Drying of mucilage causes water repellency in the rhizosphere of maize: measurements and modelling

Background and Aims

Although maize roots have been extensively studied, there is limited information on the effect of root exudates on the hydraulic properties of maize rhizosphere. Recent experiments suggested that the mucilaginous fraction of root exudates may cause water repellency of the rhizosphere. Our objectives were: 1) to investigate whether maize rhizosphere turns hydrophobic after drying and subsequent rewetting; 2) to test whether maize mucilage is hydrophobic; and 3) to find a quantitative relation between rhizosphere rewetting, particle size, soil matric potential and mucilage concentration.

Methods

Maize plants were grown in aluminum containers filled with a sandy soil. When the plants were 3-weeks-old, the soil was let dry and then it was irrigated. The soil water content during irrigation was imaged using neutron radiography. In a parallel experiment, ten maize plants were grown in sandy soil for 5 weeks. Mucilage was collected from young brace roots growing above the soil. Mucilage was placed on glass slides and let dry. The contact angle was measured with the sessile drop method for varying mucilage concentration. Additionally, capillary rise experiments were performed in soils of varying particle size mixed with maize mucilage. We then used a pore-network model in which mucilage was randomly distributed in a cubic lattice. The general idea was that rewetting of a pore is impeded when the concentration of mucilage on the pore surface (g cm^?2) is higher than a given threshold value. The threshold value depended on soil matric potential, pore radius and contract angle. Then, we randomly distributed mucilage in the pore network and we calculated the percolation of water across a cubic lattice for varying soil particle size, mucilage concentration and matric potential.

Results

Our results showed that: 1) the rhizosphere of maize stayed temporarily dry after irrigation; 2) mucilage became water repellent after drying. Mucilage contact angle increased with mucilage surface concentration (gram of dry mucilage per surface area); 3) Water could easily cross the rhizosphere when the mucilage concentration was below a given threshold. In contrast, above a critical mucilage concentration water could not flow through the rhizosphere. The critical mucilage concentration decreased with increasing particle size and decreasing matric potential.

Conclusions

These results show the importance of mucilage exudation for the water fluxes across the root-soil interface. Our percolation model predicts at what mucilage concentration the rhizosphere turns hydrophobic depending on soil texture and matric potential. Further studies are needed to extend these results to varying soil conditions and to upscale them to the entire root system.

     

Quantitative evaluation of the role of organic acid exudation in the mobilization of rock phosphate by rape

Phosphorus-deficient rape plants appear to acidify part of their rhizosphere by exuding malic and citric acid. A simulation model was used to evaluate the effect of measured exudation rates on phosphate uptake from Mali rock phosphate. The model used was one on nutrient uptake, extended to include both the effect of ion uptake and exudation on rhizosphere pH and the effect of rhizosphere pH on the solubilization of rock phosphate. Only the youngest zones of the root system were assumed to exude organic acids. The transport of protons released by organic acids was described by mass flow and diffusion. An experimentally determined relation was used describing pH and phosphate concentration in the soil solution as a function of total soil acid concentration. Model parameters were determined in experiments on organic acid exudation and on the uptake of phosphate by rape from a mixture of quartz sand and rock phosphate. Results based on simulation calculations indicated that the exudation rates measured in rape plants deficient in phosphorus can provide the roots with more phosphate than is actually taken up. Presence of root hairs enhanced the effect of organic acid exudation on calculated phosphate uptake. However, increase of root hair length without exudation as an alternative strategy to increase phosphate uptake from rock phosphate appeared to be less effective than exudation of organic acids. It was concluded that organic acid exudation is a highly effective strategy to increase phosphate uptake from rock phosphate, and that it unlikely that other rhizosphere processes play an important role in rock phosphate mobilization by rape.     

