Mineral nutrition and reduction processes in the rhizosphere of rice

Summary The influence of nitrogen, phosphorus, and potassium on the reduction processes in the rhizosphere of rice grown in solution culture and of rice under lowland conditions was studied. In solution culture the redox potential in the complete nutrient solution was highest, indicating that fully nourished roots have the highest oxidizing power. When the supply of only one element was interrupted, the lack of potassium in the nutrient solution caused the greatest decline in redox potential. Redox potential was further decreased when, besides nitrogen, either phosphorus or potassium was discontinued. Simultaneous deficiencies of nitrogen and potassium lowered redox potential even more severely than did deficiency of all three elements. A long-term nitrogen fertilizer trial under lowland conditions, however, revealed that an abundant supply of nitrogen can decrease redox potential. Redox potential was higher in the soil near plants than in the soil away from plants. In solution culture, at low Eh levels, the increase in iron reducing power of the solution was correlated with the decrease in redox potential. The total number of bacteria and iron reducing bacteria increased almost parallel to the decrease in redox potential and increase in iron reducing power. These relationships show that the nutritional status of the rice plant essentially influences bacterial activity and, thus, oxidation-reduction conditions around the roots. Since sufficient potassium nutrition seems important in maintaining the oxidising power of rice roots, root growth in a potassium deficient soil with K application was compared with root growth without K application. Without potassium the fine lateral roots far from the root base showed black coloration due to ferrous sulfide, indicating a loss of oxidising power. With increasing potassium supply, this phenomenon disappeared and the iron content of the rice plants decreased. re]19751208     

Rhizosphere soil nitrogenase (C2H2 reduction) as influenced by plant density in intermediate deep water rice

Summary Nitrogenase activity in the rhizosphere soil of intermediate deep water rice was investigated employing gas chromatographic acetylene reduction assay. A raise in the plant density decreased the rhizosphere nitrogenase. Moreover, nitrogen fixation in the rhizosphere soil varied among the three rice varieties under intermediate deep water situations. Results indicate that nitrogen fixation is affected by plant density and the rice variety.     

Effect of Rice Plants on Nitrogenase Activity of Flooded Soils

In samples of flooded soil containing blue-green algae (cyanobacteria), the presence of rice plants did not influence the nitrogenase activity of the algae. Nitrogenase activity of heterotrophic bacteria was enhanced by the presence of rice plants, but this activity was not affected by changes in plant density. The rate of nitrogen fixation in the rhizosphere, however, varied significantly among the 16 rice varieties tested. A simple method was devised to test the nitrogen-fixing activity in the root zone of rice varieties, and data were obtained showing marked differences in the activities of the 16 varieties. In tests of two varieties with dissimilar rates of nitrogen fixation in their rhizospheres, the variety which had the greater root weight and lesser shoot weight and which supported greater methane formation had the greater nitrogenase activity.     

Ontogenetic Changes in Root Nodule Subpopulations of Common Bean (Phaseolus vulgaris L.): I. NITROGENASE ACTIVITY AND RESPIRATION DURING POD-FILLING

Nitrogenase activity is commonly measured on a whole plant basis,or only on parts of the root system. In the present paper, activityin different root nodule subpopulations was followed throughoutreproductive growth, in order to characterize the pod-fillingdecline in nitrogenase activity. Inoculated common bean plantswere grown to maturity under controlled environment conditions.Nitrogenase activity (H₂ evolution in air) and nodule respiration(CO₂ evolution) were measured in three separate zones of theroot system with a non-destructive, open flow, gas-exchangesystem. Nitrogenase activity in the top zone drastically declinedat the initiation of pod-filling, whereas nitrogenase activityin the mid zone was stable during the same period. Hence, thepod-filling decline was limited to a certain nodule subpopulationand not of a systemic type. Nodule respiration showed a similar,but less pronounced pattern. The sharp decline in nitrogenaseactivity was not paralleled in nitrogenase specific activity.Nitrogenase activity is not likely to be limited by the availabilityof oxygen or carbohydrates at the onset of pod-filling becausespecific nodule respiration did not change significantly atthis time. In the top zone, nitrogenase specific activity declinedgradually throughout the measurements, whereas in the mid-partof the root system specific activity peaked and gradually declined2-4 weeks later. The dissimilarities between specific and totalnitrogenase activity were explained by differences in nodulegrowth rates. The data suggest that the oldest nodule populationloses activity at the onset of pod-filling. At the same time,nodules grow and nitrogenase activity increases in younger distalparts of the root system. Estimating total nitrogen fixationin this symbiosis by partial sampling of nodulated root systemsis likely to be very misleading. Key words: Nitrogen fixation, respiration, pod-filling decline, Phaseolus vulgaris, ontogeny     

