Flooding and salinity effects on growth and survival of four common forested wetland species

The survival, growth, and biomass of baldcypress (Taxodium distichum (L.) Rich.), water tupelo (Nyssa aquatica L.), Chinese tallow (Sapium sebiferum (L.) Roxb.), and green ash (Fraxinus pennsylvanica Marsh.) seedlings were examined in an experiment varying water levels (watered, flooded) and salinity levels (0, 2, and 10 ppt, plus a simulated storm surge with 32 ppt saltwater). All seedlings, except for those flooded with 10 ppt saltwater, survived to the end of the experiment. In 10 ppt saltwater, flooded baldcypress, water tupelo, and green ash survived two weeks whereas Chinese tallow survived for 6 weeks. However, a second set of slightly older baldcypress, water tupelo, and Chinese tallow seedlings survived eight weeks of flooding with 10 ppt saltwater. When carried through the winter to the beginning of the second growing season, flooded baldcypress and Chinese tallow seedlings from the 0 and 2 ppt treatments leafed out, but only Chinese tallow recovered from the saltwater surge treatment. The diameter and growth (height) of each species was not affected when watered with 2 ppt saltwater, except for the effects of the height growth of baldcypress. Growth was reduced for all species when watered with 10 ppt saltwater. The diameter growth of green ash was reduced by freshwater flooding. The diameter growth of baldcypress and water tupelo was greater when flooded with fresh water. Flooding with 2 ppt saltwater caused a significant reduction in diameter growth in water tupelo, green ash, and Chinese tallow, but not in baldcypress. Root and stem biomass values were not significantly different for any species between the 0 and 2 ppt salinity watering treatments. However, seedlings watered with 10 ppt saltwater had significantly lower root and stem biomass values, except for baldcypress roots and green ash stems. Baldcypress was least affected by flooding with 0 and 2 ppt saltwater, although there were slight reductions in root biomass with increasing salinity. Because of the susceptibility of the seedlings of these four species to increases in flooding and salinity, their regeneration may be limited in the future, thereby causing shifts in species composition.     

Interactive effects of substrate,hydroperiod, and nutrients on seedling growth of Salix nigra and Taxodium distichum

The large river swamps of Louisiana have complex topography and hydrology, characterized by black willow (Salix nigra) dominance on accreting alluvial sediments and vast areas of baldcypress (Taxodium distichum) deepwater swamps with highly organic substrates. Seedling survival of these two wetland tree species is influenced by their growth rate in relation to the height and duration of annual flooding in riverine environments. This study examines the interactive effects of substrate, hydroperiod, and nutrients on growth rates of black willow and baldcypress seedlings. In a greenhouse experiment with a split-split-plot design, 1-year seedlings of black willow and baldcypress were subjected to two nutrient treatments (unfertilized versus fertilized), two hydroperiods (continuously flooded versus twice daily flooding/draining), and two substrates (sand versus commercial peat mix). Response variables included height, diameter, lateral branch count, biomass, and root:stem ratio. Black willow growth in height and diameter, as well as all biomass components, were significantly greater in peat substrate than in sand. Black willow showed a significant hydroperiod–nutrient interaction wherein fertilizer increased stem and root biomass under drained conditions, but flooded plants did not respond to fertilization. Baldcypress diameter and root biomass were higher in peat than in sand, and the same two variables increased with fertilization in flooded as well as drained treatments. These results can be used in Louisiana wetland forest models as inputs of seedling growth and survival, regeneration potential, and biomass accumulation rates of black willow and baldcypress.     

Effects of flooding and salinity on photosynthesis and water relations of four Southeastern Coastal Plain forest species

