Photosynthetic response of Chlamydomonas reinhardtii and Chlamydomonas sp. 1710 to zinc toxicity

Zinc (Zn) is an essential trace element but can lead to water contamination and ecological deterioration when present in excessive amounts. Therefore, investigating the photosynthetic response of microalgae to Zn stress is of great significance. In this study, we assessed the photosynthetic responses of neutrophilic Chlamydomonas reinhardtii and acidophilic Chlamydomonas sp. 1710 to Zn exposure for 96 h. The specific growth rate (μ), chlorophyll-a (Chl-a) content, and chlorophyll fluorescence parameters were determined. The results demonstrated that Chlamydomonas sp. 1710 was much more tolerant to Zn than C. reinhardtii, with the half-maximal inhibitory concentration (IC50) values of 225.4 mg/L and 23.4  mg/L, respectively. The μ and Chl-a content of C. reinhardtii decreased in the presence of 15  mg/L Zn, whereas those of Chlamydomonas sp. 1710 were unaffected by as high as 100  mg/L Zn. Chlorophyll fluorescence parameters indicated that the regulation of energy dissipation, including non-photochemical quenching, played a crucial role in Zn stress resistance for both Chlamydomonas strains. However, in the case of C. reinhardtii, non-photochemical quenching was inhibited by 5  mg/L Zn in the first 48 h, whereas for Chlamydomonas sp. 1710, it remained unaffected under 100  mg/L Zn. Chlamydomonas sp. 1710 also exhibited a 20 times stronger capacity for regulating the electron transfer rate than C. reinhardtii under Zn stress. The light energy utilization efficiency (α) of Chlamydomonas sp. 1710 had the most highly non-linear correlation with μ, indicating the energy utilization and regulation process of Chlamydomonas sp. 1710 was well protected under Zn stress. Collectively, our findings demonstrate that the photosystem of Chlamydomonas sp. 1710 is much more resilient and tolerant than that of C. reinhardtii under Zn stress.

Application of Chlorophyll Fluorescence Analysis Technique in Studying the Response of Lettuce (Lactuca sativa L.) to Cadmium Stress

To reveal the impact of cadmium stress on the physiological mechanism of lettuce, simultaneous determination and correlation analyses of chlorophyll content and photosynthetic function were conducted using lettuce seedlings as the research subject. The changes in relative chlorophyll content, rapid chlorophyll fluorescence induction kinetics curve, and related chlorophyll fluorescence parameters of lettuce seedling leaves under cadmium stress were detected and analyzed. Furthermore, a model for estimating relative chlorophyll content was established. The results showed that cadmium stress at 1 mg/kg and 5 mg/kg had a promoting effect on the relative chlorophyll content, while cadmium stress at 10 mg/kg and 20 mg/kg had an inhibitory effect on the relative chlorophyll content. Moreover, with the extension of time, the inhibitory effect became more pronounced. Cadmium stress affects both the donor and acceptor sides of photosystem II in lettuce seedling leaves, damaging the electron transfer chain and reducing energy transfer in the photosynthetic system. It also inhibits water photolysis and decreases electron transfer efficiency, leading to a decline in photosynthesis. However, lettuce seedling leaves can mitigate photosystem II damage caused by cadmium stress through increased thermal dissipation. The model established based on the energy captured by a reaction center for electron transfer can effectively estimate the relative chlorophyll content of leaves. This study demonstrates that chlorophyll fluorescence techniques have great potential in elucidating the physiological mechanism of cadmium stress in lettuce, as well as in achieving synchronized determination and correlation analyses of chlorophyll content and photosynthetic function.

A classification method of stress in plants using unsupervised learning algorithm and chlorophyll fluorescence technology

Introduction

Chilling injury is one of the most common meteorological disasters affecting cucumber production. For implementing remedial measures as soon as possible to minimize production loss, a timely and precise assessment of chilling injury is crucial.

Methods

To evaluate the possibility of detecting cucumber chilling injury using chlorophyll fluorescence (ChlF) technology, we investigated the continuous changes in ChlF parameters under various low-temperature conditions and created the criteria for evaluating chilling injury. The ChlF induction curves were first collected before low-temperature as unstressed samples and daily 1 to 5 days after low-temperature as chilling injury samples. Principal component analysis was employed to investigate the public information on ChlF parameters and evaluate the differences between samples with different degrees of chilling injury. The parameters (F v/F m, Y(NO), qP, and F o) accounted for a large proportion in the principal components and could characterize chilling injury. Uniform manifold approximation and projection method was employed to extract new features (Feature 1, Feature 2, Feature 3, and Feature 4) from ChlF parameters for subsequent classification model. Taking four features as input, a classification model based on the Fuzzy C-means clustering algorithm was constructed in order to identify the chilling injury classes of cucumber seedlings. The cucumber seedlings with different chilling injury classes were analyzed for ChlF images, rapid light curves, and malondialdehyde content.

