Severe Environmental Stress Research Articles

Environmental hazard and migration intentions in a coastal area in Ghana: a case of sea flooding

Recently, there has been significant debate about whether ‘environmental migration’ can constitute a form of adaptation to environmental change, as opposed to forced or flight migration. The Foresight Report on Migration and Environmental Change (2011) suggested environmental factors are one driver of migration, as well as political, social, economic and demographic drivers, and that—under the right conditions—migration can be a form of adaptation to changing climatic conditions. However, this is dependent on migrants having adequate social and financial capital to undertake beneficial types of migration; it further argues that environmental change may result in ‘trapped populations’ whereby people who lack the necessary resources to re-establish livelihoods elsewhere may be left exposed to increasingly severe environmental shocks and stresses in situ. Research on the climate-migration nexus in West Africa has largely focused on out-migration from the semi-arid Sahel with more limited evidence about how sea flooding interacts with migration flows. This paper attempts to help fill this knowledge gap. Using data from a representative survey of households across three coastal communities in Ghana’s Volta River Delta, this paper concludes that exposure to sea flooding may not be a primary cause of out-migration as other community, economic and political factors influence migration intentions and decisions. Thus, it is important for planned adaptation interventions to be strengthened in situ to enable households, particularly farming households, sustain their livelihoods.

Environmental stress effects on reproduction and sexual dimorphism in the gynodioecious species Silene acaulis

In gynodioecious species, hermaphrodite plants invest both in seed and pollen production, whereas female plants only produce fruits. For both sexes to coexist, such unbalanced investment is expected to translate in some kind of reproductive compensation, particularly under severe nutrient shortage or environmental stress. This study investigates 1) whether females of the gynodioecious boreo-alpine plant Silene acaulis show any overall compensation in fitness components; 2) the existence of sexual dimorphism in flowers that favor different modes in each sex; 3) whether environmental severity, characterized by means of a Species Distribution Model, strength these patterns. Flower size, fruit and seed production, and germination success were recorded in six populations across the Pyrenees and the Alps. Fruit set was significantly higher in female (59%) than in hermaphrodite plants (17%), and female organs (ovary and style) were larger in female than in hermaphrodite plants, but female flowers were smaller. Contrary to expectations, environment severity was not related with most of the biological traits studied and only germination was associated to environmental gradients. These results confirm that fruit production plays an important role as a compensatory strategy between sexes and, consequently, may favor the presence of both sexes in a gynodioecious species.

Arbuscular mycorrhiza effects on plant performance under osmotic stress.

At present, drought and soil salinity are among the most severe environmental stresses that affect the growth of plants through marked reduction of water uptake which lowers water potential, leading to osmotic stress. In general, osmotic stress causes a series of morphological, physiological, biochemical, and molecular changes that affect plant performance. Several studies have found that diverse types of soil microorganisms improve plant growth, especially when plants are under stressful conditions. Most important are the arbuscular mycorrhizal fungi (AMF) which form arbuscular mycorrhizas (AM) with approximately 80% of plant species and are present in almost all terrestrial ecosystems. Beyond the well-known role of AM in improving plant nutrient uptake, the contributions of AM to plants coping with osmotic stress merit analysis. With this review, we describe the principal direct and indirect mechanisms by which AM modify plant responses to osmotic stress, highlighting the role of AM in photosynthetic activity, water use efficiency, osmoprotectant production, antioxidant activities, and gene expression. We also discuss the potential for using AMF to improve plant performance under osmotic stress conditions and the lines of research needed to optimize AM use in plant production.

