Suboptimal Temperature Acclimation Enhances Chilling Tolerance by Improving Photosynthetic Adaptability and Osmoregulation Ability in Watermelon

The temperature drop of plants from the optimal requirements can increase tolerance to severe chilling stress. Photosynthesis and osmoregulators were analyzed during chilling stress to explore the adaptation mechanisms that underlie the induction of chilling tolerance in response to suboptimal temperature. The relationships of these processes to suboptimal temperature acclimation in watermelon were then determined.Suboptimal temperature-acclimated watermelon plants demonstrated tolerance during chilling stress, as indicated by the decreased electrolyte leakage and malondialdehyde accumulation compared with those non-acclimated watermelon plants. Chilling-induced photoinhibition and reduction in CO2 assimilation rate were alleviated after suboptimal temperature acclimation. The xanthophyll cycle level was enhanced by improving thermal dissipation ability and avoiding light damage. Consequently, the chilling tolerance of suboptimal temperature-acclimated watermelon plants was enhanced. The osmoregulation ability induced by suboptimal temperature acclimation protected watermelon plants against chilling injury because of the accumulation of small molecular substances, such as soluble sugar and proline. The protein levels of Rubisco activase(ClRCA) and the gene expression of the Benson–Calvin cycle simultaneously increased in suboptimal temperature-acclimated watermelon plants during chilling stress. Chilling tolerance in watermelon plants induced by suboptimal temperature acclimation is associated with enhanced photosynthetic adaptability and osmoregulation ability.

Transcriptomic analysis reveals putative osmoregulation mechanisms in the kidney of euryhaline turbot Scophthalmus maximus responded to hypo-saline seawater

Turbot harbor a relatively remarkable ability to adapt to opposing osmotic challenges and are an excellent model species to study the physiological adaptations of flounder associated with osmoregulatory plasticity.The kidney transcriptome of turbot treated 24 h in water of hypo-salinity(salinity 5)and seawater(salinity 30)was sequenced and characterized.In silico analysis indicated that all unigenes had significant hits in seven databases.The functional annotation analysis of the transcriptome showed that the immune system and biological processes associated with digestion,absorption,and metabolism played an important role in the osmoregulation of turbot in response to hypo-salinity.Analysis of biological processes associated with inorganic channels and transporters indicated that mineral absorption and bile secretion contributed to iono-osmoregulation resulting in cell volume regulation and cell phenotypic plasticity.Moreover,we analyzed and predicted the mechanisms of canonical signaling transduction.Biological processes involved in renin secretion,ECM-receptor interaction,adherens junction,and focal adhesion played an important role in the plasticity phenotype in hypo-stress,while the signal transduction network composed of the MAPK signaling pathway and PI3K-Akt signaling pathway with GABAergic synapse,worked in hypoosmoregulation signal transduction in the turbot.In addition,analysis of the tissue specificity of targeted gene expression using qPCR during salinity stress was carried out.The results showed that the kidney,gill,and spleen were vital regulating organs of osmotic pressure,and the osmoregulation pattern of euryhaline fish dif fered among species.

Effect of Dietary Potassium on Growth, Nitrogen Metabolism, Osmoregulation and Immunity of Pacific White Shrimp （Litopenaeus vannamel） Reared in Low Salinity Seawater

An 8 weeks feeding experiment was conducted to determine the effect of dietary potassium on the growth and physio-logical acclimation of Pacific white shrimp （Litopenaeus vannamei） reared in diluted seawater （salinity 4）. Six semi-purified practical diets containing 0.59, 0.96, 1.26, 1.48, 1.74, and 2.17 g potassium K＋ per 100 g diet were formulated, respectively. The survival and feed conversion rate did not show significant difference among groups of shrimps given these diets （P〉0.05）. The shrimps fed the diets containing 0.96-1.48 g K＋ per 100g diet gained the highest weight, specific growth rate, and protein efficiency ratio. Their ammonium-N excretion rate as well as hemolymph concentration of Na＋ and C1- were significantly lower than those of the control （P〈0.05）, but a reverse trend was observed for their gill Na＋/K＋-ATPase. Moreover, the shrimps fed with 1.48 g K＋ per 100 g diet were the highest in hemolymph urea level, and the phenoloxidase and lysozyme activities were significantly higher than those of the control （P〈0.05）. The growth and physiological response of the test shrimps suggested that diet containing 1.48 g K＋ per 100 g diet improved the growth of L. vannamei in low-salinity seawater, and enhanced the physiological acclimation of the organism.

