Oxygen dynamics in a salt-marsh soil and in Suaeda maritima during tidal submergence

Habitats occupied by many halophytes are not only saline, but are also prone to flooding and yet surprisingly few studies have evaluated submergence tolerance in halophytes. Sediment, floodwater, and intra-plant O₂ dynamics were evaluated during tidal submergence for the leaf-succulent halophyte Suaeda maritima (L.) Dum. For S. maritima growing in soil just above the mud flat in a UK salt marsh, the soil was only moderately hypoxic just prior to tidal inundation, presumably owing to drainage and O₂ entry facilitated by frequent, large cracks. O₂ declined to very low levels following high tide. By contrast, mud flat sediment remained waterlogged, lacked cracks, and was anoxic. Plant O₂ dynamics were investigated using field-collected plants in sediment blocks transported to a controlled-submergence system in a glasshouse. Submergence during night-time resulted in anoxia within leaves, whereas during day-time O₂ was produced by underwater photosynthesis. The thin lateral roots of S. maritima presumably access some O₂ from hypoxic sediments, but could also experience transient episodes of severe hypoxia/anoxia, especially as any internal O₂ movement from shoots would be small owing to the low gas-filled porosity in roots. Fermentative metabolism to lactate, producing some ATP in the absence of O₂, might contribute to tolerance of transient O₂ deficits. Lactate was high in root tissues, whereas ethanol production (tissue and incubation medium contents) was low, both in comparison with values reported for other species. Our findings demonstrate the importance of tolerance to transient waterlogging and submergence for the halophyte S. maritima growing in a tidal salt marsh.     

Root morphology is related to the phenotypic variation in waterlogging tolerance of two populations of Suaeda salsa under salinity

The effects of waterlogging and salinity on seedling emergence, seedling growth and ion accumulation in a euhalophyte Suaeda salsa in an intertidal zone and on saline inland soil were investigated. Seedlings of S. salsa from the intertidal zone emerged more rapidly than those of the inland population under both waterlogged and drained conditions. Waterlogging and salinity had no adverse effects on seedling emergence of S. salsa from the intertidal zone, but markedly inhibited this parameter in the inland population. Waterlogging did not affect the seedling survival, shoot dry mass, and shoot height in high salinity in S. salsa from the intertidal zone, while the opposite trend was shown in the inland population. The root dry mass was higher in S. salsa from the intertidal zone as compared to the inland population, in waterlogged treatments by 1.9, 1.3, and 1.5 times in 1, 200, and 600 mM NaCl, respectively, and in drained treatments by 1.8, 2.3, and 3.0 times in 1, 200, and 600 mM NaCl, respectively. Waterlogging increased Na⁺ and K⁺ concentrations in high salinity, but waterlogging had no effect on Cl^- concentration in shoots of S. salsa from the intertidal zone. In all NaCl treatments, waterlogging had no effect on concentrations of these ions in shoots of S. salsa from the saline inland site. In a field investigation, the fresh mass of shoots and roots were lower, whereas the root/shoot ratio was 1.5 times higher in S. salsa from the intertidal zone, compared with the inland population. These findings indicate that S. salsa population from the intertidal zone is more waterlogging tolerant than the inland population. S. salsa from the intertidal zone produced relatively more root biomass and this might help anchor plants against tidal action in the intertidal zone. The physiological and morphological characteristics may determine the natural distributions of the two S. salsa populations in their different saline environments.     

Growth and physiological responses of two mangrove species (Bruguiera gymnorrhiza and Kandelia candel) to waterlogging

Effects of duration of waterlogging on growth and physiological responses of two mangrove species, Bruguiera gymnorrhiza and Kandelia candel, were investigated. The relative growth rate of B. gymnorrhiza decreased significantly with waterlogged time, with the highest value found for drained plants and the lowest in plants under 12 weeks waterlogging. On the contrary, no significant difference was found between waterlogged and drained K. candel plants. The shoot to root biomass ratio of K. candel increased when subjected to 8 or 12 weeks waterlogging but little change was recorded in B. gymnorrhiza, indicating a shift in biomass allocation from roots to shoots in K. candel under prolonged waterlogging but not in B. gymnorrhiza. These different growth responses between the two mangrove species supported the hypothesis that K. candel is more tolerant to waterlogging than B. gymnorrhiza. Under 12 weeks waterlogged treatment, root oxidase activity significantly decreased in B. gymnorrhiza but increased in K. candel. Chlorophyll contents of K. candel increased more rapidly in response to waterlogging than B. gymnorrhiza. Activities of both peroxidase and superoxide dismutase increased significantly in leaves of K. candel when the waterlogging period was longer than 8 weeks, while only the peroxidase activity of B. gymnorrhiza showed a significant increase, indicating that K. candel had stronger resistance to the oxidant damage resulting from waterlogging. These physiological indicators further supported the hypothesis that K. candel is more tolerant to waterlogging than B. gymnorrhiza.     