Copper alters hypoxia sensitivity and the behavioural emersion response in the amphibious fish Kryptolebias marmoratus

Elevated levels of metals have been reported in mangrove ecosystems worldwide. Mangrove fishes also routinely experience severe environmental stressors, such as hypoxia. In the amphibious fish Kryptolebias marmoratus (mangrove rivulus), a key behavioural response to avoid aquatic stress is to leave water (emersion). We hypothesized that copper (Cu) exposure would increase the sensitivity of this behavioural hypoxia avoidance response due to histopathological effects of Cu on gill structure and function. K. marmoratus were exposed to either control (no added Cu) or Cu (300μg/L) for 96h. Following this period, fish were exposed to an acute hypoxic challenge (decline in dissolved oxygen to ∼0% over 15min), and the emersion response was recorded. Gills were examined for histological changes. Fish exposed to Cu emersed at a higher dissolved oxygen level (7.5±0.6%), relative to the control treatment group (5.8±0.4%). Histological analysis showed that the gill surface area increased and the interlamellar cell mass (ILCM) was reduced following Cu exposure, contrary to our prediction. Overall, these data indicate that Cu induces hypoxia-like changes to gill morphology and increases the sensitivity of the hypoxia emersion response.

RpoS Affects Gene Expression in Salmonella enterica serovar Typhi Under Early Hyperosmotic Stress.

During the infectious procedure of Salmonella enterica serovar Typhi (S. Typhi), Salmonella would suffer from some severe environmental stresses, such as gastric acid stress, enteric hyperosmotic stress, bile acids stress, and oxidative stress. S. Typhi must regulate the expression of numerous genes through the complex regulatory network to adapt strict stresses. RpoS, which encodes sigma factor σs, was reported to be a very important regulator in the maximal survival of enteric pathogens including S. enterica in the stress conditions. However, the role of RpoS in S. Typhi under early hyperosmotic stress is not clear. In this study, we prepared the RpoS-deleted mutant (△RpoS) and compared the growth of △RpoS and wild-type strain. In addition, we analyzed its global transcription profile under early hyperosmotic stress by a microarray. The results showed that △RpoS grew significantly slower than wild-type strain and 24 genes displayed differential expression between the wild-type strain and ΔRpoS upon 30-min exposure in the high osmolarity. Most of these genes are associated with enzymes of metabolism. Taken together, our study firstly demonstrated that RpoS affects gene expression in S. Typhi under early hyperosmotic stress and may impact the growth of bacteria by regulating bacterial metabolic enzymes.

Granulostasis: Protein Quality Control of RNP Granules

Ribonucleoprotein (RNP) granules transport, store, or degrade messenger RNAs, thereby indirectly regulating protein synthesis. Normally, RNP granules are highly dynamic compartments. However, because of aging or severe environmental stress, RNP granules, in particular stress granules (SGs), convert into solid, aggregate-like inclusions. There is increasing evidence that such RNA-protein inclusions are associated with several age-related neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), fronto-temporal dementia (FTD) and Alzheimer’s disease (AD). Thus, understanding what triggers the conversion of RNP granules into aggregates and identifying the cellular players that control RNP granules will be critical to develop treatments for these diseases. In this review article, we discuss recent insight into RNP and SG formation. More specifically, we examine the evidence for liquid-liquid phase separation (LLPS) as an organizing principle of RNP granules and the role of aggregation-prone RNA-binding proteins (RBPs) in this process. We further discuss recent findings that liquid-like SGs can sequester misfolded proteins, which promote an aberrant conversion of liquid SGs into solid aggregates. Importantly, very recent studies show that a specific protein quality control (PQC) process prevents the accumulation of misfolding-prone proteins in SGs and, by doing so, maintains the dynamic state of SGs. This quality control process has been referred to as granulostasis and it relies on the specific action of the HSPB8-BAG3-HSP70 complex. Additional players such as p97/valosin containing protein (VCP) and other molecular chaperones (e.g., HSPB1) participate, directly or indirectly, in granulostasis, and ensure the timely elimination of defective ribosomal products and other misfolded proteins from SGs. Finally, we discuss recent findings that, in the stress recovery phase, SGs are preferentially disassembled with the assistance of chaperones, and we discuss evidence for a back-up system that targets aberrant SGs to the aggresome for autophagy-mediated clearance. Altogether the findings discussed here provide evidence for an intricate network of interactions between RNP granules and various components of the PQC machinery. Molecular chaperones in particular are emerging as key players that control the composition and dynamics of RNP granules, which may be important to protect against age-related diseases.

