Morphological and Physiological Responses of Hawaiian Hibiscus tiliaceus Populations to Light and Salinity

Hibiscus tiliaceus (Hau) is a pantropical mangrove associate that usually occurs in coastal ecosystems where substrate salinity is relatively high, but it also inhabits upland habitats in Hawaii. Cuttings from three populations on the island of Oahu, Hawaii, were collected and grown in the glasshouse under two levels of substrate salinity (0 and 335 mOsm kg-1) and three light treatments (0%, 50%, and 90% shade). Photosynthetic gas exchange, biomass allocation, and accumulation were studied in relation to salinity and light. Salinity reduced net CO2 assimilation in the upland population but had no effect or stimulated photosynthesis in the coastal populations, whereas increasing salinity decreased stomatal conductance in all populations and therefore increased water-use efficiency. The degree to which photosynthesis was inhibited by salinity was inversely proportional to the salinity of the source population, indicating a loss of salinity tolerance in upland plants. Light had a stronger effect on leaf area ratio (LAR) and leaf mass per area (LMA), whereas salinity had a stronger effect on leaf water content, internode length, and plant biomass. Salinity reduced total new biomass by 58%, 50%, and 34% in full sun, 50% shade, and 90% shade, respectively, but this response did not differ between populations. Salinity reduced the photosynthesis, but not growth, of upland plants because increased allocation to photosynthetic tissue increased LAR to compensate for inhibition of photosynthesis by salinity.     

Integrated Responses to Salinity Fluctuation

SYNOPSIS. Continuous and discontinuous intertidal waters bothexperience fluctuations in salinity frequently accompanied bychanges in temperature, levels of dissolved gases and pH. Amultidimensional approach to environmental fitness may thereforebe more representative of natural conditions in the intertidalzone. Various combinations of environmental factors are discussedincluding salinity/temperature, and salinity/O₂ tension. Salinitywill directly affect osmotic and ionic regulation and indirectlyaffect acid-base balance and various components of the respiratorysystem including ventilation, gas exchange, perfusion, O₂ transportby the respiratory pigment and utilization at the tissues. Additionalareas covered in this review article are osmotic problems facedby air-breathing aquatic intertidal animals, ontogenetic changesin osmoregulatory capabilities and a comparison of steady state(square wave) versus sinusoidal (cyclical) salinity changes.     

Differences in Salinity Tolerance and Gene Expression Between Two Populations of Atlantic Cod (Gadus morhua) in Response to Salinity Stress

Populations of marine fish, even from contrasting habitats, generally show low genetic differentiation at neutral genetic markers. Nevertheless, there is increasing evidence for differences in gene expression among populations that may be ascribed to adaptive divergence. Studying variation in salinity tolerance and gene expression among Atlantic cod (Gadus morhua) from two populations distributed across a steep salinity gradient, we observed high mortality (45% North Sea cod and 80% Baltic Sea cod) in a reciprocal common garden setup. Quantitative RT-PCR assays for expression of hsp70 and Na/K-ATPase α genes demonstrated significant differences in gene regulation within and between populations and treatment groups despite low sample sizes. Most interesting are the significant differences observed in expression of the Na/K-ATPase α gene in gill tissue between North Sea and Baltic cod. The findings strongly suggest that Atlantic cod are adapted to local saline conditions, despite relatively low levels of neutral genetic divergence between populations.     

Salinity and Salicornia bigelovii Torr. Seedling Establishment. Water Relations

Seedling establishment of Salicornia bigelovii was inhibitedwhen seeds were sown in vermiculite watered with 0 ppt NaCl(control treatment) while the presence of 10 or 30 ppt NaClhad no inhibitory effect. During most of the experimental periodthe rate of water absorption in both cotyledons and embryo-axisof the salt treatments was at least twice that of the control.The organic material decreased in the cotyledons at the sametime that it was increasing in the embryo-axis. Salinity didnot show any apparent effect on reserve mobilization from thecotyledons to the embryo-axis. Ions accumulated with the increasedsalinity of the root environment in both the cotyledons andthe embryo-axis. The former accumulated more than the latterexcept in the control treatment. After the 8th day from sowingthe cotyledon was —1•38, —1•95 and —2•74MPa for the 0, 10 and 30 ppt treatments, respectively. The gradients between external solution and the cotyledons were(0•80, 1•19 and 1•15 MPa for the 0, 10 and 30ppt, respectively. The _p of the control treatment was very low(0•02 MPa) when compared to the ones for the 10(0•53MPa) and the 30 ppt (0•99 MPa) treatments. Glycinebetaineand soluble amino nitrogen accounted for over 90% of the organiccomponent of the tissue osmolality with soluble sugars beingresponsible for the remainder. In the control the inorganicand organic components were of the same magnitude while in the10 and 30 ppt treatments the inorganic was 8•6 and 14•1fold greater than the organic component, respectively. Watermovement into the control plants was apparently inhibited bythe lack of inorganic solutes and as a consequence the cotyledonsfrom these plants did not expand and seedling growth was severelyreduced. Key words: Halophyte, seedling establishment, tissue osmolality components     

