Combined effects of simulated tidal sea-level rise and salinity on seedlings of a mangrove species, Kandelia candel (L.) Druce

To investigate the effects of the simultaneous occurrence of salt stress and tidal sea-level rise on mangroves, potted Kandelia candel seedlings were treated under deep flooding (flooded 40 cm above the soil surface for 16 h per day, inundating the entire plant) and shallow flooding (flooded just above the soil surface for 8 h per day) at salinity levels of 5, 15, and 25 ppt over 14 months. Deep flooding enhanced stem elongations at all salinity levels but increased stem biomass only at 5 ppt. Deep flooding increased both leaf production and leaf fall; leaf biomass increased at 5 ppt, but decreased at 15 and 25 ppt. Biomass ratios of root/shoot (R/S) of deep flooding treatments were significantly lower than those of shallow flooding treatments. Under deep flooding, superoxide dismutase (SOD) activities did not show significant change between 5 and 15 ppt, but increased at 25 ppt. With increasing salinity level, peroxidase (POD) activities increased, and the difference between shallow and deep flooding was enhanced. Malonaldehyde (MDA) content significantly decreased at 25 ppt with 40 cm flooding, but was not affected by other treatments. These results demonstrated that the growth and physiological responses of K. candel seedlings under deep flooding conditions varied with salinity level; growth was enhanced at low salinity level but inhibited at high salinity level. It is therefore probable that K. candel will shift from downstream to upstream, where the influence of fresher river water resources will ameliorate the effects of increased salinities that accompany deeper tidal flooding in these mangrove ecosystems.     

Water salinity and inundation control soil carbon decomposition during salt marsh restoration: An incubation experiment

Coastal wetlands are a significant carbon (C) sink since they store carbon in anoxic soils. This ecosystem service is impacted by hydrologic alteration and management of these coastal habitats. Efforts to restore tidal flow to former salt marshes have increased in recent decades and are generally associated with alteration of water inundation levels and salinity. This study examined the effect of water level and salinity changes on soil organic matter decomposition during a 60‐day incubation period. Intact soil cores from impounded fresh water marsh and salt marsh were incubated after addition of either sea water or fresh water under flooded and drained water levels. Elevating fresh water marsh salinity to 6 to 9 ppt enhanced CO₂ emission by 50%?80% and most typically decreased CH₄ emissions, whereas, decreasing the salinity from 26 ppt to 19 ppt in salt marsh soils had no effect on CO₂ or CH₄ fluxes. The effect from altering water levels was more pronounced with drained soil cores emitting ~10‐fold more CO₂ than the flooded treatment in both marsh sediments. Draining soil cores also increased dissolved organic carbon (DOC) concentrations. Stable carbon isotope analysis of CO₂ generated during the incubations of fresh water marsh cores in drained soils demonstrates that relict peat OC that accumulated when the marsh was saline was preferentially oxidized when sea water was introduced. This study suggests that restoration of tidal flow that raises the water level from drained conditions would decrease aerobic decomposition and enhance C sequestration. It is also possible that the restoration would increase soil C decomposition of deeper deposits by anaerobic oxidation, however this impact would be minimal compared to lower emissions expected due to the return of flooding conditions.     

Response of wheat to subsoil salinity and temporary water stress at different stages of the reproductive phase

