Mapping water,oxygen, and pH dynamics in the rhizosphere of young maize roots

Rhizosphere processes are highly dynamic in time and space and strongly depend on each other. Key factors influencing pH changes in the rhizosphere are root exudation, respiration, and nutrient supply, which are influenced by soil water content levels. In this study, we measured the real‐time distribution of soil water, pH changes, and oxygen distribution in the rhizosphere of young maize plants using a recently developed imaging approach. Neutron radiography was used to capture the root system and soil water distribution, while fluorescence imaging was employed to map soil pH and soil oxygen changes. Germinated seeds of maize (Zea mays L.) were planted in glass rhizotrons equipped with pH and oxygen‐sensitive sensor foils. After 20 d, the rhizotrons were wetted from the bottom and time‐lapsed images via fluorescence and neutron imaging were taken during the subsequent day and night cycles for 5 d. We found higher water content and stronger acidification in the first 0.5 mm from the root surface compared to the bulk soil, which could be a consequence of root exudation. While lateral roots only slightly acidified their rhizosphere, crown roots induced stronger acidification of up to 1 pH unit. We observed changing oxygen patterns at different soil moisture conditions and increasing towards lateral as well as crown roots while extending laterally with ongoing water logging. Our work indicates that plants alter the rhizosphere pH and oxygen also depending on root type, which may indirectly arise also from differences in age and water content changes. The results presented here were possible only by combining different imaging techniques to examine profiles at the root‐soil interface in a comprehensive way during wetting and drying.     

The dynamics of oxygen concentration, pH value, and organic acids in the rhizosphere of Juncus spp.

A novel type of planar optodes for simultaneous optical analysis of pH and oxygen dynamics in the rhizosphere is introduced. The combination of the optical, non-invasive measurement of these parameters with sterile sampling of rhizosphere solution across and along growing roots by use of a novel type of rhizobox provides a methodical step forward in the investigation of the physicochemical dynamics of the rhizosphere and its underlying matter fluxes between roots and soil. In this study, this rhizobox was used to investigate the effect of oxygen releasing roots of three Juncus species on the amount and distribution of organic acids in reductive, oxygen-deficient soils of different pH (pH 3.9-pH 5.9). Pronounced diurnal variations of oxygen concentration and pH along the roots, particularly along the elongation zone were observed. Long-term records over more than eight weeks revealed considerable spatial and temporal patterns of oxygen over a range of almost 200 μmol O₂ L⁻¹ and pH dynamics of ±1.4 pH units in the rhizosphere. A strong effect of oxidative acidification due to oxygen release by the plant roots was clearly visible for Juncus effusus, whereas the roots of Juncus articulatus alkalinized the rhizosphere. In contrast, roots of Juncus inflexus induced no effects on rhizospheric pH. Only four different organic acids (oxalate, acetate, formate and lactate) were detectable in all soil solutions. Maximal concentration of all organic acids occurred at pH 3.9, whereas the lowest concentration of each organic acid was found at pH 5.9. Hence, considering the pH-dependence of the redox potential, the acid soil provided increased reductive conditions leading to slower anaerobic degradation of organic acids to CO₂ or methane (CH₄). The concentration of organic acids decreased by up to 58% within a distance of only 4 mm from the bulk soil to the root surface, i.e. reciprocal to the pronounced O₂-gradient. The decreasing presence of organic acids toward the oxygen releasing roots is possibly due to a change in the composition of the microbial community from anaerobic to aerobic conditions. The present study highlights the dynamic interplay between O₂ concentration, pH and organic acids as key parameters of the physicochemical environment of the rhizosphere, particularly for wetland plants growing in oxygen-deficient waterlogged soils.     

Movement of metals from soil to plant roots

The rhizobox system offers quantitative information concerning within the pH changes the range of the rhizosphere. The movement of ⁶⁵Zn, ⁵⁴Mn and ⁵⁹Fe across the rhizosphere of soybean was examined. The contributions of the rhizosphere processes to heavy metals solubilization was evaluated. Changes in pH values around the roots of barley and soybean were investigated using the rhizobox. Results indicate that the extent to which the rhizosphere pH can differ from that of the bulk soil depends mainly on the plant species and initial bulk soil pH. Apparently, soybean has greater ability than barley to solubiiize heavy metals in the rhizosphere. The range of pH change was different from that of the heavy metals, indicating that the solubility of heavy metals is not only influenced by soil pH.     

