Soil salinity regulates spatial-temporal heterogeneity of seed germination and seedbank persistence of an annual diaspore-trimorphic halophyte in northern China.

BACKGROUND AND AIMS: Seed heteromorphism is a plant strategy that an individual plant produces two or more distinct types of diaspores, which have diverse morphology, dispersal ability, ecological functions and different effects on plant life history traits. The aim of this study was to test the effects of seasonal soil salinity and burial depth on the dynamics of dormancy/germination and persistence/depletion of buried trimorphic diaspores of a desert annual halophyte Atriplex centralasiatica. METHODS: We investigated the effects of salinity and seasonal fluctuations of temperature on germination, recovery of germination and mortality of types A, B, C diaspores of A. centralasiatica in the laboratory and buried diaspores in situ at four soil salinities and three depths. Diaspores were collected monthly from the seedbank from December 2016 to November 2018, and the number of viable diaspores remaining (not depleted) and their germinability were determined. RESULTS: Non-dormant type A diaspores were depleted in the low salinity "window" in the first year. Dormant diaspore types B and C germinated to high percentages at 0.3 and 0.1 mol L-1 soil salinity, respectively. High salinity and shallow burial delayed depletion of diaspore types B and C. High salinity delayed depletion time of the three diaspore types and delayed dormancy release of types B and C diaspores from autumn to spring. Soil salinity modified the response of diaspores in the seedbank by delaying seed dormancy release in autum and winter and by providing a low-salt concentration window for germination of non-dormant diaspores in spring and early summer. CONCLUSIONS: Buried trimorphic diaspores of annual desert halophyte A. centralasiatica exhibited diverse dormancy/germination behavior in respond to seasonal soil salinity fluctuation. Prolonging persistence of the seedbank and delaying depletion of diaspores under salt stress in situ primarily is due to inhibition of dormancy-break. The differences in dormancy/germination and seed persistence in the soil seedbank may be a bet-hadging strategy adapted to stressful temporal and spatial heterogeneity, and allows A. centralasiatica to persist in the unpredictable cold desert enevironment.

Habitat-specific allocations of elements in Atriplex lentiformis seeds indicate adaptation to metal toxicity.

Self-sustaining vegetation in metal-contaminated areas is essential for rebuilding ecological resilience and community stability in degraded lands. Metal-tolerant plants originating from contaminated post-mining areas may hold the key to successful plant establishment and growth. Yet, little is known about the impact of metal toxicity on reproductive strategies, metal accumulation, and allocation patterns at the seed stage. Our research focused on the metal tolerant Atriplex lentiformis. Specifically, we examined the effects of toxic metal(loid) concentration in soils on variability in its reproductive strategies, including germination patterns, elemental uptake, and allocation within the seeds. We employed advanced imaging techniques like synchrotron X-ray fluorescence microscopy (2D scans and 3D tomograms) combined with inductively coupled plasma mass spectrometry to reveal significant differences in metal(loid) concentration and distribution within the seed structures of A. lentiformis from contrasting habitats. Exclusive Zn hotspots of high concentrations were found in the seeds of the metallicolous accession, primarily in the sensitive tissues of shoot apical meristems and root zones of the seed embryos. Our findings offer novel insights into phenotypic variability and metal tolerance and accumulation in plants from extreme environments. This knowledge can be applied to enhance plant survival and performance in land restoration efforts.

Comparing Kinetics of Xylem Ion Loading and Its Regulation in Halophytes and Glycophytes.