The effect of temperature on salt uptake by tomato plants with diurnal and noctural waterlogging of salinized rootzones

Summary Tomato plants were grown at three constant temperatures (10, 20 and 28°C) with drained or waterlogged rootzones and were irrigated with saline solution (0.09M NaCl).Each increase in temperature resulted in an increase in leaf Na-ion and Cl-ion concentrations in plants grown with drained rootzones. However, with plants grown with waterlogged rootzones, maximum leaf concentrations of Na-ions and Cl-ions occurred at 20°C.At 10°C there were no differences between Na-ion and Cl-ion concentrations for drained or waterlogged treatments. At 20 and 28°C, waterlogging of the rootzone resulted in significantly higher concentrations of Na-ions and Cl-ions in leaf and stem tissues than occurred with drained rootzones.There were no differences in Na-ions and Cl-ions and Cl-ions in plant tops if plants were waterlogged with saline solution during the day or night.Transpiration increased significantly with each increase in temperature but showed no other treatment dependent responses.     

Copper uptake kinetics in hydroponically-grown durum wheat (Triticum turgidum durum L.) as compared with soil’s ability to supply copper

This paper focuses on the causes of zonation on agricultural land affected by secondary salinity between two halophytic grasses, puccinellia (Puccinellia ciliata Bor. cv. Menemen) and tall wheatgrass (Thinopyrum ponticum (Podp.) Z.-W. Liu & R.R.-C. Wang cv. Tyrrell). We hypothesized that the differences in zonation of puccinellia and tall wheatgrass were caused primarily by differences in the tolerance of these two species to waterlogging under saline conditions. This hypothesis was tested by conducting experiments in the field and in the glasshouse in irrigated sand cultures. At a saltland field site, locations dominated by puccinellia had EC_e values that were consistently higher (11–12 dS/m in early spring, and 5–9 dS/m in late summer) than locations dominated by tall wheatgrass. However locations dominated by puccinellia also had a watertable that was shallower (0.07–0.09 m in the high rainfall season; 0.11–0.13 m in the low rainfall season) than locations dominated by tall wheatgrass. In the glasshouse both species had similar growth responses to salinity under drained conditions, with a 50% decrease in shoot dry mass (DM) at ～300 mM NaCl. However, the combination of salinity (250 mM NaCl) and waterlogging increased puccinellia shoot DM by 150% but decreased shoot DM of tall wheatgrass by 90% (compared with salinity alone). Under saline/waterlogged conditions, puccinellia showed better exclusion of Na⁺ and maintenance of K⁺/Na⁺ in the shoots than tall wheatgrass. We conclude that the zonation of puccinellia and tall wheatgrass is associated with differences in their ion regulation which leads to substantial differences in their growth under saline/waterlogged conditions.     

High phenotypic plasticity of Suaeda maritima observed under hypoxic conditions in relation to its physiological basis

Background and Aims

Phenotypic plasticity, the potential of specific traits of a genotype to respond to different environmental conditions, is an important adaptive mechanism for minimizing potentially adverse effects of environmental fluctuations in space and time. Suaeda maritima shows morphologically different forms on high and low areas of the same salt marsh. Our aims were to examine whether these phenotypic differences occurred as a result of plastic responses to the environment. Soil redox state, indicative of oxygen supply, was examined as a factor causing the observed morphological and physiological differences.

Methods

Reciprocal transplantation of seedlings was carried out between high and low marsh sites on a salt marsh and in simulated tidal-flow tanks in a glasshouse. Plants from the same seed source were grown in aerated or hypoxic solution, and roots were assayed for lactate dehydrogenase (LDH) and alcohol dehydrogenase, and changes in their proteome.