Root and rhizosphere responses of iron‐efficient or ‐inefficient apple genotypes to solution pH

Solution culture with four pH levels was employed in this experiment to evaluate root and rhizosphere responses of Malus xiaojinensis [iron (Fe)‐efficient species] and M. baccata (Fe‐inefficient species) in order to pursue some of their physiological mechanism for Fe absorption. The results showed that M. xiaojinensis had a higher fresh weight per seedling than M. baccata at any of the solution pH levels tested and the differences were significant between the two species with increasing of the solution pH levels, particularly at the pH of 7.4 or 8.4. The reducing abilities of root exudates for the two species under test were decreased with increasing of the solution pH from 5.4 to 8.4, in which the reducing abilities for M. xiaojinensis were always more than two times higher than those for M baccata. The significant decrease of the reducing ability was found only at pH of 8.4 for M. xiaojinensis or at both 7.4 and 8.4 for M. baccata, respectively. Malus xiaojinensis had significantly higher respiration rates than M. baccata at the higher solution pH levels. Both rhizosphere pH and rhizosphere redox potential were influenced by the solution pH levels remarkably in distances of 0–4 mm to root surface or in distances of 5–10 mm along the root from the root tip, respectively. Genotypic differences in these two parameters were clearly showed at the solution pH of 7.4, in which rhizosphere pH of M xiaojinensis was clearly lower than that of M. baccata, while the rhizosphere redox potential of the former was much higher than that of the latter.     

Phosphate aquisition by red clover and black mustard on a humic podzol

Recent investigations have shown that phosphate (P) mobilization by root exudates is an important feature of genotypes to acquire P even in soils of low‐P availability. We, therefore, investigated P mobilization processes in the rhizosphere of red clover (Trifolium pratense L.) and black mustard (Brassica nigra L.) on a humic podzol. As measured by the Kuchenbuch‐Jungk method (Kuchenbuch and Jungk, 1982), both species accumulated similar quantities of citrate (12 μmol/g soil) in the rhizosphere in about 1 mm distance from the soil‐root interface. Despite of similar concentrations of P‐mobilizing citrate in the rhizosphere of both species, red clover took up nearly the two‐fold of P compared to black mustard. Differences in rhizosphere pH were determined between both species. Black mustard did not acidify the rhizosphere, whereas red clover decreased the pH in the rhizosphere from 5.8 to about 4.0 (in 0.01M CaCl₂). The simultaneous acidification and excretion of citrate compared to citrate excretion alone had consequences for P mobilization processes in the rhizosphere. Phosphate mobilization from the soil solid phase was higher at higher pH. Thus, the citrate‐induced P desorption was not the limiting step in P acquisition by red clover and black mustard. Calculations of P distribution in the soil solution between free ortho‐P and humic‐associated P showed that at higher pH most of the P was associated with dissolved humic substances, whereas at pH < 5, most of the P was present as free ortho‐P. These P species can readily be taken up by the roots whereas humic‐associated P must probably be desorbed from the humic surface before uptake. Phosphate species calculations, therefore, explained the higher P uptake of red clover compared to black mustard. Aluminum species distribution calculations in the soil solution further show that even at pH < 5.0 in the soil solution, citrate strongly complex Al and thereby reduce the activity of monomeric Al species. The excretion of citrate can, therefore, counteract the root induced acidification of the rhizosphere with respect to Al toxicity.     

Novel micro–suction‐cup design for sampling soil solution at defined distances from roots

Micro–suction cups made of nylon membranes and polyacrylic tubes with planar geometry of the membrane were designed for repeated sampling of rhizosphere solution at defined distances from a root monolayer. Adsorption tests revealed that the materials used (nylon membrane, polyacrylic tube) have little influence on the concentration of heavy metals in the sample solution, whereas some organic acids are partly retained by the suction cup. A sampling protocol was developed for collecting extremely small solution volumes (i.e., droplets of 28.3±2.46 μl) for subsequent measurements of trace elements using ICP‐SFMS. A homogeneity test showed that soil‐solution concentrations of Ca, K, Mg, and Ni could be reproduced independent of the suction‐cup position in a rhizobox experiment without plants. In a similar experiment, the rhizobox was planted with the Ni hyperaccumulator Thlaspi goesingense. Compared to more distant soil layers, an increase of Ni and a concurrent decrease of Ca, K, and Mg at 1 mm distance from the root plane was found. These changes can be related to plant uptake and mobilization processes. Our results show that the novel micro–suction cups are a valuable tool for elucidating rhizosphere processes.     

