Lower Photosynthetic Rates Research Articles

The Potential of Helichsryum splendidum (Thunb.) Less. for the Restoration of Sites Polluted with Coal Fly Ash.

The disposal of coal fly ash (CFA) generated from coal-fired power stations has serious impact on the ecosystem, by converting large pieces of land to barren ash dams with the potential to contaminate groundwater, surface water, air and soil. The aim of this study was to clarify the potential of phytoremediation using Helichrysum splendidum (Thunb.) Less. in areas polluted by CFA through conduction of pot trial experiments for 14 weeks. Plants of the same age were cultivated in CFA to assess their growth, photosynthetic rate and tolerance towards metal toxicity. This study revealed that the CFA was moderately polluted with heavy metals, and a lower photosynthetic rate was recorded for the CFA plants in comparison to the controls (plants grown in soil). Although the CO2 assimilation rate was lower for the CFA plants, increased growth was recorded for all the plants tested. Inductively coupled plasma mass spectrometry (ICP-MS) was used to quantify the amount of trace elements in samples and parameters including translocation factor (TF) and bioconcentration factor (BCF) were used to evaluate the phytoremediation potential of H. splendidum (Thunb.) Less. The results revealed that higher concentrations of Cd, Co, Cr, Cu, Mn and Pb were accumulated in the roots, while As, Ni and Zn were found in the shoots. Elements including As, Cr and Zn reported TF values above 1, indicating the plants' phytoextraction potential. The BCF values for As, Cu and Zn were 1.22, 1.19 and 1.03, indicating effectiveness in the phytostabilization processes. A removal rate efficiency ranging from 18.0 to 56.7% was recorded confirming that, H. splendidum (Thunb.) Less. can be employed for restoration of CFA dams.

The chloroplast pentatricopeptide repeat protein RCN22 regulates tiller number in rice by affecting sugar levels via the TB1–RCN22–RbcL module

As an important yield component, rice tiller number controls panicle number and determines grain yield. Regulation of rice tiller number by chloroplast pentatricopeptide repeat (PPR) proteins has not been reported previously. Here, we report the rice reduced culm number22 (rcn22) mutant, which produces few tillers owing to suppressed tiller bud elongation. Map-based cloning revealed that RCN22 encodes a chloroplast-localized P-type PPR protein. We found that RCN22 specifically binds to the 5′ UTR of RbcL mRNA (encoding the large subunit of Rubisco) and enhances its stability. The reduced abundance of RbcL mRNA in rcn22 leads to a lower photosynthetic rate and decreased sugar levels. Consequently, transcript levels of DWARF3 (D3) and TEOSINTE BRANCHED1 (TB1) (which encode negative regulators of tiller bud elongation) are increased, whereas protein levels of the positive regulator DWARF53 (D53) are decreased. Furthermore, high concentrations of sucrose can rescue the tiller bud growth defect of the rcn22 mutant. On the other hand, TB1 directly binds to the RCN22 promoter and downregulates its expression. The tb1/rcn22 double mutant shows a tillering phenotype similar to that of rcn22. Our results suggest that the TB1–RCN22–RbcL module plays a vital role in rice tiller bud elongation by affecting sugar levels.

Biological nitrogen fixation maintains carbon/nitrogen balance and photosynthesis at elevated CO2.

Understanding crop responses to elevated CO2 is necessary to meet increasing agricultural demands. Crops may not achieve maximum potential yields at high CO2 due to photosynthetic downregulation, often associated with nitrogen limitation. Legumes have been proposed to have an advantage at elevated CO2 due to their ability to exchange carbon for nitrogen. Here, the effects of biological nitrogen fixation (BNF) on the physiological and gene expression responses to elevated CO2 were examined at multiple nitrogen levels by comparing alfalfa mutants incapable of nitrogen fixation to wild-type. Elemental analysis revealed a role for BNF in maintaining shoot carbon/nitrogen (C/N) balance under all nitrogen treatments at elevated CO2, whereas the effect of BNF on biomass was only observed at elevated CO2 and the lowest nitrogen dose. Lower photosynthetic rates at were associated with the imbalance in shoot C/N. Genome-wide transcriptional responses were used to identify carbon and nitrogen metabolism genes underlying the traits. Transcription factors important to C/N signalling were identified from inferred regulatory networks. This work supports the hypothesis that maintenance of C/N homoeostasis at elevated CO2 can be achieved in plants capable of BNF and revealed important regulators in the underlying networks including an alfalfa (Golden2-like) GLK ortholog.