The “isohydric trap”: A proposed feedback between water shortage,stomatal regulation,and nutrient acquisition drives differential growth and survival of European pines under climatic dryness

Climatic dryness imposes limitations on vascular plant growth by reducing stomatal conductance, thereby decreasing CO₂ uptake and transpiration. Given that transpiration‐driven water flow is required for nutrient uptake, climatic stress‐induced nutrient deficit could be a key mechanism for decreased plant performance under prolonged drought. We propose the existence of an “isohydric trap,” a dryness‐induced detrimental feedback leading to nutrient deficit and stoichiometry imbalance in strict isohydric species. We tested this framework in a common garden experiment with 840 individuals of four ecologically contrasting European pines (Pinus halepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature and low soil water availability. We measured growth, survival, photochemical efficiency, stem water potentials, leaf isotopic composition (δ¹³C, δ¹⁸O), and nutrient concentrations (C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower δ¹⁸O and higher δ¹³C values than the other species, indicating higher time‐integrated transpiration and water‐use efficiency (WUE), along with lower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greater dry‐biomass (up to 63‐fold) and survival (100%). Conversely, the more mesic mountain pine species showed higher leaf δ¹⁸O and lower δ¹³C, indicating lower transpiration and WUE, higher water potentials, severe P and K deficiencies and N:P and N:K imbalances, and poorer photochemical efficiency, growth, and survival. These results support our hypothesis that vascular plant species with tight stomatal regulation of transpiration can become trapped in a feedback cycle of nutrient deficit and imbalance that exacerbates the detrimental impacts of climatic dryness on performance. This overlooked feedback mechanism may hamper the ability of isohydric species to respond to ongoing global change, by aggravating the interactive impacts of stoichiometric imbalance and water stress caused by anthropogenic N deposition and hotter droughts, respectively.     

Root exudation and rhizoplane bacterial abundance of barley (Hordeum vulgare L.) in relation to nitrogen fertilization and root growth

The abundance of bacteria in the rhizoplane of barley varieties was investigated at different soil nitrogen levels. Increased amendments of nitrogen resulted in higher bacterial numbers in the rhizoplane of barley seedlings of different varieties. A negative correlation was found between nitrogen level in the soil and the growth rate of the seedling roots. The effect of nitrogen on the bacterial abundances could be indirect through changed root growth and thereby changed exudation. The exudation of soluble organic carbon componds from barley seedling roots were measured in hydroponic culture. The effect of natural variation in root growth rate and of different concentrations of nitrogen in the nutrient solution was investigated. The amount of exudates consituted 2–66% of the dry weight increase in root biomass, depending on the root growth. Slower growing roots released considerably more organic carbon per unit root weight than faster growing roots. The variation in root exudation appeared to be mainly explained by differences in root growth, rather than of the nitrogen concentration in the nutrient solution. A significantly higher exudation rate was found during day time compared to night.     

Upscaling from Rhizosphere to Whole Root System: Modelling the Effects of Phospholipid Surfactants on Water and Nutrient Uptake

While the rhizosphere presents a different chemical, physical and biological environment to bulk soil, most experimental and modelling investigations of plant growth and productivity are based on bulk soil parameters. In this study, water and nutrient acquisition by wheat (Triticum aestivum L.) roots was investigated using rhizosphere- and root-system-scale modelling. The physical and chemical properties of rhizosphere soil could be influenced by phospholipid surfactants in the root mucilage. Two models were compared: a 2-dimensional (2D) Finite Element Method rhizosphere model, and a 3-dimensional (3D) root architecture model, ROOTMAP. ROOTMAP was parameterised to reproduce the results of the detailed 2D model, and was modified to include a rhizosphere soil volume. Lecithin (a phospholipid surfactant) could be exuded into the rhizosphere soil volume, decreasing soil water content and hydraulic conductivity at any given soil water potential, and decreasing phosphate adsorption to soil particles. The rhizosphere-scale modelling (5 × 5 mm² soil area, 10 mm root length, uptake over 12 h) predicted a reduction in water uptake (up to 16% at 30 kPa) and an increase in phosphate uptake (up to 4%) with lecithin exudation into the rhizosphere, but little effect on nitrate uptake, with only a small reduction in dry soil (1.6% at 200 kPa). The 3D root model reproduced the water (y = 1.013x, R² = 0.996), nitrate (y = 1x, R² = 1) and phosphate (y = 0.978x, R² = 0.998) uptake predictions of the rhizosphere model, providing confidence that a whole root system model could reproduce the dynamics simulated by a Finite Element Method rhizosphere model. The 3D root architecture model was then used to scale-up the rhizosphere dynamics, simulating the effect of lecithin exudation on water, nitrate and phosphate acquisition by a wheat root system, growing over 41 d. When applied to growing and responsive roots, lecithin exudation increased P acquisition by up to 13% in nutrient-rich, and 49% in relatively nutrient-poor soil. A comparison of wheat (Triticum aestivum L.) and lupin (Lupinus angustifolius L.) root architectures, suggested an interaction between the P acquisition benefit of rhizosphere lecithin and root architecture, with the more highly-branched wheat root structure acquiring relatively more P in the presence of lecithin than the sparsely-branched lupin root system.     