Root-induced changes in the rhizosphere: Importance for the mineral nutrition of plants

Root-induced changes in the rhizosphere may affect mineral nutrition of plants in various ways. Examples for this are changes in rhizosphere pH in response to the source of nitrogen (NH₄-N versus NO₃-N), and iron and phosphorus deficiency. These pH changes can readily be demonstrated by infiltration of the soil with agar containing a pH indicator. The rhizosphere pH may be as much as 2 units higher or lower than the pH of the bulk soil. Also along the roots distinct differences in rhizosphere pH exist. In response to iron deficiency most plant species in their apical root zones increase the rate of H⁺ net excretion (acidification), the reducing capacity, the rate of Fe^III reduction and iron uptake. Also manganese reduction and uptake is increased several-fold, leading to high manganese concentrations in iron deficient plants. Low-molecular-weight root exudates may enhance mobilization of mineral nutrients in the rhizosphere. In response to iron deficiency, roots of grass species release non-proteinogenic amino acids (?phytosiderophores”?) which dissolve inorganic iron compounds by chelation of Fe^III and also mediate the plasma membrane transport of this chelated iron into the roots. A particular mechanism of mobilization of phosphorus in the rhizosphere exists in white lupin (Lupinus albus L.). In this species, phosphorus deficiency induces the formation of so-called proteoid roots. In these root zones sparingly soluble iron and aluminium phosphates are mobilized by the exudation of chelating substances (probably citrate), net excretion of H⁺ and increase in the reducing capacity. In mixed culture with white lupin, phosphorus uptake per unit root length of wheat (Triticum aestivum L.) plants from a soil low in available P is increased, indicating that wheat can take up phosphorus mobilized in the proteoid root zones of lupin. At the rhizoplane and in the root (root homogenates) of several plant species grown in different soils, of the total number of bacteria less than 1 % are N₂-fixing (diazotrophe) bacteria, mainly Enterobacter and Klebsiella. The proportion of the diazotroph bacteria is higher in the rhizosphere soil. This discrimination of diazotroph bacteria in the rhizosphere is increased with foliar application of combined nitrogen. Inoculation with the diazotroph bacteria Azospirillum increases root length and enhances formation of lateral roots and root hairs similarly as does application of auxin (IAA). Thus rhizosphere bacteria such as Azospirillum may affect mineral nutrition and plant growth indirectly rather than by supply of nitrogen.     

Effects of different N sources on N nutrient and microbial distribution across soybean rhizosphere

Abstract

Nitrogen is the most important nutrient for plant growth. In the present study, investigations were carried out on the effects of sodium nitrate, ammonium sulfate, urea, and two types of controlled-release coated urea (LP-40 and LP-70) fertilizers on the NO₃ ^?-N, NH₄ ⁺-N concentrations, and microbial numbers as well as pH distribution across the rhizosphere of soybean (Glycine max L. Merrill, var. Heinong 35). The study was conducted on a typical black soil using a rhizobox system. The results showed that NO₃ ^?-N was the main source of nitrogen, which was deficient in the rhizosphere in the treatments of ammonium sulfate, urea, LP-40, and LP-70, but accumulated considerably in the sodium nitrate treatment. The NH₄ ⁺-N concentration slightly increased in the rhizosphere in the ammonium sulfate treatment, and decreased in the rhizosphere when the other four kinds of N fertilizers were supplied. In an the treatments, bacterial and fungal numbers were highest in the central compartment (C.C.) of the rhizoboxes where the soybean root system was confined, but the rhizosphere width estimated from the increase in the microbial abundance differed among different N fertilizers. The experimental results also indicated that the fungal composition in the C.C. was less diverse than in other parts of the rhizobox compartments, and that the majority of fungal groups was represented by Penicillium spp., suggesting that the microbial distribution across the soybean rhizosphere differed both quantitatively and qualitatively.     

