser and co-workers, enables comparison of the physiological status of plants grown on polluted vs. control areas. The paper shows that the Chl a measurements are very useful tool in evaluating of heavy metal ions influence on perennial grasses, tested as potential phytoremediators. Among 5 cultivars tested, the highest concentration of Cd and Zn ions, not associated with the yield reduction, was detected in the biomass of tall fescue cv. Rahela. Chl a fluorescence interpreted as double normalized curves pointed out Rahela as the outstanding cultivar under the HM ions stress.

Photosynthetic proton and electron transport in wheat leaves under prolonged moderate drought stress

In conditions of long-lasting moderate drought stress, we have studied the photoprotective responses in leaves of wheat (Triticum aestivum L., cv. Katya) related to the photosynthetic electron and proton transport. The dark-interval relaxation kinetics of electrochromic bandshift (ECS) indicated a decrease of electric and an increase of osmotic component of the proton motive force in drought stressed leaves, but neither the total proton motive force (pmf) nor the thylakoid proton conductance (gH(+)) were affected. We observed the enhanced protection against overreduction of PSI acceptor side in leaves of drought stressed plants. This was obviously achieved by the rapid buildup of transthylakoid pH gradient at relatively low light intensities, directly associated to the steep increase of NPQ and the down-regulation of linear electron transport. It was further accompanied by the steep increase of redox poise at PSII acceptor side and PSI donor side. The early responses related to thylakoid lumen acidification in drought-stressed leaves could be associated with the activity of an enhanced fraction of PSI not involved in linear electron flow, which may have led to enhanced cyclic electron pathway even in relatively low light intensities, as well as to the drought-induced decrease of IP-amplitude in fast chlorophyll fluorescence kinetics.

Response of bean (Vicia faba L.) plants to low sink demand by measuring the gas exchange rates and chlorophyll a fluorescence kinetics

Background

The decline of photosynthesis in plants under low sink demand is well known. Previous studies focused on the relationship between stomatal conductance (g_s) and net photosynthetic rate (P_n). These studies investigated the effect of changes in Photosystem II (PSII) function on the P_n decline under low sink demand. However, little is known about its effects on different limiting steps of electron transport chain in PSII under this condition.

Methodology/Principal Finding

Two-month-old bean plants were processed by removing pods and flowers (low sink demand). On the 1^st day after low sink demand treatment, a decline of P_n was accompanied by a decrease in g_s and internal-to-ambient CO₂ concentration ratio (C_i/C_a). From the 3^rd to 9^th day, P_n and g_s declined continuously while C_i/C_a ratio remained stable in the treatment. Moreover, these values were lower than that of control. W_k (a parameter reflecting the damage to oxygen evolving complex of the donor side of PSII) values in the treatment were significantly higher than their corresponding control values. However, RC_QA (a parameter reflecting the number of active RCs per excited cross-section of PSII) values in the treatment were significantly lower than control from the 5^th day. From the 11^th to 21^st day, P_n and g_s of the treatment continued to decline and were lower than control. This was accompanied by a decrease of RC_QA, and an increase of W_k. Furthermore, the quantum yield parameters φ_Po, φ_Eo and ψ_Eo in the treatment were lower than in control; however, C_i/C_a values in the treatment gradually increased and were significantly higher than control on the 21^st day.

Conclusions

Stomatal limitation during the early stage, whereas a combination of stomatal and non-stomatal limitation during the middle stage might be responsible for the reduction of P_n under low sink demand. Non-stomatal limitation during the late stages after the removal of the sink of roots and pods may also cause P_n reduction. The non-stomatal limitation was associated with the inhibition of PSII electron transport chain. Our data suggests that the donor side of PSII was the most sensitive to low sink demand followed by the reaction center of PSII. The acceptor side of PSII may be the least sensitive.