High Light Conditions Research Articles

Divergent trait responses to nitrogen addition in tall and short species

Abstract Asymmetrical light competition and direct detrimental effect of nitrogen have been proposed as two main mechanisms driving species richness declines following nitrogen (N) addition. N addition is also known to alter functional trait composition towards increased dominance of tall and fast‐growing species. However, whether trait changes vary between species and, in particular, how the traits of tall and short species respond to N addition has rarely been studied. Understanding whether different trait changes occur for tall and short species would provide insight into mechanisms underlying N addition effects. Based on a long‐term N addition experiment, we measured the natural height of 44 plant species and the photosynthetic active radiation (PAR) at seven heights in the vegetation and identified a clear stratification of the plant community into tall species (>30 cm tall), which experienced high light conditions, and short species that grew under the canopy. We also measured four functional traits, including maximum plant height, specific leaf area, leaf dry matter content and leaf thickness, on 35 species that occurred in fertilized and unfertilized plots. Structural equation modelling revealed that nitrogen (N) addition significantly reduced species richness by enhancing light asymmetry and had no direct effects, suggesting that N detrimental effects are negligible in our system. Consistent with this, we found different responses of traits and diversity for the tall and short species. Specifically, N addition reduced the number of short species but increased the number of tall species. In addition, specific leaf area increased, and leaf dry matter content decreased, for short species only, suggesting that they shifted to a fast growth strategy to cope with lower light levels. In contrast, tall species increased their height further to capture more light at the top of the canopy. Synthesis. The divergent trait responses observed for tall and short species show that although certain traits, like height and leaf traits, show correlated responses at the whole community level, these correlations may not be consistent across all species. Together, our results highlight that light competition is the main mechanism driving species loss following N addition.

High productivity of fucoxanthin and eicosapentaenoic acid in a marine diatom Chaetoceros gracilis by perfusion culture under high irradiance

For a high production rate of microalgae, a high-cell-density culture is an effective strategy for reducing the light intensity per dry weight in the culture to avoid photoinhibition under high light conditions. However, under high cell density microalgae are exposed to nutrient-deficient conditions, resulting in a reduced biomass production rate. To enhance the production rate of high-value bioactives such as fucoxanthin and eicosatetraenoic acid (EPA) by the diatom Chaetoceros gracilis, a perfusion culture was operated at harvesting rates of 10–40 % under light intensities of 300–2000 μmol m−2 s−1. At a harvesting rate of 20 % under a high light intensity of 2000 μmol m−2 s−1, the dry weight increased, and the areal productivity reached 15.6 ± 1.3 g-dry weight (dw) m−2 d−1, which is comparable to the maximum value previously reported in diatoms. The high biomass production, even at a low harvesting rate of 20 %, could result from sufficient nutrient supply by perfusion culture. At a harvesting rate of 20 %, the EPA areal productivity achieved 183 mg-dw m−2 d−1 even under 2000 μmol m−2 s−1 due to the increase in the dry weight using the perfusion culture. However, the fucoxanthin content decreased under 2000 μmol m−2 s−1. A strong relationship was observed between the specific light intensity per dry weight and fucoxanthin content in C. gracilis. This relationship indicates that the fucoxanthin content does not increase unless the specific light intensity per dry weight is adjusted below 20 μmol g-dw−1 s−1. Thus, perfusion culture proved to be useful to control the biomass-specific light intensity for the effective production of EPA and fucoxanthin in C. gracilis under high light conditions expected for outdoor culture.

A point mutation in the gene encoding magnesium chelatase I subunit influences strawberry leaf color and metabolism.