Citrate, Malate, and Succinate Concentration in Exudates from P-Sufficient and P-Stressed Medicago sativa L. Seedlings

Under certain stress conditions roots exude organic molecules, which may facilitate the uptake of nutrients. The objective of this research was to identify and measure the effect of low P upon the exudation of organic acids by roots of alfalfa (Medicago sativa L.) seedlings. Surface-sterilized alfalfa seeds were grown aseptically in sterile sand using an apparatus specially designed for the addition of ±P nutrient solutions and for the collection of root exudates. Citric, malic, and succinic acids were detected in the root exudates of 24-day-old alfalfa seedlings. Citrate exudation from the roots of P-stressed alfalfa was 182% that of plants receiving a complete nutrient solution. The increased release of citrate may provide a mechanism by which P-stressed plants enhance the availability of P in the rhizosphere.     

Increased P diffusion as an explanation of increased p availability in flooded rice soils

Summary Phosphorus supply factors (capacity, kinetic, intensity, and diffusivity) and plant growth were the approaches used to assess P supply of flooded rice soils. Increases in the capacity, intensity, and kinetic factors, as measured by E-value, solution P concentration, and soil P release rate to a distilled water sink respectively, were unpronouced and infrequent upon water-saturation of ten soils. However, increases in the diffusitivity factor, as measured by ³²P diffusion coefficients, were at least ten-fold as soil moisture increased. The greatest increases in P diffusion occurred as soil moisture increased beyond one-third bar.Using a P fertilized soil or P treated powdered cellulose as the P source and a minus P nutrient solution to nourish a split root system with water and nutrients, data were obtained which suggested that P uptake and rice-shoot growth (indicators of P availability) increased with increasing moisture level. Phosphorus uptake and rice-shoot growth were greatst when the soil or P treated cellulose were water-saturated. These data indicate that increased soil P availability upon flooding can be attributed to an increase in the diffusivity factor.Paper Number 4532 of the Journal Series of the North Carolina Agricultural Experiment Station.Paper Number 4532 of the Journal Series of the North Carolina Agricultural Experiment Station.     

Heterogeneous distribution of phosphorus and potassium in soil influences wheat growth and nutrient uptake

Heterogeneous distribution of mineral nutrients in soil profiles is a norm in agricultural lands, but its influence on nutrient uptake and crop growth is poorly documented. In this study, we examined the effects of varying phosphorus (P) and potassium (K) distribution on plant growth and nutrient uptake by wheat (Triticum aestivum L.) grown in a layered or split soil culture in glasshouse conditions. In the layered pot system the upper soil was supplied with P and either kept watered or allowed to dry or left P-deficient but watered, whereas the lower soil was watered and fertilised with K. Greater reductions in shoot growth, root length and dry weight in the upper soil layer occurred in −P/wet than in +P/dry upper soil treatment. Shoot P concentration and total P content were reduced by P deficiency but not by upper soil drying. Genotypic responses showed that K-efficient cv. Nyabing grew better and took up more P and K than K-inefficient cv. Gutha in well-watered condition, but the differences decreased when the upper soil layer was dry. In the split-root system, shoot dry weight and shoot P and K contents were similar when P and K were applied together in one compartment or separated into two compartments. In comparison, root growth was stimulated and plants took up more P and K in the treatment with the two nutrients supplied together compared with the treatment in which the two nutrients were separated. Roots proliferated in the compartment applied with either P or K at the expense of root growth in the adjoining compartment with neither P nor K. Heterogeneous nutrient distribution has a direct decreasing effect on root growth in deficient patches, and nutrient redistribution within the plant is unlikely to meet the demand of roots grown in such patches.     