Toxic effect of mercury on arbuscular mycorrhizal fungi colonisation and physiological status of three seed-based Miscanthus hybrids

BackgroundCurrently, mercury pollution is a widespread problem in the world. As mercury is difficult to remove from the environment, it has long-term negative effects on soil health and human life. One of the techniques to stabilise Hg is phytostabilisation, which can be supported by arbuscular mycorrhizal fungi (AMF). MethodsIn a 4-month pot experiment, we investigated the suitability of three seed-based Miscanthus hybrids (GNT3, GNT34, GNT43) for growth on soils heavily polluted with mercury (6795.7 mg kg-1). During the experiment, the effects of high soil contamination with mercury on physiological parameters and colonisation of roots of seed-based Miscanthus hybrids by indigenous AMF from Hg-contaminated and uncontaminated soils were investigated. ResultsA high pseudo-total Hg concentration (6795.75 mg kg-1) in soil was found. The Hg content in the aerial part of GNT34 grown on Hg-contaminated soil was 1.5 times and 3 times higher than GNT3 and GNT43, respectively. The Hg content in the roots of GNT3 on Hg-contaminated soil was 25% and 10% lower than that of GNT34 and GNT43, respectively. The N content in the aboveground part of GNT34 in the Hg variant was 13.5% lower compared to the control soil. The P and K content in the shoots of the Miscanthus hybrids was lower in the plants grown on Hg-contaminated soil. The P content in GNT43 in the Hg variant was 33% and 19% lower than in GNT34 and GNT3, respectively. The K content in GNT34 in the Hg variant was 24.7% and 31.4% higher than in GNT43 and GNT3, respectively. The dry weight of the shoots and roots as well as the shoot height of the Miscanthus hybrids were lower in Hg-contaminated soil. Lower values of AMF root colonisation parameters (F, M) were observed in the plants in the Hg variant. In the Hg variant, a lower photosynthetic rate and a decrease in chlorophyll content were observed in the leaves of the Miscanthus hybrids. In the Hg variant, an increase in the content of flavonols was observed. The strongest toxic effect of mercury on the light phase of photosynthesis was measured in GNT34. ConclusionSoils heavily contaminated with mercury negatively affected the physiological parameters of Miscanthus, as evidenced by a decrease in photosynthetic rate and biomass. The ability of indigenous AMF from Hg-contaminated soils to colonise the roots of seed-based Miscanthus hybrids was limited.

Trocas gasosas ao longo do dia de plantas de soja submetidas ao encharcamento do solo e restrição luminosa

ABSTRACT: Aerobic respiration declines in the root tissue of soybean plants exposed to waterlogged soil with a low oxygen content, causing metabolic disorders that negatively affect gas exchange and photosynthetic activity, hampering growth and production. As such, this study to investigated daytime gas exchange, relative chlorophyll content, specific leaf area (SLA) and photosynthetic activity curves in response to photosynthetic photon flux density (PPFD) in soybean plants grown under different conditions: no stress (control), shading, waterlogged soil, and waterlogged soil + shading. Soybean plants exposed to either waterlogging or shading and both conditions simultaneously exhibited reduced photosynthesis linked to lower stomatal opening. Plants submitted to shading showed an increase in SLA, quantum yield and photosynthetic rates, while those grown in waterlogged soil, in full sun, exhibited chlorotic leaves and low apparent quantum yield, resulting in low photosynthetic rates under shading and high light levels. Thus, waterlogging and shading in isolation or combined, compromise daytime gas exchange and alter photosynthetic activity in plants.

Biofertilizers from wastewater treatment as a potential source of mineral nutrients for growth of amaranth plants.