Although control of xylem ion loading is essential to confer salinity stress tolerance, specific details behind this process remain elusive. In this work, we compared the kinetics of xylem Na+ and K+ loading between two halophytes (Atriplex lentiformis and quinoa) and two glycophyte (pea and beans) species, to understand the mechanistic basis of the above process. Halophyte plants had high initial amounts of Na+ in the leaf, even when grown in the absence of the salt stress. This was matched by 7-fold higher xylem sap Na+ concentration compared with glycophyte plants. Upon salinity exposure, the xylem sap Na+ concentration increased rapidly but transiently in halophytes, while in glycophytes this increase was much delayed. Electrophysiological experiments using the microelectrode ion flux measuring technique showed that glycophyte plants tend to re-absorb Na+ back into the stele, thus reducing xylem Na+ load at the early stages of salinity exposure. The halophyte plants, however, were capable to release Na+ even in the presence of high Na+ concentrations in the xylem. The presence of hydrogen peroxide (H2O2) [mimicking NaCl stress-induced reactive oxygen species (ROS) accumulation in the root] caused a massive Na+ and Ca2+ uptake into the root stele, while triggering a substantial K+ efflux from the cytosol into apoplast in glycophyte but not halophytes species. The peak in H2O2 production was achieved faster in halophytes (30 min vs 4 h) and was attributed to the increased transcript levels of RbohE. Pharmacological data suggested that non-selective cation channels are unlikely to play a major role in ROS-mediated xylem Na+ loading.

Dynamics of the diaspore and germination stages of the life history of an annual diaspore-trimorphic species in a temperate salt desert.

MAIN CONCLUSION: Individuals of the annual halophyte Atriplex centralasiatica produce three kinds of diaspores that differ in dispersal, dormancy/germination response and type of seed bank formed, which likely is a bet-hedging strategy in the rainfall-unpredictable environment on the semi-arid, saline Ordos Plateau in Inner Mongolia, China. Seasonal fluctuations in environmental conditions provide germination cues for the establishment of seedlings at the right time and place to ensure plant survival and population regeneration. Diaspore heteromorphism is a phenomenon in which diaspores with stark qualitative differences in morphology and ecology are produced by the same maternal plant. Germination responses and dispersal times of the annual halophyte Atriplex centralasiatica were examined to determine the role of diaspore heteromorphism in its adaptation to salt desert conditions. A. centralasiatica is a tumbleweed that produces three types of diaspores that differ in morphology and ecophysiology. The relative potential dispersal ability and intensity of dormancy of the three diaspore types was type A (fan-shaped diaspores with yellow fruits) < type B (fan-shaped diaspores with black fruits) < type C (globular diaspores with black fruits). In the field, type A retained high germinability, but all of them were depleted from the (transient) soil seedbank in the first growing season. Types B and C cycled between dormancy and nondormancy, and 0 and > 90.0% remained in the soil seedbank 2 years after dispersal, respectively. The dormancy, dispersal and salt tolerance of type B diaspores were intermediate between those of A and C. Type A exhibited low dispersal-nondormancy, type B exhibited intermediate dispersal-intermediate dormancy and type C exhibited high dispersal ability-high dormancy. In the unpredictable salt desert habitat, the functional differences in germination and dispersal of the three diaspores act as a bet-hedging mechanism and ensure population establishment in different years by spreading germination over time and space.

The role of seed appendage in improving the adaptation of a species in definite seasons: a case study of Atriplex centralasiatica.

BACKGROUND: As a common accompanying dispersal structure, specialized seed appendages play a critical role in the successful germination and dispersal of many plants, and are regarded as an adaptation character for plants survival in diverse environments. However, little is known about how the appendages modulate the linkage between germination and environmental factors. Here, we tested the responses of germination to seasonal environmental signals (temperature and humidity) via seed appendages using Atriplex centralasiatica, which is widely distributed in salt marshlands with dry-cold winter in northern China. Three types of heteromorphic diaspores that differ in morphology of persistent bracteole and dormancy levels are produced in an individual plant of A. centralasiatica. RESULTS: Except for the nondormant diaspore (type A, with a brown seed enclosed in a persistent bracteole), bracteoles regulated inner seed dormancy of the other two dormant diaspore types, i.e., type B (flat diaspore with a black inner seed) and type C (globular diaspore with a black inner seed). For types B and C, germination of bracteole-free seeds was higher than that of intact diaspores, and was limited severely when incubated in the bracteole-soaking solution. Dormancy was released at a low temperature (< 10 °C) and suitable humidity (5-15%) condition. Oppositely, high temperature and unfit humidity induced secondary dormancy via inhibitors released by bracteoles. Type C with deeper dormancy needed more stringent conditions for dormancy release and was easier for dormancy inducement than type B. The germination windows were broadened and the time needed for dormancy release decreased after the bracteole flushing for the two dormant types in the field condition. CONCLUSIONS: Bracteoles determine the germination adaptation by bridging seeds and environmental signals and promising seedlings establishment only in proper seasons, which may also restrict species geographical distribution and shift species distributing ranges under the global climate change scenarios.

