Elemental distribution in tissue components of N2-fixing nodules of Psoralea pinnata plants growing naturally in wetland and upland conditions in the Cape Fynbos of South Africa

There is little information on in situ distribution of nutrient elements in N₂-fixing nodules. The aim of this study was to quantify elemental distribution in tissue components of N₂-fixing nodules harvested from Psoralea pinnata plants grown naturally in wetland and upland conditions in the Cape Fynbos. The data obtained from particle-induced X-ray emission revealed the occurrence of 20 elements (Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, As, Br, Rb, Sr, Y, Zr, Mo and Ba) in nodule components. Although, in upland plants, the concentrations of S, Fe, Si, Mn and Cu showed a steady increase from the middle cortex to the medulla region of P. pinnata nodules, in wetland plants, only S, Fe and Mn showed an increase in concentration from the middle cortex to the bacteria-infected medulla of P. pinnata nodules. By contrast, the concentrations of Cl, K, Ca, Zn and Sr decreased from middle cortex to nodule medulla. The alkaline earth, alkali and transition elements Rb, Sr, Y and Zr, never before reported in N₂-fixing nodules, were found to occur in root nodules of P. pinnata plants grown in both wetland and upland conditions.     

潜流人工湿地中植物对氮磷净化影响效应的研究

采用潜流人工湿地系统,配制以NH⁺₄-N、NO^-₃-N和PO^3-₄-P为主要成分的模拟污水,通过间歇运行方式,考察了芦苇和小叶章的生长情况、生理生态学特性及其对污水中N、P净化效能的影响,并研究了植物对湿地系统pH变化、NO^-₃-N和NH⁺₄-N净化效率的影响。结果表明,当水力停留时间为7d时,小叶章和芦苇湿地对TN的去除率分别为65.1%和99.6%,去除负荷分别为1.66g · m^-3 · d^-1和2.53g · m^-3 · d^-1。小叶章和芦苇对去除TN的贡献率分别为14.7%、61.7%,对去除TP的贡献率分别为11.7%和12.9%;芦苇植株内N、P浓度分别为29.2mg/g和3.41mg/g。芦苇湿地的净化效能高于小叶章湿地。湿地系统中pH值先升高后降低的拐点可作为氨氧化反应结束的指示参数。     

Response of Two Dominant Boreal Freshwater Wetland Plants to Manipulated Warming and Altered Precipitation

This study characterized the morphological and photosynthetic responses of two wetland plant species when they were subject to 2–6°C fluctuations in growth temperature and ±50% of precipitation, in order to predict the evolution of natural wetlands in Sanjiang Plain of North-eastern China. We investigated the morphological and photosynthetic responses of two dominant and competitive boreal freshwater wetland plants in Northeastern China to manipulation of warming (ambient, +2.0°C, +4.0°C, +6.0°C) and altered precipitation (−50%, ambient, +50%) simultaneously by incubating the plants from seedling to senescence within climate-controlled environmental chambers. Post-harvest, secondary growth of C. angustifolia was observed to explore intergenerational effects. The results indicated that C. angustifolia demonstrated a greater acclimated capacity than G. spiculosa to respond to climate change due to higher resistance to temperature and precipitation manipulations. The accumulated effect on aboveground biomass of post-harvest secondary growth of C. angustifolia was significant. These results explain the expansion of C. angustifolia during last 40 years and indicate the further expansion in natural boreal wetlands under a warmer and wetter future. Stability of the natural surface water table is critical for the conservation and restoration of G. spiculosa populations reacting to encroachment stress from C. angustifolia expansion.     