Osmoregulation Mechanism of Drought Stress and Genetic Engineering Strategies for Improving Drought Resistance in Plants

Drought, one of the main adverse environmental factors, obviously affected plant growth and development. Many adaptive strategies have been developed in plants for coping with drought or water stress, among which osmoregulation is one of the important factors of plant drought tolerance. Many substances play important roles in plant osmoregulation for drought resistance, including proline, glycine betaine, Lea proteins and soluble sugars such as levan, trehalose, sucrose, etc. The osmoregulation mechanism and the genetic engineering of plant drought-tolerance are reviewed in this paper.

New insights into the influence of myo-inositol on carbohydrate metabolism during osmoregulation in Nile tilapia (Oreochromis niloticus)

A two-factor(23)orthogonal testwas conducted to investigate the effects of dietary myo-inositol(MI)on the osmoregulation and carbohydrate metabolism of euryhaline fish tilapia(Oreochromis niloticus)under sustained hypertonic stress(20 practical salinity units[psu]).6 diets containing either normal carbohydrate(NC,30%)or high carbohydrate(HC,45%)levels,with 3 levels(0,400 and 1,200 mg/kg diet)of MI,respectively,were fed to 540 fish under 20 psu for 8 weeks.Dietary MI supplementation significantly improved growth performance and crude protein content of whole fish,and decreased the content of crude lipid of whole fish(P<0.05).Curled,disordered gill lamella and cracked gill filament cartilage were observed in the gill of fish fed diets without MI supplementation.The ion transport capacity in gill was significantly improved in the 1,200 mg/kg MI supplementation groups compared with the 0 mg/kg MI groups(P<0.05).Moreover,the contents of Na^(+),K^(+),Cl^(-)in serum weremarkedly reduced with the dietary MI supplementation(P<0.05).The fish fed 1,200 mg/kg MI supplementation had the highest MI content in the gills and the lowest MI content in the serum(P<0.05).Additionally,the fish fed with 1,200 mg/kg MI supplementation had the highest MI synthesis capacity in gills and brain(P<0.05).Dietary MI markedly promoted the ability of carbohydrate metabolism in liver(P<0.05).Moreover,fish in the 1,200 mg/kg MI groups had the highest antioxidant capacity(P<0.05).This study indicated that high dietary carbohydrate would intensify stress,and impair the ability of osmoregulation in tilapia under a long-term hypersaline exposure.The supplementation of MI at 1,200 mg/kg in the high carbohydrate diet could promote carbohydrate utilization and improve the osmoregulation capacity of tilapia under long-term hypertonic stress.

Diurnal feeding as a potential mechanism of osmoregulation in aphids

Diurnal variation in phloem sap composition has a strong infuence on aphid performance.The sugar-rich phloem sap serves as the sole diet for aphids and a suite of physiological mechanisms and behaviors allowv them to tolerate the high osmotic stress.Here,we tested the hypothesis that night-time feeding by aphids is a behavior that takes advantage of the low sugar diet in the night to compensate for osmotic stress incurred while feeding on high sugar diet during the day.Using the electrical penetration graph(EPG)technique.we examined the eiects of diurmal rhythm on feeding behaviors of bird cherry-oat aphid(Rhopalosiphurm padi L.)on wheat.A strong diurmal rhythm in aphids as indicated by the presence of a cyclical pattern of expression in a core clock gene did not impact aphid feeding and similar feeding behaviors were observed during day and night.The major difference observed between day and night feeding was that aphids spent significantly longer time in phloem salivation during the night compared to the day.In contrast,aphid hydration was reduced at the end of the day-time feeding compared to end of the night-time fepding.Gene expression analysis of R.padi osmoregulatory genes indicated that sugar break down and water transport into the aphid gut was reduced at night.These data suggest that while diumal variation occurs in phloem sap composition,aphids use night time feeding to overcome the high osmotic stress incurred while feeding on sugar-rich phloem sap during the day.