Identification of candidate genes related to salt tolerance of the secretohalophyte Atriplex canescens by transcriptomic analysis.

BACKGROUND: Atriplex canescens is a typical C4 secretohalophyte with salt bladders on the leaves. Accumulating excessive Na+ in tissues and salt bladders, maintaining intracellular K+ homeostasis and increasing leaf organic solutes are crucial for A. canescens survival in harsh saline environments, and enhanced photosynthetic activity and water balance promote its adaptation to salt. However, the molecular basis for these physiological mechanisms is poorly understood. Four-week-old A. canescens seedlings were treated with 100 mM NaCl for 6 and 24 h, and differentially expressed genes in leaves and roots were identified, respectively, with Illumina sequencing. RESULTS: In A. canescens treated with 100 mM NaCl, the transcripts of genes encoding transporters/channels for important nutrient elements, which affect growth under salinity, significantly increased, and genes involved in exclusion, uptake and vacuolar compartmentalization of Na+ in leaves might play vital roles in Na+ accumulation in salt bladders. Moreover, NaCl treatment upregulated the transcripts of key genes related to leaf organic osmolytes synthesis, which are conducive to osmotic adjustment. Correspondingly, aquaporin-encoding genes in leaves showed increased transcripts under NaCl treatment, which might facilitate water balance maintenance of A. canescens seedlings in a low water potential condition. Additionally, the transcripts of many genes involved in photosynthetic electron transport and the C4 pathway was rapidly induced, while other genes related to chlorophyll biosynthesis, electron transport and C3 carbon fixation were later upregulated by 100 mM NaCl. CONCLUSIONS: We identified many important candidate genes involved in the primary physiological mechanisms of A. canescens salt tolerance. This study provides excellent gene resources for genetic improvement of salt tolerance of important crops and forages.

Evaluation of the photosynthetic parameters, emission of volatile organic compounds and ultrastructure of common green leafy vegetables after exposure to non-steroidal anti-inflammatory drugs (NSAIDs).

Understanding the effects of many essential non-steroidal anti-inflammatory drugs (NSAIDs) on plants is still limited, especially at environmentally realistic concentrations. This paper presents the influence of three of the most frequently used NSAIDs (diclofenac, ibuprofen, and naproxen) at environmentally realistic concentrations on the autochthonous green leafy vegetables: orache (Atriplex patula L.), spinach (Spinacia oleracea L.) and lettuce (Lactuca sativa L.). Our research was focused on the determination of the photosynthetic parameters, the emission rate of volatile organic compounds, and the evaluation of the ultrastructure of leaves of studied vegetables after exposure to abiotic stress induced by environmental pollutants, namely NSAIDs. The data obtained indicate a moderate reduction of foliage physiological activity as a response to the stress induced by NSAIDs to the selected green leafy vegetables. The increase of the 3-hexenal and monoterpene emission rates with increasing NSAIDs concentration could be used as a sensitive and a rapid indicator to assess the toxicity of the NSAIDs. Microscopic analysis showed that the green leafy vegetables were affected by the selected NSAIDs. In comparison to the controls, the green leafy vegetables treated with NSAIDs presented irregular growth of glandular trichomes on the surface of the adaxial side of the leaves, less stomata, cells with less cytoplasm, irregular cell walls and randomly distributed chloroplasts. Of the three NSAIDs investigated in this study, ibuprofen presented the highest influence. The results obtained in this study can be used to better estimate the impact of drugs on the environment and to improve awareness on the importance of the responsible use of drugs.

Maturation of Atriplex halimus L. leaves involves changes in the molecular regulation of stomatal conductance under high evaporative demand and high but not low soil water content.