Exploring physiological plasticity and local thermal adaptation in an intertidal crab along a latitudinal cline

Intertidal organisms have evolved physiological mechanisms that enable them to maintain performance and survive during periods of severe environmental stress with temperatures close to their tolerance limits. The level of these adaptive responses in thermal physiology can vary among populations of broadly distributed species depending on their particular environmental context and genetic backgrounds. Here we examined thermal performances and reaction norms for metabolic rate (MR) and heart rate (HR) of seven populations of the porcelanid crab Petrolisthes violaceus from markedly different thermal environments across the latitudinal gradient of ~3000km. Physiological responses of this intertidal crab under common-garden conditions suggest the absence of local thermal adaptation along the geographic gradient (i.e., lack of latitudinal compensation). Moreover, thermal physiological sensitivities and performances in response to increased temperatures evidenced the existence of some level of: i) metabolic rate control or depression during warm temperature exposures; and ii) homeostasis/canalization (i.e., absence or low levels of plasticity) in physiological traits that may reflect some sort of buffering mechanism in most of the populations. Nevertheless, our results indicate that elevated temperatures can reduce cardiac function but not metabolic rate in high latitude crabs. The lack of congruence between HR and MR supports the idea that energy metabolism in marine invertebrates cannot be inferred from HR and different conclusions regarding geographic differentiation in energy metabolism can be obtained from both physiological traits. Integrating thermal physiology and species range extent can contribute to a better understanding of the likely effects of climate change on natural populations of marine ectotherms.

Life history trade-offs, the intensity of competition, and coexistence in novel and evolving communities under climate change.

The consequences of climate change for local biodiversity are little understood in process or mechanism, but these changes are likely to reflect both changing regional species pools and changing competitive interactions. Previous empirical work largely supports the idea that competition will intensify under climate change, promoting competitive exclusions and local extinctions, while theory and conceptual work indicate that relaxed competition may in fact buffer communities from biodiversity losses that are typically witnessed at broader spatial scales. In this review, we apply life history theory to understand the conditions under which these alternative scenarios may play out in the context of a range-shifting biota undergoing rapid evolutionary and environmental change, and at both leading-edge and trailing-edge communities. We conclude that, in general, warming temperatures are likely to reduce life history variation among competitors, intensifying competition in both established and novel communities. However, longer growing seasons, severe environmental stress and increased climatic variability associated with climate change may buffer these communities against intensified competition. The role of life history plasticity and evolution has been previously underappreciated in community ecology, but may hold the key to understanding changing species interactions and local biodiversity under changing climates.This article is part of the themed issue 'Human influences on evolution, and the ecological and societal consequences'.

Physiological and molecular responses to drought and salinity in soybean

Drought and salinity are severe environmental stresses and limit soybean growth. In this study, a comparative analysis of physiological and molecular responses of two soybean (Glycine max L.) genotypes to these stresses was carried out. Plants of drought-tolerant genotype RD (cv. FD92) and sensitive genotype SD (cv. Z1303) were exposed to 15 % (m/v) PEG 6000, which simulated drought stress, or 150 mM NaCl. The RD plants maintained larger leaf area and higher net photosynthetic rate, chlorophyll content, stomatal conductance, and relative water content compared with the SD plants. Leaf proline content increased under both stresses more in RD than in SD. The drought tolerance of RD plants was also correlated with greater antioxidant activity and lower content of hydrogen peroxide and malondialdehyde under stress conditions. Amounts of abscisic acid, jasmonic acid, and salicylic acid under stress increased to a greater extent in RD than in SD plants. At the molecular level, the effects of 20-d stress treatments were manifested by relatively higher expression of drought- or salt-related genes: GmP5CS, GmDREB1a, GmGOLS, GmBADH, and GmNCED1 in RD plants than in SD plants. These results form the basis for understanding the mechanisms of the drought- and salt-stress tolerance in soybean.