Results and discussion

The results demonstrated that the variations in these indicators among the different chilling injury classes supported the validity of the classification model. Our findings provide a better understanding of the relationship between ChlF parameters and the impact of low-temperature treatment on cucumber seedlings. This finding offers an additional perspective that can be used to evaluate the responses and damage that plants experience under stress.

Effect of Zinc Foliar Fertilization Alone and Combined with Trehalose on Maize (Zea mays L.) Growth under the Drought

Maize (Zea mays L.) is one of the most widely grown cereals in the world. Its cultivation is affected by abiotic stress caused by climate change, in particular, drought. Zinc (Zn) supplied by foliar nutrition can increase plant resistance to water stress by enhancing physiological and enzymatic antioxidant defence mechanisms. One of the possibilities to reduce the effect of drought on plant production is also the utilization of trehalose. In order to confirm the effect of the foliar application of selected forms of Zn (0.1% w/v solution)-zinc oxide micro- (ZnO) and nanoparticles (ZnO^NP), zinc sulphate (ZnSO₄) and zinc chelate (ZnEDTA)-a pot experiment in controlled conditions was conducted in combination with trehalose (1% w/v solution) on selected growth parameters of maize exposed to the drought stress. A significant effect of coapplication of Zn and trehalose on chlorophyll content, chlorophyll fluorescence parameters, root electrical capacity, weight of maize aboveground biomass (AGB) and Zn content in AGB was found. At the same time, the hypothesis of a positive effect of carbohydrates on increasing the uptake of foliar-applied Zn was confirmed, especially for the ZnEDTA and ZnSO₄. This paper presents the first empirical evidence of the trehalose addition to sprays for zinc foliar fertilization of maize proving to be an effective way of increasing the resistance of maize grown under drought stress conditions.

The 5-Aminolevulinic Acid (5-ALA) Supplement Enhances PSII Photochemical Activity and Antioxidant Activity in the Late Growth Promotion of Pseudostellaria heterophylla

This study focused on the physiological regulation and mechanism of exogenous 5-aminolevulinic acid (5-ALA) in the late growth of P. heterophylla. In the middle of May, different concentrations of 5-ALA (0, 10, 20, 50 mg·L^-1) were sprayed on the leaves. The effects of 5-ALA on tuberous root growth, antioxidant enzyme system, gas exchange, photosynthetic pigment contents and photosynthetic characteristics were measured from 23 May to 13 June. A concentration of 20 mg·L^-1 of 5-ALA led to a significant increase in the yield of fresh root and biomass allocation at 38.12% and 25.07%, respectively, in comparation with the control (0 mg·L^-1). The moderate concentration of 5-ALA statistically stimulated antioxidation activities. 5-ALA treatment enhanced photosynthetic activity and reduced photodamage. Compared to the control, there were increases in the chlorophyll fluorescence parameters of P. heterophylla under 5-ALA treatment. Moreover, 20 mg·L^-1 of 5-ALA significantly changed the kinetic parameters of fluorescence. It enhanced the light absorption and distribution efficiency of PSII and the activities of leaves, resulting in alleviating photoinhibition by the excess excitation energy. The correlation indicated that there was a significant positive correlation between the yield of tuberous roots and biomass allocation, P_n and catalase (CAT), and a negative correlation between the yield of tuberous roots and malondialdehyde (MDA). The appropriate 5-ALA concentration in the late growth stage of P. heterophylla effectively enhanced the net photosynthetic capacity, mainly resulting from the enhancement of PSII photochemical activity to promote the increases in excitation energy absorption, capture and electron transfer efficiency of the leaves. Finally, 5-ALA treatment can increase the photochemical activity of PSII in the whole leaf and ultimately delay the senescence of P. heterophylla.