Comparison of Antipredator Responses of Two Related Water Striders to a Common Predator

This study compares the antipredator responses of two congeneric water striders that live in different habitats yet share a common predator. Aquarius nebularis and A. remigis are stream-dwelling species of water striders found in close association in central Kentucky. Aquarius nebularis typically inhabit third- and fourth-order streams that support diverse and complex fish communities; A. remigis inhabit adjacent first- and second-order tributaries that tend to have either no fish or relatively few fish patchily distributed. The ecological conditions for these two species are distinctly different, yet the green sunfish (Lepomis cyanellus) is a surface-feeding predator and a potential risk to both species. In experimental pools with sunfish, an initial fish strike caused A. remigis to move to the edge of the pool and become inactive. For the rest of the observation period, A. remigis reduced behaviour patterns that attract predators such as skating on open water, mating and male–male aggression. The proportion of A. remigis missing from sunfish pools was 5 times higher than in pools lacking fish, suggesting that 20% of water striders were killed by predation. In contrast, an initial fish strike caused A. nebularis to exhibit a burst of activity as individuals erratically skated over the open water. They then resumed the less active pre-attack behaviours a short time later. Aquarius nebularis showed none of the antipredator behaviours typical for A. remigis. Furthermore, the proportion of A. nebularis missing from sunfish pools did not differ from that in pools without fish. Although antipredator behaviour has been examined for relatively few species of water striders, this study suggests that the behaviour of the most commonly studied species, A. remigis, may not be indicative for water striders in general., Corresponding author: James J. Krupa, School of Biological Sciences, University of Kentucky, Lexington, KY 40506-0225, U.S.A. E-mail: bio149@pop.uky.edu bio149@pop.uky.edu     

Responses of Atriplex spongiosa and Suaeda monoica to Salinity

The growth and tissue water, K⁺, Na⁺, Cl⁻, proline and glycinebetaine contents of the shoots and roots of two Chenopodiaceae, Atriplex spongiosa and Suaeda monoica have been measured over a range of external NaCl salinities. Both species showed some fresh weight response to low salinity mainly due to increased succulence. S. monoica showed both a greater increase in succulence (at low salinities) and tolerance of high salinities than A. spongiosa. Both species had high affinities for Na⁺ and maintained constant but low shoot K⁺ contents with increasing salinity. These trends were more marked with S. monoica in which Na⁺ stimulated the accumulation of K⁺ in roots. An association between high leaf Na⁺ accumulation, high osmotic pressure, succulence, and a positive growth response at low salinities was noted. Proline accumulation was observed in shoot tissues with suboptimal water contents. High glycinebetaine contents were found in the shoots of both species. These correlated closely with the sap osmotic pressure and it is suggested that glycinebetaine is the major cytoplasmic osmoticum (with K⁺ salts) in these species at high salinities. Na⁺ salts may be preferentially utilized as vacuolar osmotica.     

Growth and Water Relations of Lotus Creticus Creticus Plants as Affected by Salinity

Young plants of Lotus creticus creticus growing in a hydroponic culture were submitted to 0, 70 and 140 mM NaCl treatments for 28 d and the growth and ecophysiological characteristics of these plants have been studied. The growth of Lotus plants was not affected by salinity when applied for a short period (about 15 d); however, 140 mM NaCl induced a decrease in shoot RGR at the end of the treatment. The root growth was not decreased, even it was stimulated by 140 mM NaCl. The osmotic adjustment of Lotus plants at 70 and 140 mM NaCl maintained constant pressure potential, avoiding the visual wilting. For a similar leaf water potential, cuticular transpiration of salinized plants was lower than in control plants due to the salinity effect on the cuticle. Moreover, the presence of hairy leaves (60 and 160 trichomes per mm² in young and adult leaves, respectively) allows keeping almost 81 % of sprayed water and absorbing the 9 % of the water retained, decreased the epidermal conductance to water vapour diffusion.     