Phosphorus Efficiency of Winter Canola Cultivars and Rhizosphere Properties in Rhizobox Technique

Production of oilseed crops requires balanced fertilization, while environmental problems of applied fertilizers must be considered. We evaluated seven winter canola cultivars (Brassica napus L.) for their relative efficiency to use or acquire phosphorus (P) under deficient and sufficient conditions. Average root mean diameter (RMD), total root length (RL) and root surface area (RA), of plants were measured as well as rhizosphere properties in rhizobox technique. Water-soluble P (WSP) and phosphatase activity of treatments containing plant were higher than control. WSP increased by 2.86 mg kg^?1 soil in Gabriela and 2.63 mg kg^?1 soil in Elvis at P deficient condition, compared to the control soil. The Olsen extractable P of the treatments decreased compared to control. Variations in total dry weights of cultivars were mainly explained by the differences in P solubilizing bacteria (PSB) population, pH, and phosphatase activity. The responsible mechanism for the P efficient (PE) cultivars can be higher P uptake through larger root and changes in rhizosphere properties.     

Effect of slow releasing nitrogen fertilizers on nutrient status across wheat rhizosphere grown on volcanic ash

Abstract

The volcanic ash of the Mount Pinatubo in Philippines was used in this study. The major drawbacks of this ash for growing agricultural crops are nitrogen (N) and iron (Fe) deficiencies with low organic matter contents. The main objective of this study is to investigate the wheat grown on the volcanic ash to and determine the nutrient status across its rhizosphere using a rhizobox system. Either oxamide or polyolefinresin‐coated urea (PORCU) along with potash and phosphate fertilizers was applied to each rhizobox containing the volcanic ash. Plants were grown on the central compartment (CC) of the rhizobox. The nutrient status was examined by the assessment of distribution patterns of NH₄ ⁺‐N, NO₃ ^‐‐N, Ca²⁺, Mg²⁺, K⁺, Cl^‐, SO₄ ^2‐, PO₄ ^3‐, and associated pH. Although NH₄ ⁺‐N in both oxamide and PORCU treatments was accumulated to a somewhat considerable extent of the rhizosphere. The rhizosphere effect was more prominent in oxamide treatment. While NO₃ ^‐‐N concentrations in either of the treatment was low with a mere rhizosphere effect in PORCU treatment and a rugged distribution in oxamide treatment. Dominant anions and cations accumulated in the CC and the near by compartments of oxamide treatment were Cl^‐, SO₄ ^2‐ and Ca²⁺, respectively. In contrast, SO₄ ^2‐ and K⁺ were accumulated in the CC and the adjacent PORCU treated compartments. Thus the overall distribution of nutrients and pH across the wheat rhizosphere was rugged. Despite of this, it seems that with a rugged nutrient distribution and pH, the effects of slow releasing N fertilizers may well ensure the N benefit on plants while growing on the volcanic ash under circumstances of low N content.     

Ecophysiological Differentiation of Two Mosla Species in Response to Nitrogen and Water Levels

Terrestrial ecosystems are experiencing increased inputs of nitrogen (N) and temporal fluctuations in precipitation, causing flooding or drought, and this could strongly affect the fate of terrestrial plant species, as they might have different abilities to adapt to the changing environment. We grew Mosla dianthera (a widespread species) and M. hangchowensis (an endangered species) under three water treatments (drought, sufficient water, and waterlogging) in combination with three levels of N supply (low, intermediate, and sufficient N) to study the ecophysiological responses of the congeneric species to those simulated environmental changes. The two species showed different responses to waterlogging and drought treatments, particularly when there was abundant N supply in the system. For example, under sufficient N but drought or waterlogging conditions, M. dianthera increased root mass ratio (RMR) and decreased leaf mass ratio (LMR), total leaf area (LA), and leaf area ratio (LAR); such changes can enhance water acquisition and reduce water loss under both drought and waterlogging conditions, in contrast to the general lack of change in those parameters with M. hangchowensis. These differentiations in traits suggest that increased N availability might worsen drought and waterlogging injury to M. hangchowensis and thus accelerate the decline of this population. However, M. dianthera maybe better adapted to high N availability and both drought and waterlogging conditions. We hypothesize that the different adaptive abilities to high N availability and drought and waterlogging conditions are partly responsible for the ecological differentiation observed between these two species in the field and may determine their fate in their native habitat. Further research should test this hypothesis in field experiments.     