The impact of an extreme flood in spring/summer 2010 on phytoplankton communities in oxbow lakes of the lower Vistula River, central Poland

In 2010, central Europe was hit by a massive flood. The main objective of this study was to assess the impact of this natural disaster on phytoplankton communities in four oxbow lakes of the lower Vistula River. In order to explain changes caused by the flood, the data from the year 2010 were compared with the data from the previous year. In summer 2009, the abundance and biomass of phytoplankton as well as chlorophyll a content in small isolated oxbow lakes were relatively low. Small nanoplankton flagellates prevailed. Much higher abundance and biomass of phytoplankton and chlorophyll a content were recorded in a large oxbow lake, characterized by the predominance of filamentous cyanobacteria. After the flood wave, the abundance and biomass of phytoplankton in the small oxbow lakes increased dramatically (18 and 7 times, respectively), but the number of species decreased. Filamentous cyanobacteria (Dolichospermum), euglenophyta and dinoflagellates were identified. In the large oxbow lake, the number of species increased but the abundance and biomass of phytoplankton decreased (by approximately 60 and 75%). There was a shift in dominant algal species. Cyanobacteria were replaced mainly by cryptophyta, small chlorophyta (Chlamydomonas) and xanthophyta (Heterothrix). The least significant changes were recorded in the Vistula River. Here, before and after the flood, the abundance and biomass of phytoplankton as well as chlorophyll a content and the dominant algal species remained unchanged. The massive flood caused severe environmental stress, which disturbed phytoplankton development and led to shifts between clear water and turbid states in the lakes.

Drought-resistant and water-saving plants: from laboratory to field

Drought is one of the most severe environmental stresses affecting plant growth and development. It has profound impacts on agriculture worldwide, causing enormous yield reduction and increasing production cost every year. Global climate change will likely worsen this situation in the future. Therefore, it is imperative for us to study the mechanisms of drought resistance and identify candidate genes that are useful for breeding drought-resistant and water-saving crops. Plants have evolved an array of different mechanisms to cope with drought stress and adapt to the changing environment. Here, in the context of our progress in drought-resistant crop improvement within the past decade, we briefly review the latest advances in plant drought resistance and potential applications in drought-resistant crop breeding in terms of drought avoidance and drought tolerance and discuss issues in drought resistance.

Climate Change, Desertification, and Societal Responses along the Mu Us Desert Margin during the Ming Dynasty

Abstract Historical records for the Mu Us Desert margin during the Ming dynasty (1368–1644) and corresponding high-resolution climate proxy records have prompted studies on societal responses to climatic changes in this region. The Mu Us Desert margin is highly sensitive to changes in desertification and biological productivity controlled in part by Asian monsoon variations. Here the existing historical temperature and precipitation records are examined to understand spatiotemporal climate variations and to identify potential mechanisms that have driven desertification in the region over the past 500 years. The focus here is on three severe desertification events that occurred in 1529–46, the 1570s, and 1601–50. The relationships among temperature, precipitation, and desertification indicate that a cold/drought-prone climate drives the desertification process. During the Ming dynasty, this region was one of nine important military districts, where the frontier wall (the Great Wall) and other fortifications were constructed. To maintain the defense system, military officers made a valiant effort to decrease the influence of desertification. However, the human-waged war against nature was largely futile, and local rebellions in the stricken region were spawned by the inability of the government to cope with the severe environmental stresses associated with rapid desertification.

Knocking Down the Expression of GMPase Gene OsVTC1-1 Decreases Salt Tolerance of Rice at Seedling and Reproductive Stages.