Root aerobic respiration and growth characteristics of three Typha species in response to hypoxia

The responses of root aerobic respiration to hypoxia in three common Typha species were examined. Typha latifolia L., T. orientalis Presl, and T. angustifolia L. were hydroponically cultivated under both aerobic and hypoxic growth conditions to measure root oxygen consumption rates. Hypoxia significantly enhanced the root aerobic respiration capacity of the two deep-water species, T. orientalis and T. angustifolia, while it did not affect that of the shallow-water species, T. latifolia. T. angustifolia increased its root porosity and root mass ratio, while T. latifolia increased its root diameter under the hypoxic growth conditions. The relative growth rates in biomass of T. orientalis and T. angustifolia were 59 and 39% higher, respectively, under the hypoxic growth conditions than under the aerobic growth conditions. In contrast, that of T. latifolia did not differ between the two conditions. In T. orientalis and T. angustifolia, enhanced root aerobic respiration rates under the hypoxic growth conditions would have increased the nutrient uptake, and thus higher relative growth rates were obtained. For the deep-water species, T. orientalis and T. angustifolia, the root aerobic respiration capacity was enhanced, probably in order to maintain the generation of respiratory energy under hypoxia.     

Characterization of phosphorus uptake in slow- and fast-growing southern pine seedlings grown in solution culture

The effects of low-P growth conditions on growth and net P acquisition were examined in two species of pine that are indigenous to P-deficient soils of the Atlantic Coastal Plain: pond pine (Pinus serotina Michx.), a moderately-fast growing pine, and a slow-growing seed source of loblolly pine P. taeda L.) from Texas. Short-term ³²P uptake experiments were conducted using intact nonmycorrhizal seedlings that had been grown for 7 weeks in continuously-flowing solution culture at 5 or 100 µM P. Growth and P uptake of pond pine were more responsive to a higher P supply than the slow-growing loblolly pine. Pond pine seedlings in the 100 µM P treatment were twice the size of those grown in 5 µM P and accumulated almost five times as much seedling P. In contrast, seedling biomass of loblolly pine increased by only 8% under high-P growth conditions, and seedlings accumulated twice as much P, reflecting the higher P concentrations in shoot and root tissues. Although rates of unidirectional influx of ³²P were 22 and 61% higher under low-P growth conditions in pond and loblolly pine, respectively, net uptake rates in seedlings from the 5 µM P treatment were over three times those of seedlings grown in 100 µM P. These results suggest that unidirectional efflux out of the root was controlling net uptake of P as much, if not more, than unidirectional influx. Efflux of³² P out of root tissue, particularly older tissue, decreased in seedlings grown under low-P conditions, possibly due to a reduction in the size of the phosphorus pool available for efflux, i.e. the soluble P_i pool. Over 75% of the total root P in both loblolly and pond pine seedlings grown in 100 µM P treatment was present as organic P, suggesting that organic P, particularly phytate, may represent important storage pools in roots of woody species. Within each species, higher rates of influx and net uptake in seedlings from the low-P treatment were associated with lower P concentrations in shoot and root tissue, and shoot FW:root FW ratios. Efflux may represent a short-term means of regulating net P uptake, while the demand for P created by growth and storage may represent a long-term regulation.     

Waterlogging tolerance in the tribe Triticeae: the adventitious roots of Critesion marinum have a relatively high porosity and a barrier to radial oxygen loss

Nine species from the tribe Triticeae – three crop, three pasture and three ‘wild’ wetland species – were evaluated for tolerance to growth in stagnant deoxygenated nutrient solution and also for traits that enhance longitudinal O₂ movement within the roots. Critesion marinum (syn. Hordeum marinum) was the only species evaluated that had a strong barrier to radial O₂ loss (ROL) in the basal regions of its adventitious roots. Barriers to ROL have previously been documented in roots of several wetland species, although not in any close relatives of dryland crop species. Moreover, the porosity in adventitious roots of C. marinum was relatively high: 14% and 25% in plants grown in aerated and stagnant solutions, respectively. The porosity of C. marinum roots in the aerated solution was 1·8–5·4‐fold greater, and in the stagnant solution 1·2–2·8‐fold greater, than in the eight other species when grown under the same conditions. These traits presumably contributed to C. marinum having a 1·4–3 times greater adventitious root length than the other species when grown in deoxygenated stagnant nutrient solution or in waterlogged soil. The length of the adventitious roots and ROL profiles of C. marinum grown in waterlogged soil were comparable to those of the extremely waterlogging‐tolerant species Echinochloa crus‐galli L. (P. Beauv.). The superior tolerance of C. marinum, as compared to Hordeum vulgare (the closest cultivated relative), was confirmed in pots of soil waterlogged for 21 d; H. vulgare suffered severe reductions in shoot and adventitious root dry mass (81% and 67%, respectively), whereas C. marinum shoot mass was only reduced by 38% and adventitious root mass was not affected.     