Effect of low pO2 on colonization of maize roots by a genetically altered Pseudomonas putida [PH6(L1019)]

Among the factors which may affect colonization of roots by soil bacteria is that of rhizosphere oxygen partial pressure (pO₂). The oxygen concentration in the root zone influences both microbes and roots. Roots exposed to low pO₂, as might occur during flooding and waterlogging of the soil, become more leaky and loss of soluble carbon increases. To determine whether periods of low pO₂ increased root colonization by a genetically altered pseudomonad we inoculated 3- to 4-week-old maize plants, grown in soil and transferred to a hydroponic system or grown in fritted clay, with Pseudomonas putida PH6(L1019)(lacZY+) following exposure of the roots to air or cylinder N₂. Numbers of heterotrophs and the marked pseudomonad were determined by dilution plating. Low pO₂ generally increased the numbers of bacteria associated with roots exposed to the treatments in solution or in undisturbed fritted clay rooting medium. Under low pO₂ in a hydroponic system, roots of intact maize plants tended also to have higher soluble organic C and hexose (anthrone-detectable sugars) than roots exposed to air. The effect of low pO₂ was most pronounced in the fritted clay where low pO₂ favored colonization by the marked strain; numbers were 3- to 96-fold greater than those on roots flushed with air but accounted for only 0.06–0.61% of the total population. Roots exposed to low pO₂ tended to accumulate more C. Results suggest that in the fritted clay, the pseudomonad was able to exploit the increased C supply and to achieve greater numbers on roots exposed to low pO₂, whereas the dilution of carbon released from roots in the hydroponic apparatus did not allow for the same magnitude of increase on roots. Received: 2 December 1996     

Nitrogen accumulation in nodulated and non‐nodulated pea plants,grown in a sandy soil at different acidities

Abstract

The growth of nitrate‐supplied and dinitrogen‐fixing pea plants was studied in a pot experiment with a sandy soil in a pH‐H_?O range from 3.4 to 5.6. Optimum growth in both treatments occurred at pH 5.0. At low pH, N₂‐plants yielded significantly less than NO₃‐plants. Planting of nodulated seedlings did not enhance yield in comparison with sowing in inoculated soil, indicating that nodulation was not the most sensitive process in restricting yield. Comparison of the nitrogen contents of shoots of planted and sown N₂‐plants allowed the suggestion that the synthesis of nitrogenous compounds was also not limiting yield. At low pH, root growth was severely reduced in dinitrogen‐fixing plants in comparison with nitrate‐supplied plants. This difference could be explained by the influence of the form of nitrogen nutrition on the cation‐anion uptake pattern of the plant and the resulting pH‐shift in the rhizosphere. It is to be expected that in an acid soil under field conditions the indirect effect of nitrate on root growth and nodulation via increase of the pH is more extensive than its direct negative effect on nodulation.     

Clonal and seasonal shifts in communities of saprotrophic microfungi and soil enzyme activities in the mycorrhizosphere of Salix spp.

The species‐specific microbial root and rhizosphere colonization contributes essentially to the plant nutrient supply. The species number and colonization densities of cultivable saprotrophic microfungi and the activities of nutrient‐releasing soil enzymes (protease, acid and alkaline phosphatase, arylsulfatase) were investigated in the rhizosphere of one low mycorrhizal (Salix viminalis) and one higher mycorrhizal (S. × dasyclados) willow clone at a Eutric Cambisol in N Germany. After soil washing, in total 32 and 28 saprotrophic microfungal species were isolated and identified microscopically from the rhizosphere of S. viminalis and S. × dasyclados, respectively. The fungal species composition changed within the growing season but the species number was always lower under S. × dasyclados than under S. viminalis. Under both willow clones, the fungal colonization density was largest in spring, and the species number was largest in autumn. Acid‐phosphatase activity (p < 0.001) and protease activity (p < 0.003) were significantly affected by the Salix clone, whereas arylsulfatase and alkaline‐phosphatase activities did not show clone‐specific differences. All enzyme activities reached their maxima in the summer sampling. Rhizosphere colonization with Acremonium butyri, Cladosporium herbarum, and Penicillium janthinellum contributed significantly to explain the activities of acid phosphatase. Rhizosphere colonization with Cylindrocarpon destructans, Penicillium spinulosum, Plectosphaerella cucumerina, and Trichoderma polysporum contributed significantly to explain the arylsulfatase activities. Effects of the saprotrophic fungal colonization densities on the protease activities in the rhizosphere were low. Acid‐ and alkaline‐phosphatase and arylsulfatase activities in the rhizosphere soil were stronger affected by the composition of the saprotrophic fungal communities than by the Salix clone itself. In conclusion, the colonization density of some saprotrophic microfungi in the rhizosphere contributed to explain shifts in soil‐enzyme activities of the P and S cycles under different willow clones.     