Magnesium chelatase (MgCh) catalyzes the insertion of magnesium into protoporphyrin IX, a vital step in chlorophyll (Chl) biogenesis. The enzyme consists of 3 subunits, MgCh I subunit (CHLI), MgCh D subunit (CHLD), and MgCh H subunit (CHLH). The CHLI subunit is an ATPase that mediates catalysis. Previous studies on CHLI have mainly focused on model plant species, and its functions in other species have not been well described, especially with regard to leaf coloration and metabolism. In this study, we identified and characterized a CHLI mutant in strawberry species Fragaria pentaphylla. The mutant, noted as p240, exhibits yellow-green leaves and a low Chl level. RNA-Seq identified a mutation in the 186th amino acid of the CHLI subunit, a base conserved in most photosynthetic organisms. Transient transformation of wild-type CHLI into p240 leaves complemented the mutant phenotype. Further mutants generated from RNA-interference (RNAi) and CRISPR/Cas9 gene editing recapitulated the mutant phenotype. Notably, heterozygous chli mutants accumulated more Chl under low light conditions compared with high light conditions. Metabolite analysis of null mutants under high light conditions revealed substantial changes in both nitrogen and carbon metabolism. Further analysis indicated that mutation in Glu186 of CHLI does not affect its subcellular localization nor the interaction between CHLI and CHLD. However, intramolecular interactions were impaired, leading to reduced ATPase and MgCh activity. These findings demonstrate that Glu186 plays a key role in enzyme function, affecting leaf coloration via the formation of the hexameric ring itself, and that manipulation of CHLI may be a means to improve strawberry plant fitness and photosynthetic efficiency under low light conditions.

Invasion of a non‐native anuran likely disrupts pond ecosystems

Abstract Freshwater ecosystems worldwide are under increasing pressure from multiple threats, including invasive species and climate change, with ponds being particularly vulnerable because of their shallow depth and dynamic hydrology. Australian brown tree frogs Litoria ewingii, introduced to New Zealand in 1875, have spread across both main islands, breeding in a large range of shallow fishless ponds. Since native frogs do not produce aquatic tadpoles, any tadpole influence will be novel, so these ecosystems may be especially vulnerable to such influences. To determine the functional role of L. ewingii tadpoles and to uncover potential effects on pond community structure and function, we conducted two 3‐week‐long mesocosm experiments. The first crossed tadpole presence with predatory invertebrate presence. Mesocosms containing tadpoles had a lower biomass of periphyton and macrophytes, and lower abundance of two families of small‐sized invertebrates (Culicidae and Chironomidae) compared to mesocosms without tadpoles, possibly as a consequence of grazing, predation or reduced dipteran oviposition. However, predatory invertebrates did not affect tadpoles except at their smallest stage, so L. ewingii probably are not subject to strong top‐down control by invertebrates in fishless ponds. We evaluated the effects of tadpole density on ecosystem processes in the second experiment where half the mesocosms also were shaded to simulate permanent pond hydrological status; shading stabilised the temperature fluctuations typical of temporary ponds. In shaded mesocosms tadpole density did not change phytoplankton, whereas in unshaded mesocosms, phytoplankton biomass increased with tadpole density, possibly because tadpole excretion enhanced algal growth in high light conditions. Higher densities of tadpoles also decreased dissolved oxygen concentration regardless of shading during the day—potentially an indirect effect of grazing on macrophytes and periphyton reducing photosynthesis. Overall, these non‐native tadpoles are likely to be having large influences on ecosystem processes such as nutrient cycling in small ponds, especially when at high densities. In light of these potential effects on pond ecosystems, and the generalist traits of these frogs which could see them spread to even remote alpine ponds, control measures to prevent further range expansion should be considered in New Zealand.

An algal-bacterial symbiotic system of carbon fixation using formate as a carbon source

The commercial implementation of algal cultures for efficient capture of CO2 from fossil-derived flue gases is not a reality yet due to several major challenges, particularly low gas-liquid mass transport efficiency along with loss of productivity due to pests. This study has explored an algal-bacterial symbiotic system to utilize formate as a carbon source. The algal-bacterial assemblage, after a 400-day adaptive evolution using the formate medium, has demonstrated a new capability to assimilate both formate and CO2 to promote biomass production. The formate tolerant culture system also addresses CO2 mass transfer limitation under high light conditions. Continuous cultivation of the assemblage on formate led to a carbon capture efficiency of 90 % with a biomass concentration of 0.92 g L−1 and biomass productivity of 0.31 g L−1 day−1, and the system did not require CO2 aeration. In addition, 13C isotope tracing, proteomics, and microbial community analysis reveal a uniquely evolved community for formate utilizations and robust biomass synthesis, in which Chlorella sorokiniana is a dominant phototroph companying with highly diverse bacterial subpopulations. In the presence of formate, evidence has shown that Chlorella is able to form mutualism with formate utilizing bacteria in semi-continuous cultures, leading to a highly effective photomixotrophic metabolism. This study demonstrates a new route of using electrochemical-derived formate to support mutualistic algae-bacteria biorefinery while the formate as an alternative carbon source could repel pests for outdoor algal cultivations.