Key Results

Transplanted (away) seedlings and those that remained in their home position developed the morphology characteristic of the home or away site. Shoot Na⁺, Cl⁻ and K⁺ concentrations were significantly different in plants in the high and low marsh sites, but with no significant difference between home and away plants at each site. High LDH activity in roots of plants grown in aeration and in hypoxia indicated pre-adaptation to fluctuating root aeration and could be a factor in the phenotypic plasticity and growth of S. maritima over the full tidal range of the salt marsh environment. Twenty-six proteins were upregulated under hypoxic conditions.

Conclusions

Plasticity of morphological traits for growth form at extremes of the soil oxygenation spectrum of the tidal salt marsh did not correlate with the lack of physiological plasticity in the constitutively high LDH found in the roots.     

Effect of salinity and waterlogging on growth,anatomical and antioxidative responses in <Emphasis Type="Italic">Mentha aquatica</Emphasis> L.

Salinity and waterlogging are two stresses which in nature often occur simultaneously. In this work, effects of combined waterlogging and salinity stresses are studied on the anatomical alteration, changes of enzymatic antioxidant system and lipid peroxidation in Mentha aquatica L. plants. Seedlings were cultured in half-strength Hoagland medium 50 days after sowing, and were treated under combination of three waterlogging levels (well drained, moderately drained and waterlogging) and NaCl (0, 50, 100, 150 mM) for 30 days. Moderately drained and waterlogging conditions induced differently aerenchyma formation in roots of M. aquatica salt-treated and untreated plants. Moreover, stele diameter and endodermis layer were also affected by salt stress and waterlogging. Salt stress significantly decreased growth, relative water content (RWC), protein level, catalase (CAT) and polyphenol oxidase (PPO) activities, and increased proline content, MDA content, H₂O₂ level and activities of superoxide dismutase (SOD), peroxidase (POX), and ascorbate peroxidase (APX). Waterlogging in salt-untreated plants increased significantly growth parameters, RWC, protein content, antioxidant enzyme activity, and decreased proline content, H₂O₂ and MDA levels. In salt-treated plant, waterlogging caused strong induction of antioxidant enzymes activities especially at severe stress condition. These results suggest M. aquatica is a waterlogging tolerant plant due to significant increase of antioxidant activity, membrane stability and growth under water stress. High antioxidant capacity under waterlogging can be a protective strategy against oxidative damage, and help to salt stress alleviation.     

Antioxidative responses and morpho-anatomical adaptations to waterlogging in Sesbania virgata

Sesbania virgata (Leguminosae) is tolerant of long periods of soil inundation. However, its morphological adaptations to anoxia and its response to possible damage from oxidative stress are still unknown. Here, we provide new information that helps to explain the ability of S. virgata plants to grow in flooded environments. Plants containing six expanded leaves were placed in masonry tanks and were subjected to the following conditions: control (well watered), soil waterlogging (water to the setup level of 1 cm above the soil surface—roots and parts of the stems flooded), and complete submergence (whole plant flooded). Plants exposed to flooding (soil waterlogging and complete submergence) significantly increased their production of hydrogen peroxide (H₂O₂), indicating the extent of oxidative injury posed by stress conditions. We demonstrate that plants exposed to flooding develop an efficient scavenger of ROS (generated during stress) in the roots through the coordinated action of nonenzymatic ascorbic acid (Asc) and dehydroascorbate (DHA) as well as the enzymatic antioxidants superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) that are present in the tissues. Moreover, we observed the development of morpho-anatomical structures such as adventitious roots, lenticels, and cracks in the stem of plants under soil waterlogging. The secondary root of plants under soil waterlogging showed a thinner cortex and larger number of elements of small diameter vessels. Numerous aerenchymas were observed in the newly formed in the adventitious roots. We conclude that these antioxidative responses and morpho-anatomical adaptations in the roots are part of a suite of adaptations that allow S. virgata plants to survive long periods of flooding, notably under waterlogged conditions.     

Waterlogging Stress and Ethylene Production in the Dune Slack Plant, Scirpus americanus

The sedge, Scirpus americanus Pers., grows in dune slacks andother freshwater and brackish water wetland communities. Whenwaterlogged in a greenhouse, the concentration of ethylene increased4-fold in stems of S. americanus plants. This increase was associatedwith a decrease in plant height and an increase in aerenchymaas exhibited under waterlogged conditions. Endogenous ethyleneproduction in S. americanus was compared to that in anotherdune slack species, Panicum amarulum, and also to Spartina aherniflorafrom a salt marsh. These species did not respond by increasingendogenous ethylene upon waterlogging. In the field, a 16 cmrainfall significantly increased the endogenous ethylene productionin S. americanus. As the water table subsided the concentrationof accumulated ethylene in stem tissue decreased. Exposure ofS. americanus to exogenous ethylene inhibited stem extensionand increased aerenchyma formation, thus linking ethylene tothe morphological characteristics of waterlogged plants of thisspecies. These experiments support the hypothesis that ethylenemodulates S. americanus morphology in natural waterlogged environmentsand may be of importance in adapting this species to life inthe wetland environment. Key words: Dune slack, waterlogging, ethylene     