Effect of buckwheat (Fagopyrum esculentum) on soil‐phosphorus availability and organic acids

As a cover crop, buckwheat (Fagopyrum esculentum) may increase soil‐P availability. Buckwheat was grown in low‐P and P‐fertilized field plots, and organic anions were measured in rhizosphere soil. Soil‐P availability was not affected by buckwheat, but the concentration of rhizosphere tartrate^2– was significantly higher (p < 0.005) in low‐P vs. P‐fertilized plots. This suggests that organic‐anion root exudation may have a role in buckwheat‐rhizosphere P dynamics.     

Role of Phosphatases,Iron Reducing,and Solubilizing Activity on the Nutrient Acquisition in Mixed Cropped Peanut and Barley

The effect of interspecific complementary and competitive root interactions and rhizosphere effects on primarily phosphorus (P) and iron (Fe) but also nitrogen (N), potassium (K), calcium (Ca), zinc (Zn), and manganese (Mn) nutrition between mixed cropped peanut (Arachis hypogaea L.) and barley (Hordeum vulgare L.). In order to provide more physiological evidence on the mechanisms of interspecific facilitation, phosphatase activities in plant and rhizosphere, root ferric reducing capacity (FR), Fe-solubilizing activity (Fe-SA), and rhizosphere pH were determined. The results of the experiment revealed that biomass yield of peanut and barley was decreased by associated plant species as compared to their monoculture. Rhizosphere chemistry was strongly and differentially modified by the roots of peanut and barley and their mixed culture. In the mixed cropping of peanut/barley, intracellular alkaline and acid phosphatases (AlPase and APase), root secreted acid phosphatases (S-APase), acid phosphatases activity in rhizosphere (RS-APase), and bulk soil (BS-APase) were higher than that of monocultured barley. Regardless of plant species and cropping system, the rhizosphere pH was acidified and concomitantly to this available P and Fe concentrations in the rhizosphere were also increased. The secretion Fe-solubilizing activity (Fe-SA) and ferric reducing (FR) capacity of the roots were generally higher in mixed culture relative to that in monoculture treatments which may improve Fe and Zn nutrition of peanut. Furthermore, mixed cropping improved N and K nutrition of peanut plants, while Ca nutrition was negatively affected by mixed cropping.     

一株在微生物燃料电池中可以利用喹啉发电的产电假单胞杆菌Q1的产电特性研究

A rhizobox experiment was conducted to compare iron (Fe) oxidation and changes of pH, redox potential (Eh) and fractions of zinc (Zn) and lead (Pb) in rhizosphere and non-rhizosphere soils of four emergent-rooted wetland plants (Echinodorus macrophyllus, Eleocharis geniculata, Hydrocotyle vulgaris and Veronica serpyllifolia) with different radial oxygen loss (ROL) from roots. The results indicated that all these wetland plants decreased pH and concentration of Fe(Ⅱ) but increased the Eh in the rhizosphere soils. Pb and Zn were transformed from unstable fractions to more stable fractions in the rhizosphere soils, so decreasing their potential metal mobility factors (MF). Among the four plants, E. macrophyllus, with the highest ROL and root biomass, possessed the greatest ability in formation of Fe plaque and in the reduction of heavy metal MFs in the rhizosphere soil. Wetland plants, with higher ROLs and root biomass, may thus be effective in decreasing potential long-term heavy metal bioavailabilities.     