A dual porosity model of nutrient uptake by root hairs

? The importance of root hairs in the uptake of sparingly soluble nutrients is understood qualitatively, but not quantitatively, and this limits efforts to breed plants tolerant of nutrient-deficient soils. ? Here, we develop a mathematical model of nutrient uptake by root hairs allowing for hair geometry and the details of nutrient transport through soil, including diffusion within and between soil particles. We give illustrative results for phosphate uptake. ? Compared with conventional 'single porosity' models, this 'dual porosity' model predicts greater root uptake because more nutrient is available by slow release from within soil particles. Also the effect of soil moisture is less important with the dual porosity model because the effective volume available for diffusion in the soil is larger, and the predicted effects of hair length and density are different. ? Consistent with experimental observations, with the dual porosity model, increases in hair length give greater increases in uptake than increases in hair density per unit main root length. The effect of hair density is less in dry soil because the minimum concentration in solution for net influx is reached more rapidly. The effect of hair length is much less sensitive to soil moisture.     

Mobilization of Cu and Zn by root exudates of dicotyledonous plants in resin-buffered solutions and in soil

It has been frequently suggested that root exudates play a role in trace metal mobilization and uptake by plants, but there is little in vivo evidence. We studied root exudation of dicotyledonous plants in relation to mobilization and uptake of Cu and Zn in nutrient solutions and in a calcareous soil at varying Cu and Zn supply. Spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) were grown on resin-buffered nutrient solutions at varying free ion activities of Cu (pCu 13.0–10.4) and Zn (pZn 10.1–6.6). The Cu and Zn concentrations in the nutrient solution increased with time, except in plant-free controls, indicating that the plant roots released organic ligands that mobilized Cu and Zn from the resin. At same pCu, soluble Cu increased more at low Zn supply, as long as Zn deficiency effects on growth were small. Zinc deficiency was observed in most treatment solutions with pZn ≥ 9.3, but not in nutrient solutions of a smaller volume/plant ratio in which higher Zn concentrations were observed at same pZn. Root exudates of Zn-deficient plants showed higher specific UV absorbance (SUVA, an indicator of aromaticity and metal affinity) than those of non-deficient plants. Measurement of the metal diffusion flux with the DGT technique showed that the Cu and Zn complexes in the nutrient solutions were highly labile. Diffusive transport (through the unstirred layer surrounding the roots) of the free ion only could not explain the observed plant uptake of Cu and of Zn at low Zn²⁺ activity. The Cu and Zn uptake by the plants was well explained if it was assumed that the complexes with root exudates contributed 0.4% (Cu) or 20% (Zn) relative to the free ion. In the soil experiment, metal concentrations and organic C concentrations were larger in the solution of planted soils than in unplanted controls. The SUVA of the soil solution after plant growth was higher for unamended soils, on which the plants were Zn-deficient, than for Zn-amended soils. In conclusion, root exudates of dicotyledonous plants are able to mobilize Cu and Zn, and plants appear to respond to Zn deficiency by exuding root exudates with higher metal affinity.     

A new approach for the quantification of root-cap mucilage exudation in the soil

Direct evidence on the functions of root-cap mucilage during plant root growth in soil is limited mainly due to the lack of a method for in situ measurements. In this paper, we offer a method that facilitates the measurement of mucilage exudation when roots are growing in soil. We observed the mucilage exudation directly through a transparent panel located on the side of a root box in which plant roots were growing. We used a CCD camera attached to a microscope to observe and record mucilage exudation. Using image analysis, the activity of mucilage exudation was evaluated based on the area occupied by the mucilage on the root tip. The area of mucilage observed on the root tips after 1-h growth in soil corresponded with the weight of mucilage that was originally observed on the tips before they were transplanted. This relationship suggests that the observed area on root tip relates to total exudation. The area of mucilage exudation on the root tips was high (0.48 mm²) at night and low (0.35 mm²) at midday, suggesting that the activity of mucilage exudation follows diurnal changes. Furthermore, the mucilage exudation positively correlated with the root elongation rate, implying that fast-growing roots exude more mucilage.     