Physiological and Transcriptomic Analysis Reveals the Responses and Difference to High Temperature and Humidity Stress in Two Melon Genotypes

Due to the frequent occurrence of continuous high temperatures and heavy rain in summer, extremely high-temperature and high-humidity environments occur, which seriously harms crop growth. High temperature and humidity (HTH) stress have become the main environmental factors of combined stress in summer. The responses of morphological indexes, physiological and biochemical indexes, gas exchange parameters, and chlorophyll fluorescence parameters were measured and combined with chloroplast ultrastructure and transcriptome sequencing to analyze the reasons for the difference in tolerance to HTH stress in HTH-sensitive 'JIN TAI LANG' and HTH-tolerant 'JIN DI' varieties. The results showed that with the extension of stress time, the superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) activities of the two melon varieties increased rapidly, the leaf water content increased, and the tolerant varieties showed stronger antioxidant capacity. Among the sensitive cultivars, Pn, Fv/Fm, photosystem II, and photosystem I chlorophyll fluorescence parameters were severely inhibited and decreased rapidly with the extension of stress time, while the HTH-tolerant cultivars slightly decreased. The cell membrane and chloroplast damage in sensitive cultivars were more severe, and Lhca1, Lhca3, and Lhca4 proteins in photosystem II and Lhcb1-Lhcb6 proteins in photosystem I were inhibited compared with those in the tolerant cultivar. These conclusions may be the main reason for the different tolerances of the two cultivars. These findings will provide new insights into the response of other crops to HTH stress and also provide a basis for future research on the mechanism of HTH resistance in melon.

Combined Effects of Allelopathic Polyphenols on Microcystis aeruginosa and Response of Different Chlorophyll Fluorescence Parameters

Polyphenols are allelochemicals secreted by aquatic plants that effectively control cyanobacteria blooms. In this study, sensitive response parameters (including CFPs) of Microcystis aeruginosa were explored under the stress of different polyphenols individually and their combination. The combined effects on M. aeruginosa were investigated based on the most sensitive parameter and cell densities. For pyrogallic acid (PA) and gallic acid (GA), the sensitivity order of parameters based on the EC₅₀ values (from 0.73 to 3.40 mg L^–1 for PA and from 1.05 to 2.68 mg L^–1 for GA) and the results of the hierarchical cluster analysis showed that non-photochemical quenching parameters [NPQ, q_N, q_N(rel) and q_CN] > photochemical quenching parameters [YII, q_P, q_P(rel) and q_L] or others [F_v/F_m, F’_v/F’_m, q_TQ and UQF_(rel)] > cell densities. CFPs were not sensitive to ellagic acid (EA) and (+)-catechin (CA). The sensitivity order of parameters for M. aeruginosa with PA-GA mixture was similar to that under PA and GA stress. The quantitative (Toxicity Index, TI) and qualitative (Isobologram representation) methods were employed to evaluate the combined effects of PA, GA, and CA on M. aeruginosa based on cell densities and NPQ. TI values based on the EC_{50 cells} suggested the additive effects of binary and multiple polyphenols, but synergistic and additive effects according to the EC_{50 NPQ} (varied from 0.16 to 1.94). In terms of NPQ of M. aeruginosa, the binary polyphenols exhibited synergistic effects when the proportion of high toxic polyphenols PA or GA was lower than 40%, and the three polyphenols showed a synergistic effect only at the ratio of 1:1:1. Similar results were also found by isobologram representation. The results showed that increasing the ratio of high toxic polyphenols would not enhance the allelopathic effects, and the property, proportion and concentrations of polyphenols played an important role in the combined effects. Compared with cell densities, NPQ was a more suitable parameter as evaluating indicators in the combined effects of polyphenols on M. aeruginosa. These results could provide a method to screen the allelochemicals of polyphenols inhibiting cyanobacteria and improve the inhibitory effects by different polyphenols combined modes.

A Computation Method Based on the Combination of Chlorophyll Fluorescence Parameters to Improve the Discrimination of Visually Similar Phenotypes Induced by Bacterial Virulence Factors

Phenotyping biotic stresses in plant-pathogen interactions studies is often hindered by phenotypes that can hardly be discriminated by visual assessment. Particularly, single gene mutants in virulence factors could lack visible phenotypes. Chlorophyll fluorescence (CF) imaging is a valuable tool to monitor plant-pathogen interactions. However, while numerous CF parameters can be measured, studies on plant-pathogen interactions often focus on a restricted number of parameters. It could result in limited abilities to discriminate visually similar phenotypes. In this study, we assess the ability of the combination of multiple CF parameters to improve the discrimination of such phenotypes. Such an approach could be of interest for screening and discriminating the impact of bacterial virulence factors without prior knowledge. A computation method was developed, based on the combination of multiple CF parameters, without any parameter selection. It involves histogram Bhattacharyya distance calculations and hierarchical clustering, with a normalization approach to take into account the inter-leaves and intra-phenotypes heterogeneities. To assess the efficiency of the method, two datasets were analyzed the same way. The first dataset featured single gene mutants of a Xanthomonas strain which differed only by their abilities to secrete bacterial virulence proteins. This dataset displayed expected phenotypes at 6 days post-inoculation and was used as ground truth dataset to setup the method. The efficiency of the computation method was demonstrated by the relevant discrimination of phenotypes at 3 days post-inoculation. A second dataset was composed of transient expression (agrotransformation) of Type 3 Effectors. This second dataset displayed phenotypes that cannot be discriminated by visual assessment and no prior knowledge can be made on the respective impact of each Type 3 Effectors on leaf tissues. Using the computation method resulted in clustering the leaf samples according to the Type 3 Effectors, thereby demonstrating an improvement of the discrimination of the visually similar phenotypes. The relevant discrimination of visually similar phenotypes induced by bacterial strains differing only by one virulence factor illustrated the importance of using a combination of CF parameters to monitor plant-pathogen interactions. It opens a perspective for the identification of specific signatures of biotic stresses.