Variation in the incidence of H2-oxidizing chemolithotrophic bacteria in rice grown under different cultivation conditions

Summary The incidence of H₂-oxidizing chemolithotrophic bacteria associated with rice grown under continuous wetland, upland, and rainfed wetland conditions was studied by¹⁴C-autoradiographic technique in a neutral soil at IRRI (Maahas) and an acid rainfed wetland soil (Luisiana).In Maahas soil, H₂-oxidizing chemolithotrophic bacteria were not detected in the endorhizosphere, rhizosphere, and nonrhizosphere soil of rice grown under dryland conditions. Under continuously flooded conditions a very large population of these bacteria were found in the endorhizosphere but not in the oxidized and reduced soil.A very low population of these bacteria were found in the endorhizosphere and basal culm of rice grown under rainfed wetland conditions at Luisiana. Bacteria isolated from Maahas wetland rice and inoculated to rice seedling planted in Luisiana soil failed to establish.Both Maahas and Luisiana soils consumed externally supplied H₂ and produced H₂ and CH₄ almost at the same rate when they were amended with rice straw or sucrose. This paper discusses possible causes of variation in the number of these bacteria and their distribution in rice grown under different cultural and soil conditions.     

Population of Aerobic Heterotrophic Nitrogen-Fixing Bacteria Associated with Wetland and Dryland Rice

Nitrogen-fixing activity and populations of nitrogen-fixing bacteria associated with two varieties of rice grown in dryland and wetland conditions were measured at various growth stages during the dry season. Acetylene reduction activities were measured both in the field and for the hydroponically grown rice, which was transferred from the field to water culture 1 day before assay. The activities measured by both methods were higher in wetland than in dryland rice. The population of nitrogen-fixing heterotrophic bacteria associated with rhizosphere soil, root, and basal shoots was determined by the most probable number method with semisolid glucose-yeast extract and semisolid malate-yeast extract media. The number of nitrogen-fixing bacteria was higher in wetland conditions than in dryland conditions. The difference between two conditions was most pronounced in the population associated with the basal shoot. The glucose medium gave higher counts than did the malate medium. Colonies were picked from tryptic soy agar plates, and their nitrogen-fixing activity was tested on a semisolid glucose-yeast extract medium. The incidence of nitrogen-fixing bacteria among aerobic heterotrophic bacteria in association with rhizosphere soil, root, and basal shoots was much lower in dryland rice than in wetland rice.     

Accumulation properties of As,Cd, Cu,Pb and Zn by four wetland plant species growing on submerged mine tailings

Plants may reduce element leakage from submerged mine tailings by phytostabilisation. However, high shoot concentrations of elements might disperse them and could be harmful to grazing animals. The aim of this investigation was to find out which of the three properties; low-accumulation, root accumulation or shoot accumulation of elements, occur in four of the most common wetland species growing on an old submerged mine tailings and if their properties can be determined by a hydroponic experiment. Above- and below-ground parts of Salix (mixed tissue from S. phylicifolia and S. borealis), Carex rostrata, Eriophorum angustifolium, and Phragmites australis were sampled and analysed for Cd, Cu, Zn, Pb and As. Differences in uptake and translocation properties of the same plant species were observed between field-grown plants and plants grown in hydroponics. These differences were probably due to processes in the soil–root interface. Thus, hydroponic screening studies should not be used to find suitable species for vegetation of wet-covered mine tailings. Most species were found to have restricted translocation of elements to the shoot, i.e. they were root accumulators, and only the shoot concentrations of Salix for Cd and Zn and E. angustifolium for Pb might be toxic to grazing animals. Thus, plant establishment on submerged tailings can be a safe method to stabilise the metals.     