Review of wheat improvement for waterlogging tolerance in Australia and India: the importance of anaerobiosis and element toxicities associated with different soils

Background and Aims

The lack of knowledge about key traits in field environments is a major constraint to germplasm improvement and crop management because waterlogging-prone environments are highly diverse and complex, and the mechanisms of tolerance to waterlogging include a large range of traits. A model is proposed that waterlogging tolerance is a product of tolerance to anaerobiosis and high microelement concentrations. This is further evaluated with the aim of prioritizing traits required for waterlogging tolerance of wheat in the field.

Methods

Waterlogging tolerance mechanisms of wheat are evaluated in a range of diverse environments through a review of past research in Australia and India; this includes selected soils and plant data, including plant growth under waterlogged and drained conditions in different environments. Measurements focus on changes in redox potential and concentrations of diverse elements in soils and plants during waterlogging.

Key Results

(a) Waterlogging tolerance of wheat in one location often does not relate to another, and (b) element toxicities are often a major constraint in waterlogged environments. Important element toxicities in different soils during waterlogging include Mn, Fe, Na, Al and B. This is the first time that Al and B toxicities have been indicated for wheat in waterlogged soils in India. These results support and extend the well-known interactions of salinity/Na and waterlogging/hypoxia tolerance.

Conclusions

Diverse element toxicities (or deficiencies) that are exacerbated during waterlogging are proposed as a major reason why waterlogging tolerance at one site is often not replicated at another. Recommendations for germplasm improvement for waterlogging tolerance include use of inductively coupled plasma analyses of soils and plants.Key words: Waterlogging, microelements, toxicity, redox potential, wheat, anaerobiosis

‘No grain is ever produced without water, but too much water tends to spoil the grain and inundation is as injurious to growth as dearth of water.’ Narada Smriti XI, 19; circa 3000 bc.‘Waterlogging’ is defined as a condition of the soil where excess water limits gas diffusion; while ‘waterlogging tolerance’ is defined as survival or the maintenance of high growth rates, biomass accumulation or grain yield under waterlogging relative to non waterlogged (usually drained soil) conditions (Setter and Waters, 2003).

     

Effects of waterlogging and salinity on plant-water relations and on the accumulation of solutes in three mangrove species

Dispersal units of Avicennia marina (Forsk.) Vierh., Rhizophora mucronata Lam. and Bruguiera gymnorrhiza (L.) Lam. were cultivated in mangrove sediment under greenhouse conditions. After 7 months plants were subjected to the following waterlogging and salinity treatments for 60 days: drained non-saline, drained saline, waterlogged non-saline and waterlogged saline. Measurements were made of stomatal resistance (r_s), tissue water potentials, relative water content, and the concentration of Na, K, Ca and Mg in plant organs. Measurements were made 4 days before the termination of waterlogging and continued for 12 days thereafter. Generally, the highest stomatal resistance occurred in the waterlogged saline treatments and the least stomatal resistance occurred in the drained non-saline treatments. Water potentials were usually lower in the saline treatments than in the non-saline treatments. Depression in water potential was greatest in Avicennia and least in Bruguiera. In all 3 species, the concentration of cations was high in the roots. Salinity had greater effects on the concentration of ions in Avicennia than in Rhizophora or Bruguiera. The low stomatal resistance, low tissue water potentials, high relative water content and high tissue cation concentration in Avicennia suggest that it is ideally suited as the pioneer in the mangrove association. Generally, Rhizophora was more tolerant of salinity than Bruguiera. The responses of the plants reveal the remarkable adaptability of mangroves to the saline environment. The adaptive features of each species determine its characteristic habitat in intertidal areas.     

The influence of water level on the growth and photosynthesis of Hydrocotyle ranunculoides L.fil.