[Regulation of extracellular ATP onchlorophyll content and fluorescence characteristics of Angelica sinensis seedlings under drought and low temperature stress]

As an important signal molecule, extracellular ATP(eATP) can regulate many physiological and biochemical responses to plant stress. In this study, the regulation of extracellular ATP(eATP) on chlorophyll content and chlorophyll fluorescence parameters of Angelica sinensis seedlings were studied under drought and low temperature stress. The results showed that all the chlorophyll content, the actual photochemical efficiency [Y(Ⅱ)], the electron transfer rate(ETR), the photochemical quenching coefficient(qP and qL) of A. sinensis leaves were significantly decreased under drought and low temperature stress, respectively. At the same time, non-photochemical quenching(NPQ and qN) were also all significantly increased, respectively. The application of eATP alleviated the decrease of chlorophyll content, Y(Ⅱ), ETR, qP and qL of A. sinensis leaves under drought and low temperature stress, and eliminated the increase of qN and NPQ. The results indicated that eATP could effectively increase the open ratio of PSⅡ reaction centers, and improve the electron transfer rate and light energy conversion efficiency of PSⅡ of A. sinensis leaves under drought and low temperature stress. It is beneficial to enhance the chlorophyll synthesis and the adaptability of PSⅡ about A. sinensis seedlings to drought and low temperature stress.

Comparative Proteomic Analysis during the Involvement of Nitric Oxide in Hydrogen Gas-Improved Postharvest Freshness in Cut Lilies

Our previous studies suggested that both hydrogen gas (H₂) and nitric oxide (NO) could enhance the postharvest freshness of cut flowers. However, the crosstalk of H₂ and NO during that process is unknown. Here, cut lilies (Lilium "Manissa") were used to investigate the relationship between H₂ and NO and to identify differentially accumulated proteins during postharvest freshness. The results revealed that 1% hydrogen-rich water (HRW) and 150 μM sodium nitroprusside (SNP) significantly extended the vase life and quality, while NO inhibitors suppressed the positive effects of HRW. Proteomics analysis found 50 differentially accumulated proteins in lilies leaves which were classified into seven functional categories. Among them, ATP synthase CF1 alpha subunit (chloroplast) (AtpA) was up-regulated by HRW and down-regulated by NO inhibitor. The expression level of LlatpA gene was consistent with the result of proteomics analysis. The positive effect of HRW and SNP on ATP synthase activity was inhibited by NO inhibitor. Meanwhile, the physiological-level analysis of chlorophyll fluorescence and photosynthetic parameters also agreed with the expression of AtpA regulated by HRW and SNP. Altogether, our results suggested that NO might be involved in H₂-improved freshness of cut lilies, and AtpA protein may play important roles during that process.

Effect of UV-B radiation on the content of UV-B absorbing compounds and photosynthetic parameters in Parmotrema austrosinense from two contrasting habitats

We studied the resistance of Parmotrema austrosinense to UV-B stress. We focused on the effects of a high dose UV-B radiation on the content of chlorophylls, carotenoids and UV-B screening compounds. Photosynthetic parameters were measured by chlorophyll fluorescence (potential and effective quantum yields, photochemical and non-photochemical quenching) and evaluated in control and UV-B-treated lichens. Lichens from two different locations in Cordoba, Argentina, were selected: (i) high altitude and dry plots at (Los Gigantes) and (ii) lowland high salinity plots (Salinas Grandes). UV-B treatment led to a decrease in the content of photosynthetic pigments and UV-B screens (absorbance decrease in 220-350 nm) in the samples from Salinas Grandes, while in Los Gigantes samples, an increase in UV-B screen content was observed. Chlorophyll fluorescence parameters showed a UV-B-induced decline in F_V /F_M , Φ_PSII and qP indicating limitation of primary photosynthetic processes in photosystem II (PSII) of symbiotic alga, more pronounced in Salinas Grandes samples. Protective mechanism of PSII were activated by the UV-B treatment to a higher extent in samples from Salinas Grandes (NPQ 0.48) than in Los Gigantes samples (NPQ 0.26). We concluded that site-related characteristics, and in particular different UV-B radiation regimen, had a strong effect on resistance of the photosynthetic apparatus of P. austrosinense to UV-B radiation.