Marine heatwaves and decreased light availability interactto erode the ecophysiological performance of habitat-forming kelp species.

Coastal marine ecosystems are threatened by a range of anthropogenic stressors, operating at global, local, and temporal scales. We investigated the impact of marine heatwaves (MHWs) combined with decreased light availability over two seasons on the ecophysiological responses of three kelp species (Laminaria digitata, L. hyperborea, and L. ochroleuca). These species function as important habitat-forming foundation organisms in the northeast Atlantic and have distinct but overlapping latitudinal distributions and thermal niches. Under low-light conditions, summertime MHWs induced significant declines in biomass, blade surface area, and Fv/Fm values (a measure of photosynthetic efficiency) in the cool-water kelps L. digitata and L. hyperborea, albeit to varying degrees. Under high-light conditions, all species were largely resistant to simulated MHW activity. In springtime, MHWs had minimal impacts and in some cases promoted kelp performance, while reduced light availability resulted in lower growth rates. While some species were negatively affected by summer MHWs under low-light conditions (particularly L. digitata), they were generally resilient to MHWs under high-light conditions. As such, maintaining good environmental quality and water clarity may increase resilience of populations to summertime MHWs. Our study informs predictions of how habitat-forming foundation kelp species will be affected by interacting, concurrent stressors, typical of compound events that are intensifying under anthropogenic climate change.

Accurate flux predictions using tissue-specific gene expression in plant metabolic modeling.

The accurate prediction of complex phenotypes such as metabolic fluxes in living systems is a grand challenge for systems biology and central to efficiently identifying biotechnological interventions that can address pressing industrial needs. The application of gene expression data to improve the accuracy of metabolic flux predictions using mechanistic modeling methods such as flux balance analysis (FBA) has not been previously demonstrated in multi-tissue systems, despite their biotechnological importance. We hypothesized that a method for generating metabolic flux predictions informed by relative expression levels between tissues would improve prediction accuracy. Relative gene expression levels derived from multiple transcriptomic and proteomic datasets were integrated into FBA predictions of a multi-tissue, diel model of Arabidopsis thaliana's central metabolism. This integration dramatically improved the agreement of flux predictions with experimentally based flux maps from 13C metabolic flux analysis compared with a standard parsimonious FBA approach. Disagreement between FBA predictions and MFA flux maps was measured using weighted averaged percent error values, and for parsimonious FBA this was169%-180% for high light conditions and 94%-103% for low light conditions, depending on the gene expression dataset used. This fell to 10%-13% and 9%-11% upon incorporating expression data into the modeling process, which also substantially altered the predicted carbon and energy economy of the plant. Code and data generated as part of this study are available from https://github.com/Gibberella/ArabidopsisGeneExpressionWeights.

Ultrasound treatment enhanced the functional properties of phycocyanin with phlorotannin from Ascophyllum nodosum.

Phycocyanin offers advantageous biological effects, including immune-regulatory, anticancer, antioxidant, and anti-inflammation capabilities. While PC, as a natural pigment molecule, is different from synthetic pigment, it can be easily degradable under high temperature and light conditions. In this work, the impact of ultrasound treatment on the complex of PC and phlorotannin structural and functional characteristics was carefully investigated. The interaction between PC and phlorotannin after ultrasound treatment was studied by UV-Vis, fluorescence spectroscopy, circular dichroism (CD) spectroscopy, fourier transform infrared (FTIR) spectroscopy. Additionally, the antioxidant potential and in vitro digestibility of the complexes were assessed. The result was manifested as the UV-Vis spectrum reduction effect, fluorescence quenching effect and weak conformational change of the CD spectrum of PC. PC was identified as amorphous based on the X-ray diffraction (XRD) data and that phlorotannin was embedded into the PC matrix. The differential scanning calorimetry (DSC) results showed that ultrasound treatment and the addition of phlorotannin could improve the denaturation peak temperatures (Td) of PC to 78.7°C. In vitro digestion and free radical scavenging experiments showed that appropriate ultrasound treatment and the addition of phlorotannin were more resistant to simulated gastrointestinal conditions and could improve DPPH and ABTS+ free radical scavenging performance. Ultrasound treatment and the addition of phlorotannin changed the structural and functional properties of PC. These results demonstrated the feasibility of ultrasound-assisted phlorotannin from A. nodosum in improving the functional properties of PC and provided a possibility for the application of PC-polyphenol complexes as functional food ingredients or as bioactive materials.