Exploring alternative fertilizers is crucial in agriculture due to the cost and environmental impact of inorganic options. This study investigated the potential of sewage-derived biofertilizers on the growth and physiology of Amaranthus cruentus plants. Various treatments were compared, including control treatments with inorganic fertilizer and treatments with biofertilizers composed of microalgae, biosolids and reclaimed water. The following traits were investigated: photosynthetic pigments, gas exchange, growth, and leaf nutrient concentrations. The results showed that the concentrations of N, P, Cu, Fe Zn and Na nutrients, in the dry microalgae and biosolids, were quite high for the needs of the plants. The wet microalgae presented high concentration of Cu, Fe and Zn nutrients while reclaimed water contained high concentration of N, K, Ca and S. Na and Zn nutrients increased in the leaf of plants treated with dry microalgae and biosolid, respectively. At the beginning of the flowering phase, total chlorophyll and carotenoids contents were lower for plants grown with wet microalgae while for plants grown with higher doses of biosolid or reclaimed water total chlorophyll was increased, and carotenoids were not affected. Lower photosynthetic pigments under wet microalgae resulted in lower photosynthetic rates. On the other hand, amendments with dry microalgae and biosolid increased photosynthetic rates with the biosolid being the most effective. Higher applications of biosolid, wet and dry microalgae produced a considerable increase in shoot biomass of amaranth, with the dry microalgae being the most effective. Additionally, reclaimed water obtained after tertiary treatment of sewage with microalgae and biosolids applied alone showed promising effects on plant growth. Overall, these findings suggest that organic fertilizers derived from sewage treatment have the potential to enhance plant growth and contribute to sustainable agricultural practices.

Exploring the Impact of Potassium on Growth, Photosynthetic Performance, and Nutritional Status of Lemon Trees (cv. Adamopoulou) Grafted onto Sour Orange and Volkamer Lemon Rootstocks

(1) Background: This study investigates the effects of potassium (K) and rootstock on the growth, photosynthetic activity, and mineral nutrition of lemon trees; (2) Methods: Lemon trees (cv. Adamopoulou) grafted onto sour orange (Citrus aurantium) (SO) or Volkamer lemon (Citrus volkameriana) (VL) rootstock were cultivated hydroponically under 0.00, 0.75, 1.50, 3.00, and 6.00 mM K. Plant growth and nutrition parameters, as well as leaf photosynthetic rate, stomatal conductance, transpiration rate, intracellular CO2, chlorophyll, and carotenoid concentration were assessed; (3) Results: Under K deficiency (0 mM K), plants exhibited chlorotic and necrotic symptoms, more pronounced in older leaves. Potassium deficiency adversely affected various physiological processes in lemon leaves, including a decrease in photosynthesis rate, stomatal conductance, transpiration rate, intercellular CO2 concentration, water use efficiency, CO2 utilization efficiency, chlorophyll a/b ratio, and carotenoid concentrations (some effects were rootstock-dependent). Low photosynthetic rates under K deficiency were due to both stomatal- and non-stomatal limitations. Elevated K in the nutrient solution consistently reduced the total plant uptake of P, Ca, Mg, B, Mn, and Zn, resulting in nutrient imbalances, as evidenced by the significant decrease in P, Ca, Mg, and Zn concentrations found in scion tissues (especially at the 6 mM K treatment). Rootstock-dependent responses were also observed in scion leaf and stem growth and in mineral nutrient concentrations, uptake, and distribution across plant parts; (4) Conclusions: Our study reveals interesting aspects on how to optimize K fertilization in lemon trees in the context of sustainable agriculture by considering nutrient interactions and rootstock-dependent effects. Understanding these complex interactions and improving K fertilization practices is expected to improve lemon tree performance, yield, and fruit quality.