Soil and root populations of fluorescent Pseudomonas spp. associated with seedlings and field-grown plants are affected by sorghum genotype

Sorghum [Sorghum bicolor (L.) Moench] is valued for bioenergy, feed and food. Potential of sorghum genotypes to support differing populations of root- and soil-associated fluorescent Pseudomonas spp. or Fusarium spp., in two soils, was assessed. Culturable pseudomonads were enumerated from roots and soil of sorghum (Redlan and RTx433) and wheat (Lewjain) seedlings repeatedly grown in cycled soils in the growth chamber. Pseudomonads and Fusarium spp. were assessed from roots and soil of field-grown sorghum along with biological control traits hydrogen cyanide (HCN) and 2,4-diacetylphlorogluconol (phl) production. After four 4-week cycles, soil associated with Redlan seedlings had greater numbers of fluorescent pseudomonads than Lewjain. In dryland field conditions, RTx433 roots had greater numbers of pseudomonads than Redlan before anthesis but similar numbers after. There were no differences in numbers of pseudomonads from dryland soil or roots or soil of irrigated plants. Percentages of HCN-producing root isolates and phl soil isolates declined on irrigated Redlan plants, but percentages of HCN-producers increased in dryland conditions. Redlan roots had greater percentages of Fusarium isolates in the Gibberella fujikuroi complex. Results indicated that sorghum genotype affected root-associated populations of fluorescent Pseudomonas spp. and Fusarium spp. across soil environments.     

Monoterpene synthesis in shoots regenerated from callus cultures

-Shoots regenerated from two-year old callus cultures of Lavandula angustifolia and Rosmarinus officinalis accumulated monoterpenes characteristic of the parent tissue. No such compounds could be detected in undifferentiated callus maintained under a variety of conditions.     

Potassium uptake efficiency and dynamics in the rhizosphere of maize (Zea mays L.), wheat (Triticum aestivum L.), and sugar beet (Beta vulgaris L.) evaluated with a mechanistic model

Plant species differ in nutrient uptake efficiency. With a pot experiment, we evaluated potassium (K) uptake efficiency of maize (Zea mays L.), wheat (Triticum aestivum L.), and sugar beet (Beta vulgaris L.) grown on a low-K soil. Sugar beet and wheat maintained higher shoot K concentrations, indicating higher K uptake efficiency. Wheat acquired more K because of a greater root length to shoot dry weight ratio. Sugar beet accumulated more shoot K as a result of a 3- to 4-fold higher K influx as compared to wheat and maize, respectively. Nutrient uptake model NST 3.0 closely predicted K influx when 250 mg K kg^?1 were added to the soil, but under-predicted K influx under low K supply. Sensitivity analysis showed that increasing soil solution K concentration (C_Li) by a factor of 1.6–3.5 or buffer power (b) 10- to 50-fold resulted in 100% prediction of K influx. When both maximum influx (I_max) and b were increased by a factor of 2.5 in maize and wheat and 25 in sugar beet, the model could predict measured K influx 100%. In general, the parameter changes affected mostly calculated K influx of root hairs, demonstrating their possible important role in plant K efficiency.     

NaCl胁迫下沙枣幼苗生长和阳离子吸收、运输与分配特性

沙枣(Elaeagnus angustifolia L.)耐盐性强,是我国北方生态脆弱地区造林绿化的一个先锋树种。为探讨沙枣的盐适应机制,研究了不同浓度NaCl(0、100和200 mmol/L)胁迫30d对其水培幼苗生物量累积以及不同组织(根、茎、叶)K+、Na+、Ca2+和Mg2+吸收、运输与分配的影响。结果表明:盐胁迫不同程度地促进了沙枣苗根系生长;100 mmol/L NaCl胁迫对幼苗生物量累积无明显影响,而200 mmol/L则显著抑制了生物量累积;盐胁迫幼苗根、茎、叶中Na+含量以及K+-Na+选择性运输系数(S K,Na)和Ca2+-Na+选择性运输系数(S Ca,Na)显著或大幅度增加,而K+、Ca2+、Mg2+含量以及K+/Na+、Ca2+/Na+和Mg2+/Na+比值则显著或大幅度下降;200 mmol/L NaCl胁迫沙枣根Na+含量和根Na+净累积量分别为22.15 mg/g干重和1.87 mg/株(是对照的16.20倍和20.06倍),根成为Na+净累积量增加幅度最大的组织和Na+含量最高的组织;200 mmol/L NaCl胁迫沙枣茎、叶中的Na+含量以及冠组织Na+净累积量分别高达5.15、7.71 mg/g干重和3.29 mg/株(是对照的7.22倍、9.58倍和5.45倍),但幼苗仍能正常生长。综合分析认为,沙枣的盐适应机制是根系拒盐和冠组织耐盐,主要通过根系的补偿生长效应、根系对Na+的聚积与限制作用以及冠组织对Na+的忍耐来实现的,同时也与根、茎和叶对K+、Ca2+选择性运输能力显著增强有关。     

Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands

In constructed wetlands, solids accumulation may have two consequences with opposing effects on treatment efficiency: it decreases the longevity by reducing void space and it enhances biological activity by favoring biofilm development. The goal of our study was to estimate the effect of plants (presence and species) and artificial aeration on solids accumulation (volatile and inorganic). The horizontal and vertical distribution of solids was sampled using solids traps in 12 constructed wetland mesocosms (5 years old). Microbial density and activity were estimated in the biological fraction of the sampled solids. The effect of plant presence reduced accumulated solids by 26% and sulphide content by 50% sulphide content. There was more solids accumulation in Typha angustifolia units than in Phragmites australis. Also, T. angustifolia generated more biological activities at the surface and close to the inlet while conditions were more homogeneous throughout P. australis units. Aeration (1) stimulated biofilm development at the inlet of planted beds, (2) seemed to reduce mineral matter accumulation and (3) generated the same pattern of activities in planted beds enabling to reach a total nitrogen removal rate of up to 0.65 g N m^?2 d^?1.     

Soil properties and actinorhizal vegetation influence nodulation of Alnus glutinosa and Elaeagnus angustifolia by Frankia

Nodulation (mean number of nodules per seedling) was 5 times greater for Elaeagnus angustifolia than for Alnus glutinosa overall when seedlings were grown in pots containing either an upland or an alluvial soil from central Illinois, USA. However, the upland Alfisol had 1.3 times greater nodulation capacity for A. glutinosa than for E. angustifolia. The presence of A. glutinosa trees on either soil was associated with a two-fold increase in nodulation capacity for E. angustifolia. Nodulation increases for soils under A. glutinosa were obtained for A. glutinosa seedlings in the Alfisol, but decreased nodulation for A. glutinosa seedlings occurred in the Mollisol. Greatest nodulation of E. angustifolia seedlings occurred near pH 6.6 for soil pH values ranging from 4.9 to 7.1, while greatest nodulation of A. glutinosa occurred at pH 4.9 over the same pH range. Nodulation was not affected by total soil nitrogen concentrations ranging from 0.09 to 0.20%. Mollisol pH was significantly lower under A. glutinosa trees than under E. angustifolia trees. For 4- to 8-year-old field-grown trees, A. glutinosa nodule weights were negatively correlated with soil pH, while for similar aged E. angustifolia trees nodulation in the acidic Alfisol was not detected.     

Nitrogen utilization by Typha latifolia L. as affected by temperature and rate of nitrogen application

Greenhouse and growth chamber studies were conducted to evaluate growth and N utilization by Typha latifolia L. in flooded organic soil under varying temperatures and rates of N additions. Elevation of temperature from 10 to 25°C increased shoot biomass yields by 275%. Root biomass yields were lowest at 10°C and increased linearly as a function of temperature. Shoot/root ratios were low (0.72–0.82) at lower temperatures (10–15°C) and ratios increased by about three times at higher temperatures (20–30°C). Biomass yields were increased by addition of N fertilizers, while the shoot/root ratios were directly related to plant-available N present in the soil.Fertilizer ¹⁵N uptake (expressed as % of applied N) by the whole plant was 5.3% at 10°C, 37.5% at 20°C and at 30°C decreased to 20.8%. Fertilizer N accumulation in shoots was 2.1–29.8% of applied N, while roots accumulated 3.2–7.7%. Under greenhouse conditions, N uptake by T. latifolia was found to increase with increased rate of N application. Fertilizer N uptake by both shoots and roots was in the range of 61–77%. Plants cultured in growth chambers were affected by low light conditions resulting in poor growth and low fertilizer ¹⁵N uptake, as compared to plants grown under greenhouse conditions. Added fertilizer N was the major source of N during the early part of the growing season, while soil organic N was the major and perhaps the sole source of N during the latter part of the growing season.     