To examine the effects of subsoil NaCl salinity in relation to water stress imposed at different growth stages, wheat was grown in a heavy texture clay soil (vertosol) under glasshouse conditions in polythene lined cylindrical PVC pots (100 cm long with 10.5 cm diameter) with very low salinity level (EC_e 1.0 dS/m; ESP 1.0 and Cl 30 mg/kg soil) in top 10 cm soil (10–20 cm pot zone) and low salinity level (EC_e 2.5 dS/m, ESP 5, and Cl 100 mg/kg soil) in top 10–20 cm soil (20–30 cm pot zone). The plants were exposed to three subsoil salinity levels in the 20–90 cm subsoil (30–100 cm pot zone) namely low salinity (EC_e: 2.5 dS/m, ESP: 5, Cl: 100 mg/kg soil), medium salinity (EC_e: 4.0 dS/m, ESP: 10, Cl: 400 mg/kg) and high salinity (EC_e: 11.5 dS/m, ESP: 20, Cl: 1950 mg/kg) in the subsoil (20–90 cm soil layer: 30–100 cm pot zone). Watering of plants was withheld for 20 days commencing at either early booting or anthesis or mid grain filling, and then resumed until maturity, and these treatments were compared with no water stress. Water stress commencing at anthesis stage had the most depressing effect on grain yield and water use efficiency of wheat followed by water stress at grain filling stage and early booting stage. High subsoil salinity reduced grain yield by 39.1, 24.3%, and 13.4% respectively in plants water-stressed around anthesis, early booting, and mid grain filling compared with 36.6% in well-watered plants. There was a significant reduction in root biomass, rooting depth, water uptake and water use efficiency of wheat with increasing subsoil salinity irrespective of water regimes. Plants at high subsoil salinity had 64% of their root biomass in the top 0–30 cm soil and there was a marked reduction in subsoil water uptake. Roots also penetrated below the non-saline surface into salinised subsoil and led to attain high concentration of Na and Cl and reduced Ca/Na and K/Na ratio of flag leaf at anthesis stage. Results suggest that high subsoil salinity affects root growth and water uptake, grain yield and water use efficiency even in well water plants. Water stress at anthesis stage had the most depressing effect on wheat.     

培养基质、储藏方式和盐度对三种海滨植物种子萌发的影响

研究了培养基质、储藏方式和盐度对3种海滨植物互花米草(Spartina alterniflora)、盐地碱蓬(Suaeda salsa)和芦苇(Phragmites australis)种子萌发的影响,探索在潮间带环境下海滨植物种子萌发适应策略。结果表明:3种植物的干藏种子和湿藏芦苇种子随着盐度的升高,萌发率和萌发速率均显著下降,湿藏互花米草和盐地碱蓬种子在各盐度下萌发率和萌发速率差异不显著。各盐度-土培-干藏互花米草,中、高盐度-土培-干藏盐地碱蓬,土培各处理,中、高盐度-水培-干藏,高盐度-水培-湿藏芦苇种子萌发失败。湿藏提高了各盐度处理下土培互花米草,中、高盐度-水培和土培盐地碱蓬,淡水、中盐度-水培芦苇种子的萌发率和萌发速率。干藏互花米草种子在中、高盐度和土埋条件下种子的萌发受到抑制,限制了互花米草向高潮带与潮上带的扩展;而经常受潮水浸淹保持湿润的种子能抵抗高盐和泥沙沉积,导致互花米草种群逐步向低潮带方向发展;湿藏芦苇种子在淡水中萌发率和萌发速率最高,当潮上带盐度降低时,芦苇具有很强的竞争优势,但是对盐度和土埋敏感,限制了其向海的拓展;盐地碱蓬在中、高盐度和土培条件下萌发速率最高,快速萌发的适应策略和广适应性在盐地碱蓬占据高潮带和中潮带广大区域的过程中起到了重要的作用。     

Disentangling the effects of drought,salinity, and sulfate on baldcypress growth in a coastal plain restored wetland

Because of their dominance in swamps of the southeastern United States, baldcypress (Taxodium distichum) trees are commonly used in wetland restoration. Though baldcypress are known to tolerate moderate flooding and salinity, their growth has been shown to decrease when they experience drought and high salinity. This study examined the effects of drought and elevated salinity on the growth of baldcypress seedlings and saplings. In a restored wetland in North Carolina, we examined the growth of 8‐year‐old baldcypress by measuring height and diameter at breast height (DBH) along salinity, nutrient, and flooding gradients. In a greenhouse, we placed 1‐year‐old baldcypress seedlings in either drought or saturated conditions and applied different water treatments: fresh water, sulfate, and artificial salt water (5 ppt [parts per thousand]). Over 26 weeks, we measured diameter at root collar, height, and biomass. In the field, chloride concentrations in soil solution had a negative effect on DBH and height (51 and 36% decrease, respectively); high water levels had a negative effect on height (47% decrease) and DBH (46% decrease). In the greenhouse, both drought and salinity decreased diameter growth (43 and 61% decline, respectively) and height (64 and 43% decline, respectively). Sulfate did not have a significant effect on diameter growth, but caused a 14% decrease in height. Our results suggest that both drought and salinity (even as low as 1 ppt) can lead to a 20–60% decline in baldcypress growth. Restoration practitioners should consider the negative consequences of both drought and increased salinity on baldcypress growth when planning for wetland restoration.     