Genome‑wide analysis of the maize LACS gene family and functional characterization of the ZmLACS9 responses to heat stress

Long-chain acyl-coenzyme A (CoA) synthetases (LACSs) are capable of catalyzing fatty acids to form fatty acyl-CoA thioesters involved in lipid catabolism, serving a crucial role in plant development and adaptation to environmental stresses. However, comprehensive knowledge on LACS family members in maize is lacking to date. In this study, total 11 ZmLACS members were identified in maize, and were divided into four subfamilies based on their phylogenetic relationships and conserved protein motifs. Analysis of their cis-regulatory elements and expression profiles revealed that certain of these ZmLACS genes exhibited high tissue specificity and were regulated by various environmental stimuli - ZmLACS9 in particular was markedly induced by heat and salt stress. The ZmLACS9 protein was found to localize to the chloroplasts, and a ZmLACS9 loss-of-expression mutation resulted in decreased chlorophyll content and fewer thylakoid grana layers compared to the wild type. When exposed to heat stress, the zmlacs9 mutant exhibited a more sensitive phenotype, more damage to chloroplasts, lower photosynthetic rate, and higher reactive oxygen species accumulation. Furthermore, the activation of genes involved in glycolipid synthesis under stress was notably dampened in the zmlacs9 mutant, and ZmLACS9 affected the contents of different lipid classes and conferred tolerance to heat stress in transgenic yeast. Together, these findings offer valuable information towards the functional study of the ZmLACS gene family and highlight ZmLACS9 as an excellent candidate gene for heat resistance in maize.

SlMYB41 positively regulates tomato thermotolerance by activating the expression of SlHSP90.3

High-temperature stress has become a major abiotic factor that dramatically limits plant growth and crop yield. Plants have evolved complex mechanisms to cope with high-temperature stress, but the factors that regulate plant thermotolerance remain to be discovered. Here, a high temperature-induced MYB transcription factor SlMYB41 was cloned from tomato (Solanum lycopersicum). Two individual SlMYB41-RNA interference (RNAi) lines (MR) and one CRISPR/Cas9 mediated myb41 mutant (MC) were obtained to investigate the function of SlMYB41 in tomato thermotolerance. Under high-temperature stress, we found that the MR and MC lines showed more wilting than the wild type (WT), with more ion leakage, more MDA accumulation, lower contents of osmotic adjustment substances, and more accumulation of reactive oxygen species (ROS) which was resulted from lower antioxidative enzyme activities. In addition, the photosynthetic capacity and complex of MR and MC lines were damaged more seriously than WT plants under high-temperature stress, mainly manifested in lower photosynthetic rate and Fv/Fm. Moreover, heat stress-related genes, such as SlHSP17.6, SlHSP17.7, and SlHSP90.3 were downregulated in MR and MC lines. Importantly, Y1H and LUC analysis indicated that SlMYB41 can directly activate the transcription of SlHSP90.3. Together, our study suggest that SlMYB41 positively regulates tomato thermotolerance by activating the expression of SlHSP90.3.

Different hydraulic and photosynthetic responses to summer drought between newly sprouted and established Moso bamboo culms.

The subtropical regions in China are prone to recurrent summer droughts induced by the Western Pacific Subtropical High-Pressure, which has induced the death of tens of millions of culms of Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), a widely distributed giant bamboo with high economic and ecological values. In the future, the intensity and frequency of the summer drought are projected to increase in these areas due to global climate change, which may lead to significant age-specific mortality of Moso bamboo. So far, it is still unclear about the age-specific response mechanisms of hydraulic traits and carbon balance of Moso bamboo when it is suffering to an ongoing summer drought. This study aimed to investigate the hydraulic and photosynthetic responses of newly sprouted (1 year old) and established (2-5 years old) culms of Moso bamboo to summer drought, which was manipulated by throughfall reduction in Lin'an of Zhejiang. The results showed that both newly sprouted and established culms had a gradually weakening hydraulic conductivity and photosynthesis during the whole drought process. In the early stage of the manipulated drought, the established culms had more loss of hydraulic conductivity than the newly sprouted culms. However, the newly sprouted culms had significant more loss of hydraulic conductivity and lower photosynthetic rates and stomatal conductance in the middle and late stages of the manipulated drought. The results suggest that the newly sprouted culms were more susceptible to summer drought than established culms due to the combined effects of hydraulic damage and photosynthetic restriction, explaining why the newly sprouted culms have higher mortality than elder culms when subjected to extreme drought. These findings provided insights into the mechanisms of Moso bamboo's age-specific drought-induced mortality, which will help for the anti-drought management of bamboo.