Foliar demand and resource economy of nutrients in dry tropical forest species

Important phenological activities in seasonally dry tropical forest species occur within the hot‐dry period when soil water is limiting, while the subsequent wet period is utilized for carbon accumulation. Leaf emergence and leaf area expansion in most of these tree species precedes the rainy season when the weather is very dry and hot and the soil cannot support nutrient uptake by the plants. The nutrient requirement for leaf expansion during the dry summer period, however, is substantial in these species. We tested the hypothesis that the nutrients withdrawn from the senescing leaves support the emergence and expansion of leaves in dry tropical woody species to a significant extent. We examined the leaf traits (with parameters such as leaf life span, leaf nutrient content and retranslocation of nutrients during senescence) in eight selected tree species in northern India. The concentrations of N, P and K declined in the senescing foliage while those of Na and Ca increased. Time series observations on foliar nutrients indicated a substantial amount of nutrient resorption before senescence and a ‘tight nutrient budgeting’. The resorbed N‐mass could potentially support 50 to 100% and 46 to 80% of the leaf growth in terms of area and weight, respectively, across the eight species studied. Corresponding values for P were 29 to 100% and 20 to 91%, for K 29 to 100% and 20 to 57%, for Na 3 to 100% and 1 to 54%, and for Ca 0 to 32% and 0 to 30%. The species differed significantly with respect to their efficiency in nutrient resorption. Such interspecific differences in leaf nutrient economy enhance the conservative utilization of soil nutrients by the dry forest community. This reflects an adaptational strategy of the species growing on seasonally dry, nutrient‐poor soils as they tend to depend more or less on efficient internal cycling and, thus, utilize the retranslocated nutrients for the production of new foliage biomass in summer when the availability of soil moisture and nutrients is severely limited.     

Spatial and temporal variation in organic acid anion exudation and nutrient anion uptake in the rhizosphere of Lupinus albus L.

We investigated in situ the temporal patterns and spatial extent of organic acid anion exudation into the rhizosphere solution of Lupinus albus, and its relation with the nutrient anions phosphate, nitrate and sulfate by means of a rhizobox micro suction cup method under P sufficient conditions. We compared the soil solution in the rhizosphere of cluster roots with that in the vicinity of normal roots, nodules and bulk soil. Compared to the other rhizosphere and soil compartments, concentrations of organic acid anions were higher in the vicinity of cluster roots during the exudative burst (citrate, oxalate) and nodules (acetate, malate), while concentrations of inorganic nutrient anions were highest in the bulk soil. Both active cluster roots and nodules were most efficient in taking up nitrate and phosphate. The intensity of citrate exudation by cluster roots was highly variable. The overall temporal patterns during the lifetime of cluster roots were overlaid by a diurnal pattern, i.e. in most cases, the exudation burst consisted of one or more peaks occurring in the afternoon. Multiple exudation peaks occurred daily or were separated by 1 or 2 days. Although citrate concentrations decreased with distance from the cluster root apex, they were still significantly higher at a distance of 6 to 10 mm than in the bulk soil. Phosphate concentrations were extremely variable in the proximity of cluster roots. While our results indicate that under P sufficient conditions cluster roots take up phosphate during their entire life time, the influence of citrate exudation on phosphate mobilization from soil could not be assessed conclusively because of the complex interactions between P uptake, organic acid anion exudation and P mobilization. However, we observed indications of P mobilization concurrent with the highest measured citrate concentrations. In conclusion, this study provides semiquantitative in situ data on the reactivity of different root segments of L. albus L. in terms of root exudation and nutrient uptake under nutrient sufficient conditions, in particular on the temporal variability during the lifetime of cluster roots.     