Salinity is a severe environmental stress that greatly impairs production of crops worldwide. Previous studies have shown that GMPase plays an important role in tolerance of plants to salt stress at vegetative stage. However, the function of GMPase in plant responses to salt stress at reproductive stage remains unclear. Studies have shown that heterologous expression of rice GMPase OsVTC1-1 enhanced salt tolerance of tobacco seedlings, but the native role of OsVTC1-1 in salt stress tolerance of rice is unknown. To illustrate the native function of GMPase in response of rice to salt stress, OsVTC1-1 expression was suppressed using RNAi-mediated gene silencing. Suppressing OsVTC1-1 expression obviously decreased salt tolerance of rice varieties at vegetative stage. Intriguingly, grain yield of OsVTC1-1 RNAi rice was also significantly reduced under salt stress, indicating that OsVTC1-1 plays an important role in salt tolerance of rice at both seedling and reproductive stages. OsVTC1-1 RNAi rice accumulated more ROS under salt stress, and supplying exogenous ascorbic acid restored salt tolerance of OsVTC1-1 RNAi lines, suggesting that OsVTC1-1 is involved in salt tolerance of rice through the biosynthesis regulation of ascorbic acid. Altogether, results of present study showed that rice GMPase gene OsVTC1-1 plays a critical role in salt tolerance of rice at both vegetative and reproductive stages through AsA scavenging of excess ROS.

The effect of HMB ingestion on the IGF-I and IGF binding protein response to high intensity military training

ObjectiveInsulin-like growth factor-I (IGF-I) is a metabolic and anabolic biomarker that has been proposed to reflect physiological adaptations resulting from multistressor environments. The bioactivity of IGF-I is regulated by seven different insulin-like growth factor binding proteins (IGFBPs) which act not only as carriers of IGF-1, but also function as a modulator of IGF-I availability and activity. Supplementing with β-hydroxy-β-methylbutyrate (HMB) has been shown to enhance physiological outcomes associated with intense training, and has been reported to augment the IGF-1 response. The purpose of this study was to examine the effect of 23days of HMB supplementation on circulating levels of IGF-I and IGFBPs in combat soldiers during highly intense military training. MethodsThirteen male soldiers from an elite infantry unit volunteered to participate in this double-blind, parallel design study. Soldiers were provided 3g·day−1 of either HMB (n=6) or placebo (PL; n=7). During the study soldiers performed advanced military training with periods of restricted sleep and severe environmental stressors. Blood samples were obtained prior to (PRE) and approximately 18h following the final supplement consumption (POST). ResultsNo significant differences were observed for circulating IGF-1 concentrations between HMB and PL (p=0.568). In addition, no differences were seen between the groups for IGFBP-1 (p=1.000), IGFBP-2 (p=0.855), IGFBP-3 (p=0.520), IGFBP-4 (p=0.103), IGFBP-5 (p=0.886), or IGFBP-6 (p=0.775). A significant difference was noted between HMB (169.9±23.0ng·ml−1) and PL (207.2±28.0ng·ml−1) for IGFBP-7 at POST (p=0.042). ConclusionsAlthough the results of this study do not support the influence of HMB supplementation on circulating concentrations of IGF-1 or IGFBPs1–6 during high intensity military training, it does present initial evidence that it may lower circulating IGFBP-7 concentrations. This may provide some indication of a reduced stress response, but further investigation on the physiological role of IGFBP-7 and military training is needed.

Functional characterization of blue-light-induced responses and PHOTOTROPIN 1 gene in Welwitschia mirabilis.