Leaf physiology and histopathology of the interaction between the opportunistic phytopathogen Fusarium equiseti and Gossypium hirsutum plants

Fungal diseases constitute a major challenge in the cultivation of the world's most important textile plant, cotton. Opportunistic fungi attack plants and promote physiological alterations and tissue damage, thereby causing losses in productivity. Herein, the hypothesis that Gossypium hirsutum L. plants are physiologically affected, depending on the phenological stage and site of infection (root system or leaf), when affected by the opportunistic phytopathogen Fusarium equiseti was tested. To this end, the photosynthetic pigment production, gas exchange, chlorophyll a fluorescence, and histopathological mechanisms involved in the plant-phytopathogen interaction were evaluated. This study aimed to diagnose the most aggressive route of infection by comparing infection via root with infection via the leaf, and to identify the developmental stage with the highest susceptibility by comparing different phenological stages. Cotton plants show metabolic losses at the maturation stage, resulting in low photosynthetic pigment concentrations, low photosynthetic rates, and photochemical stress. However, when infected by F. equiseti in the root region, the photosynthesis and photochemistry of these plants are more affected than plants inoculated in the leaf region. Low net photosynthesis (A) and water use efficiency (WUE) rates, but high light absorption flux (ABS/RC) and energy dissipation flux (DI0/RC) values were observed. Regardless of the phenological stage, the presence of F. equiseti in the roots of cotton plants caused high transpiratory rates in the plants, suggesting non-stomatic water loss and photochemical damage. In contrast, non-inoculated plants produced more photosynthetic pigments and showed higher WUE and photochemical efficiency, as indicated by the high PIABS values. Leaf histopathology assessment revealed more fragile regions of the epidermis, such as the contact region between the guard cells and adjacent epidermal cells, as well as the basal region of glandular trichomes as important access routes for F. equiseti to the inner leaf layers at all developmental stages. This study confirmed that the root colonization of cotton plants with F. equiseti can more intensely impact the physiological quality of the plants, regardless of the developmental stage.

Interactive effect of branch source-sink ratio and leaf aging on photosynthesis in pistachio.

Tree source-sink ratio has a predominant and complex impact on tree performance and can affect multiple physiological processes including vegetative and reproductive growth, water and nutrient use, photosynthesis, and productivity. In this study, we manipulated the branch level source-sink ratio by reduction of photosynthetic activity (partial branch defoliation) or thinning branch fruit load early in the growing season (after fruit set) in pistachio (Pistacia vera) trees. We then characterized the leaf photosynthetic light response curves through leaf aging. In addition, we determined changes in leaf non-structural carbohydrates (NSC) and nitrogen (N) concentrations. In leaves with high source-sink ratios, there was a gradual decrease in maximum net photosynthetic rate (ANmax) over the growing season, while in branches with low source-sink ratios, there was a sharp decline in ANmax in the first two weeks of August. Branches with high-sink showed an up-regulation (increase) in photosynthesis toward the end of July (at 1,500 growing degree days) during the period of rapid kernel growth rate and increased sink strength, with ANmax being about 7 μmol m-1 s-1 higher than in branches with low-sink. In August, low source-sink ratios precipitated leaf senescence, resulting in a drastic ANmax decline, from 25 to 8 μmol m-1 s-1 (70% drop in two weeks). This reduction was associated with the accumulation of NSC in the leaves from 20 to 30 mg g-1. The mechanisms of ANmax reduction differ between the two treatments. Lower photosynthetic rates of 8-10 μmol m-1 s-1 late in the season were associated with lower N levels in high-sink branches, suggesting N remobilization to the kernels. Lower photosynthesis late in the season was associated with lower respiration rates in low-source branches, indicating prioritization of assimilates to storage. These results can facilitate the adaptation of management practices to tree crop load changes in alternate bearing species.