Salinity effects on greenhouse gas emissions from wetland soils are contingent upon hydrologic setting: a microcosm experiment

Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO₄^2?). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO₄^2?, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO₄^2?, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO₂ production, in both drought and flooded treatments. Contrary to our expectations, CH₄ fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO₄^2? treatments respectively. In the drought treatments, we saw virtually no CH₄ release from any core, while artificial seawater with sulfate increased N₂O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N₂O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology.     

Mycorrhizal symbiosis and response of sorghum plants to combined drought and salinity stresses

Arbuscular mycorrhizal (AM) symbiosis can confer increased host resistance to drought stress, although the effect is unpredictable. Since AM symbiosis also frequently increases host resistance to salinity stress, and since drought and salinity stress are often linked in drying soils, we speculated that the AM influence on plant drought response may be partially the result of AM influence on salinity stress. We tested the hypothesis that AM-induced effects on drought responses would be more pronounced when plants of comparable size are exposed to drought in salinized soils. In two greenhouse experiments, several water relations characteristics were measured in sorghum plants colonized by Glomus intraradices (Gi), Gigaspora margarita (Gm) or a mixture of AM species, during a sustained drought following exposure to salinity treatments (NaCl stress, osmotic stress via concentrated macronutrients, or soil leaching). The presence of excess salt in soils widened the difference in drought responses between AM and nonAM plants in just two instances. Days required for plants to reach stomatal closure were similar for Gi and nonAM plants exposed to drought alone, but with exposure to combined NaCl and drought stress, stomates of Gi plants remained open 17-22% longer than in nonAM plants. Promotion of stomatal conductance by Gm occurred with exposure to NaCl/drought stress but not with drought alone or with soil leaching before drought. In other instances, however, the addition of salt tended to nullify an AM-induced change in drought response. Our findings confirm that AM fungi can alter host response to drought but do not lend much support to the idea that AM-induced salt resistance might help explain why AM plants can be more resilient to drought stress than their nonAM counterparts.     

Morphological and physiological responses of Triglochin buchenaui K?cke, Mering & Kadereit to various combinations of water and salinity: implications for resilience to climate change

Impacts of abiotic factors on low lying salt marsh species have been closely evaluated as unique stressors, but few studies have determined the physiological and morphological responses to predicted climate change conditions. This study investigated the effects of water state (tidal, flooded and drought) and salinity on Triglochin buchenaui K?cke, Mering and Kadereit, newly separated from the Triglochin bulbosa L. complex. Growth, proline concentration, relative water content, electrolyte leakage, chlorophyll concentration and morphological analyses were done following 3?months of treatments. Results showed significant variation in height (7.57?±?0.5?C29?±?1.55?cm, p?<?0.005, df?=?55), leaf area increments and relative growth rates which decreased with increasing salinity under all water states. There was almost a cessation of growth under submergence which reduces the plant??s regeneration potential under these conditions. Proline accumulation (1.84?±?0.23?C3.36?±?0.38?mg?g^?1 DW), response of photosynthetic pigments and electrolyte leakage (8.17?±?0.80?C38.36?±?7.42?%???S?g^?1?FW) were fundamental to osmotic and membrane response regulation. Plants survived in all water states at salinity up to 45?ppt, but the optimum range was 0?C18?ppt, and best water state was the tidal condition. Viable rhizomes were produced under drought conditions but in the absence of seed banks, regeneration of the species following prolonged submergence may not be possible due to absence of both rhizomes and seeds. In their natural habitat, highly succulent T. buchenaui plants (29?±?1.55?cm tall and above) are indicative of freshwater conditions.     