The influence of flooding and salinity on photosynthesis and water relations was examined for four common coastal tree species [green ash (Fraxinus pennsylvanica Marshall), water tupelo (Nyssa aquatica L.). Chinese tallow (Sapium sebiferum (L.) Roxb.), and baldcypress (Taxodium distichum (L.) Richard)]. Both chronic (as might be associated with sea level rise) and acute (similar to hurricane storm surges) exposures to these stresses were examined. Chronic freshwater flooding of green ash, water tupelo, and Chinese tallow seedlings reduced photosynthesis (A) relative to that of watered seedlings, while baldcypress was unaffected. Chinese tallow A declined with increasing length of flooding. A salinity increase of the floodwater to 2 ppt decreased A of baldcypress and water tupelo, but not A of green ash and Chinese tallow, which was already severely reduced by freshwater flooding. All seedlings of the four species died within 2 to 6 weeks when flooded with 10 ppt saltwater. Photosynthesis of all four species did not differ between 0 and 2 ppt watering. Watering with 10 ppt salinity initially reduced A of all four species, but the seedlings recovered over time. Photosynthesis was severely decreased for all species when flooded with 21 ppt salinity for 48 hours. Reduced A continued following the treatment. Photosynthesis of only green ash and water tupelo was reduced by watering with 21 ppt salinity for 48 hours. Flooding of low-lying areas with increased salinity would lead to shifts in species composition of coastal forests due to these differential tolerances.     

Effects of Flooding on Susceptibility of Taxodium distichum L. Seedlings to Drought

Responses of baldcypress (Taxodium distichum) seedlings to soil moisture were studied to test the hypothesis that flooding may lead to seedling's higher susceptibility to drought. Treatments included a well-watered but drained control (C), continuously flooded (CF), control followed by drought (CD), and flooded followed by drought (FD). Gas exchange values revealed no significant effects on net photosynthetic rate (P _N) in response to flooding. In contrast, after the onset of drought, P _N was significantly reduced in CD and FD plants. Significant growth reductions under mild drought conditions indicated that baldcypress seedlings were drought sensitive. However, comparison of gas exchange rates and growth responses between CD and FD plants indicated that prior flooding had no detectable effect on subsequent sensitivity of baldcypress to drought. These findings explain baldcypress persistence in wetland habitats characterized by periodic flooding and mild drought.     

Use of shelter tubes,grass‐specific herbicide,and herbivore exclosures to reduce stressors and improve restoration of semiarid thornscrub forests

In semiarid south Texas, land conversion has reduced thornscrub forests by greater than 95%, and stressors, including competition with invasive grasses, mammalian herbivory, and drought, threaten the success of restoration efforts. This study assessed the effectiveness of multiple restoration treatments aimed at improving survival and growth of thornscrub forest seedlings planted in old agricultural fields. In January 2013, we treated greater than 1,100 seedlings with grass‐specific herbicide, herbivore exclosures, and shelter tubes, used separately or combined. We further evaluated the effects of shelter tube duration (0, 6, 12, and 18 months). For each seedling, we quantified surrounding invasive grass cover, browse intensity, height, and basal diameter every 4 months until September 2014. Herbicide application decreased invasive grass cover approximately 5‐fold and increased seedling survival (23%) and basal diameter (26%). Shelter tube application for 12 and 18 months increased seedling survival (10%) and height (43 and 74%, respectively), whereas seedlings treated with tubes for only 6 months performed similar to those left untreated. Exclosures had no impact on seedling survival but increased seedling height (23%) and basal diameter (26%). We found no significant interactive effects of treatments. Overall, herbicide most effectively increased seedling survival and basal diameter growth, whereas shelter tubes proved most useful for promoting height growth. Combined, these treatments increased implementation and maintenance costs 2‐fold, but minimized seedling mortality and maximized restoration potential. These findings highlight the necessity of post‐planting seedling management to reduce stress from invasive grasses, mammalian herbivory, and drought and improve restoration potential in semiarid thornscrub forests.     

Threshhold for recovery in the marsh halophyte Spartina alterniflora grown under the combined effects of salinity and soil drying

A study quantifying the physiological threshhold at which Spartina alterniflora plants are able to tolerate the interactive effects of salinity and soil drying was conducted in a climate controlled greenhouse. The experiment consisted of two levels of salinity (3-5 ppt, L and 35-38 ppt, H) as well as four dynamic water levels: flooding (water level maintained 3-5 cm above the soil surface at high tide and 10 cm below the soil surface at low tide for entire study duration, F), 8-day drought (water level maintained at least 20 cm below the soil surface at high tide for 8 days then flooded, 8 days), 16-day drought (water level maintained at least 20 cm below the soil surface at high tide for 16 days then flooded, 16 days), and 24-day drought (water level maintained at least 20 cm below the soil surface at high tide for 24 days then flooded, 24 days). Plant gas exchange and growth responses were measured along with soil conditions of redox potential and water potential. Significant decreases were seen in plant gas exchange and growth in response to increases in salinity and soil drying. Survival was 100% for all flooded treatments while increased salinity combined with soil drying decreased survival to 86% in both low salt/24-day drought plants (LD24) and high salt/16-day drought plants (HD16). The lowest survival rate was seen in the high salt/24-day drought treatment (HD24) at 29%. Therefore, it appears that the critical time for recovery from the combined effects of increased salinity and soil drying may greatly diminish after two weeks from the onset of stress conditions. Consequently, if salinity continues to increase along the MRDP, marshes dominated by S. alterniflora may be more susceptible to short-term drought and likewise large-scale marsh browning.     