Redox basis of photosynthesis inhibition at supra-optimal bicarbonate in mesophyll protoplasts of Arabidopsis thaliana

We examined the patterns of photosynthetic O2 evolution at 1 mM (optimal) and 10 mM (supra-optimal) bicarbonate in mesophyll protoplasts of Arabidopsis thaliana. The photosynthetic rate of protoplasts reached the maximum at an optimal concentration of 1 mM bicarbonate and got suppressed at supra-optimal levels of bicarbonate. We examined the basis of such photosynthesis inhibition by mesophyll protoplasts at supra-optimal bicarbonate. The wild-type protoplasts exposed to supra-optimal bicarbonate showed up signs of oxidative stress. Besides the wild-type, two mutants were used: nadp-mdh (deficient in chloroplastic NADP-MDH) and vtc1 (deficient in mitochondrial ascorbate biosynthesis). The protoplasts of the nadp-mdh mutant exhibited a higher photosynthetic rate and greater sensitivity to supra-optimal bicarbonate than the wild-type. The ascorbate-deficient vtc1 mutant had a low photosynthetic rate and no significant inhibition at high bicarbonate. The nadp-mdh mutants had elevated activities, protein, and transcript levels of key antioxidant enzymes. On the other hand, the antioxidant enzyme systems in vtc1 mutants were not much affected at supra-optimal bicarbonate. We propose that the inhibition of photosynthesis at supra-optimal bicarbonate depends on the redox state of mesophyll protoplasts. The robust antioxidant enzyme systems in protoplasts of nadp-mdh mutant might be priming the plants to sustain high photosynthesis at supra-optimal bicarbonate.

Wheat Transformation with ScTPS1-TPS2 Bifunctional Enzyme for Trehalose Biosynthesis Protects Photosynthesis during Drought Stress

Wheat cultivation makes an important contribution to human nutrition. Trehalose synthesis plays a role in the tolerance to drought stress. A bifunctional TPS-TPP enzyme gene from yeast was used to obtain transgenic wheat plants to increase trehalose synthesis. Mature wheat embryos were transformed using pGreen rd29A::TPS1-TPS2 or pGreen 35S::TPS1-TPS2 constructs. The transgene presence in mature leaves of T3 plants was confirmed by sequencing a PCR fragment of the inserted transgene. Transgenic and NT plants were submitted to drought stress for eight days. Transformed wheat lines retained a higher relative water content than NT plants during drought stress, and the Rubisco activity was unaffected. Plants transformed with the 35S construct showed a lower photosynthetic rate and lower fructose 1–6-bisphosphatase (FBPase) activity during drought, suggesting that constitutive trehalose and sucrose synthesis caused a reduced ribulose 1,5-bisphosphate (RuBP) regeneration. Lines transformed with the rd29A promoter showed a higher photosynthetic rate after eight days of drought, as the RuBP regeneration was unaffected. Transgenic wheat plants had higher biomass and grain weight than NT plants after drought. These results suggest that trehalose synthesis improves photosynthesis during stress and induces changes in the activity of some Calvin-cycle enzymes, reflected in plant metabolism and growth.

Proteomic, transcriptomic, biochemical, and physio-anatomical analyses provide insights into energy metabolism, light usage, and photosynthesis in cigar tobacco under different light intensities

Low light (LL) resulting in shading are among the detrimental abiotic stresses limiting plant growth and suppressing crop productivity. Shading is a key cultivation technique in cigar wrapper tobacco production but little is known about its impact on transcriptional and translational regulatory networks. Here, we integrate transcriptomic and proteomic profiling with physio-biochemical and anatomical analyses under different light intensities [T200 (200 μmol m−2 s−1), T100 (100 μmol m−2 s−1), and T50 (50 μmol m−2 s−1)] to uncover the underlying molecular response mechanisms of tobacco plants. We found that the leaf anatomical structure impacts photosynthetic capacity and that LL intensities (particularly T50) decrease leaf and palisade thickness (42%) and spongy tissues (16%), which leads to a lower photosynthetic rate (84%) compared with T200. Furthermore, we identified 3045 and 590 significantly differentially expressed genes (DEGs) and differentially expressed proteins (DEPs) in the transcriptome and proteome of cigar tobacco, respectively. A total of 110 pairs were correlated which were upregulated in photosynthesis-antenna proteins, photosynthesis, and defense/detoxification-related pathways, according to integrated omics analyses, and downregulated in tyrosine metabolism, starch and sucrose metabolism, mitochondrial electron transport chain, and glycolysis pathways; and associated with decreased activities of glyceraldehyde-3-phosphate dehydrogenase (31%), starch phosphorylase (25%), and pyruvate kinase (24%) enzymes related to glycolysis. Our results show that cigar tobacco efficiently utilizes low light to reconfigure its energy metabolism, and offer profound insights into the response mechanisms at the physio-biochemical, anatomical, and molecular levels. This study thus represents a valuable resource of genes and proteins for future functional studies underlying LL response.