Osmoregulation determines sperm cell geometry and integrity for double fertilization in flowering plants

Distinct from the motile flagellated sperm of animals and early land plants,the non-motile sperm cells of flowering plants are carried in the pollen grain to the female pistil.After pollination,a pair of sperm cells are delivered into the embryo sac by pollen tube growth and rupture.Unlike other walled plant cells with an equilibrium between internal turgor pressure and mechanical constraints of the cell walls,sperm cells wrapped inside the cytoplasm of a pollen vegetative cell have only thin and discontinuous cell walls.The sperm cells are uniquely ellipsoid in shape,although it is unclear how they maintain this shape within the pollen tubes and after release.In this study,we found that genetic disruption of three endomembrane-associated cation/H+exchangers specifically causes sperm cells to become spheroidal in hydrated pollens of Arabidopsis.Moreover,the released mutant sperm cells are vulnerable and rupture before double fertilization,leading to failed seed set,which can be partially rescued by depletion of the sperm-expressed vacuolar water channel.These results suggest a critical role of cell-autonomous osmoregulation in adjusting the sperm cell shape for successful double fertilization in flowering plants.

The Effects of Arbuscular Mycorrhizal Fungi on Reactive Oxyradical Scavenging System of Tomato Under Salt Tolerance

The effects of arbuscular mycorrhizal fungi （AMF）, Glomus mosseae, on oxygen radical scavenging system of tomato under salt stress were studied in potted culture experiments. The response of tomato （Lycopersieon eseulentum L.） cultivar Zhongza 9 seedlings with AMF inoculation and control to salt stress （0, 0.5 and 1.0% NaCl solution, respectively） was investigated. The results showed that the salt stress significantly reduced the dry matter content of roots, stems and leaves, and also the leaf area as compared with the control treatment. However, arbuscular mycorrhizal-inoculated （AM） significantly improved the dry matter and the leaf area in the salt-stressed plants. The effect of AMF on dry matter was more pronounced in aerial bromass than in root biomass which might be due to AM colonization. The activities of SOD, POD, ASA-POD, and CAT in leaves and roots of mycorrhizal and non-mycorrhizal treatment of tomato plants were increased and had different rules under different NaCl concentrations （solution of 0, 0.5 and 1% NaCl）, but all enzymes had a rise in the beginning of treatment under salt stress conditions. The AMF did not change the rule of tomato itself under salt stress, but AMF increased these enzyme activities in different levels. The AMF treatment significantly increased SOD, POD and ASA-POD activities in leaves and roots, whereas it had little effects on CAT in root. O2- production rate and MDA content in leaves increased continuously, which showed a positive line correlation with salt stress concentration. O2- production rate and MDA content in tomato plants significantly decreased by AM treatment compared with nonmycorrhizal treatment. In conclusion, AM could alleviate the growth limitations imposed by saline conditions, and thereby play a very important role in promoting plant growth under salt stress in tomato.

Claroideoglomus etunicatum improved the growth and saline-alkaline tolerance of Potentilla anserina by altering physiological and biochemical properties