Urea hydrolysis in some tea soils

The influence of application of hexachlorocyclohexane (HCH=gamma BHC), to a submerged tropical field soil at rates equivalent to recommended field rates (1–2.5kg a.i./ha) and twice this level, upon the rhizosphere soil nitrogenase, nitrogen fixers, and soil redox potential (Eh) was investigated. The rhizosphere soil from HCH-treated field exhibited significantly higher nitrogenase activity than that from untreated fields. HCH retarded the drop in redox potential of the field soil upto 80 days after transplantation under submerged conditions. Populations of nitrogen-fixingAzospirillum sp. and Azotobacter, to a greater extent, and anaerobic organisms, to a lesser extent, were stimulated in HCH-treated soils. Results indicate the stimulation of heterotrophic nitrogen-fixing bacteria by HCH in submerged paddy soils.     

Nitrogen fixation by Anabaena cylindrica

Summary Blending Anabaena cylindrica cultures results in a loss of nitrogenase activity which is correlated with the breakage of the filaments at the junctions between heterocysts and vegetative cells. Oxygen inhibition of nitrogen fixation was significant only above atmospheric concentrations. Nitrogen-fixation activities in the dark were up to 50% of those observed in the light and were dependent on oxygen (10 to 20% was optimal). Nitrogenase activity was lost in about 3 h when cells were incubated aerobically in the dark. Re-exposure to light resulted in recovery of nitrogenase activity within 2 h. Blending, oxygen, or dark pre-incubation had similar effects upon cultures grown under air or nitrogen and did not inhibit light-dependent CO₂ fixation. We conclude that heterocysts are the sites of nitrogenase activity and propose a model for nitrogen fixation by Anabaena cylindrica.     

The effect of sediment redox potential and soil acidity on nitrogen uptake,anaerobic root respiration,and growth of rice (Oryza sativa)

Summary Oryza sativa Loisel cultivar Mars., a common lowland rice variety was grown under controlled soil redox conditions (Eh) and acidity (pH). The effect of two variables (Eh and pH) on growth, anaerobic root respiration, and uptake of added labelled nitrogen, was investigated. Plant growth, estimated by dry weight showed significantly higher growth under reducing sediment redox potentials (−200 mV and 0 mV) and at a soil pH of 6.5 Using the activity of the inducible enzyme alcohol dehydrogenase (ADH) as an indicator of anaerobic root respiration, a decrease in redox potential resulted in an increase in root ADH. However, growth paralled increases in anaerobic root respiration suggesting nitrogen transformation in the soil to be a primary parameter governing growth. Labelled nitrogen uptake which was greater under anaerobic conditions apparently led to greater growth of lowland rice in the highly reduced or anaerobic soil treatments.     