The blue light (BL) receptor phototropin (phot) is specifically found in green plants; it regulates various BL-induced responses such as phototropism, chloroplast movement, stomatal opening, and leaf flattening. In Arabidopsis thaliana, two phototropins--phot1 and phot2--respond to blue light in overlapping but distinct ways. These BL-receptor-mediated responses enhance the photosynthetic activity of plants under weak light and minimize photodamage under strong light conditions. Welwitschia mirabilis Hook.f. found in the Namib Desert, and it has adapted to severe environmental stresses such as limiting water and strong sunlight. Although the plant has physiologically and ecologically unique features, it is unknown whether phototropin is functional in this plant. In this study, we assessed the functioning of phot-mediated BL responses in W. mirabilis. BL-dependent phototropism and stomatal opening was observed but light-dependent chloroplast movement was not detected. We performed a functional analysis of the PHOT1 gene of W. mirabilis, WmPHOT1, in Arabidopsis thaliana. We generated transgenic A. thaliana lines expressing WmPHOT1 in a phot1 phot2 double mutant background. Several Wmphot1 transgenic plants showed normal growth, although phot1 phot2 double mutant plants showed stunted growth. Furthermore, Wmphot1 transgenic plants showed normal phot1-mediated responses including phototropism, chloroplast accumulation, stomatal opening, and leaf flattening, but lacked the chloroplast avoidance response that is specifically mediated by phot2. Thus, our findings indicate that W. mirabilis possesses typical phot-mediated BL responses that were at least partially mediated by functional phototropin 1, an ortholog of Atphot1.

Effect of sowing time on buckwheat (Fagopyrum esculentum Moench) growth and yield in central Korea

Buckwheat (Fagopyrum esculentum MOENCH) is a traditional Korean crop that gaining global attention as a health food due to its rich nutrition. Climate change effects such as high temperature, cold weather, flooding, and drought induce plant stress, reduced production yield of upland crops. In general, buckwheat is grown during the fall season in Korea. Flooding is a severe environmental stress factor, and flooding during the early growth stage of buckwheat causes large reductions in yield. Therefore, we established optimum growth parameters to maximize buckwheat yield. The optimum temperature for harvesting of buckwheat was approximately 1,083 ~ 1,515°C. The number of growing degree days (G.D.D.) from flowering to harvest was 509 ~ 673°C. Plant hight, stem diameter, and dry matter weight were reduced as planting date was delayed (p < 0.05). We investigated buckwheat sowing time during fall, and established an optimum cropping system according to the local climate conditions in central Korea. The growth period of buckwheat cultivar ‘Yangjueol’ was approximately 56 ~ 60 days during fall, and yield was higher than that of cultivar ‘Daesan’. The results indicated that buckwheat yield in central Korea was maximized by sowing cultivar on 25th August and cultivar ‘Daesan’ on 15th August. The yield of Yangjeol sown in spring was 119 kg 10a-1, whereas the yield of Daesan sown in summer was 158kg 10a-1. The results of this study established the effects of sowing time on growth and yield in central Korea. We propose an optimum cropping system for improving quality.

Molecular importance of prawn large heat shock proteins 60, 70 and 90

Considering the importance of heat shock proteins (HSPs) in the innate immune system of prawn, a comparative molecular approach was proposed to study the crustacean large HSPs 60, 70 and 90. Three different large HSPs were identified from freshwater prawn Macrobrachium rosenbergii (Mr) cDNA library during screening. The structural and functional characteristic features of HSPs were studied using various bioinformatics tools. Also, their gene expression and mRNA regulation upon various pathogenic infections was studied by relative quantification using 2(-ΔΔCT) method. MrHSP60 contains a long chaperonin 60 domain at 46-547 which carries a chaperonin 60 signature motif between 427 and 438, whereas MrHSP70 contains a long HSP70 domain at 21-624 and MrHSP90 carries a HSP90 domain at 188-719. The two dimensional analysis showed that MrHSP60 contains more amino acids (52%) in helices, whereas MrHSP70 (40.6%) and MrHSP90 (51.8%) carried more residues in coils. Gene expression results showed significant (P<0.05) expression of MrHSP60, 70 and 90 in haemocyte, gill and hepatopancreas, respectively. Further, the expression level was up-regulated upon bacterial (Aeromonas hydrophilla and Vibrio harveyi) and viral [white spot syndrome virus (WSSV) and M.rosenbergii nodo virus (MrNV)] infections during various time periods. The gene expression results exhibited the potential involvement of these three HSPs in the immune system of prawn. The study indicated the potentiality of these molecules, thereby protecting cells against pathogens as well as severe cellular and environmental stresses in crustaceans.