The impact of salinity exposure on survival and temperature tolerance of the Antarctic collembolan Cryptopygus antarcticus

The collembolan Cryptopygus antarcticus Willem is potentially exposed to habitat salinities equal to (or greater than) sea water, as a result of sea spray, drying of littoral habitats, dispersal or temporary entrapment on the surface of sea water, or exposure to localized salt deposits from dense vertebrate populations on terrestrial habitats. To test the impact of this exposure on C. antarcticus, the tolerance of the collembolan to being placed on the surface of sea water and solutions of higher salt concentrations is investigated. The effects of acclimation to exposure to liquids of different salinities [44, 100 and 200 parts per thousand (ppt) sea salt] on cold and heat tolerance, as well as thermal activity thresholds, are also explored. Cryptopygus antarcticus shows > 75% survival after 10 days of exposure to both sea water and 100‐ppt salt, whereas it exhibits significantly lower survival after 5 days (60% survival) and 10 days (40%) of exposure to a 200‐ppt solution. Body water content also decreases after exposure to all salinities, and particularly to the 200‐ppt solution, in which > 50% of body water is lost after 10 days. Acclimation results in greater cold tolerance, although heat tolerance at 33, 35 and 37 °C is either unaltered or reduced. The thermal activity thresholds of C. antarcticus at both high and low temperatures are also negatively affected by saline exposure. The data demonstrate the capacity of C. antarcticus to tolerate periods of exposure to saline conditions, and also show that this exposure can enhance cross‐tolerance to low temperatures. The present study also demonstrates that salinity‐associated stress at moderately low and high temperatures narrows the thermal range of activity, thus reducing the ability of collembolans to forage, develop and reproduce. © 2013 The Royal Entomological Society     

Water relations and leaf expansion: importance of time scale

The role of leaf water relations in controlling cell expansion in leaves of water-stressed maize and barley depends on time scale. Sudden changes in leaf water status, induced by sudden changes in humidity, light and soil salinity, greatly affect leaf elongation rate, but often only transiently. With sufficiently large changes in salinity, leaf elongation rates are persistently reduced. When plants are kept fully turgid throughout such sudden environmental changes, by placing their roots in a pressure chamber and raising the pressure so that the leaf xylem sap is maintained at atmospheric pressure, both the transient and persistent changes in leaf elongation rate disappear. All these responses show that water relations are responsible for the sudden changes in leaf elongation rate resulting from sudden changes in water stress and putative root signals play no part. However, at a time scale of days, pressurization fails to maintain high rates of leaf elongation of plants in either saline or drying soil, indicating that root signals are overriding water relations effects. In both saline and drying soil, pressurization does raise the growth rate during the light period, but a subsequent decrease during the dark results in no net effect on leaf growth over a 24 h period. When transpirational demand is very high, however, growth-promoting effects of pressurization during the light period outweigh any reductions in the dark, resulting in a net increase in growth of pressurized plants over 24 h. Thus leaf water status can limit leaf expansion rates during periods of high transpiration despite the control exercised by hormonal effects on a 24 h basis.     

Effects of salinity and flooding on seedlings of cabbage palm (Sabal palmetto)