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.     

Comparative studies on physiological and biochemical adaptation of Taxodium distichum and Taxodium ascendens seedlings to different soil water regimes

Responses of baldcypress (Taxodium distichum) and pondcypress (Taxodium ascendens) seedlings in leaf gas exchange and root metabolism to a wide range of hydrological regimes were studied. Four water treatments included control (C), mild drought (MD), wet soil (WS) and flooding (FL). Both species showed no significant change in net photosynthetic rate (Pn) in response to flooding in contrast to significant reduction in Pn under mild drought conditions. In baldcypress, Pn was also maintained at control level compared to significant increase of Pn in pondcypress when subjected to the wet soil treatment. In flooding, both species showed significant increase in contents of malate and shikimate in their lateral roots and further in total roots, as compared to control. However, different adaptation to flooding was demonstrated in their taproots in contents of malate and shikimate. Furthermore, baldcypress seedlings showed significant reductions in lateral and total root biomass, in contrast with no significant change in that of pondcypress seedlings in response to flooding. In wet soil, no significant effects were revealed in contents of malate and shikimate in roots (including different root portions) of baldcypress seedlings in contrast to significant increase of their contents in lateral roots of pondcypress seedlings. No significant effects on root biomass were detected in either baldcypress or pondcypress seedlings in response to wet soil. When subjected to mild drought, baldcypress seedlings displayed no significant change in contents of malate and shikimate in roots, whereas pondcypress seedlings exhibited significant reduction in content of shikimate in lateral roots and total roots. In addition, root biomass of baldcypress seedlings showed no significant change, while lateral and total root biomass of pondcypress seedlings demonstrated significant increase when mild drought was imposed. These findings indicate that baldcypress and pondcypress appear to be promising candidates for reforestation in the Three Gorges Reservoir region due to their characteristics of water-tolerance and mild drought endurance. However, pondcypress is more resilient than baldcypress to the continuum of water availability conditions expected in the Three Gorges Reservoir region.     

Effects of drought stress and N supply on the growth, biomass partitioning and water-use efficiency of Sophora davidii seedlings

A greenhouse experiment was conducted in order to understand the adaptation responses to different water and N conditions, and further explore if additional N supply could improve the water-use efficiency (WUE) and adaptability of Sophora davidii seedlings under dry conditions. Two-month-old seedlings were subjected to a completely random design with three water (80, 40 and 20% water field capacity (FC)) and three N supply (N0: 0, Nl: 92 and Nh:184 mg N kg⁻¹ soil) regimes. Drought stress dramatically decreased seedlings height, basal diameter, leaf number, leaf area, root length, and biomass production. An increase in below-ground biomass was observed indicating a higher root/shoot ratio (R/S) under drought stress conditions, and drought further decreased relative water content (RWC) and WUE. On the other hand, S. davidii seedlings exhibited strong responses to N supply, but the responses were inconsistent with the various N supply levels. Low N supply (Nl) increased seedlings height, basal diameter, leaf number, leaf area, and biomass production, but decreased root length. In contrast, high N supply (Nh) decreased or had little effect on these growth characteristics. N supply increased leaf percentages, but decreased fine root percentages. In addition, Nl rather than the other two N treatments increased leaf area ratio (LAR), leaf/fine root mass ratio (L/FR), R/S and RWC under severe drought stress (20% FC), even though these parameters could increase with the Nh treatment under well-watered condition (80% FC). Moreover, Nl also increased WUE under three water conditions, but Nh had little effect on WUE under drought stress conditions (40% FC and 20% FC). The results suggested that water and N co-limited the growth of S. davidii seedlings, and the seedlings exhibited great positive responses to Nl in this study. Appropriate or low N supply, therefore, would be recommended to stimulate growth, enhance WUE, alleviate drought stress, and consequently contribute to S. davidii seedling establishment under dry condition, but excess N supply should be avoided.     