Oxygen and the light–dark cycle of nitrogenase activity in two unicellular cyanobacteria

Cyanobacteria capable of fixing dinitrogen exhibit various strategies to protect nitrogenase from inactivation by oxygen. The marine Crocosphaera watsonii WH8501 and the terrestrial Gloeothece sp. PCC6909 are unicellular diazotrophic cyanobacteria that are capable of aerobic nitrogen fixation. These cyanobacteria separate the incompatible processes of oxygenic photosynthesis and nitrogen fixation temporally, confining the latter to the dark. Although these cyanobacteria thrive in fully aerobic environments and can be cultivated diazotrophically under aerobic conditions, the effect of oxygen is not precisely known due to methodological limitations. Here we report the characteristics of nitrogenase activity with respect to well‐defined levels of oxygen to which the organisms are exposed, using an online and near real‐time acetylene reduction assay combined with sensitive laser‐based photoacoustic ethylene detection. The cultures were grown under an alternating 12–12 h light–dark cycle and acetylene reduction was recorded continuously. Acetylene reduction was assayed at 20%, 15%, 10%, 7.5%, 5% and 0% oxygen and at photon flux densities of 30 and 76 μmol m^?2 s^?1 provided at the same light–dark cycle as during cultivation. Nitrogenase activity was predominantly but not exclusively confined to the dark. At 0% oxygen nitrogenase activity in Gloeothece sp. was not detected during the dark and was shifted completely to the light period, while C. watsonii did not exhibit nitrogenase activity at all. Oxygen concentrations of 15% and higher did not support nitrogenase activity in either of the two cyanobacteria. The highest nitrogenase activities were at 5–7.5% oxygen. The highest nitrogenase activities in C. watsonii and Gloeothece sp. were observed at 29°C. At 31°C and above, nitrogenase activity was not detected in C. watsonii while the same was the case at 41°C and above in Gloeothece sp. The differences in the behaviour of nitrogenase activity in these cyanobacteria are discussed with respect to their presumed physiological strategies to protect nitrogenase from oxygen inactivation and to the environment in which they thrive.     

Effect of soil temperature on nitrogen fixation on roots of rice and reed

Summary The relation of nitrogenase activity (ethylene evolution) to soil temperature or incubation temperature of roots was determined on two genera of swamp plants, namely rice (Oryza sativa) cultivated in tropical climate and reed (Phragmites communis) grown in temperate regions. For both intact rice plants and excised rice roots the optimum temperature was 35°C. On excised roots nitrogenase activity responded more sensitivity to changes in temperature. In contrast to intact rice plants no ethylene evolution occurred on excised roots at 17 and 44°C. On reed roots temperature optimum was between 26 and 30°C which is clearly lower than on rice (35°C). The temperature range in which nitrogen fixation occurred was, however, similar to that of rice, although on a lower level. The results suggest a higher potential of the tropics for associative N₂ fixation, while in cooler climates the lower temperatures appear to be a major limiting factor.     

Long-term drought stress alters nitrogenase activity and carbon translocation in split-root cultured Alnus incana

Split-root cultured grey alder, Alnus incana (L.) Moench., was grown in sand in cuvettes with a continuous supply of nutrient solution. During the drought treatment for up to 9 days the supply of solution was withheld from one of the split-root halves. After 2–3 days of treatment, soil water became depleted and the unwatered root halves were at a constant drought stress, water potential (Ψ_nodules) = -1.1 to -1.6 MPa. Nitrogenase activity in the dry half decreased to about 70% of the initial value during the first 2–3 days and then stayed at this level. The water supply to the shoot from the wet root half was high and only a temporary slight decrease in photosynthesis and stomatal conductance was found in drought-stressed split-root plants. Labelling studies showed a reduced translocation of photoassimilates to the dry nodules. The fixation of CO₂ in the nodules seemed to be more tolerant to drought than nitrogenase activity. During the drought treatment there was an osmotic adjustment from -0.9 to -1.7 MPa, but no change in the storage of starch in the nodules. In alders where parts of the root system is kept dry these roots acclimate and continue a persistent nitrogenase activity.     