Floating Pennywort (Hydrocotyle ranunculoides L.fil.), a native to North America and naturalized in Central and South America, is an invasive aquatic weed in western Europe and several other regions worldwide. H. ranunculoides settles primarily in stagnant to slow-flowing waters (e.g. ditches, canals, rivers, lakes and ponds). The species prefers sunny and nutrient-rich sites and forms dense interwoven mats, which can quickly cover the surface of infested waters. In this study, the effect of three different water levels on growth of Floating Pennywort was investigated. Plants were cultivated on high-nutrient soils under waterlogged, semi-drained and drained conditions. Highest relative growth rates (RGR) of 0.097±0.004 g g⁻¹ dw d⁻¹ were reached under waterlogged conditions. This was significantly higher than RGR of plants cultivated semi-drained and drained. Floating Pennywort showed some phenological adaptations to drained soil conditions, including significant differences in the relative amounts of leaf, petiole and shoot biomass, whilst the relative amount of root biomass was not significantly influenced by the water level. Furthermore, Floating Pennywort reached under drained conditions lower relative water contents (RWC) of leaves, petioles and shoots, a significant shorter length of internodes, a significant lower extent of shoot porosity (POR), a lower chlorophyll content and an increased Chl_a:Chl_b ratio. In addition, maximum gas exchange of drained cultivated plants is significantly lower, due to strongly decreased leaf conductance under reduced water availability. Overall, H. ranunculoides showed ability to grow under different water levels, but performed best under waterlogged conditions.     

Constraints of root response to waterlogging in Alisma triviale

To understand the economics of root aerenchyma formation in wetland plants, we investigated in detail the response of Alisma triviale to waterlogging. We hypothesized costs being associated with development of a large root air space. In three out-door pot experiments, seedlings (1 experiment) and mature plants (2 experiments) were grown under waterlogged and drained conditions for up to 2?months. Waterlogging promoted growth, and was associated with increased root porosity and decreased root density (fresh mass per volume). The increased formation of aerenchyma was associated with a higher root dry matter content for a given root density. Despite improved growth and earlier flowering, the waterlogged plants also showed signs of being constrained by the anoxic substrate, such as shallower roots, and a higher leaf dry matter content. The formation of aerenchyma was associated with costs, such as increased root dry matter content and reduced metaxylem vessel diameter. The faster growth of the seedlings under the waterlogged conditions, despite some signs of being stressed, was possibly a result of decreased requirements to allocate biomass below ground. In mature plants the increased aerenchyma allowed deeper root penetration, and ameliorated the effects of anoxia, reducing the differences in plant traits between the treatments.     

Roots of willow (Salix viminalis L.) show marked tolerance to oxygen shortage in flooded soils and in solution culture

Responses to soil flooding and oxygen shortage were studied in field, glasshouse and controlled environment conditions. Established stools ofSalix viminalis L., were compared at five field sites in close proximity but with contrasting water table levels and flooding intensities during the preceding winter. There was no marked effect of site on shoot extension rate, time to half maximum length or final length attained. When rooted cuttings were waterlogged for 4 weeks in a glasshouse, soil redox potentials quickly decreased to below zero. Shoot extension was slowed after a delay of 20 d, while, in the upper 100 mm of soil, formation and outgrowth of unbranched adventitious roots with enhanced aerenchyma development was promoted after 7 d. At depths of 100–200 mm and 200–300 mm, extension by existing root axes was halted by soil flooding, while adventitious roots from above failed to penetrate these deeper zones. After 4 weeks waterlogging, all arrested root tips recommenced elongation when the soil was drained; their extension rates exceeding those of roots that were well-drained throughout. Growth in fresh mass was also stimulated. The additional aerenchyma found in adventitious roots in the upper 100 mm of soil may have been ethylene regulated since gas space development was inhibited by silver nitrate, an ethylene action inhibitor. The effectiveness of aerenchyma was tested by blocking the entry of atmospheric oxygen into plants with lanolin applied to lenticels of woody shoots of plants grown in solution culture. Root extension was halved, while shoot growth remained unaffected. H Lambers Section editor     

Effects of Waterlogging on Water Relations of Actively-growing and Dormant Sitka Spruce Seedlings