Rehabilitation of Swamp Paperbark (<Emphasis Type="Italic">Melaleuca ericifolia</Emphasis>) wetlands in south-eastern Australia: effects of hydrology,microtopography, plant age and planting technique on the success of community-based revegetation trials

Wetlands dominated by Swamp Paperbarks (Melaleuca spp., Myrtaceae) are common in coastal regions across Australia. Many of these wetlands have been filled in for coastal development or otherwise degraded as a consequence of altered water regimes and increased salinity. Substantial resources, often involving community groups, are now being allocated to revegetating the remaining wetland sites, yet only rarely is the effectiveness of the rehabilitation strategies or on-ground procedures robustly assessed. As part of a larger project investigating the condition and rehabilitation of brackish-water wetlands of the Gippsland Lakes, we overlaid a scientifically informed experimental design on a set of community-based planting trials to test the effects of water depth, microtopography, plant age and planting method on the survival and growth of seedlings of Melaleuca ericifolia Sm. in Dowd Morass, a degraded, Ramsar-listed wetland in south-eastern Australia. Although previous laboratory and greenhouse studies have shown M. ericifolia seedlings to be salt tolerant, the strongly interactive effects of waterlogging and salinity resulted in high seedling mortality (>90%) in the field-based revegetation trials. Seedlings survived best if planted on naturally raised hummocks vegetated with Paspalum distichum L. (Gramineae), but their height was reduced compared with seedlings planted in shallowly flooded environments. Age of plants and depth of water were important factors in the survival and growth of M. ericifolia seedlings, whereas planting method seemed to have little effect on survival. Improved testing of revegetation methods and reporting of success or otherwise of revegetation trials will improve the effectiveness and accountability of projects aiming to rehabilitate degraded coastal wetlands.     

Tolerance of Pinus taeda and Pinus serotina to low salinity and flooding: Implications for equilibrium vegetation dynamics

Questions: 1. Do pine seedlings in estuarine environments display discrete or continuous ranges of physiological tolerance to flooding and salinity? 2. What is the tolerance of Pinus taeda and P. serotina to low salinity and varying hydrologic conditions? 3. Are the assumptions for ecological equilibrium met for modeling plant community migration in response to sea‐level rise? Location: Albemarle Peninsula, North Carolina, USA. Methods: In situ observations were made to quantify natural pine regeneration and grass cover along a salinity stress gradient (from marsh, dying or dead forest, to healthy forest). A full‐factorial greenhouse experiment was set up to investigate mortality and carbon allocation of Pinus taeda and P. serotina to low‐salinity conditions and two hydrology treatments over 6 months. Treatments consisted of freshwater and two salinity levels (4 ppt and 8 ppt) under either permanently flooded or periodically flushed hydrologic conditions. Results: Natural pine regeneration was common (5–12 seedlings per m²) in moderate to well‐drained soils where salinity concentrations were below ca. 3.5 ppt. Pine regeneration was generally absent in flooded soils, and cumulative mortality was 100% for 4 and 8 ppt salinity levels under flooded conditions in the greenhouse study. Under weekly flushing conditions, mortality was not significantly different between 0 and 4 ppt, confirming field observations. Biomass accumulation was higher for P. taeda, but for both pine species, the root to shoot ratio was suppressed under the 8 ppt drained treatment, reflecting increased below‐ground stress. Conclusions: While Pinus taeda and P. serotina are commonly found in estuarine ecosystems, these species display a range of physiological tolerance to low‐salinity conditions. Our results suggest that the rate of forest migration may lag relative to gradual sea‐level rise and concomitant alterations in hydrology and salinity. Current bioclimate or landscape simulation models assume discrete thresholds in the range of plant tolerance to stress, especially in coastal environments, and consequently, they may overestimate the rate, extent, and timing of plant community response to sea‐level rise.     