蚯蚓对湿地植物光合特性及净化污水能力的影响

以香蒲、芦苇和美人蕉为研究对象,并以土壤+沙子+有机质混合物为供试基质模拟人工湿地处理污水,采用向基质中加入蚯蚓与未加入蚯蚓2种处理。研究加入蚯蚓后,香蒲、芦苇和美人蕉光合速率、蒸腾速率、SPAD值和水分蒸发、蒸腾量的变化及其对净化污水能力的影响。结果表明:与未加入蚯蚓相比,加入蚯蚓后,香蒲、芦苇和美人蕉的净光合速率、蒸腾速率、SPAD值和水分蒸发、蒸腾量均增加,其中芦苇的净光合速率、蒸腾速率和水分蒸发、蒸腾量增加达到显著水平(P <0.05),而香蒲的水分蒸发、蒸腾量增加也达到显著水平(P <0.05);加入蚯蚓后,香蒲、芦苇和美人蕉对COD_Mn、NH₄⁺-N、NO₃^--N、TN和TP的去除率均增加,且香蒲和芦苇对COD_Mn的去除率显著增加 (P <0.05)。加入蚯蚓后,香蒲、芦苇和美人蕉的SPAD值均增加,说明蚯蚓能提高湿地植物对氮的吸收,增加植株中的氮含量,促进湿地植物的光合速率和蒸腾速率从而提高对污水的净化能力。     

Abstracts of Platform Presentations from the Phytoremediation Session at the 14th Annual West Coast Conference on Contaminated Soils,Sediments, and Water (March 15–18, San Diego,CA)

Previous field studies indicate that zucchini (Cucurbita pepo) has a unique ability to phytoextract persistent organic pollutants from soil. It is unlikely that C. pepo evolved a unique mechanism favoring POP extraction and uptake, but all plants have evolved means to facilitate nutrient acquisition from soil. We have hypothesized that the exudation of organic acids as a means to acquire phosphorus could facilitate the uptake of persistent organic pollutants by increasing contaminant bioavailability to the plants. In one study, we assessed DDE uptake and organic acid exudation by zucchini (an uptaker of POPs) and cucumber (a non-uptaker of POPs) under various cultivation and nutrient conditions. Under dense planting (5 plants in a 5-kg pot of DDE-contaminated soil), zucchini accumulated significant and expected amounts of DDE but surprisingly, under these stressed conditions, cucumber phytoextracted greater amounts of DDE. The cucumber rhizosphere concentrations of organic acids were significantly higher than that of zucchini, suggesting that the increased organic acid exudation promoted DDE uptake by cucumber. Conversely, under non-stressed conditions zucchini phytoextracted significantly greater quantities of pollutant than cucumber but no differences in organic acid content of the rhizosphere of the two species were observed. Separately, zucchini and other species were grown under field conditions and weekly amendments of different nutrients were made (nitrogen, phosphorus, nitrogen/phosphorus, aluminum sulfate to bind phosphorus in the soil). The uptake and translocation of the weathered pollutant and inorganic elements was found to vary with nutrient amendments. Lastly, data will be presented from rhizotron units constructed to facilitate not only the direct in situ isolation of exuded organic acids but also the isolation of xylem sap and rhizosphere soil pore water from individual plants. The role of cultivation conditions and nutrient availability in controlling root morphology, organic acid exudation, and contaminant uptake will be discussed.     