Two-year-old Sitka spruce [Picea sitchensis (Bong.) Carr.] seedlings,either actively growing or dormant, were waterlogged in a growthroom at 15 °C. Shoot and root growth, transpiration andleaf water potential were observed. In actively-growing plants shoot extension continued after waterlogging,though at a reduced rate, and shoots of dormant plants brokebud and extended during the waterlogging period. Root growthwas suppressed by waterlogging in both types of plant. The 22day waterlogging treatment eventually killed the actively-growingplants but plants which were dormant at the time of waterloggingwere more tolerant. Changes in plant water relations after waterloggingwere entirely different depending on the condition of the plantswhen the soil was flooded. Dormant plants showed a gradual reductionin transpiration and increased water stress over the waterloggingperiod; after the soil was drained leaf water potential increasedto equal the value of control plants which had been maintainedin a freely drained condition, but transpiration did not increaseuntil root growth began. Actively-growing plants exhibited amore complex behaviour, characterized by a very rapid reductionin transpiration after waterlogging, accompanied by a briefperiod of water stress, followed by a period of increasing transpirationrate in the absence of water stress. Finally a second reductionin transpiration occurred and water stress increased as theseedlings died. The importance of the stage of activity of theroot system to the response of plants to waterlogging is discussed. Picea sitchensis (Bong.) Carr., Sitka spruce, waterlogging, water relations, dormancy, transpiration, water potential     

Nutritional and chlorophyll fluorescence responses of lucerne (<Emphasis Type="Italic">Medicago sativa</Emphasis>) to waterlogging and subsequent recovery

Periodic flooding of perennial crops such as lucerne (Medicago sativa,L) is a major cause of lowered productivity and leads in extreme cases to plant death. In this study, effects of waterlogging and subsequent recovery on plant nutrient composition and PSII photochemistry were studied to gain a better understanding of the mechanisms of recovery as they relate to leaf photochemistry (chlorophyll fluorescence) and nutrient dynamics. Three lucerne cultivars and one breeding line were flooded for 20 d, drained and left to recover for another 16 d under glasshouse conditions. Leaf and root nutrient composition (P, K, Ca, Mg, B, Cu and Zn) of waterlogged lucerne was significantly lower than in freely drained controls, leaf N concentrations were also significantly lower in waterlogged lucerne. At the same time, there were significantly (5-fold) higher concentrations of Fe in waterlogged roots and Na in leaves (2-fold) of stressed plants. PS II photochemistry, which was impaired due to waterlogging, recovered almost fully after 16 d of free drainage in all genotypes. Alongside fluorescence recovery, concentrations of several nutrients also increased in recovered plants. Growth parameters, however, remained suppressed after draining. The latter was due to both the smaller capacity of CO₂ assimilation in previously waterlogged plants (caused in part by nutrient deficiency and associated inhibition of PSII) and the plants need to re-direct available nutrient and assimilate pools to repair the damage to the photosynthetic apparatus and roots. It is concluded, that for any lucerne-breeding program it is important to determine not only the degree of tolerance to waterlogging but also the potential for recovery of different genotypes, as well as look for outstanding individuals within each population.     

Nitrification in the rhizosphere of a flooding-resistant and a flooding-non-resistant Rumex species under drained and waterlogged conditions

Abstract The effects of the flooding-resistant plant species Rumex palustris and the non-flooding-resistant plant species Rumex acetosa on nitrification were compared. The plants were grown under drained and waterlogged conditions on a mixture of calcareous riversand and sieved grassland soil with a high potential nitrifying activity. In the shoots of R. acetosa , but not in those of R. palustris , the ratio between the amounts of accumulated carboxylates and organic nitrogen, ((CA-A)/N_org.), appeared to be a useful indicator of ammonium or nitrate consumption by tghe plant. In both plant species, the inorganic nitrogen source had no observed effect on the (C-A)/N_org. ratio in the roots.
The growth of R. acetosa , but not that of R. palustris was inhibited by waterlogging of the soil. Both the activity and the growth of the ammonium-oxidizing bacteria were repressed under drained and waterlogged conditions in soils with R. palustris , a condition that was attributed to a competitive ammonium uptake by its relatively fast growing roots. In the presence of R. acetosa , the activity and growth of the ammonium-oxidizing bacteria were inhibited under waterlogged, but not under drained, conditions. he growth and activity of nitrite-oxidizing bacteria in the absence of actively ammonium-oxidizing, nitrite-producing bacteria was likely due to organotrophic growth.     

The interaction between waterlogging and salinity in higher plants: causes,consequences and implications

This paper reviews a range of studies under controlled conditions (glasshouse and growth cabinet) focusing on the effects of the interaction between waterlogging (hypoxia) and salinity on the ion relations, growth and survival of higher plants. The literature shows that in general, waterlogging under saline conditions causes increased Na⁺ and Cl^– concentrations in the shoot, due initially to increased rates of transport. These increased concentrations in the shoots have adverse effects on plant growth and survival. It is argued that the interaction between waterlogging and salinity has major implications for saltland management, and for the selection and breeding of plants adapted to saltland.     