Differential Responses of Two Wheat Varieties Differing in Salt Tolerance to the Combined Stress of Mn and Salinity

The effect of Mn and NaCl on growth, mineral nutrients and antioxidative enzymes in two tetroploid wheat genotypes differing in salt tolerance was investigated in this study. 100 mM NaCl and Mn stress significantly inhibited plant growth, photosynthesis and Ca uptake, while stimulated ROS accumulation, MDA and proline content in wheat plants, Mn stress also increased SOD, APX, GR and DHAR activities. Durum wheat (AS780) was less affected by 100 mM NaCl and Mn stress than emmer wheat (AS847) due to more proline production, higher antioxidative enzymes activities and less-affected mineral nutrients. Application of 10 mM NaCl to Mn-stressed durum wheat alleviated Mn-induced damage by reducing Mn accumulation and translocation, while promoting proline accumulation and SOD, APX and GR activities. Irrespective of NaCl level, the combined stress of Mn and NaCl caused more severe oxidative stress, result in further reduction of photosynthetic rate and plant growth in emmer wheat as compared to Mn stress alone. The additively negative effects of NaCl and Mn stress on growth of emmer wheat results from reduced SOD and APX activities as well as Ca, Cu and Fe accumulation in both shoots and roots. These results suggest that salt-tolerant durum wheat is superior to emmer in adapting to Mn stress and the combined stress of salinity and Mn.

     

Differential Responses of Nitrogen-Fixing Cyanobacteria to Salinity and Osmotic Stresses

Two nitrogen-fixing Anabaena strains were found to be differentially tolerant to salinity and osmotic stresses. Anabaena torulosa, a brackish-water, salt-tolerant strain, was relatively osmosensitive. Anabaena sp. strain L-31, a freshwater, salt-sensitive strain, on the other hand, displayed significant osmotolerance. Salinity and osmotic stresses affected nitrogenase activity differently. Nitrogen fixation in both of the strains was severely inhibited by the ionic, but not by the osmotic, component of salinity stress. Such differential sensitivity of diazotrophy to salinity-osmotic stresses was observed irrespective of the inherent tolerance of the two strains to salt-osmotic stress. Exogenously added ammonium conferred significant protection against salinity stress but was ineffective against osmotic stress. Salinity and osmotic stresses also affected stress-induced gene expression differently. Synthesis of several proteins was repressed by salinity stress but not by equivalent or higher osmotic stress. Salinity and osmotic stresses induced many common proteins. In addition, unique salt stress- or osmotic stress-specific proteins were also induced in both strains, indicating differential regulation of protein synthesis by the two stresses. These data show that cyanobacterial sensitivity and responses to salinity and osmotic stresses are distinct, independent phenomena.     

Physiological and Optical Responses of the Harmful Dinoflagellate Heterocapsa circularisquama to a Range of Salinity

The responses of division rate, cell volume, cellular carbon (C) and nitrogen (N), cellular pigments and optical characteristics to changes in salinity were examined in the dinoflagellate Heterocapsa circularisquama. The physiological and optical characteristics of H. circularisquama were significantly affected by changes in salinity. When cells were exposed to different salinities, they exhibited physiological acclimation by adjusting their cellular properties associated with growth. This could be a beneficial tactic for ensuring proliferation and limiting damages induced by adverse environmental factors. The results of this study could be essential for assisting in the development of growth models for H. circularisquama.     

Ionic and Osmotic Effects of Salinity on Single-Leaf Photosynthesis in Two Wheat Cultivars with Different Drought Tolerance

The effects of iso-osmotic salinity and drought stresses on leaf net photosynthetic rate (P _N) in two wheat (Triticum aestivum L.) cultivars BR 8 and Norin 61, differing in drought tolerance, were compared. In drought-sensitive Norin 61, the decline of P _N was larger than that in drought-tolerant BR 8. Under NaCl treatment, P _N decreased in two phases similarly in both cultivars. In the first phase, photosynthetic depression was gradual without any photochemical changes. In the second phase, photosynthetic depression was rapid and accompanied with a decline of the energy conversion efficiency in photosystem 2 (_PS2). Our observations suggest that the osmotic factor may induce a gradual depression of photosynthesis due to stomatal closure under both stress treatments. However, under NaCl treatment, a ionic factor (uptake and accumulation of excess Na⁺) may have direct effects on electron transport and cause more severe photosynthetic depression. The drought tolerance mechanism of BR 8 was insufficient to maintain single-leaf photosynthesis under salinity.     