To investigate the effects of arbuscular mycorrhizal(AM)fungi on the growth and saline–alkaline tolerance of Potentilla anserina L.,the seedlings were inoculated with Claroideoglomus etunicatum(W.N.Becker&Gerd.)C.Walker&A.Schüßler in pot cultivation.After 90 days of culture,saline–alkaline stress was induced with NaCl and NaHCO_(3)solution according to the main salt components in saline–alkaline soils.Based on the physiological response of P.anserina to the stress in the preliminary experiment,the solution concentrations of 0 mmol/L,75 mmol/L,150 mmol/L,225 mmol/L and 300 mmol/L were treated with stress for 10 days,respectively.The mycorrhizal colonization rate,mycorrhizal dependence,chlorophyll content,malondialdehyde content,antioxidant enzyme activities,osmoregulation substances content and water status were measured.The results showed that with the increase of NaCl and NaHCO_(3)stress concentration,mycorrhizal colonization rate,colonization intensity,arbuscular abundance and vesicle abundance decreased,and reached the lowest value at 300 mmol/L.Strong mycorrhizal dependence was observed after the symbiosis with AM fungus,and the dependence was higher under NaHCO_(3)treatment.Under NaCl and NaHCO_(3)stress,inoculation with AM fungus could increase chlorophyll content,decrease malondialdehyde content,increase activities of superoxide dismutase,peroxidase and catalase,increase contents of proline,soluble sugar and soluble protein,increase tissue relative water content and decrease water saturation deficit.It was concluded that salt–alkali stress inhibited the colonization of AM fungus,but the mycorrhiza still played a positive role in maintaining the normal growth of plants under salt–alkali stress.

Effect of various Na/K ratios in low-salinity well water on growth performance and physiological response of Pacific white shrimp Litopenaeus vannamei

To investigate the influence of sodium to potassium （Na/IO ratios on the growth performance and physiological response of the Pacific white shrimp （Litopenaeus vananmei）, various concentrations of KC1 were added to low-salinity well water （salinity 4） in an 8-week culture trial. Six treatments with NWK ratios of 60：1, 42：1, 33：1, 23：1, 17：1, and 14：1 were replicated in triplicate. The highest weight-gain rate （3 506±48）% and survival rate （89.38±0.88）% was observed in well water with Na/K ratios of 23：1 and 42：1, respectively, while the feed conversion ratio （1.02～0.01）, oxygen consumption, and ammonia-N excretion rate was the lowest in the medium with a Na/K ratio of 23：1. Gill Na＋-K＋-ATPase activity, as an indicator of osmoregulation, peaked in the treatment where the Na/K ratio was 17：1. The total hemocyte count, respiratory burst, and immune-related enzyme activities （ALP, LSZ, PO, and SOD） ofL. vananmei were affected significantly by Na/K ratios （P〈0.05）. After challenged with Vibrio harveyi, the cumulative mortality of shrimp reared in a Na/K ratio of 23：1 （30±14.14）% was significantly lower than the control （75～7.07）%. In conclusion, the addition of K＋ to low-salinity well water in L. vannamei cultures is feasible. Na/K ratios ranging from 23：1 to 33：1 might improve survival and growth. Immunity and disease resistance are also closely related to the Na/K ratio of the low-salinity well water. The findings may contribute to the development of more efficient K^＋ remediation strategies for L. vananmei culture in low-salinity well water.

Changes of Water Status and Different Responses of Osmoregulants in Jatropha Curcas L. Seedlings to Air-drought Stress

[Objective] The study aimed to investigate the changes of water status and different responses of osmoregulants during air-drought stress,to better understand mechanisms of drought resistance in Jatropha Curcas L. [Methods] The 12-day-old J. curcas seedlings were held in a climate chamber at 25/20 ℃（day/night）,16 hours illumination,and 75% of relative humidity for air-drought treatment,and the changes of water potential,osmotic potential and the content of soluble sugar,proline,betaine were measured. [Results] Water potential and osmotic potential in leaves of J. curcas seedlings dropped significantly,pressure potential lost during air-drought stress,and the contents of osmoregulants soluble sugar,proline and betaine rose significantly to different extent in the leaves and stems. [Conclusion] Osmoregulants in the leaves and stems respond differently to air-drought stress,and in general leaves are much more responsive to the drought than stems of J. curcas seedlings.