Effects of environmental lighting on students’ sleep, alertness and mood: A field study in a Chinese boarding school

A field study was conducted in two classrooms to explore the effects of standard and high light levels on sleep, alertness and mood of boarding high school students in Suzhou, China. 79 participants took part in a 7-week study in winter. After 3-week baseline assessments under standard lighting, participants were exposed to 3-week high light levels, separated by a 1-week washout. One group experienced this condition in the morning and the other experienced it in the evening. Horizontal illuminance was recorded in real time during the experiment and converted to eye-level vertical illuminance and equivalent melanopic lux. Sleep wristbands and questionnaires were used. Morning or evening high light levels significantly improved acute alertness during the increased light level hours compared with standard lighting, but there was no significant difference in sleep or mood. Additionally, compared to the evening high light levels, morning condition was significantly associated with longer deep sleep duration, easier falling asleep and lower negative mood. This study is the first to directly correlate high light conditions and timing of light exposure with subjective and objective sleep, alertness and mood of Chinese boarding high school students, which can provide references for the application of healthy lighting in classrooms.

Chrysanthemum morifolium β-carotene hydroxylase overexpression promotes Arabidopsis thaliana tolerance to high light stress

The β-carotene hydroxylase gene (BCH) regulates zeaxanthin production in response to high light levels ro protect Chrysanthemum morifolium plants against light-induced damage. In this study, the Chrysanthemum morifolium CmBCH1 and CmBCH2 genes were cloned and their functional importance was assessed by overexpressing them in Arabidopsis thaliana. These transgenic plants were evaluated for gene-related changes in phenotypic characteristics, photosynthetic activity, fluorescence properties, carotenoid biosynthesis, aboveground/belowground biomass, pigment content, and the expression of light-regulated genes under conditions of high light stress relative to wild-type (WT) plants. When exposed to high light stress, WT A. thaliana leaves turned yellow and the overall biomass was reduced compared to that of the transgenic plants. WT plants exposed to high light stress also exhibited significant reductions in the net photosynthetic rate, stomatal conductance, Fv/Fm, qP, and ETR, whereas these changes were not observed in the transgenic CmBCH1 and CmBCH2 plants. Lutein and zaxanthin levels were significantly increased in the transgenic CmBCH1 and CmBCH2 lines, with progressive induction with prolonged light exposure, whereas no significant changes were observed in light-exposed WT plants. The transgenic plants also expressed higher levels of most carotenoid biosynthesis pathway genes, including phytoene synthase (AtPSY), phytoene desaturase (AtPDS), lycopene-β-cyclase (AtLYCB), and ζ-carotene desaturase (AtZDS). The elongated hypocotyl 5 (HY5) and succinate dehydrogenase (SDH) genes were significantly induced following exposure to high light conditions for 12h, whereas phytochrome-interacting factor 7 (PIF7) was significantly downregulated in these plants.

Novel Insights into the Contribution of Cyclic Electron Flow to Cotton Bracts in Response to High Light.

Cyclic electron flow around photosystem I (CEF-PSI) is shown to be an important protective mechanism to photosynthesis in cotton leaves. However, it is still unclear how CEF-PSI is regulated in non-foliar green photosynthetic tissues such as bracts. In order to learn more about the regulatory function of photoprotection in bracts, we investigated the CEF-PSI attributes in Yunnan 1 cotton genotypes (Gossypium bar-badense L.) between leaves and bracts. Our findings demonstrated that cotton bracts possessed PROTON GRADIENT REGULATION5 (PGR5)-mediated and the choroplastic NAD(P)H dehydrogenase (NDH)-mediated CEF-PSI by the same mechanism as leaves, albeit at a lower rate than in leaves. The ATP synthase activity of bracts was also lower, while the proton gradient across thylakoid membrane (ΔpH), rate of synthesis of zeaxanthin, and heat dissipation were higher than those of the leaves. These results imply that cotton leaves under high light conditions primarily depend on CEF to activate ATP synthase and optimize ATP/NADPH. In contrast, bracts mainly protect photosynthesis by establishing a ΔpH through CEF to stimulate the heat dissipation process.