Arbuscular mycorrhizal symbiosis regulates physiology and performance of <Emphasis Type="Italic">Digitaria eriantha</Emphasis> plants subjected to abiotic stresses by modulating antioxidant and jasmonate levels

This study evaluates antioxidant responses and jasmonate regulation in Digitaria eriantha cv. Sudafricana plants inoculated (AM) and non-inoculated (non-AM) with Rhizophagus irregularis and subjected to drought, cold, or salinity. Stomatal conductance, photosynthetic efficiency, biomass production, hydrogen peroxide accumulation, lipid peroxidation, antioxidants enzymes activities, and jasmonate levels were determined. Stomatal conductance and photosynthetic efficiency decreased in AM and non-AM plants under all stress conditions. However, AM plants subjected to drought, salinity, or non-stress conditions showed significantly higher stomatal conductance values. AM plants subjected to drought or non-stress conditions increased their shoot/root biomass ratios, whereas salinity and cold caused a decrease in these ratios. Hydrogen peroxide accumulation, which was high in non-AM plant roots under all treatments, increased significantly in non-AM plant shoots under cold stress and in AM plants under non-stress and drought conditions. Lipid peroxidation increased in the roots of all plants under drought conditions. In shoots, although lipid peroxidation decreased in AM plants under non-stress and cold conditions, it increased under drought and salinity. AM plants consistently showed high catalase (CAT) and ascorbate peroxidase (APX) activity under all treatments. By contrast, the glutathione reductase (GR) and superoxide dismutase (SOD) activity of AM roots was lower than that of non-AM plants and increased in shoots. The endogenous levels of cis-12-oxophytodienoc acid (OPDA), jasmonic acid (JA), and 12-OH-JA showed a significant increase in AM plants as compared to non-AM plants. 11-OH-JA content only increased in AM plants subjected to drought. Results show that D. eriantha is sensitive to drought, salinity, and cold stresses and that inoculation with AM fungi regulates its physiology and performance under such conditions, with antioxidants and jasmonates being involved in this process.     

Resistance to water stress of Alnus maritima: intraspecific variation and comparisons to other alders

Alnus maritima (seaside alder) is a rare and threatened woody species that occurs naturally as three disjunct subspecies. While its limited numbers and distribution have prompted investigations into the cause of its rarity, the aesthetic appeal and apparent flood resistance of A. maritima have led to a strong interest in its horticultural potential. The occurrence of all three subspecies of A. maritima exclusively in waterlogged soils in nature suggests that water availability may limit both the success of plants in the wild and the potential for their use in landscapes. We conducted three experiments to assess how A. maritima would respond to a wide range of root-zone moisture contents. First, we examined foliar gas exchange and growth of one clone during drought and flooding. Plants survived, grew, and avoided damage to and abscission of leaves during seven treatments that ranged from complete root-zone inundation to severe drought. Partial flooding and slight drought maximized plant dry weight, while total flooding and mild, moderate, and severe drought limited growth. High rates of photosynthesis were sustained except during severe drought. Next, we examined genotypic variation in drought resistance among the subspecies of A. maritima and compared the resistance of A. maritima to that of four other species of Alnus. Only subtle differences were detected among subspecies in how deficit irrigation influenced the physiology and development of plants. Of the five species evaluated, A. maritima and Alnus nepalensis most strongly expressed indicators of drought resistance. Lastly, a field trial with 270 seedlings of subspecies oklahomensis confirmed that A. maritima can be grown on soils with moisture contents ranging from dry (≈10% water by volume) to saturated. We conclude that, even though A. maritima thrives when a portion of its root zone is constantly inundated, it is more drought tolerant than some other species indigenous to well drained soils. The drought resistance of A. maritima can be attributed in part to its continued high rate of photosynthesis, fine control of stomatal conductance, increased specific leaf weight, and increased root:shoot ratio under conditions of drought. Considering the natural habitat to which A. maritima is restricted, our data illustrate that the niche a species occupies in nature does not necessarily represent how adapted the species may be to dissimilar environmental conditions. We also conclude that A. maritima and A. nepalensis show promise for increased use as ornamentals resistant to disparate soil water contents.