MAIN CONCLUSION: Under high water availability, the maximum gas exchange was observed at noon in the expanding and expanded leaves. The expanded leaves showed lower gas exchange capacity due to the regulation of stomatal-movement genes. Under well-watered condition, stomatal conductance (gs) and photosynthetic rate (A) of expanding and expanded leaves of Atriplex halimus peaked at noon despite the midday decline in the leaf relative water content, suggesting deviation from typical isohydric behaviour. However, the expanding leaves had higher gs and A than the expanded ones. When light intensity was temporarily increased, A and gs were enhanced in both types of leaves though to a higher level in the expanding leaves. In well-watered expanded leaves: (1) A was mainly dependent on gs rather than photosynthetic capacity; gs was controlled by internal factors, thereby limiting water loss via transpiration (E); (2) the accumulation of total soluble sugars (TSS) along with increased Rubisco protein could be a subsidiary factor limiting A; (3) TSS and ABA seem to act in co-ordination to up-regulate ABA-dependent genes controlling gs and (4) the significant induction of DREBs suggests a role in maintaining high relative water content in these leaves compared to the expanding ones. In expanding leaves of well-watered plants, high A along with Rubisco down-regulation and elevated TSS suggests that A was regulated by signals coordinating carbon and nitrogen balance and the elevated ABA could be involved in regulating the hydraulic activity to enhance cell expansion and facilitate leaf growth. Both expanded and expanding leaves behaved in typical isohydric manner under water stress, which did not involve the accumulation of ABA suggesting that stomatal closure was primarily stimulated by hydraulic rather than chemical signals.

Epidermal bladder cells confer salinity stress tolerance in the halophyte quinoa and Atriplex species.

Epidermal bladder cells (EBCs) have been postulated to assist halophytes in coping with saline environments. However, little direct supporting evidence is available. Here, Chenopodium quinoa plants were grown under saline conditions for 5 weeks. One day prior to salinity treatment, EBCs from all leaves and petioles were gently removed by using a soft cosmetic brush and physiological, ionic and metabolic changes in brushed and non-brushed leaves were compared. Gentle removal of EBC neither initiated wound metabolism nor affected the physiology and biochemistry of control-grown plants but did have a pronounced effect on salt-grown plants, resulting in a salt-sensitive phenotype. Of 91 detected metabolites, more than half were significantly affected by salinity. Removal of EBC dramatically modified these metabolic changes, with the biggest differences reported for gamma-aminobutyric acid (GABA), proline, sucrose and inositol, affecting ion transport across cellular membranes (as shown in electrophysiological experiments). This work provides the first direct evidence for a role of EBC in salt tolerance in halophytes and attributes this to (1) a key role of EBC as a salt dump for external sequestration of sodium; (2) improved K+ retention in leaf mesophyll and (3) EBC as a storage space for several metabolites known to modulate plant ionic relations.

Phytostabilization of arsenic in soils with plants of the genus Atriplex established in situ in the Atacama Desert.

In the ChiuChiu village (Atacama Desert, Chile), there is a high concentration of arsenic (As) in the soil due to natural causes related to the presence of volcanoes and geothermal activity. To compare the levels of As and the growth parameters among plants of the same genus, three species of plants were established in situ: Atriplex atacamensis (native of Chile), Atriplex halimus, and Atriplex nummularia. These soils have an As concentration of 131.2 ± 10.4 mg kg(-1), a pH of 8.6 ± 0.1, and an electrical conductivity of 7.06 ± 2.37 dS m(-1). Cuttings of Atriplex were transplanted and maintained for 5 months with periodic irrigation and without the addition of fertilizers. The sequential extraction of As indicated that the metalloid in these soils has a high bioavailability (38 %), which is attributed to the alkaline pH, low organic matter and Fe oxide content, and sandy texture. At day 90 of the assay, the As concentrations in the leaves of A. halimus (4.53 ± 1.14 mg kg(-1)) and A. nummularia (3.85 ± 0.64 mg kg(-1)) were significantly higher than that in A. atacamensis (2.46 ± 1.82 mg kg(-1)). However, the three species accumulated higher levels of As in their roots, indicating a phytostabilization capacity. At the end of the assay, A. halimus and A. nummularia generated 30 % more biomass than A. atacamensis without significant differences in the As levels in the leaves. Despite the difficult conditions in these soils, the establishment of plants of the genus Atriplex is a recommended strategy to generate a vegetative cover that prevents the metalloid from spreading in this arid area through the soil or by wind.