Effects of water stress on nitrogenase activity in Abuts incana

Tolerance to water stress was studied in plants of grey alder, Alnus incana (L.) Moench, grown in a climate chamber in pots of sand supplied with a nitrogen-free nutrient solution. The plants were subjected to a single drying and recovery cycle, during which acetylene reduction, transpiration and stomatal resistance were measured. At different stress levels the plants were placed in a closed system to equilibrate the water potential in the plant-soil system. The water potential of the plants was determined, after which they were watered and their recovery studied. Nitrogenase activity showed low tolerance to water deficit. At moderate stress (−0.6 to −0.8 MPa) acetylene reduction was reduced by half, and at more severe stress, (< −1 MPa) activity was near zero. There was a rapid decrease in nitrogenase activity coincident with stomatal closure, which indicates a continuous need for photoassimilates for nitrogenase activity. Nodules or nitrogenase activity seemed to be weak sinks for assimilates compared with root pressure bleeding. Measurements of nitrogenase activity in root nodule homogenates supplied with ATP and reductant suggested a loss of active nitrogenase in the nodules in response to water stress. The recovery from moderate stress or long dark treatment took several days, and recovery from severe stress took still longer. Shortage of assimilates and disturbances in oxygen and nitrogen balances in the nodules are discussed as reasons for the reduced nitrogenase activity in response to water stress.     

Plant-induced changes in the redox potentials of rice rhizospheres

Redox potentials in microsites of the rhizosphere of flooded rice were continuously measured for several days. Close to the root tips redox potential markedly increased. The highest increase was measured in the rhizosphere of the tips of short lateral roots. Aerobic redox conditions were reached there, except in a very strongly reduced soil. Both the extension of the oxidation zone around the root tips and the maximum redox potential reached were influenced by the reducing capacity of the soil. The radius of the redox rhizosphere varied from less than 1 mm in a strongly reduced soil up to 4 mm in a weakly reduced one. The root-induced oxidation processes in the rhizosphere depended on the atmospheric oxygen supply to the roots.     

The Physiology and Nitrogen-fixing Capability of Aquatically and Terrestrially Grown Neptunia plena: The Importance of Nodule Oxygen Supply

The aquatic legume Neptunia plena (L.) Benth. was grown in non-aeratedwater culture or vermiculite. Growth, nodulation, nitrogen fixationand nodule physiology were investigated. Over an 80-d period,plants grew and fixed nitrogen and carbon equally well in bothrooting media, although distribution of growth between plantparts varied. Total nodule dry weights and volumes were similarbut vermiculite-grown plants had three times as many (smaller)nodules than those grown in water. Oxygen diffusion resistanceof nodules exposed to 21% oxygen and 10% acetylene did not differsignificantly. Both treatments showed similar declines in rootrespiration and acetylene reduction activity (approx. 10%) whenroot systems were exposed to stepped decreases and increasesin rhizosphere oxygen concentration. However, nitrogenase activityof aquatically grown plants was irreversibly inhibited by rapidexposure of nodules to ambient air, whereas vermiculite-grownplants were unaffected. Aeration of water-cultured N. plenareduced stem length (but not mass) and number of nodules perplant. The concentration of nitrogen fixation by 163%. PossibleO₂ transport pathways from the shoot atmosphere to roots andnodules are discussed. Aquatic legume, diffusion resistance, Neptunia plena, nitrogen fixation, oxygen, root nodules     

Nitrogen fixation in seagrass meadows: Regulation, plant–bacteria interactions and significance to primary productivity

The rhizosphere sediments of seagrasses are generally a site of intense nitrogen fixation activity and this can provide a significant source of "new" nitrogen for the growth of the plants. In this paper, I review the data concerning nitrogen fixation in seagrass ecosystems, the transfer of the fixed nitrogen from the bacteria to the plants and its contribution to the overall productivity of seagrasses in different climatic zones.
The relationship between the plants and diazotrophic heterotrophic bacteria in the rhizosphere is discussed, particularly focusing on the potentially important role of nitrogen-fixing, sulphate-reducing bacteria. The regulation of nitrogen fixation rates in the rhizosphere by photosynthetically driven oxygen and fixed carbon release by the plant roots and rhizomes, and the availability of ammonium in the porewater, is assessed. Finally, the hypothesis that a mutualistic or symbiotic association exists between the seagrasses and heterotrophic nitrogen fixers in the rhizosphere, based on the mutual exchange of fixed carbon and nitrogen, is discussed.     