Effect of culture conditions at lab-scale on metabolite composition and antibacterial and antibiofilm activities of Dunaliella tertiolecta.

Dunaliella tertiolecta RCC6 was cultivated indoors in glass bubble column photobioreactors operated under batch and semi-continuous regimens and using two different conditions of light and temperature. Biomass was harvested by centrifugation, frozen, and then lyophilized. The soluble material was obtained by sequential extraction of the lyophilized biomass with solvents with a gradient of polarity (hexane, ethyl acetate, and methanol) and its metabolic composition was investigated through nuclear magnetic resonance (NMR) spectroscopy. The effect of light on chlorophyll biosynthesis was clearly shown through the relative intensities of the 1 H NMR signals due to pheophytins. The highest signal intensity was observed for the biomasses obtained at lower light intensity, resulting in a lower light availability per cell. Under high temperature and light conditions, the 1 H NMR spectra of the hexane extracts showed an incipient accumulation of triacylglycerols. In these conditions and under semi-continuous regimen, an enhancement of β-carotene and sterols production was observed. The antibacterial and antibiofilm activities of the extracts were also tested. Antibacterial activity was not detected, regardless of culture conditions. In contrast, the minimal biofilm inhibitory concentrations (MBICs) against Escherichia coli for the hexane extract obtained under semi-continuous regimen using high temperature and irradiance conditions was promising.

Towards energy-aware tinyML on battery-less IoT devices

With the advent of Tiny Machine Learning (tinyML), it is increasingly feasible to deploy optimized ML models on constrained battery-less Internet of Things (IoT) devices with minimal energy availability. Due to the unpredictable and dynamic harvesting environment, successfully running tinyML on battery-less devices is still challenging. In this paper, we present the energy-aware deployment and management of tinyML algorithms and application tasks on battery-less IoT devices. We study the trade-offs between different inference strategies, analyzing under which circumstances it is better to make the decision locally or send the data to the Cloud where the heavy-weight ML model is deployed, respecting energy, accuracy, and time constraints. To decide which of these two options is more optimal and can satisfy all constraints, we define an energy-aware tinyML optimization algorithm. Our approach is evaluated based on real experiments with a prototype for battery-less person detection, which considers two different environments: (i) a controllable setup with artificial light, and (ii) a dynamic harvesting environment based on natural light. Our results show that the local inference strategy performs best in terms of execution speed when a controllable harvesting environment is considered. It can execute 3 times as frequently as remote inference at a harvesting current of 2 mA and using a capacitor of 1.5 F. In a realistic harvesting scenario with natural light and making use of the energy-aware optimization algorithm, the device will favor remote inference under high light conditions due to the better accuracy of the Cloud-based model. Otherwise, it switches to local inference.

Suppression of the HOS1 Gene Affects the Level of ROS Depending on Light and Cold.

The E3 ubiquitin-protein ligase HOS1 is an important integrator of temperature information and developmental processes. HOS1 is a negative regulator of plant cold tolerance, and silencing HOS1 leads to increased cold tolerance. In the present work, we studied ROS levels in hos1Cas9Arabidopsis thaliana plants, in which the HOS1 gene was silenced by disruption of the open reading frame via CRISPR/Cas9 technology. Confocal imaging of intracellular reactive oxygen species (ROS) showed that the hos1 mutation moderately increased levels of ROS under both low and high light (HL) conditions, but wild-type (WT) and hos1Cas9 plants exhibited similar ROS levels in the dark. Visualization of single cells did not reveal differences in the intracellular distribution of ROS between WT and hos1Cas9 plants. The hos1Cas9 plants contained a high basal level of ascorbic acid, maintained a normal balance between reduced and oxidized glutathione (GSH and GSSG), and generated a strong antioxidant defense response against paraquat under HL conditions. Under cold exposure, the hos1 mutation decreased the ROS level and substantially increased the expression of the ascorbate peroxidase genes Apx1 and Apx2. When plants were pre-exposed to cold and further exposed to HL, the expression of the NADPH oxidase genes RbohD and RbohF was increased in the hos1Cas9 plants but not in WT plants. hos1-mediated changes in the level of ROS are cold-dependent and cold-independent, which implies different levels of regulation. Our data indicate that HOS1 is required to maintain ROS homeostasis not only under cold conditions, but also under conditions of both low and high light intensity. It is likely that HOS1 prevents the overinduction of defense mechanisms to balance growth.