Rapid regulation of the plasma membrane H⁺-ATPase activity is essential to salinity tolerance in two halophyte species, Atriplex lentiformis and Chenopodium quinoa.

BACKGROUND AND AIMS: The activity of H(+)-ATPase is essential for energizing the plasma membrane. It provides the driving force for potassium retention and uptake through voltage-gated channels and for Na(+) exclusion via Na(+)/H(+) exchangers. Both of these traits are central to plant salinity tolerance; however, whether the increased activity of H(+)-ATPase is a constitutive trait in halophyte species and whether this activity is upregulated at either the transcriptional or post-translation level remain disputed. METHODS: The kinetics of salt-induced net H(+), Na(+) and K(+) fluxes, membrane potential and AHA1/2/3 expression changes in the roots of two halophyte species, Atriplex lentiformis (saltbush) and Chenopodium quinoa (quinoa), were compared with data obtained from Arabidopsis thaliana roots. KEY RESULTS: Intrinsic (steady-state) membrane potential values were more negative in A. lentiformis and C. quinoa compared with arabidopsis (-144 ± 3·3, -138 ± 5·4 and -128 ± 3·3 mV, respectively). Treatment with 100 mm NaCl depolarized the root plasma membrane, an effect that was much stronger in arabidopsis. The extent of plasma membrane depolarization positively correlated with NaCl-induced stimulation of vanadate-sensitive H(+) efflux, Na(+) efflux and K(+) retention in roots (quinoa > saltbush > arabidopsis). NaCl-induced stimulation of H(+) efflux was most pronounced in the root elongation zone. In contrast, H(+)-ATPase AHA transcript levels were much higher in arabidopsis compared with quinoa plants, and 100 mm NaCl treatment led to a further 3-fold increase in AHA1 and AHA2 transcripts in arabidopsis but not in quinoa. CONCLUSIONS: Enhanced salinity tolerance in the halophyte species studied here is not related to the constitutively higher AHA transcript levels in the root epidermis, but to the plant's ability to rapidly upregulate plasma membrane H(+)-ATPase upon salinity treatment. This is necessary for assisting plants to maintain highly negative membrane potential values and to exclude Na(+), or enable better K(+) retention in the cytosol under saline conditions.

Comparative studies of C3 and C4 Atriplex hybrids in the genomics era: physiological assessments.

We crossed the C3 species Atriplex prostrata with the C4 species Atriplex rosea to produce F1 and F2 hybrids. All hybrids exhibited C3-like δ(13)C values, and had reduced rates of net CO2 assimilation compared with A. prostrata. The activities of the major C4 cycle enzymes PEP carboxylase, NAD-malic enzyme, and pyruvate-Pi dikinase in the hybrids were at most 36% of the C4 values. These results demonstrate the C4 metabolic cycle was disrupted in the hybrids. Photosynthetic CO2 compensation points (Г) of the hybrids were generally midway between the C3 and C4 values, and in most hybrids were accompanied by low, C3-like activities in one or more of the major C4 cycle enzymes. This supports the possibility that most hybrids use a photorespiratory glycine shuttle to concentrate CO2 into the bundle sheath cells. One hybrid exhibited a C4-like Г of 4 µmol mol(-1), indicating engagement of a C4 metabolic cycle. Consistently, this hybrid had elevated activities of all measured C4 cycle enzymes relative to the C3 parent; however, C3-like carbon isotope ratios indicate the low Г is mainly due to a photorespiratory glycine shuttle. The anatomy of the hybrids resembled that of C3-C4 intermediate species using a glycine shuttle to concentrate CO2 in the bundle sheath, and is further evidence that this physiology is the predominant, default condition of the F2 hybrids. Progeny of these hybrids should further segregate C3 and C4 traits and in doing so assist in the discovery of C4 genes using high-throughput methods of the genomics era.

[Anatomy of the vegetative organs of two species of Atriplex (Chenopodiaceae) from Venezuela]. / Anatomía de los órganos vegetativos de dos especies de Atriplex (Chenopodiaceae) de Venezuela.