Evolutionary Divergences in Root Exudate Composition among Ecologically-Contrasting Helianthus Species

Plant roots exude numerous metabolites into the soil that influence nutrient availability. Although root exudate composition is hypothesized to be under selection in low fertility soils, few studies have tested this hypothesis in a phylogenetic framework. In this study, we examined root exudates of three pairs of Helianthus species chosen as phylogenetically-independent contrasts with respect to native soil nutrient availability. Under controlled environmental conditions, seedlings were grown to the three-leaf-pair stage, then transferred to either high or low nutrient treatments. After five days of nutrient treatments, we used gas chromatography-mass spectrometry for analysis of root exudates, and detected 37 metabolites across species. When compared in the high nutrient treatment, species native to low nutrient soils exhibited overall higher exudation than their sister species native to high nutrient soils in all three species pairs, providing support for repeated evolutionary shifts in response to native soil fertility. Species native to low nutrient soils and those native to high nutrient soils responded similarly to low nutrient treatments with increased exudation of organic acids (fumaric, citric, malic acids) and glucose, potentially as a mechanism to enhance nutrition acquisition. However, species native to low nutrient soils also responded to low nutrient treatments with a larger decrease in exudation of amino acids than species native to high nutrient soils in all three species pairs. This indicates that species native to low nutrient soils have evolved a unique sensitivity to changes in nutrient availability for some, but not all, root exudates. Overall, these repeated evolutionary divergences between species native to low nutrient soils and those native to high nutrient soils provide evidence for the adaptive value of root exudation, and its plasticity, in contrasting soil environments.     

First observation of diffusion-limited plant root phosphorus uptake from nutrient solution

Diffusion towards the root surface has recently been shown to control the uptake of metal ions from solutions. The uptake flux of phosphorus (P) from solutions often approaches the maximal diffusion flux at low external concentrations, suggesting diffusion-controlled uptake also for P. Potential diffusion limitation in P uptake from nutrient solutions was investigated by measuring P uptake of Brassica napus from solutions using P-loaded Al(2) O(3) nanoparticles as mobile P buffer. At constant, low free phosphate concentration, plant P uptake increased up to eightfold and that of passive, diffusion-based samplers up to 40-fold. This study represents the first experimental evidence of diffusion-limited P uptake by plant roots from nutrient solution. The Michaelis constant of the free phosphate ion obtained in unbuffered solutions (K(m) = 10.4 μmol L(-1) ) was 20-fold larger than in the buffered system (K(m) ～0.5 μmol L(-1) ), indicating that K(m) s determined in unbuffered solutions do not represent the transporter affinity. Increases in the P uptake efficiency of plants by increasing the carrier affinity are therefore unlikely, while increased root surface area or exudation of P-solubilizing compounds are more likely to enhance P uptake. Furthermore, our results highlight the important role natural nanoparticles may have in plant P nutrition.     

大豆和水稻对铝胁迫响应的生理机制

通过水培方式,研究了铝处理对双子叶植物大豆和单子叶植物水稻根系生长、养分吸收、根系内含物及根分泌物的影响.结果表明,低铝 (10 μmol·L^-1)浸种刺激大豆种子萌发和根系的生长,对水稻无明显促进作用.铝处理增加了两种作物根系对P的吸收,降低K、Ca、Mg的吸收.水稻比大豆根系积累较少的Al和更多的P.铝胁迫条件下,大豆和水稻根系内源可溶性蛋白含量升高、可溶性酚含量下降、可溶性糖含量先上升后下降,且大豆根系内源柠檬酸含量下降明显.与大豆相比,水稻积累较低的柠檬酸和较高的可溶性蛋白、可溶性酚,但两者可溶性糖没有差异.铝处理增加大豆根系柠檬酸、可溶性蛋白、可溶性酚、可溶性糖的分泌量,且大豆分泌量显著高于水稻.在铝处理条件下,大豆根系具有较高的阳离子交换量(CEC),而水稻的CEC较低.这说明大豆和水稻对铝胁迫具有不同的生理反应,水稻的高耐铝性可能与其较高的磷吸收和较低的CEC有关,而与其根系分泌物的关系不大.     