Ethylenediurea protects against ozone phytotoxicity not by adding nitrogen or controlling stomata in a stomata-unresponsive hybrid poplar

Ozone (O3) pollution is a persistent environmental issue worldwide, which causes widespread damage to vegetation, deteriorating plant health and reducing plant productivity. Ethylenediurea (EDU) is a synthetic chemical that has been widely applied in scientific studies as a protectant against O3 phytotoxicities. Despite four decades of active research, the exact mechanisms to explain its mode of action remain unclear. Here, we aimed to reveal whether EDU's phytoprotective property is due to its control over stomatal regulation and/or its action as a nitrogen (N) fertilizer, utilizing stomatal-unresponsive plants of a hybrid poplar (Populus koreana × trichocarpa cv. Peace) grown in a free-air O3-concenctration enrichment (FACE) facility. Plants were treated with water (WAT), EDU (400 mg L−1), or EDU's constitutive amount of N every nine days, and exposed to ambient (AOZ) or elevated (EOZ) O3 during a growing season (June–September). EOZ led to extensive foliar injuries (but protected against rust disease), lower photosynthetic rate (A), impaired dynamics of responses of A to changes in light intensity, and smaller total plant leaf area. EDU protected against common phytotoxicities caused by EOZ without inducing stomatal closure, since stomatal conductance (gs) was generally unresponsive to the experimental treatments. EDU also modulated the dynamic response of A to light fluctuations under O3 stress. N addition acted as a fertilizer but did not satisfactorily protect plants against O3 phytotoxicities. The results suggest that EDU protects against O3 phytotoxicity not by adding N or controlling stomata, which provides a new insight into our understanding of the mode of action of EDU as a protectant against O3 phytotoxicity.

Understanding parasitism in Loranthaceae: Insights from plastome and mitogenome of Helicanthes elastica

Loranthaceae is the largest family of the order Santalales and includes root and stem hemiparasites. The parasites are known to exhibit reductions in the genomic features as well as relaxed or intensified selection shifts. In this study, we report plastome and mitogenome sequence of Helicanthes elastica (subtribe Amyeminae, tribe Lorantheae), an endemic, monotypic genus of Western Ghats, India growing on remarkably diverse host range. The length of plastome sequence was 1,28,805 bp while that of mitogenome was 1,65,273 bp. This is the smallest mitogenome from Loranthaceae reported till date. The plastome of Helicanthes exhibited loss of ndh genes (ψndhB), ψinfA, rps15, rps16, rpl32, trnK-UUU, trnG-UCC, trnV-UAC and trnA-UGC while mitogenome exhibited pseudogenized cox2, nad1 and nad4 genes. The comparative study of Loranthaceae plastomes revealed that the pseudogenization or loss of genes was not specific to any genus or tribe and variation was noted in the number of introns of clpP gene in the family. Several photosynthetic genes have undergone relaxed selection supporting lower photosynthetic rates in parasitic plants while some respiratory genes exhibited intensified selection supporting the idea of host-parasite arm race in Loranthaceae. The plastome gene content was found conserved in root hemiparasites compared to stem hemiparasites. The atp1 gene of mitogenome was chimeric and part of it exhibited similarities with Lamiales members. The phylogenetic analysis based on plastid genes placed Helicanthes sister to the members of subtribe Dendrophthoinae.

Response of photosynthesis, the xanthophyll cycle, and wax in Japanese yew (Taxus cuspidata L.) seedlings and saplings under high light conditions.