In Venezuela, Atriplex is represented by A. cristata and A. oestophora, the latter being endemic; they inhabit coastal areas with high temperatures, high solar radiation and sandy soils with high salt content. This work aimed to provide information to facilitate and clarify these species taxonomic delimitation, throughout the study of the anatomy of their vegetative organs; this may also clarify our understanding of their adaptability to soil and climatic conditions prevailing in areas they inhabit. The plant material was collected from at least three individuals of each species in Punta Taima Taima and Capatárida, Falcon. Segments of roots, located near the neck and towards the apex, apical, middle and basal internodes of stems, were taken; and of leaves, located in the middle portion of plants. This material was fixed in FAA (formaldehyde, acetic acid, 70% ethanol) until processing. Semipermanent and permanent microscope slides were prepared with transverse or longitudinal sections, made using a razor (free-hand) or a rotation microtome, in this latter case, after paraffin embedding; besides, additional plates were mounted with portions of leaf epidermis, obtained by the maceration technique. The sections were stained with aqueous toluidine blue (1%) or safranin-fast-green, and mounted in water-glycerin or in Canada balsam. In order to calculate the vulnerability index, the vessel diameter in the vascular rings of roots, as well as their density, were quantified. Our results revealed structural features in the different organs, that resulted of taxonomic value and allowed the distinction of the species: in the leaf, the presence of aquifer tissue, the number of vascular bundles and their organization in the midrib, and the collenchyma differentiation in this part of the leaf; in the roots, the xylem and phloem arrangement in the growth rings, the nature of conjunctive tissue, and the presence of included phloem in one species. In addition, the species showed typical anatomical features of halophytes and xerophytes, such as: high density of trichomes on leaves and young stems which act as salt secreting glands, abundant sclerenchyma in stems and roots, water storage tissue and Kranz anatomy in leaves, narrow cortical region in young roots, presence of cambial variants in stems and roots, as well as short and narrow xylem vessels. Vulnerability index calculations indicated that both species tend to assure conduction but not the efficiency of the system. Atriplex species have anatomical characters which facilitate their adaptation to the special conditions prevailing in their habitats and that may be used for taxonomic delimitation.

Polyploidy enhances the occupation of heterogeneous environments through hydraulic related trade-offs in Atriplex canescens (Chenopodiaceae).

Plant hydraulic characteristics were studied in diploid, tetraploid and hexaploid cytotypes of Atriplex canescens (Chenopodiaceae) to investigate the potential physiological basis underlying the intraspecific habitat differentiation among plants of different ploidy levels. Populations of A. canescens from different habitats of the Chihuahuan Desert (New Mexico, USA) were analyzed using flow cytometry to determine ploidy levels. Traits related to xylem water transport efficiency and safety against drought-induced hydraulic failure were measured in both stems and leaves. At the stem level, cytotypes of higher ploidy showed consistently lower leaf-specific hydraulic conductivity but greater resistance to drought-induced loss of hydraulic conductivity. At the leaf level, comparisons in hydraulics between cytotypes did not show a consistent pattern, but exhibited high plasticity to proximal environmental conditions related to soil water availability. The results suggest that a trade-off between stem hydraulic efficiency and safety across ploidy levels underlies niche differentiation among different cytotypes of A. canescens. Polyploidization may have been facilitated by environmental heterogeneity related to water availability, and variation in water-related physiology found in the present study suggests an important functional basis for the niche differentiation and coexistence of A. canescens cytotypes in desert environments.

Effects of grazing on leaf area index, fractional cover and evapotranspiration by a desert phreatophyte community at a former uranium mill site on the Colorado Plateau.

This study employed ground and remote sensing methods to monitor the effects of grazing on leaf area index (LAI), fractional cover (f(c)) and evapotranspiration (ET) of a desert phreatophyte community over an 11 year period at a former uranium mill site on the Colorado Plateau, U.S. Nitrate, ammonium and sulfate are migrating away from the mill site in a shallow alluvial aquifer. The phreatophyte community, consisting of Atriplex canescens (ATCA) and Sarcobatus vermiculatus (SAVE) shrubs, intercepts groundwater and could potentially slow the movement of the contaminant plume through evapotranspiration (ET). However, the site has been heavily grazed by livestock, reducing plant cover and LAI. We used livestock exclosures and revegetation plots to determine the effects of grazing on LAI, f(c) and ET, then projected the findings over the whole site using multi-platform remote sensing methods. We show that ET is approximately equal to annual precipitation at the site, but when ATCA and SAVE are protected from grazing they can develop high f(c) and LAI values, and ET can exceed annual precipitation, with the excess coming from groundwater discharge. Therefore, control of grazing could be an effective method to slow migration of contaminants at this and similar sites in the western U.S.