Increased spatial dominance in high nitrogen, saturated soil due to clonal architecture plasticity of the invasive wetland plant, Phalaris arundinacea

Clonality as a plant growth strategy has been a successful adaptation contributing to clonal plants being the dominant vegetation in many ecosystems and has been implicated as a significant factor contributing to invasiveness. The objective of this study was to determine if Phalaris arundinacea, an invasive wetland plant, modifies its clonal growth behavior when grown in high resource conditions. When grown in ideal conditions (high soil-N and moisture), we hypothesized that along with an increase in tiller production and robustness (biomass per tiller), P. arundinacea would increase the spatial spread of tiller placement (distance from parent and daughters). To test this we conducted a greenhouse study in which we grew P. arundinacea seedlings under two soil-N levels (no nitrate addition or 40 g N m^?2 year^?1) at two soil moisture levels (dry or saturated) for 10 weeks and recorded the placement (angle and distance from the parent plant) of each tiller produced. Total aboveground and belowground biomass, shoot/root ratio, and biomass per tiller were measured at the conclusion of the experiment. Plants grown in saturated conditions produced significantly more tillers that were more widely dispersed. Surprisingly, soil-N did not significantly affect most characteristics of spatial pattern, though soil-N did affect biomass production, shoot/root ratio, and biomass per tiller. These results indicate soil moisture and soil-N affect different aspects of the clonal growth behavior of P. arundinacea in the early stage of colonization. This new information provides a mechanism to explain how P. arundinacea aggressively competes for space in wetland habitats.     

Maintenance of 32P uptake and transport in Pinus serotina seedlings under hypoxic growth conditions

In the present study, we examined the effects of long- and short-term hypoxia on net uptake and transport of phosphorus to shoots of pond pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine, and the influence aerenchyma formation might have in maintenance of P uptake and transport. Seedlings were grown under aerobic (250 μM O₂) or hypoxic (≤50 μM O₂) solution conditions for 5.3 weeks in continuously flowing solution culture containing 100 μM P. Intact seedlings were then labeled with ³²P for up to 24 h to determine how short- and long-term hypoxic solution conditions affected rates of unidirectional influx and the accumulation of ³²P in roots and shoots. Seedlings in the long-term hypoxic treatment were grown for 5.3 weeks in hypoxic solution and also labeled in hypoxic uptake solution. The short-term hypoxic treatments included a 24-h hypoxic pretreatment followed by time in labeled hypoxic uptake solution for seedlings grown under aerobic or hypoxic conditions; in the latter case, diffusion of atmospheric O₂ entry into stem and root collar lenticels was blocked, thus removing any influence that aerenchyma formation might have had on enhancing O₂ concentrations of root tissue. Although unidirectional influx rates of ³²P in roots of seedlings grown under long-term hypoxic conditions were 1.4 times those of aerobically grown seedlings, accumulation of ³²P in roots was similar after 24 h in labeled uptake solution. These results suggest that ³²P efflux was also higher under hypoxic conditions. Higher shoot/root fresh weight ratios and lower shoot P concentrations in seedlings grown under hypoxic solution conditions suggest that the “shoot P demand” per unit root should be high. Yet accumulation of ³²P in shoots was reduced by 50% after 24 h in hypoxic uptake solution. Both short-term hypoxic treatments decreased accumulation of ³²P in roots by more than 50%. Short-term hypoxia decreased shoot accumulation in seedlings grown under aerobic and hypoxic conditions by 84 and 50%. respectively. Short- and long-term hypoxic conditions increased the percentage of root ³²P in the nucleic acid and chelated-P pools, resulting in a significantly smaller percentage of ³²P in the soluble inorganic phosphate (p_i) pool, the pool available for transport to the shoot. However, a reduction in pool size or in labeling of the pool available for transport cannot fully account for the large reduction in accumulation of ³²P in shoots, particularly in the short-term hypoxic treatment of aerobically grown seedlings. Our results suggest that both influx and transport of ³²P to shoots of pond pine seedlings are O₂-dependent processes, and that the transport of ³²P to shoots may be more sensitive to hypoxic solution conditions than influx at the cortical and epidermal plasmalemma, with aerenchyma formation supporting a substantial amount of both ³²P uptake and transport.