Effect of an Epichloë endophyte on adaptability to water stress in Festuca sinensis

As water stress, including drought and waterlogging, can severely affect plant growth, this study investigated the effects of an endophyte from the genus Epichloë on two different ecotypes of Festuca sinensis grass under five soil water conditions in a controlled greenhouse experiment. Changes in F. sinensis plants grown with (E+) and without the endophyte (E−) were evaluated as they were subjected to different water treatments (20%, 35%, 50%, 65% and 80% relative saturation moisture content, RSMC). Growth parameters such as plant height, number of tillers, blade width, stem diameter, root length, total biomass, root-shoot ratio and relative water content were determined. The results showed that drought and waterlogging significantly (P < 0.05) inhibited the growth of F. sinensis. The presence of the endophyte significantly (P < 0.05) increased plant growth and root-shoot ratio under drought and waterlogged conditions. In addition, the plant height, number of tillers, blade width, stem diameter and total biomass in seedlings of both ecotypes reached the maximum at 65% RSMC, which suggests the optimal water condition. These findings also show that moderate drought (35% and 50% RSMC) could promote root growth of grass seedlings. Therefore, endophytic infections can result in enhanced host plant resistance to drought and waterlogged conditions.     

种子大小和干旱胁迫对辽东栎幼苗生长和生理特性的影响

在温室内遮阴条件下,设置80%、60%、40%和20%田间持水量(对照、轻度、中度和重度干旱)4个处理,研究种子大小和干旱胁迫对盆栽辽东栎幼苗生长和生理特性的影响。结果表明: 大种子(3.05±0.38 g)幼苗的单株叶面积、总干质量和根冠比在所有处理均显著大于小种子(1.46±0.27 g)幼苗,前者的株高、基径、叶片数、比叶面积、相对生长率和净同化率等生长参数在轻度、中度和重度干旱处理均不同程度大于后者。大种子幼苗叶片过氧化物酶(POD)、过氧化氢酶(CAT)和超氧化物歧化酶(SOD)活性均高于小种子幼苗,前者叶片丙二醛(MDA)、可溶性蛋白、游离脯氨酸含量和叶绿素总量在部分干旱处理显著大于后者。除根冠比外,其他生长参数均随干旱胁迫增强逐渐减小,重度干旱处理大、小种子幼苗总干质量分别比对照降低19.4%和20.0%。POD、CAT和SOD活性均随干旱胁迫增强先升后降,在中度干旱处理,大、小种子幼苗POD活性分别显著高于对照126.7%和142.1%,CAT活性分别显著高于对照170.0%和151.9%。在重度干旱处理,大、小种子幼苗MDA含量分别显著高于对照86.5%和68.9%。可溶性蛋白、游离脯氨酸含量和叶绿素总量均随干旱胁迫增强先升后降,在中度干旱处理,大、小种子幼苗可溶性蛋白含量分别显著高于对照320.7%和352.7%。辽东栎大种子幼苗可依赖其生长和生理方面的优势比小种子幼苗具有更强的干旱耐受性,在退化次生林人工辅助实生更新中应优先选用抗逆性更强的大种子幼苗。     

Responses to drought preconditioning in<Emphasis Type="Italic"> Eucalyptus globulus</Emphasis> Labill. provenances

Shoot water relations and morphological responses to drought preconditioning were studied by subjecting 5-month-old seedlings of three provenances of Eucalyptus globulus to different water regimes for 36 days in a greenhouse pot study. Moderately stressed plants were watered every 6 days and severely stressed plants were watered every 9 days. Control plants were watered daily. Drought cycles induced significant changes in morphological and physiological characteristics. Preconditioned seedlings were smaller in size, root collar diameter, height, and leaf area than control seedlings. Shoot/root ratio was not affected by drought. Osmotic potential at full turgor (ψπ_FT) and osmotic potential at turgor loss point (ψπ_TLP) were significantly lower and the magnitude of osmotic adjustment was significantly higher under the severe than under the moderate stress treatment. In severely stressed plants a decrease of turgid mass/dry mass contributed to osmotic adjustment. In a subsequent acclimation test, preconditioned seedlings showed higher values of stomatal conductance, predawn relative water content and water potential and lower mortality than control plants. These variables were significantly related to ψπ_FT. We assume that the reduced leaf area and osmotic adjustment observed in preconditioned seedlings contributed to drought acclimation in the selected E. globulus provenances leading to better rates of gas exchange and improved water status than non-conditioned plants. Provenances exhibited differences in their responses to drought, albeit mainly morphological differences. E. globulus subsp. bicostata from Tumbarumba grew more quickly (larger diameter and height relative growth rate) than the other provenances, implying a greater ability to tolerate water stress. It can be expected that preconditioned seedlings will display greater tolerance of water stress than non-conditioned plants and perform better during early establishment (higher survival and early growth).     