Sabal palmetto (Walt.) Lodd. ex Schultes (cabbage palm) dominates the coastal limit of many forests in North Florida and Georgia, United States. Changes in saltwater flooding due to sea level rise have been credicted with pushing the coastal limit of cabbage palms inland, eliminating regeneration before causing death of mature trees. Localized freshwater discharge along the coast causes different forest stands to experience tidal flooding with waters that differ in salinity. To elucidate the effect of such variation on regeneration failure under tidal flooding, we examined relative effects of flooding and salinity on the performance of cabbage palm seedlings. We examined the relationship between seedling establishment and degree of tidal inundation in the field, compared the ability of seedlings to withstand tidal flooding at two coastal sites that differed in tidal water salinity, and investigated the physiological responses of cabbage palm seedlings to salinity and flooding in a factorial greenhouse experiment. Seedling survival was inversely correlated with depth and frequency of tidal flooding. Survival of seedlings at a coastal site flooded by waters low in salinity [c. 3 parts per thousand (ppt)] was greater than that at a site flooded by waters higher in salinity (up to 23 ppt). Greenhouse experiments revealed that leaves of seedlings in pots flushed twice daily with salt solutions of 0 ppt and 8 ppt exhibited little difference in midmorning net CO₂ assimilation rates; those flushed with solutions of 15 ppt and 22 ppt, in contrast, had such low rates that they could not be detected. Net CO₂ assimilation rates also declined with increasing salinity for seedlings in pots that were continuously inundated. Continuous root zone inundation appeared to ameliorate effects of salinity on photosynthesis, presumably due to increased salt concentrations and possibly water deficits in periodically flushed pots. Such problems associated with periodic flushing by salt water may play a role in the mortality of cabbage palm seedlings in the field. The salinity range in which plant performance plummeted in the greenhouse was consistent with the salinity difference found between our two coastal study sites, suggesting that variation in tidal water salinity along the coast plays an important role in the ability of cabbage palm seedlings to withstand tidal flooding.     

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na⁺ and Cl^? content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.     

Effect of environmental factors on seed germination and emergence of Lepidium vesicarium

Lepidium vesicarium is a weed species with a wide distribution in the rangelands and dry‐land farming in East Azarbaijan, Iran. The experiments were undertaken to assay the effects of light, temperature, pH, osmotic potential, NaCl concentration and burial depth on seed germination and emergence of L. vesicarium. Germination was maintained at high levels (> 80%) over a wide day/night temperature range (10/5 to 30/20°C), but a severe reduction in the germination rate of L. vesicarium was found below 20/10°C. Germination of L. vesicarium was influenced by different light/dark regimes, as the germination rate was highest at 16 h light for the all treatments (0, 8, 12, 16 and 24 h light). Germination was 92–95% over a wide range of pH (2‐10). Germination was >50% at a water potential of ?0.7 MPa and salinity of 21 dS/m, indicating that drought and salt conditions have a minimal impact on seed germination. With increasing burial depth from 0 to 2 cm, the number of days required for 50% emergence increased and no germination was observed at burial depths deeper than 3 cm. This suggests that L. vesicarium would become troublesome in the rangelands and for growers in reduced‐tillage cropping systems. The ability to emerge from shallow depths, coupled with tolerance of a wide pH range, drought and salinity at germination, should be taken into account when managing this weed species.     

Whole-plant gas exchange responses of Spartina alterniflora (Poaceae) to a range of constant and transient salinities

A two-chamber-system was used to study whole-plant gas exchange responses of Spartina alterniflora to long-term and transient salinity treatments over the range of 5 to 40 ppt NaCl. Lower photosynthetic rates, leaf water vapor conductances, belowground respiration rates, and higher aboveground respiration rates in plants adapted to 40 ppt NaCl were observed. Area-specific leaf weight increased with salinity, although the salt content of leaf tissues did not. A reduced rate of gross photosynthesis and higher aboveground respiration rate in 40-ppt NaCl plants significantly lowered the net whole-plant CO₂ gain below that of 5-ppt NaCl plants, while the net CO₂ gain of 25-ppt NaCl plants was intermediate. Within 6 hr of increasing the salinity of 5- and 25-ppt NaCl plants by 20 and 15 ppt NaCl, S. alterniflora responded by reducing leaf water vapor conductance, which in turn reduced the photosynthetic rate. This response was reversed by returning the plants to their original salinity, which indicates that S. alterniflora adjusts water loss and gas exchange in response to transient salinity stress by regulating stomatal aperture. On the other hand, decreasing salinity of the growth media of plants cultured at 25 and 40 ppt NaCl had little or no effect on gas exchange characteristics. This suggests that S. alterniflora adapts to constant salinity through fixed, salinity-dependent structural modifications, such as stomatal density.     