Evaluation of Polarimetric SAR Decomposition for Classifying Wetland Vegetation Types

The Florida Everglades is the largest subtropical wetland system in the United States and, as with subtropical and tropical wetlands elsewhere, has been threatened by severe environmental stresses. It is very important to monitor such wetlands to inform management on the status of these fragile ecosystems. This study aims to examine the applicability of TerraSAR-X quadruple polarimetric (quad-pol) synthetic aperture radar (PolSAR) data for classifying wetland vegetation in the Everglades. We processed quad-pol data using the Hong &amp; Wdowinski four-component decomposition, which accounts for double bounce scattering in the cross-polarization signal. The calculated decomposition images consist of four scattering mechanisms (single, co- and cross-pol double, and volume scattering). We applied an object-oriented image analysis approach to classify vegetation types with the decomposition results. We also used a high-resolution multispectral optical RapidEye image to compare statistics and classification results with Synthetic Aperture Radar (SAR) observations. The calculated classification accuracy was higher than 85%, suggesting that the TerraSAR-X quad-pol SAR signal had a high potential for distinguishing different vegetation types. Scattering components from SAR acquisition were particularly advantageous for classifying mangroves along tidal channels. We conclude that the typical scattering behaviors from model-based decomposition are useful for discriminating among different wetland vegetation types.

The hypoxia that developed in a microtidal estuary following an extreme storm produced dramatic changes in the benthos

Runoff from an extreme storm on 22 March 2010 led, during the next 3 months, to the formation of a pronounced halocline and underlying hypoxia in the upper reaches of the microtidal Swan–Canning Estuary. Benthic macroinvertebrates were sampled between January 2010 and October 2011 at five sites along 10km of this region. By mid-April, the number of species, total density, Simpson’s evenness index and taxonomic distinctness had declined markedly, crustaceans had disappeared and the densities of annelids and molluscs had declined slightly. These faunal attributes (except Simpson’s index) and species composition did not recover until after the end of the hypoxia. The survival of annelids and loss of crustaceans in this period reflects different sensitivities of these taxa to severe environmental stress. The results emphasise that microtidal estuaries with long residence times are highly vulnerable to the effects of environmental perturbations, particularly during warmer periods of the year.

Seasonal trends in photosynthesis and electron transport during the Mediterranean summer drought in leaves of deciduous oaks.

The California Mediterranean savanna has harsh summer conditions with minimal soil moisture, high temperature, high incoming solar radiation and little or no precipitation. Deciduous blue oaks, Quercus douglasii Hook. and Arn., are winter-deciduous obligate phreatophytes, transpiring mostly groundwater throughout the summer drought. The objective of this work is to fully characterize the seasonal trends of photosynthesis in blue oaks as well as the mechanistic relationships between leaf structure and function. We estimate radiative load of the leaves via the FLiES model and perform in situ measurements of leaf water potential, leaf nitrogen content, an index of chlorophyll content (SPAD readings), photosynthetic and electron transport capacity, and instantaneous rates of CO2 assimilation and electron transport. We measured multiple trees over 3 years providing data from a range of conditions. Our study included one individual that demonstrated strong drought stress as indicated by changes in SPAD readings, leaf nitrogen and all measures of leaf functioning. In the year following severe environmental stress, one individual altered foliation patterns on the crown but did not die. In all other individuals, we found that net carbon assimilation and photosynthetic capacity decreased during the summer drought. SPAD values, electron transport rate (ETR) and quantum yield of photosystem II (PSII) did not show a strong decrease during the summer drought. In most individuals, PSII activity and SPAD readings did not indicate leaf structural or functional damage throughout the season. While net carbon assimilation was tightly coupled to stomatal conductance, the coupling was not as tight with ETR possibly due to contributions from photorespiration or other protective processes. Our work demonstrates that the blue oaks avoid structural damage by maintaining the capacity to convert and dissipate incoming solar radiation during the hot summer drought and are effective at fixing carbon by maximizing rates during the mild spring conditions.