Changes in Plasma Osmolality, Cortisol and Amino Acid Levels of Tongue Sole(Cynoglossus semilaevis) at Different Salinities

A serial of salinity transferring treatments were performed to investigate the osmoregulation of tongue sole(Cynoglossus semilaevis). Juvenile tongue sole were directly transferred from a salinity of 30 to 0, 10, 20, 30, 40 and 50. Blood sampling was performed for each treatment after 0, 1, 6 and 12 h, as well as after 1, 2, 4, 8, 16 and 32 d. The plasma osmolality, cortisol and free amino acids were assessed. Under the experimental conditions, no fish died after acute salinity transfer. The plasma cortisol level increased 1 h after the abrupt transfer from a salinity of 30 to that of 0, 40 and 50, and decreased from 6 h to 8 d after transfer. Similar trends were observed in the changes of plasma osmolality. The plasma free amino acids concentration showed a ‘U-shaped' relationship with salinity after being transferred to different salinities for 4 days. More obvious changes of plasma free amino acid concentration occurred under hyper-osmotic conditions than under hypo-osmotic conditions. The concentrations of valine, isoleucine, lysine, glutamic acid, glycine, proline and taurine increased with rising salinity. The plasma levels of threonine, leucine, arginine, serine, and alanine showed a ‘U-shaped' relationship with salinity. The results of this study suggested that free amino acids might have important effects on osmotic acclimation in tongue sole.

Transcriptome Analysis of Pacific White Shrimp (Litopenaeus vannamei) Under Prolonged High-Salinity Stress

The Pacific white shrimp(Litopenaeus vannamei)is a marine species commonly farmed worldwide.In northern China,it has been increasingly cultured in high-salinity waters(>40),but exhibits poor growth performance.In this study,postlarval shrimps were acclimated to salinity 55,cultivated for 3 months at this salinity,and compared with a control group reared at general salinity 25.Subsequently,high-throughput RNA sequencing was applied to compare the transcriptomic responses in the gills and hepatopancreas of the shrimps in the control group and the treatment group,while the weights of the shrimps in these two groups were significantly different.The results revealed that 11834 and 2115 genes were significantly differentially expressed in the gills and hepatopancreas,respectively.Additionally,enrichment analysis of the differentially expressed genes indicated that osmoregula-tion-associated Gene Ontology terms and KEGG pathways were similar between the two subgroups of the shrimp maintained at high salinity,suggesting that the growth rate of shrimp at high salinity is independent of osmoregulation.Furthermore,examination of the shrimp with different growth rates(i.e.,weights)at high salinity revealed molt-associated processes,namely,increased expression of ecdysone response genes and downstream effector genes in the gills and hepatopancreas of slow-growing shrimp,suggesting a role of the molt-associated processes in the regulation of shrimp growth at high salinity.Thus,we not only report adaptive transcriptomic responses of L.vannamei to prolonged high-salinity stress,but also provide new insights into the shrimp growth regulation at high salinity.

Influence of Osmoregulators on Osmotolerant Yeast Candida krusei for the Production of Glycerol

Candida krusei was osmotolerant yeast for the production of glycerol. Extracellular osmotic pressure was one of the key factors to induce the enzyme activity of glycerol-3-phosphate dehydrogenase（GPDH） which severely affected the glycerol productivity. Three osmoregulators such as NaCl, PEG4000 and glycerol were used to investigate their effects on yeast growth, glucose consumption and glycerol production. In order to determine the effect of extracellular glycerol concentration, different amounts of glycerol were initially supplemented as an osmoregulator to increase glycerol production. The maximum glycerol concentration attained 179g·L^-1 with initial glycerol concentration of 80g·L^-1 in medium, compared with 41g·L^-1 in the control experiment. These results were successfully reoroduced for fed-batch orocess in an air-lift reactor.