In order to understand the adaptative changes of the Japanese yew (Taxus cuspidate L.) to high light conditions, this study investigated gas-exchange, chlorophyll fluorescence, chlorophyll, and the impact of epicuticular wax on the gas-exchange and photoinhibition of Japanese yew seedlings and saplings. The chlorophyll content per unit area and photosynthetic rate in seedling leaves were significantly lower than in sapling leaves. When leaves from seedlings and saplings were exposed to 1,200 μmol·m-2·s-1 photon flux density (PFD) for 2 h, seedling leaves exhibited a greater down-regulation of maximum quantum yield (Fv/Fm) and actual photosystem II efficiency ( PSII). Non-photochemical quenching (NPQ) and high energy quenching (qE) in sapling leaves were much higher than in seedling leaves when both were exposed to 1,200 μmol·m-2·s-1 PFD for 2 h. At a low level of O2, the photorespiration rate (Pr) and the ratio of photorespiration/gross photosynthetic rate (Pr/Pg) in seedling leaves were lower than in sapling leaves when both were exposed to 1,200 μmol·m-2·s-1 PFD, but this difference did not reach statistical significance (P < 0.05). Compared with sapling leaves, seedling leaves exhibited lower levels of xanthophyll pool. Epicuticular wax content on seedling leaves was significantly lower than on sapling leaves. The results of this study showed that wax coverage on the leaf surface decreased the photosynthetic rate in sapling leaves as a consequence of decreased stomatal conductance. Epicuticular wax is related to tree age and photoinhibition prevention in the Japanese yew. It is possible that lower photosynthetic rate, lower NPQ depending on the xanthophyll cycle, and lower deposition of epicuticular wax results in seedling plants that are not adapted to high light conditions.

Overexpression of the Poplar WRKY51 Transcription Factor Enhances Salt Tolerance in Arabidopsis thaliana

Salt is a severe environmental stressor that affects growth and development in plants. It is significant to enhance the salt tolerance in plants. In this study, a salt-responsive WRKY transcription factor PtrWRKY51 was isolated from Populus trichocarpa (clone ‘Nisqually-1′). PtrWRKY51 was highly expressed in mature leaves and root and induced by salt stress. The PtrWRKY51 was overexpressed in Arabidopsis to investigate its biological functions. Compared with Col-0 lines, Overexpressed lines had an increase in germination rate of seed, root length, higher photosynthetic rate, instantaneous leaf WUE, chlorophyll content to improve salt tolerance under salt stress conditions. In contrast, compared to overexpressed and Col-0 lines, the mutant wrky51 was more sensitive to salt stress with lower photosynthetic rate and WUE. Additionally, it was found that the complementary lines (wrky51/ PtrWRKY51) had almost the same salt response as Col-0. In conclusion, PtrWRKY51 is a potential target in the enhancement of poplar tolerance by genetic engineering strategies.

Metabolic responses of Halodule wrightii to hyposalinity

While seagrass growth is inhibited under acute high or low salinities, Halodule wrightii (shoal grass) is an exception for its wide salinity tolerances. Salinity reduction experiments were performed to determine how H. wrightii acclimates to hyposalinity. In laboratory experiments over 12 days, two salinity reduction scenarios (moderate-S15 (32→26→20→15) and severe-S5 (32→23→15→5)) were applied to H. wrightii. Periodic measurements of leaf growth, photosynthesis, quantum efficiency (Fv/Fm), and leaf respiration were made to assess the influence of moderate (S15) and severe (S5) hyposalinity treatments compared to a control (32). Regardless of salinity treatment, leaf respiration rates increased significantly with decreasing salinity. Significant depression of photosynthetic rates and Fv/Fm were found in the S5 treatment with greatest changes in the S5 plants compared to S32 controls. These results suggest that hyposalinity impairs photosynthesis and that H. wrightii survives severe short-term hyposalinity by increasing respiration rates to supplement its energetic requirements. Simultaneous low photosynthetic and high respiration rates could reduce the resiliency of H. wrightii under hyposalinity.