Effect of Salinity on Water Relations, Sodium Accumulation, Chlorophyll Content and Proteolytic Enzymes in a Wild Wheat

The effect of 50 to 200 mM NaCl on two lines (CP with solid stem and CV with hollow stem) of ×Haynaldoticum sardoum was studied. NaCl significantly reduced root and shoot fresh and dry masses, root length and less markedly shoot length of CP and CV plants. The sodium accumulated in the leaves in relation to the concentration of NaCl and length of the treatment; CP leaves contained twice as much sodium as CV leaves. The leaf chlorophyll a/b ratio was not affected by NaCl. NaCl decreased the leaf water and osmotic potentials. The pressure potential increased due to the increased concentration of dissolved solutes in the leaf, particularly sodium. The proteinase and exopeptidase activities increased during NaCl treatment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response of Melanthera biflora to Salinity and Water Stress

Melanthera biflora (Asteraceae) is a moderately salt-tolerantplant from the Indo-Pacific region. In laboratory studies itsgrowth was inhibited by salt above 50 mol m^–3, but itwas able to survive salinities approaching that of seawater,namely 400 mol m^–3. Shoot potassium concentrations weremaintained over a range of salinities up to 400 mol m^–3,while sodium and chloride accumulation followed closely theincrease in external osmotic pressure. In contrast, the increasein osmotic pressure of the leaf sap of Melanthera biflora, subjectedto water stress, was due mainly to a decrease in the ratio offresh weight/dry weight. 3-dimethylsulphoniopropionate (3-DMSP)and glycinebetaine were identified by fast atom bombardmentmass and ¹H -NMR spectroscopy, with 3-DMSP being the main oniumcompound and glycinebetaine absent in some accessions. Onium(quaternary ammonium and/or tertiary sulphonium) compounds andproline increased during salt and water stress due mainly toa decrease in the fresh weight/dry weight ratio of tissue, althoughpart of the increase in salt-stressed tissue was due to an increasein the accumulation of the onium compound. This salt-inducedincrease in 3-DMSP was inhibited in conditions of low sulphursupply and there was no compensatory increase in proline. Key words: Melanthera biflora, Asteraceae, salinity, glycinebetaine, 3-dimethylsulphonioproprionate     

Photosynthetic and Stomatal Responses of Two Mangrove Species, Aegiceras corniculatum and Avicennia marina, to Long Term Salinity and Humidity Conditions

Gas exchange characteristics were studied in two mangrove species, Aegiceras corniculatum (L.) Blanco and Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke, grown under a variety of salinity and humidity conditions. The assimilation rate was measured as a function of the intercellular CO₂ concentration [A(c_i) curve]. The photosynthetic capacity decreased with increase in salinity from 50 to 500 millimolar NaCl, as shown by decline in both the initial linear slope and the upper plateau of the A(c_i) curve, with A. corniculatum being the more sensitive species. The decline in photosynthetic capacity was enhanced by increase in the leaf to air vapor pressure difference from 6 to 24 millibars, but this treatment caused a decrease in only the upper plateau of the A(c_i) curve. Stomatal conductance was such that the intercellular CO₂ concentration obtaining under normal atmospheric conditions occurred near the transition between the lower linear and upper plateau portions of the A(c_i) curves. Thus, stomatal conductance and photosynthetic capacity together co-limited the assimilation rate, which declined with increasing salinity and decreasing humidity. The marginal water cost of carbon assimilation was similar in most treatments, despite variation in the water loss/carbon gain ratio.     

Soil Microbial Responses to Increased Moisture and Organic Resources along a Salinity Gradient in a Polar Desert

Microbial communities in extreme environments often have low diversity and specialized physiologies suggesting a limited resistance to change. The McMurdo Dry Valleys (MDV) are a microbially dominated, extreme ecosystem currently undergoing climate change-induced disturbances, including the melting of massive buried ice, cutting through of permafrost by streams, and warming events. These processes are increasing moisture across the landscape, altering conditions for soil communities by mobilizing nutrients and salts and stimulating autotrophic carbon inputs to soils. The goal of this study was to determine the effects of resource addition (water/organic matter) on the composition and function of microbial communities in the MDV along a natural salinity gradient representing an additional gradient of stress in an already extreme environment. Soil respiration and the activity of carbon-acquiring extracellular enzymes increased significantly (P < 0.05) with the addition of resources at the low- and moderate-salinity sites but not the high-salinity site. The bacterial community composition was altered, with an increase in Proteobacteria and Firmicutes with water and organic matter additions at the low- and moderate-salinity sites and a near dominance of Firmicutes at the high-salinity site. Principal coordinate analyses of all samples using a phylogenetically informed distance matrix (UniFrac) demonstrated discrete clustering among sites (analysis of similarity [ANOSIM], P < 0.05 and R > 0.40) and among most treatments within sites. The results from this experimental work suggest that microbial communities in this environment will undergo rapid change in response to the altered resources resulting from climate change impacts occurring in this region.     