Treatment of Black-Odorous Water Using Submerged Plants: The Physiological Response of Vallisneria natans

The main aim of this research was to determine the physiological response of the submerged macrophyte Vallisneria natans (V. natans) to black water with a foul odor. V. natans was chosen as the experimental plant species to investigate the morphological response and ecophysiological adaptation methods in response to varying light depths and black-odorous water. V. natans was planted in tap water (D), two types of black-odorous water (E and F), and under three distinct light conditions (low light, medium light, and high light). In the high-light condition with black-odorous water (E), the biomass content of V. natans declined from 1.78 g on the 14th day to 1.49 g on the 28th day, demonstrating that the black-odorous water inhibited the growth of V. natans. Under the stress of black-odorous water, the chlorophyll content of V. natans increased greatly in the early period but reduced during the latter experimental period. However, on the 21st day, maximum chlorophyll content of 1.30 mg/g (E) and 1.18 mg/g (F) was observed. In addition, the malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activity was monitored and reported under black-odorous water stress in V. natans. The experimental results of this work demonstrated conclusively that odorous black water exerts a certain stress on the physiological development of V. natans. Future research should incorporate the evaluation of several plant species and vary the process and environmental conditions to produce field-relevant, dependable results.

Effect of the Enhanced Production of Chlorophyll b on the Light Acclimation of Tomato.

Tomato (Solanum lycopersicum Mill.) is one of the widely cultured vegetables under protected cultivation, in which insufficient light is one of the major factors that limit its growth, yield, and quality. Chlorophyll b (Chl b) is exclusively present in the light-harvesting complex (LHC) of photosystems, while its synthesis is strictly regulated in response to light conditions in order to control the antenna size. Chlorophyllide a oxygenase (CAO) is the sole enzyme that converts Chl a to Chl b for Chl b biosynthesis. Previous studies have shown that overexpressing CAO without the regulating domain (A domain) in Arabidopsis overproduced Chl b. However, the growth characteristics of the Chl b overproduced plants under different light environmental conditions are not well studied. Considering tomatoes are light-loving plants and sensitive to low light stress, this study aimed to uncover the growth character of tomatoes with enhanced production of Chl b. The A domain deleted Arabidopsis CAO fused with the FLAG tag (BCF) was overexpressed in tomatoes. The BCF overexpressed plants accumulated a significantly higher Chl b content, resulting in a significantly lower Chl a/b ratio than WT. Additionally, BCF plants possessed a lower maximal photochemical efficiency of photosystem II (Fv/Fm) and anthocyanin content than WT plants. The growth rate of BCF plants was significantly faster than WT plants under low-light (LL) conditions with light intensity at 50-70 µmol photons m-2 s-1, while BCF plants grew slower than WT plants under high-light (HL) conditions. Our results revealed that Chl b overproduced tomato plants could better adapt to LL conditions by absorbing more light for photosynthesis but adapt poorly to excess light conditions by accumulating more ROS and fewer anthocyanins. Enhanced production of Chl b is able to improve the growth rate of tomatoes that are grown under LL conditions, indicating the prospect of employing Chl b overproduced light-loving crops and ornamental plants for protected or indoor cultivation.

Effect of DIN and DON sources on the nitrogen uptake of the seagrass Zostera japonica and the macroalgae Ulva pertusa previously grown in different light levels