Trehalose and Sucrose Osmolytes Accumulated by Algae as Potential Raw Material for Bioethanol

Currently, obtaining sustainable fuels, such as biodiesel and bioethanol, from cheap and renewable materials is a challenge. In recent years, a new approach being developed consists of producing, sugars from algae by photosynthesis. Sugar accumulation can be increased under osmotic stress (osmoregulation). The aim of this study is to show the pro-duction of sugars from algae, isolated from natural sources, and the effect of osmotic stress on fermentable sugars ac-cumulation. Strain isolation, production of sugars from each alga and the effect of osmotic stress on growth and sugar production are described. Twelve algal strains were isolated, showing growths between 0.6 and 1.8 g of biomass dry weight /L, all with production of intracellular and extracellular sugars. The strain identified as Chlorella sp. showed an increase in sugar production from 23.64 to 421 mg of sugars/g of biomass dry weight after 24 h of osmotic stress with 0.4 M NaCl. Sucrose and trehalose, both fermentable sugars, were the compatible osmolytes accumulated in response to the osmotic stress. The isolated strains are potential producers of fermentable sugars, using the photosynthetic pathway and osmotic stress.

Influence of Osmoregulators on Osmotolerant Yeast Candida krusei for the Production of Glycerol

Candida krusei was osmotolerant yeast for the production of glycerol. Extracellular osmotic pressure was one of the key factors to induce the enzyme activity of glycerol-3-phosphate dehydrogenase(GPDH) which se- verely affected the glycerol productivity. Three osmoregulators such as NaCl, PEG4000 and glycerol were used to investigate their effects on yeast growth, glucose consumption and glycerol production. In order to determine the effect of extracellular glycerol concentration, different amounts of glycerol were initially supplemented as an os- moregulator to increase glycerol production. The maximum glycerol concentration attained 179g L-1 with initial glycerol concentration of 80g L-1 in medium, compared with 41g L-1 in the control experiment. These results were successfully reproduced for fed-batch process in an air-lift reactor.

Change of osmoregulatory and hematological parameters in tilapia(Oreochromis niloticus)after exposure to sublethal mercury concentrations

The effects of Hg exposure on blood parameters and gill physiology of tilapia(Oreochromis niloticus)were analyzed.Fish maintained in freshwater were exposed for 7 days(d)to sublethal mercury concentrations(0.1 and 1 mg/L).Blood serum osmolality(SO),sodium(Nat),potassium(K^+)and chloride(Cl^-)ionic concentrations,and hematological parameters were assessed after 1 up to 7 d of exposure.Serum osmolality and ionic concentrations of exposed fish appeared differently affected throughout the experimental period compared to the controls.Osmolality was reduced at the 2 tested concentrations but Nat and Cl^-contents were only altered at 1 mg/L of Hg after 1 d of exposure and values rapidly returned to the control values thereafter.K^+content was also modified and significantly increased at both concentrations after 1 d of exposure but returned to the control values after 3 d of exposure.Red blood cell(RBC),white blood cell(WBC)and hemoglobin(Hb)levels were significantly increased throughout the experiment but returned to control values after 7 d of exposure only for the 0.1 mg/L concentration.The hematocrit(Ht)levels remained unaffected due to Hg exposure.Therefore,tilapias exposed to sublethal concentrations of Hg present a marked osmotic imbalance with ionic and hematological disorders that are rapidly compensated.

Inhibition of α-glucosidase activity by N-deoxynojirimycin analogs in several insect phloem sap feeders

Secondary metabolites and synthetic iminosugars that structurally resemble monosaccharides are potent inhibitors of a-glucosidase activity. The enzyme is core in cleaving sucrose in phloem feeding insects and it also plays a crucial role of reducing osmotic stress via the formation of oligosaccharides. Inhibition of hydrolysis by iminosug- ars should result in nutritional deficiencies and/or disruption of normal osmoregulation. Deoxynojirimycin （DNJ） and 2 N-alkylated analogs [N-butyl DNJ （NB-DNJ） and N-nonyl DNJ （NN-DNJ）] were the major iminosugars used throughout the study. The extensive experiments conducted with a-glucosidase of the whitefly Bemisia tabaci indicated the competitive nature of inhibition and that the hydrophilic DNJ is a potent inhibitor in com- parison to the more hydrophobic NB-DNJ and NN-DNJ compounds. The same inhibitory pattern was observed with the psyllid Cacopsylla bidens a-glucosidase. In contrast to the above pattern, enzymes of the aphids, Myzus persicae and Aphis gossypii were more sen- sitive to the hydrophobic iminosugars as compared to DNJ. In vivo experiments in which adult B. tabaci were fed dietary iminosugars, show that the hydrophilic DNJ was far less toxic than the lipophilic NB-DNJ and NN-DNJ. It is proposed that this pattern is attributed to the better accessibility of the hydrophobic NN-DNJ to the a-glucosidase membrane- bound compartment in the midgut. Based on the inhibitory effects of certain polyhydroxy N-alkylated iminosugars, a-glucosidase of phloem feeding hemipterans could serve as an attractive target site for developing novel pest control agents.