Iron deficiency stress responses of five gram‐inaceous monocots

Plant growth, leaf chlorosis, root reductive capacity, rhizosphere pH, and phytosiderophore release capacity were used as indices to evaluate the responses of maize (Zea mays L. cv ‘clipper'), millet (Pennisetum glaucum L. cv. ‘Dwarf Gero'), sorghum (Sorghum bicolor L. cv. YG 5760), barnyard grass (Echinochloa crus galli L. cv: unknown), wheat (Triticum aestivum L. cv. ‘tonic'), and white lupin (Lupinus albus L. cv ‘lucky') to iron‐deficiency stress. Generally, root and shoot dry matter increased with iron treatment and leaves became less chlorotic. Neither the order nor the magnitude of the root reductive capacities of the monocots studied was affected by iron deprivation, but these reductive capacities and the changes in rhizosphere pH differed markedly. Significant iron stress‐induced phytosiderophore release was observed only in wheat and sorghum in which accompanying increases in rhizosphere pH were also evident. Such phytosiderophore release matched the severity of leaf chlorosis and iron uptake and depended on the form in which the element was supplied. These results, from experiments conducted in non‐axenic hydroponic cultures, indicate that in iron‐ deficiency stress mechanisms ‐ similar to those found in dicots ‐could account for iron uptake in some graminaceous monocots, and that strategy II‐type response proposed for all in this category of plants would be an over simplification.     

Effects of drought stress on photosynthesis,rhizosphere respiration,and fine‐root characteristics of beech saplings: A rhizotron field study

Soil drought influences the C turnover as well as the fine‐root system of tree saplings. Particularly during the period of establishment, the susceptibility to drought stress of saplings is increased because of incompletely developed root systems and reduced access to soil water. Here, we subjected beech saplings (Fagus sylvatica L.) to different levels of drought stress. Beech saplings were planted in rhizotrons, which were installed in the soil of a Norway spruce forest before bud burst. Soil moisture was manipulated in the following year during May to September. We measured photosynthetic net CO₂ uptake, volume production of fine roots, and rhizosphere respiration during the growing season. Biometric parameters of the fine‐root system, biomass, and nonstructural carbohydrates were analyzed upon harvest in October. Photosynthesis and rhizosphere respiration decreased with increasing drought‐stress dose (cumulated soil water potential), and cumulative rhizosphere respiration was significantly negatively correlated with drought‐stress dose. Fine‐root length and volume production were highest at moderate soil drought, but decreased at severe soil drought. The proportion of fine‐roots diameter < 0.2 mm and the root‐to‐shoot ratio increased whereas the live‐to‐dead ratio of fine roots decreased with increasing drought‐stress dose. We conclude that the belowground C allocation as well as the relative water‐uptake efficiency of beech saplings is increased under drought.     

Growth,phosphorus uptake,and rhizosphere microbial‐community composition of a phosphorus‐efficient wheat cultivar in soils differing in pH

In a pot experiment, the P‐efficient wheat (Triticum aestivum L.) cultivar Goldmark was grown in ten soils from South Australia covering a wide range of pH (four acidic, two neutral, and four alkaline soils) with low to moderate P availability. Phosphorus (100 mg P kg^–1) was supplied as FePO₄ to acidic soils, CaHPO₄ to alkaline, and 1:1 mixture of FePO₄ and CaHPO₄ to neutral soils. Phosphorus uptake was correlated with P availability measured by anion‐exchange resin and microbial biomass P in the rhizosphere. Growth and P uptake were best in the neutral soils, lower in the acidic, and poorest in the alkaline soils. The good growth in the neutral soils could be explained by a combination of extensive soil exploitation by the roots and high phosphatase activity in the rhizosphere, indicating microbial facilitation of organic‐P mineralization. The plant effect (soil exploitation by roots) appeared to dominate in the acidic soils. Alkaline phosphatase and diesterase activities in acidic soils were lower than in neutral soils, but strongly increased in the rhizosphere compared with the bulk soil, suggesting that microorganisms contribute to P uptake in these acidic soils. Shoot and root growth and P uptake per unit root length were lowest in the alkaline soils. Despite high alkaline phosphatase and diesterase activities in the alkaline soils, microbial biomass P was low, suggesting that the enzymes could not mineralize sufficient organic P to meet the demands of plants and microorganisms. Microbial‐community composition, assessed by fatty acid methylester (FAME) analysis, was strongly dependent on soil pH, whereas other soil properties (organic‐C or CaCO₃ content) were less important or not important at all (soil texture).