Plant responses to heterogeneous salinity: growth of the halophyte Atriplex nummularia is determined by the root-weighted mean salinity of the root zone.

Soil salinity is generally spatially heterogeneous, but our understanding of halophyte physiology under such conditions is limited. The growth and physiology of the dicotyledonous halophyte Atriplex nummularia was evaluated in split-root experiments to test whether growth is determined by: (i) the lowest; (ii) the highest; or (iii) the mean salinity of the root zone. In two experiments, plants were grown with uniform salinities or horizontally heterogeneous salinities (10-450 mM NaCl in the low-salt side and 670 mM in the high-salt side, or 10 mM NaCl in the low-salt side and 500-1500 mM in the high-salt side). The combined data showed that growth and gas exchange parameters responded most closely to the root-weighted mean salinity rather than to the lowest, mean, or highest salinity in the root zone. In contrast, midday shoot water potentials were determined by the lowest salinity in the root zone, consistent with most water being taken from the least negative water potential source. With uniform salinity, maximum shoot growth was at 120-230 mM NaCl; ~90% of maximum growth occurred at 10 mM and 450 mM NaCl. Exposure of part of the roots to 1500 mM NaCl resulted in an enhanced (+40%) root growth on the low-salt side, which lowered root-weighted mean salinity and enabled the maintenance of shoot growth. Atriplex nummularia grew even with extreme salinity in part of the roots, as long as the root-weighted mean salinity of the root zone was within the 10-450 mM range.

Differential salt tolerance in seedlings derived from dimorphic seeds of Atriplex centralasiatica: from physiology to molecular analysis.

Seed dimorphism provides plants with alternative strategies for survival in unfavorable environments. Here, we investigated the physiological responses and differential gene expression caused by salinity exposure in Atriplex centralasiatica plants grown from the two different seed morphs. Seedlings derived from yellow seeds (YS) showed a greater salt tolerance than those derived from brown seeds (BS). Salt treatment induced nitric oxide (NO) synthesis in roots, and seedlings derived from YS produced greater amounts of NO than did those from BS. Analyses of NO scavenging during salt stress revealed that NO contributed to the differential salt tolerance in seedlings derived from the two seed morphs by modulating antioxidative enzyme activity, hydrogen peroxide accumulation and the ion equilibrium. We also applied transcriptomics and subsequent microarray analysis to evaluate the differential gene expression during salt treatment. These genes encoded proteins related to osmotic and ionic homeostasis, redox equilibrium and signal transduction. A select group of genes including GH3.3, CAT1/2, TIP1, SIHP1 and EXP1 were further confirmed with RT-PCR analysis. These results revealed that the enhanced salt tolerance of seedlings from YS appeared to be governed by a superior ability to achieve ionic homeostasis and redox equilibrium, a rapid response to salt stress, and ultimately better growth potential. NO serves as a vital regulator in these processes.

The role of pollen limitation on the coexistence of two dioecious, wind-pollinated, closely related shrubs in a fluctuating environment.