互花米草生物量年动态及其与滩涂生境的关系

本文研究了不同滩涂生境对互花米草(Spartina alterniflora)群落特征及生物量的影响。其结果是：建立在小潮高潮线附近滩面的互花米草群落,其密度、高度、地上和地下生物量(本文生物量均为现存生物量),均比建立在偏低、偏高位置滩面的群落为高。在淡水、肥水来源丰富的河口潮滩上建立的互花米草群落,其群落特征更突出,生物量比建立在土壤肥力低、含盐量高的非河口潮滩上群落高。潮滩上的互花米草群落地下生物量随龄级的增加而增长,但地上生物量和总生物量以三年龄群落为最高,四年龄群落却比三年龄群落低。     

Morphological and physiological responses of Scaevola sericea (Goodeniaceae) seedlings to salt spray and substrate salinity

The effects of substrate salinity and salt spray upon seedlings of Scaevola sericea were examined in this study. Three levels of substrate salinity: 0.0 ppt, 3.0 ppt, and 10.0 ppt were examined in conjunction with three levels of salt spray: zero, medium (200 mg m^-2mdd^-1), and high (1200–1500 mg-m^-2mdd^-1). Leaf surface area, root to shoot ratio, as well as leaf, stem, and root mass decreased significantly (P 0.05) with increasing substrate salinity. Biomass accumulation was very low at 10.0 ppt substrate salinity, suggesting that higher levels of substrate salinity cannot be tolerated by the seedlings. Salt spray had a substantial effect on several of these variables, however its effects were less pronounced than those of substrate salinity. Cell sap osmolarity, leaf thickness, and leaf specific mass increased significantly (P 0.05) with both increasing substrate salinity and salt spray levels. Leaf carbon isotope ratios (δ^l3C) became more positive with increasing salinity, indicating an enhancement of the intrinsic water use efficiency of the seedlings at higher salinities. Scaevola sericea is one of the dominant plants found at the leading edge of strand communities in the Hawaiian archipelago and throughout much of the tropical Pacific. Since substrate salinity and salt spray increase with proximity to the ocean, the two factors may act together to limit the seaward expansion of S. sericea in coastal habitats.     

黄河三角洲海岸带湿地柽柳在干旱年份的水分利用策略

以黄河三角洲海岸带贝壳堤湿地灌木群落主要建群种柽柳为对象,利用稳定同位素技术测定柽柳木质部和潜在水源δ¹⁸O值的时空变化,采用IsoSource模型计算潜在水源对柽柳的贡献比例,研究海岸带不同生境中柽柳对不同水分条件的适应机制.结果表明: 在降水较少的干旱年份,相对于不稳定的降水,柽柳倾向于利用稳定的土壤水和浅层地下水,但是在不同微地形生境下柽柳的水分利用策略有所差异.滩脊的柽柳72.6%～95.4%水分来源于浅层地下水和含水量相对较高的深层土壤水(40～100 cm);高潮线附近的柽柳有40.7%～97.3%的水分来源于上层土壤水(0～40 cm),以避免海水和浅层地下水的盐分胁迫.柽柳对外界水盐条件变化具有较强的适应性,在海岸带可利用水资源缺乏的恶劣生境中具有更强的种间竞争优势,从而导致柽柳单优灌木群落的形成.     