Growth rates of baldcypress (Taxodium distichum) seedlings in a treated effluent assimilation marsh

Wetlands have proven effective at improving water quality of treated wastewater effluent, which in turn promotes increased primary productivity and vertical accretion. Baldcypress (Taxodium distichum) seedlings grown under different conditions (bare root and potted) were planted in four subunits of an effluent assimilation marsh and a control marsh in southeast Louisiana, USA, and basal diameter growth was monitored over one growing season. Mean basal diameter growth for seedlings in the assimilation subunits ranged from 16.1 (±1.4) mm to 9.5 (±0.9) mm, whereas growth for seedlings planted in the control marsh was 6.4 (±0.9) mm. Seedlings planted nearest the outfall experienced greater basal diameter growth (18.1 ± 2.6) compared to those planted 700 m away (8.0 ± 0.9), with growth generally decreasing with distance. Potted seedlings experienced greater growth (19.1 ± 1.0 and 20.6 ± 1.0 for five-month-olds and ten-month olds, respectively) than bare root seedlings (4.6 ± 0.6 and 4.0 ± 0.4 for one-year-olds and two-year olds, respectively). Planting assimilation marshes with baldcypress seedlings can be an effective restoration tool for coastal Louisiana, which will provide hurricane protection and improved surface water quality. Wastewater treatment wetlands may offer an effective tool for restoring coastal baldcypress (T. distichum)-water tupelo (Nyssa aquatic) swamps in Louisiana.     

The effect of spawning salinity on eggs of spotted seatrout (Cynoscion nebulosus, Cuvier) from two bays with historically different salinity regimes

Spotted seatrout are capable of spawning in a wide range of salinities. Along the Texas Gulf Coast, bay salinities increase from an average of 14 ppt in Galveston Bay to an average of 40 ppt in Lower Laguna Madre due to the negative gradient of freshwater inflow from north to south. Tagging studies have shown that the majority of spotted seatrout do not migrate between adjacent bay systems. Spawning salinity has been shown to affect many properties of eggs including the diameter and salinity of neutral buoyancy. Spotted seatrout from two historically different salinity regimes (Matagorda Bay (MB) and Upper Laguna Madre (ULM)) were kept in the laboratory and induced to spawn in three salinities: 20, 30, and 40 ppt. The purpose of this study was to evaluate eggs at each of the three salinities and between the two bay systems. Two-way ANOVA showed a significant effect on the egg diameter of bay and spawning salinity, and a significant interaction between bays and spawning salinity. No significant difference in size at hatch was found between spawning salinities or between bays. Hatch rates in spawning salinity were >90% in all cases. Regression of wet weight on spawning salinity was highly significant for both bays. Eggs spawned in 20 ppt have the largest wet weight and eggs spawned in 40 ppt have the smallest wet weight, irrespective of parental bay origin. Percentage of water varied between 92% for 20 ppt spawned eggs and 86% in 40 ppt spawned eggs. Neutral Buoyancy Salinity (NBS) of eggs increased with increasing spawning salinity. Eggs spawned by the Upper Laguna Madre fish held at 20 ppt were not positively buoyant at 20 ppt. The results of this study suggest that spotted seatrout are locally adapted to the prevailing salinity regime within an estuary.     

Expression of multiple resistance genes enhances tolerance to environmental stressors in transgenic poplar (Populus × euramericana 'Guariento')