Elucidating the mechanisms of species coexistence is a crucial goal in ecology. Theory suggests that, when resource abundance fluctuates, coexistence can be achieved if each species in a competing pair is better at exploiting resources at opposite extremes of a fluctuating resource spectrum. Nonetheless, the proximal mechanisms allowing coexistence remain largely unexplored. In a previous paper, we showed that the coexistence of two Atriplex species was facilitated by their varying demographic response (in survival, growth and recruitment) to fluctuation in water availability. Here we explore the effect of spatial distribution, and pollen and resource limitation on the reproductive success (production of viable seeds) of the same two species. An analysis of their spatial distribution showed that Atriplex acanthocarpa had a clumped distribution, which is thought to increase the effectiveness of pollination in wind-pollinated plants, while Atriplex canescens had a random distribution, a pattern expected to restrict wind-pollination success. A pollen and resource (water and nutrients) addition experiment implemented through a repeated-measures design demonstrated that seed viability of A. canescens was both pollen and resource limited, but that these effects were negligible in A. acanthocarpa. Under natural conditions, pollen limitation restricted seed number in A. canescens to only one-third of that recorded when manual pollination was performed. By decreasing its fecundity (and consequent potential seedling recruitment), pollen limitation reverses the competitive advantage of A. canescens over A. acanthocarpa when the limiting resource (water) is abundant and seedling recruitment takes place. To our knowledge, our study of this congeneric pair in the Chihuahuan Desert is the first to document a link between pollen limitation and species coexistence.

Influence of vesicular trichomes of Atriplex nummularia on photosynthesis, osmotic adjustment, cell wall elasticity and enzymatic activity.

Vesicular trichomes play a key role in excluding toxic ions from some halophyte species, preventing the essential processes and functions of plants from being altered. Thus, the present study aimed to evaluate the influence of these structures on Atriplex nummularia irrigated using waters with three levels of osmotic potential (-0.1, -1.4 and -2.7 MPa), formulated with NaCl in plants with vesicular trichomes and plants with partial removal of trichomes. The experiment was conducted in a protected environment and plants were evaluated for physiological parameters (water, osmotic and pressure potentials, relative water content, osmotic adjustment, pressure-volume curve, gas exchange), electrolyte leakage, lipid peroxidation and enzymatic activity (superoxide dismutase, ascorbate peroxidase, catalase). The results obtained made it possible to identify the strong contribution of vesicular trichomes to physiological and biochemical parameters, with indication of cell wall stiffening and maintenance of turgor. Furthermore, the evaluation of the osmotic potentials obtained in the study suggests that the contribution of vesicular trichomes to the salinity tolerance of the species is greater than that of osmotic adjustment. Furthermore, gas exchange results suggest that the presence of trichomes was able to regulate stomatal processes so that the plant maintains its photosynthetic performance. Evaluation of electrolyte leakage, together with the increase in malondialdehyde content, showed that the maintenance of trichomes reduces the probability of oxidative stress. The activity of antioxidant enzymes was efficient in eliminating reactive oxygen species, especially the activity of ascorbate peroxidase, which stood out in terms of hydrogen peroxide detoxification.

Abscisic acid has contrasting effects on salt excretion and polyamine concentrations of an inland and a coastal population of the Mediterranean xero-halophyte species Atriplex halimus.

BACKGROUND AND AIMS: Different populations of the Mediterranean xerohalophyte species Atriplex halimus exhibit different levels of resistance to salt and osmotic stress depending on the nature of the osmocompatible solute they accumulate. There is, however, no conclusive description of the involvement of abscisic acid (ABA) in the plant response to NaCl or osmotic stress in this species. METHODS: Seedlings issued from an inland water-stress-resistant population (Sbikha) and from a coastal salt-resistant one (Monastir) were exposed in nutrient solutions to NaCl (40 or 160 mm) or to 15 % PEG for 1 d and 10 d in the presence or absence of 50 microm ABA. KEY RESULTS: Plants from Sbikha accumulated higher amounts of ABA in response to osmotic stress than those of Monastir, while an opposite trend was recorded for NaCl exposure. Exogenous ABA improved osmotic stress resistance in Monastir through an improvement in the efficiency of stomatal conductance regulation. It also improved NaCl resistance in Sbikha through an increase in sodium excretion through the external bladders. It is suggested that polyamines (spermidine and spermine) are involved in the salt excretion process and that ABA contributes to polyamine synthesis as well as to the conversion from the bound and conjugated to the free soluble forms of polyamine. Proline accumulated in response to osmotic stress and slightly increased in response to ABA treatment while glycinebetaine accumulated in response to salinity and was not influenced by ABA. CONCLUSIONS: It is concluded that ABA is involved in both salt and osmotic stress resistance in the xerohalophyte species Atriplex halimus but that it acts on different physiological cues in response to those distinct environmental constraints.