Effects of low-salinity on the growth and development of spotted halibut Verasper variegatus in the larva-juvenile transformation period with reference to pituitary prolactin and gill chloride cells responses

Low-salinity adaptability was investigated in a flatfish spotted halibut Verasper variegatus during the period from late metamorphic larvae to early juveniles by a 20-day rearing experiment under different salinity regimes (1, 4, 8, 16 and 32 ppt). Effects of low-salinity on growth and development were examined and the changes in the prolactin (PRL) production level in the pituitary and the gill chloride cell morphology were examined as physiological backgrounds for low salinity adaptation. PRL cells and chloride cells were identified by immunocytochemistry with a specific antiserum for PRL₁₈₈ and Na⁺,K⁺-ATPase. Most of the fish exposed to over 4 ppt survived for 20 days, but all the fish exposed to 1 ppt died within 5 days. Fish kept in intermediate salinities (8, 16 ppt) grew significantly better than those in the control group (32 ppt). Fish exposed to 4 ppt attained almost the same body length as the control group at 20 days after transfer, although these fish showed an abnormally dark body color as well as delayed development. These results suggested that spotted halibut has a high-adaptability to low-salinity environments and prefers an intermediate salinity near iso-osmolality (about 12 ppt) from the late metamorphic larval stage, but does not completely adapt to a hypoosmotic of 4 ppt salinity or less than half of the osmolality. The percentage of PRL-cell volume to pituitary volume was significantly higher at 4 ppt than in the control group. The chloride cells in gill filaments were significantly larger at 4 ppt than in the control group. These results suggest that juveniles could adapt to a low-salinity environment due to the activation of PRL production and enlargement of chloride cells. These laboratory findings suggest that late metamorphic larvae and early juveniles of spotted halibut may utilize a low salinity environment such as estuarine tidal flats or very shallow coastal areas as their nursery grounds in the sea.     

Effect of soil mechanical impedance on root growth of two rice varieties under field drought stress

Two upland rice varieties, Azucena and Bala, were screened for root growth under droughted and irrigated treatments in two field sites at the West Africa Rice Development Association (WARDA) experimental farm, Côte d’Ivoire, during the dry season of 1999/2000. The sites were chosen to represent contrasting soil profile penetration resistance (PR) characteristics on upland sites, although both were relatively impeding. The number of nodal root axes per unit area passing through horizontal transects (root density) was counted at 35, 56, 77 and 98 days after sowing (DAS) at 10 cm depth intervals. Azucena consistently maintained a greater root density than Bala and a greater proportion of Azucena roots grew to 30 cm depth (22.7% vs. 8.4% at 77 DAS). There was little detectable effect of water regime on root distribution but evidence of lower root numbers at depths below 20 cm in the higher PR site was revealed. A site by variety by soil depth interaction suggests that Azucena roots are more strongly affected by very high PR than those of Bala. PR between 0–30 cm depth increased greatly with decreasing soil water content during the drought as the soil dried. This increase is likely to have prevented or greatly impaired further nodal root growth within this layer. At 40 cm depth, PR was high (3–4 MPa) but did not increase during the drought. At this depth root growth rate was likely to be greatly reduced despite the availability of water. These results demonstrate that varietal differences in root morphology characterised in the laboratory can be also detected in impeding field soils as differences in the density of roots at depth. Relatively poor root growth in these fields in the absence of drought was probably due to the high mechanical impedance and/or the physiological stress of the plants in the dry season. Our results indicate that high mechanical impedance was a more fundamental constraint on root growth than soil water availability during the drought. Thus, varietal differences in root penetration ability might be very important for drought avoidance in soils of this type.     

The responses of Briza media and Koeleria macrantha to drought and re-watering

1. To test whether differences in response to drought can help to explain the differing distributions of Briza media and Koeleria macrantha , changes in their leaf growth and water relations during soil drying were measured. After droughting, plants were re-watered and recovery recorded.
2. Leaf growth of the two species showed a similar sensitivity to drought with respect to duration of soil drying and soil moisture content.
3. In both species tiller base relative water content (RWC) was maintained at similar levels to controls until soil moisture content had fallen to less than 9%. This may aid survival in habitats subject to periodic droughting by preventing damage to the meristems.
4. Briza media did not respond to re-watering. However, plants of K. macrantha re-grew after periods of over 20 days of drought, when tiller base RWCs had fallen as low as 13%. This ability to resurrect may explain the occurrence of K. macrantha in xeric calcicolous grasslands subject to episodes of severe droughting, from which B. media is absent.