The role of ecotypic variation in driving worldwide colonization by a cosmopolitan plant

For almost 100 years now, ecotypic differentiation of plant species has been a major topic of research. In changing environments, the question needs to be answered as to how long it takes to adapt, and which parameters are subject to this fast adaptation. Short-living colonizing plant species are excellent examples, especially when they are selfing. Shepherd's Purse Capsella bursa-pastoris (Brassicaceae) is one of the most wide-spread flowering species on earth and avoids only the hot and humid tropics. Many studies demonstrated the ecotypic differentiation of C. bursa-pastoris in various regions of the world but ecotypic differentiation regarding adaptability of anatomy and physiology of rosette leaves so far remained less recognized. However, the leaves are relevant for subsequent seed set; in particular, winter-annual accessions require a robust rosette to survive adverse conditions. Leaf-related traits such as the thickness of the mesophyll and epidermis, stomatal density, photosynthetic capacity and the ability to withstand and even use high light conditions were therefore analysed in provenances from various climatic zones. Photosynthetic capacity depends on leaf anatomy and cellular physiological parameters. In particular, the ability to dynamically adjust the photosynthetic capacity to changing environmental conditions results in higher fitness. Here, we attempt to relate these results to the four Mendelian leaf types according to Shull.

[Combined effects of 24-epibrassinolide and salinity on the growth and physiological perfor-mance of Ulva prolifera]

In order to study the combined effects of brassinosteroids and salinity on the growth and physiological performance of Ulva prolifera under low temperature condition, we investigated the growth, chlorophyll content, chlorophyll fluorescence parameters, soluble protein and carbohydrate contents in this algae, which was grown under three salinity in the presence or absence of 24-epibrassinolide. The results showed that, compared to control salinity (25) treatment, the growth rate of U. prolifera was enhanced by 45.9% under the moderate hyposaline condition (10), but decreased under low salinity (5) treatment, which showed high contents of chlorophyll and soluble protein. However, the presence of EBR (0.2 mg·L^-1) significantly reduced the growth of U. prolifera, especially under the control salinity (25) treatment, under which the fresh mass decreased and more spores were released. Additionally, the effective quantum yield (F_v'/F_m'), the activity of SOD and the content of soluble carbohydrate also decreased, but the soluble protein content increased under the control salinity treatment in the presence of EBR. In conclusion, moderatehypo-saline condition could be used to enhance the growth of U. prolifera at 15 ℃, and under normal salinity (25), the EBR could be used to promote the release of spores and produce more materials for mass production of U. prolifera.

Functional properties and expression quantitative trait loci for phosphate transporter GmPT1 in soybean

Phosphate (Pi) remobilization within a plant is critical for plant survival under Pi-limiting conditions. In this paper, a soybean Pi transporter gene, GmPT1, was characterized. A marked induction of GmPT1 transcript was observed in young leaves, mature leaves and lateral roots during long-term Pi starvation. Transgenic tobacco plants containing the GmPT1 gene were obtained using an Agrobacterium-mediated transformation system. Compared with wild-type plants, transgenic plants showed significant increases in phosphorus-use efficiency (PUE), photosystem II (PSII) function, total dry weight and seed weight under Pi-deficient conditions. GmPT1 expression levels and PUE were determined in a soybean recombinant inbred line population during a pot experiment that was conducted to measure chlorophyll fluorescence parameters, photosynthetic rate (PN ) and seed yield. Correlation analysis revealed that GmPT1 expression levels had significantly positive correlations with seed yield, PUE, PN and the quantum yield of PSII primary photochemistry (ΦPSII ). Expression quantitative trait loci (eQTL) mapping for GmPT1 revealed two eQTLs, one of which coincided with both the physical location of GmPT1 and a QTL associated with seed yield. These results suggest that GmPT1 plays a role in Pi remobilization, and it may be possible to improve soybean seed yields under Pi-limiting conditions by modulating GmPT1 expression levels.