The Ionic Relations of Acetabularia mediterranea

The concentrations of K⁺, Na⁺, and Cl^– in the cytoplasmand the vacuole of Acetabularia mediterranea have been measured,as have the vacuolar concentrations of SO₄^–– andoxalate. The electrical potential difference between externalsolution, and vacuole and cytoplasm has been measured. The resultsindicate that Cl^– and SO₄^–– are probably transportedactively into the cell, and that active transport of Na⁺ isoutwards. The results for K⁺ are equivocal. The fluxes of K⁺,Na⁺, Cl^–, and S0₄^–– into the cell and theeffluxes of Na⁺ and Cl^– have been determined. The Cl^–fluxes are extremely large. In all cases the plasmalemma isthe rate-limiting membrane for ion movement. A technique isdescribed for the preparation of large, completely viable cellfragments containing only cytoplasm, with no vacuole.     

Responses of Baltic Sea Ice and Open-Water Natural Bacterial Communities to Salinity Change

To investigate the responses of Baltic Sea wintertime bacterial communities to changing salinity (5 to 26 practical salinity units), an experimental study was conducted. Bacterial communities of Baltic seawater and sea ice from a coastal site in southwest Finland were used in two batch culture experiments run for 17 or 18 days at 0°C. Bacterial abundance, cell volume, and leucine and thymidine incorporation were measured during the experiments. The bacterial community structure was assessed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified partial 16S rRNA genes with sequencing of DGGE bands from initial communities and communities of day 10 or 13 of the experiment. The sea ice-derived bacterial community was metabolically more active than the open-water community at the start of the experiment. Ice-derived bacterial communities were able to adapt to salinity change with smaller effects on physiology and community structure, whereas in the open-water bacterial communities, the bacterial cell volume evolution, bacterial abundance, and community structure responses indicated the presence of salinity stress. The closest relatives for all eight partial 16S rRNA gene sequences obtained were either organisms found in polar sea ice and other cold habitats or those found in summertime Baltic seawater. All sequences except one were associated with the α- and γ-proteobacteria or the Cytophaga-Flavobacterium-Bacteroides group. The overall physiological and community structure responses were parallel in ice-derived and open-water bacterial assemblages, which points to a linkage between community structure and physiology. These results support previous assumptions of the role of salinity fluctuation as a major selective factor shaping the sea ice bacterial community structure.     

Plant Responses to Water Stress

This Special Issue comprises a series of papers that developsthe theme of plant responses to water stress, encompassing recentdevelopments at the molecular level, through responses of photosynthesisand metabolism, to their application in crop selection and yield.The consideration of water deficits is particularly timely,given the huge developments in this area in the past decade.This issue specifically sets out to place molecular and physiologicalprocesses and their agronomic applications in an environmentalcontext.     

Morpho-Physiological Responses of Alamo Switchgrass During Germination and Early Seedling Stage Under Salinity or Water Stress Conditions

Switchgrass (Panicum virgatum L.) is a warm perennial grass with valuable characteristics as a biofuel crop. To avoid competition with food crops, biofuel crops will be likely relegated to less productive soils such as marginal lands. Consequently, the salinity and water scarcity problems that commonly affect marginal lands compromise biofuel crop germination, emergence, and seedling establishment. The aims of this study were to study the germination and seedling growth of switchgrass under salinity and water stress and to describe the morpho-anatomical responses of the roots and leaves in the seedlings to these stress conditions. The effect of salt and water stress was assessed using sodium chloride (NaCl) and polyethylene glycol (PEG) 8000 at the same water potentials of ??0.8, ??1.0, and ??1.2 MPa. Seeds were moist prechilled for 7 days at 5 °C and germinated at 30/15 °C (8 h light/16 h dark). NaCl treatments (??0.8 and ??1.0 MPa) delayed germination rates but did not reduce the final germination percentage, whereas at a lower potential (??1.2 MPa), the final germination percentage was diminished. The effects of PEG (??1.0 and ??1.2 MPa) on the germination rate and final percentage were more detrimental than those induced by isosmotic concentrations of NaCl. PEG and NaCl reduced significantly the vigor index of ??0.8 to ??1.2 MPa. The morpho-anatomical changes such as the reduction in the root cross-sectional area and the thickening of the endodermis walls for both stress conditions and aerenchyma formation in the cortex under salinity could significantly contribute in the survival and tolerance during the early seedling stages.