Physiological adaptations of small mammals to desert ecosystems

Adaptations of animals to the xeric environment have been studied in various taxonomic groups and across several deserts.Despite the impressive data that have been accumulated,the focus in most of these studies is mainly on the significance of one variable at a time.Here,we attempt to integrate between responses of several physiological systems,challenged by increasing diet and water salinity and extreme temperatures,acquired in different studies of thermo and osmo-regulatory adaptations,of small rodents,to the xeric environment.Studies have shown differen-tial thermoregulatory responses to increased dietary salinity,which were attributed to habitat and habits of the relevant species.In the thermoregulatory studies,a potential adaptive significance of low metabolic rate was demonstrated.From an evolutionary point of view,the most important adaptation is in the timing of reproduction,as it enables the transfer of genetic properties to the next generation in an unpredictable ecosystem,where repro-duction might not occur every year.Results in this aspect show that increased dietary salinity,through an increase in vasopressin plasma levels,plays an important role as a regulator of the reproductive system.We assume that the amount of food existing in the habitat and the amount of reserves in the animal in the form of white adipose tissue are important for reproduction.Photoperiod affects all studied physiological responses,emphasizing the impor-tance of pre-acclimation to seasonal characteristics.We summarize the existing data and suggest neuro-endocrine pathways,which have a central role in these adaptations by affecting thermoregulation,osmoregulation and repro-duction to create the optimal response to xeric conditions.These hypotheses can be used as the basis for future studies.

Morphological changes and variations in Na+/K+-ATPase activity in the gills of juvenile large yellow croaker (Larimichthys crocea) at low salinity

Gill morphological changes and physiological responses in juvenile large yellow croaker(Larimichthys crocea)were examined upon exposure to low salinity after indoor culture of the fish at salinities of 2,4,6,8,and 24‰(control group).The thickness of the lamellae was significantly higher in the low-salinity groups than in the control group;in contrast,the interlamellar space was significantly lower in the low-salinity groups than in the control group.Additionally,a significant negative correlation was found between the thickness of lamellae and interlamellar spaces(P<0.01).Mitochondria-rich cells(MRCs)were mainly found in the filament at 24‰salinity and proliferated in the lamellae at lower salinities,suggesting that filament and lamellar MRCs are responsible for ion secretion and absorption,respectively.Meanwhile,the activity of Na+/K+-ATPase(NKA)was significantly elevated with the decrease in salinity from 24‰to 4‰(P<0.05),which was consistent with MRCs proliferation.Finally,the activity of NKA declined at 2‰salinity(P<0.05),indicating the limit of osmoregulation,which was consistent with the degeneration and necrosis of the lamellae.Additionally,different levels of gill histopathological lesions,including pavement cell(PVC)exfoliation,lamellar epithelial lifting,edema,fusion,aneurism,and necrosis,were observed from salinities of 24 to 4‰,likely reducing the respiratory efficiency and compromising the health of juvenile fish.In conclusion,large yellow croaker juveniles could improve the osmoregulatory capacity by increasing lamellar MRCs and NKA activity with the decrease in salinity from 24 to 4‰.However,the associated histopathological lesions are likely to negatively influence the fish by affecting respiration and osmoregulation,especially when the salinity is below 4‰.