Nutrient uptake by tupelo gum and bald cypress from saturated or unsaturated soil

Summary Seedlings of tupelo gum (Nyssa aquatica L.) and bald cypress (Taxodium distichum L. Rich.) were grown in pots containing a sphagnum moss-peat soil mix. Plants approximately 20 to 25 cm tall were subjected to three moisture treatments, saturated-aerated, saturated, and unsaturated soil; and three nitrogen fertilization treatments, control (no N added), urea (a reduced N source), and nitrate (an oxidized N source).Data include dry weights (g/culture) of leaves, stems, and roots; concentrations (percentage of dry weight) and contents (mg/culture) of N, P, K, Ca, and Mg in leaves, stems, and roots. Total dry weight was greater for plants grown in saturated-aerated soil than in either saturated or unsaturated soil. Differences in nutrient absorption and distribution between the plants and among the water treatments were principally the result of growth differences produced by the water treatments. Element contents and often the concentrations of P, K, Ca, or Mg were highest in both species when grown on the saturated-aerated soil and lowest when grown on unsaturated soil. The low levels of N in plants grown on saturated soils were probably the result of denitrification, as shown by the greater content of N in plants grown on soil fertilized with urea as opposed to nitrate. Thus, urea would appear to be a better N source than nitrate for fertilization in swamp forests. Growth of, and nutrient uptake by cypress was restricted less than that of tupelo when the plants were grown on saturated as compared to saturated-aerated soil. Thus, cypress appeared more tolerant than tupelo to the anaerobic root environment found in saturated soil.     

Heterotrophic utilization of extracellular products of phytoplankton in a tropical estuary

Bacterial uptake of algal exudates has been estimated in a tropicalestuary, Dona Paula, where the seasonal fluctuations in hydrographicand nutrient parameters as well as dissolved organic matterconcentrations and phytoplankton species composition are dominatedby the monsoon regime. A close coupling existed between algaland bacterial trophic levels. Algal exudation products wererapidly assimilated with short turnover times. An average 80%of the excreted material was removed by heterotrophic bacteriaand there was a significant correlation between algal extracellularproduction and net bacterial uptake of algal exudates.     

Uptake and transport of N,P and K in tomato supplied with nettle water and nutrient solution

Water extract of stinging nettle (Urtica dioica) has a growth stimulating effect on plants. This investigation elucidated effects of nettle water on uptake and transport of N, P and K. Tomato plants (Solanum lycopersicum L. cv. Dansk export) were grown in sand culture 6–8 weeks. Plants were supplied with nettle water and nutrient solution was used as a control medium. Uptake and transport of N, P and K⁺ were determined with isotopes (¹⁵N,³²P and⁸⁶Rb⁺ as a tracer for K⁺) and ion-selective electrodes and in exudation experiments. A 15% higher uptake of nitrogen (¹⁵N assay) was found after nettle water treatment compared with the nutrient solution control. The total amount of nitrogen was also higher in plants cultivated with nettle water. Transport of inorganic and organic nitrogen, measured in exudation experiments, was more than 50% higher for plants supplied with nettle water compared with plants supplied with nutrient solution. In contrast, nettle water had no effect on uptake, transport or total amount of phosphorus and potassium in the plants. Experiments in hydroculture showed that nettle water had a strong pH-elevating effect. Uptake of NH ₄ ⁺ was strongly stimulated by nettle water compared with nutrient solution. By holding pH at a constant level during the uptake period for 6 h, the uptake of NH ₄ ⁺ from nettle water was significantly lower when no adjustment of pH was made. Consequently a good deal of the NH ₄ ⁺ uptake enhancement by nettle water could be explained by pH-stimulation. Assays with the uncoupler/inhibitor 2,4-dinitrophenol (DNP) and dichlorophenyl-dimethyl-urea (DCMU) showed that uptake of nitrogen from nettle water was less metabolically-linked than uptake from a corresponding nutrient solution. All together, nettle water seems to stimulate the uptake of nitrogen, but not phosphorus or potassium.