This study quantified the absorption ability of the seagrass Zostera japonica and the macroalgae Ulva pertusa for dissolved inorganic nitrogen (DIN) (ammonium and nitrate) and dissolved organic nitrogen (DON) (urea and glycine) under different light conditions. The plants were cultured in filtered seawater (31‰, 25°C) for 2 weeks under three light levels. Macroalgae and the above- and belowground parts of seagrasses were separately placed into four different manmade seawater solutions with DIN (ammonium and nitrate) and DON (urea and glycine) stable isotopes for 1 h. The results showed that macroalgae had higher absorption rates for ammonium and nitrate after higher light (14.67 ± 2.50 and 1.29 ± 0.16 mg−1 dry weight (DW) h−1) than after lower light (4.52 ± 0.95 and 0.18 ± 0.12 mg−1 DW h−1) treatments. Compared to the belowground seagrass portions that had previously been grown in high and low light conditions, the seagrass leaves assimilated ammonium more quickly. Z. japonica preferred glycine to nitrate and urea after the high- and low-light treatments; that is, the absorption rates of the belowground seagrass parts for glycine were 14.71 ± 1.85 and 6.38 ± 0.52 mg−1 DW h−1 after the high- and low-light treatments, respectively, which were higher than those of ammonium, nitrate, and urea. The absorption rates of algae were lower than those for ammonium previously grown under medium- and low-light treatments. These results indicate that light reduction can impact the assimilation of DIN by Z. japonica and U. pertusa, and both have the ability to directly assimilate DON. This study provides information that could help reduce the negative effects of eutrophication on macroalgae and seagrasses in order to protect seagrass meadows.

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.

Mechanistic Insights on Salicylic Acid Mediated Enhancement of Photosystem II Function in Oregano Seedlings Subjected to Moderate Drought Stress.

Dramatic climate change has led to an increase in the intensity and frequency of drought episodes and, together with the high light conditions of the Mediterranean area, detrimentally influences crop production. Salicylic acid (SA) has been shown to supress phototoxicity, offering photosystem II (PSII) photoprotection. In the current study, we attempted to reveal the mechanism by which SA is improving PSII efficiency in oregano seedlings under moderate drought stress (MoDS). Foliar application of SA decreased chlorophyll content under normal growth conditions, but under MoDS increased chlorophyll content, compared to H2O-sprayed oregano seedlings. SA improved the PSII efficiency of oregano seedlings under normal growth conditions at high light (HL), and under MoDS, at both low light (LL) and HL. The mechanism by which, under normal growth conditions and HL, SA sprayed oregano seedlings compared to H2O-sprayed exhibited a more efficient PSII photochemistry, was the increased (17%) fraction of open PSII reaction centers (qp), and the increased (7%) efficiency of these open reaction centers (Fv'/Fm'), which resulted in an enhanced (24%) electron transport rate (ETR). SA application under MoDS, by modulating chlorophyll content, resulted in optimized antenna size and enhanced effective quantum yield of PSII photochemistry (ΦPSII) under both LL (7%) and HL (25%), compared to non-SA-sprayed oregano seedlings. This increased effective quantum yield of PSII photochemistry (ΦPSII) was due to the enhanced efficiency of the oxygen evolving complex (OEC), and the increased fraction of open PSII reaction centers (qp), which resulted in an increased electron transport rate (ETR) and a lower amount of singlet oxygen (1O2) production with less excess excitation energy (EXC).

Differences in leaf photoprotection strategies of tree species at different successional stages in subtropical forests under seasonal climate change and their relationship to construction cost strategies

Differences in response patterns to spatio-temporal changes in environmental stressors mediated by seasonal climatic differences reflect the underlying adaptation strategies of plants, and can provide key information on how tree species respond to seasonal climatic changes at different successional stages. However, in forests dominated by subtropical broadleaf evergreen species, the relationship between leaf photoprotection strategies and their construction cost strategies in different seasons remains unclear. It limits the assessment of the impact of seasonal climate change on community growth and exploration of potential drivers of community succession. In this study, we measured photoprotective substances, chlorophyll fluorescence, photosynthetic pigments and leaf construction cost (CC) in leaves of three mid-successional and three late-successional tree species in a subtropical broadleaf evergreen forest in different seasons. Our data showed that the young leaves of mid-successional trees had higher photoprotection ability in summer compared to winter, and were therefore better adapted to high temperature and high light conditions in summer, but not to low temperature and high light conditions in winter. The young leaves of late-successional trees had strong and flexible photoprotection in both summer and winter, which ensured normal growth of the young leaves under high temperature and high light in summer, and under low temperature and high light in winter. In addition, the photoprotection of the leaves was positively correlated with CC, suggesting that an increase in photoprotective capacity of plants was accompanied by an increase in its investment in CC. This was a response of the plants to environmental stress and the sacrifices it needed to make to maintain its survival. The differential growth patterns of late-successional species, which could grow normally in summer and winter, and mid-successional species, which were adapted to grow in summer but not in winter, may be key clues to the gradual replacement of mid-successional species by late-successional species.