Commercial and non-commercial plants face a variety of environmental stressors that often cannot be controlled. In this study, transgenic hybrid poplar (Populus × euramericana 'Guariento') harboring five effector genes (vgb, SacB, JERF36, BtCry3A and OC-I) were subjected to drought, salinity, waterlogging and insect stressors in greenhouse or laboratory conditions. Field trials were also conducted to investigate long-term effects of transgenic trees on insects and salt tolerance in the transformants. In greenhouse studies, two transgenic lines D5-20 and D5-21 showed improved growth, as evidenced by greater height and basal diameter increments and total biomass relative to the control plants after drought or salt stress treatments. The improved tolerance to drought and salt was primarily attributed to greater instantaneous water use efficiency (WUEi) in the transgenic trees. The chlorophyll concentrations tended to be higher in the transgenic lines under drought or saline conditions. Transformed trees in drought conditions accumulated more fructan and proline and had increased Fv/Fm ratios (maximum quantum yield of photosystem II) under waterlogging stress. Insect-feeding assays in the laboratory revealed a higher total mortality rate and lower exuviation index of leaf beetle [Plagiodera versicolora (Laicharting)] larvae fed with D5-21 leaves, suggesting enhanced insect resistance in the transgenic poplar. In field trials, the dominance of targeted insects on 2-year-old D5-21 transgenic trees was substantially lower than that of the controls, indicating enhanced resistance to Coleoptera. The average height and DBH (diameter at breast height) of 2.5-year-old transgenic trees growing in naturally saline soil were 3.80% and 4.12% greater than those of the control trees, but these increases were not significant. These results suggested that multiple stress-resistance properties in important crop tree species could be simultaneously improved, although additional research is needed to fully understand the relationships between the altered phenotypes and the function of each transgene in multigene transformants.     

BALDCYPRESS SEEDLING GROWTH IN THERMALLY ALTERED HABITATS

Thermal discharges from nuclear production reactors killed large areas of baldcypress-water tupelo (Taxodium distichum-Nyssa aquatica) forests along streams on the Savannah River Plant in South Carolina. Recolonization by baldcypress has been slow in areas where present stream temperatures do not exceed ambient by more than 10 C and has not occurred at all at higher temperatures. Since it has been previously shown that cypress germination and establishment are not inhibited at stream temperature below 42 C, the growth of 1-yr-old baldcypress seedlings introduced into ambient (T_max = 32 C) and ambient + 10 C (T_max = 42 C) stream temperatures was examined during a 16-month period to determine the response of this growth stage. Growth in height, diameter and biomass of these seedlings was enhanced in a stream habitat with maximum water temperatures of 42 C. Allocation of dry weight into root and shoot tissues was modified by stream temperature, with a lower root/shoot ratio in plants at the 42 C location. In thermally altered (-42 C) areas, neither establishment nor growth of established cypress seedlings is limited by stream temperature. Thus, slow reinvasion of these sites by baldcypress is thought to be due to a lack of a seed source and/or competitive exclusion.     

白刺花幼苗对不同强度干旱胁迫的形态与生理响应

为了探讨白刺花 (Sophora davidii (Franch.) Skeels) 幼苗对持续干旱胁迫的适应能力及对策,用盆栽方法人工模拟土壤干旱条件,设置土壤田间持水量 (WHC) 100%、80%、60%、40%和20% 5个干旱胁迫处理,研究了幼苗生长、生物量分配、水分利用效率 (WUE) 、叶形态解剖结构以及光合色素等在不同干旱胁迫强度下的变化特点.结果显示,胁迫处理96d后,100% WHC条件下白刺花幼苗的总叶面积、分枝数、基径及最大根长等均最大;80% WHC对幼苗产生了轻度胁迫,随着干旱胁迫强度的进一步增加,幼苗生长显著减小.干旱胁迫限制了新生叶发生与单叶面积扩展,导致冠层总叶面积减小,从而引起幼苗光合能力及生物量积累均降低.随着胁迫强度的增加,叶生物量所占的比例及叶面积/根生物量的比值都明显减小,而根生物量所占的比例增大,这说明叶生长对干旱胁迫反应比根更加敏感.另外,干旱胁迫下WUE随着生物量与耗水量的减小而降低,表明幼苗具有浪费型水分利用对策.叶绿素 (Chla、Chlb和Chla+b) 及类胡萝卜素含量 (Car) 都随着干旱胁迫增强而呈增大趋势,但Chla/b及 Chl/Ca变化趋势相反.干旱胁迫对叶片解剖结构影响较小,土壤水分减少时仅栅栏组织厚度略有增厚,海绵组织变薄.研究结果证明,60% WHC是幼苗生长、生物量积累、WUE和光捕获复合体活性受到明显抑制的干旱胁迫强度阈值;20% WHC胁迫处理对幼苗产生了严重危害.但是在实验过程中,即使在20%WHC条件下也未出现叶片凋落及幼苗死亡,表明当年生白刺花具有较强的干旱忍受能力,幼苗通过减小地上蒸发面积、增加地下生长及叶绿素含量等多种形态与生理策略适应干旱胁迫.