Root-induced changes in radial oxygen loss, rhizosphere oxygen profile, and nitrification of two rice cultivars in Chinese red soil regions

Aims

To evaluate the external and internal morphological differences of roots that might influence rice root radial oxygen loss (ROL) and the corresponding rhizosphere nitrification activity, growth characteristics and nitrogen nutrition of rice.

Methods

The root ROL and rhizosphere oxygen profile were determined using a miniaturised Clark-type oxygen microelectrode system, and the rhizosphere nitrification activity was studied with a short-term nitrification activity assay.

Results

The rice biomass, nitrogen accumulation and nitrogen use efficiency (NUE) of ZH (high yield) were significantly higher than those of HS (low yield). The root biomass, number, diameter and porosity of ZH were also much greater than those of HS. The inner and surface oxygen concentrations of the root of ZH were significantly higher than those of HS. The order of paddy soil oxygen penetration depth was ZH?>?HS?>?CK, and the order of the oxygen concentrations detected in the water layer and rhizosphere soil was the same. The rhizosphere nitrification activity and nitrate concentration of ZH were significantly higher than those of HS.

Conclusions

More porous and thicker roots improved the individual root ROL, and more adventitious root numbers enhanced the entire plant ROL and correspondingly improved the rhizosphere nitrification activity, which might influence the growth and nitrogen nutrition of rice.     

Effect of lyophilization on the nitrogenase activity of Azotobacter vinelandii]

The activity of nitrogenase in the cells of Azobacter vinelandii grown from lyophilized and non-lyophilized cultures depends on the donor of hydrogen and the concentration of oxygen in the gaseous phase. The lyophilized cells are more sensitive to oxygen (O2 optimum for nitrogen fixation is ca. 1 percent) than the non-lyophilized cells (ca. 5 percent). The determination of acetylene reduction in the course of the culture growth has shown that nitrogen fixation in the lyophilized cells takes place after a lag-period (about six hours) at a rate lower than that of the non-lyophilized cells. The results obtained suggest that lyophilization increases the sensitivity of the cells to oxygen and decreases their nitrogenase activity which is however restored after a while.     

Selection of mixed inoculants exhibiting growth-promoting activity in rice plants from undefined consortia obtained by continuous enrichment

Aims

The main objective of this study was to test the hypothesis that isolating and characterizing bacterial suspensions (undefined consortia) from the root/rhizosphere of rice will contribute to the selection of mixtures of rhizobacteria with better growth-promoting activity of rice plants.

Methods

Bacterial consortia were obtained from roots/rhizosphere soil samples of rice plants grown under upland and irrigated production systems. Those undefined consortia were subjected to five consecutive passes every 7 days in NFb (N-free broth) semisolid medium. Thereafter, strains of each growth-promoting consortia were isolated by plating on three different culture media. Then, undefined consortia, as well as mix and single bacterial strains, were characterized in terms of indoleacetic acid production, nitrogen fixation capacity, and growth promotion of rice plants.

Results

Of the 72 consortia analyzed, 41.7 % and 50.0 % increased nitrogenase activity and the production of indolic compounds, respectively, after 5 continuous passes in NFB medium. Three undefined consortia, 11 single strains and 5 strain mixtures, exhibited plant growth promotion in rice plants under greenhouse conditions.

Conclusions

Continuous enrichment in Nfb medium of undefined consortia from root/rhizosphere soil is a good strategy for the selection of plant growth-promoting bacteria for rice plants.     

Nitrogen-fixing Bacillus sp. associated with Douglas-fir tuberculate ectomycorrhizae

Nitrogenase activities, measured by acetylene reduction, were detected under microaerophilic field conditions in Douglas-fir tuberculate ectomycorrhizae. Tuberculate ectomycorrhizae consist of densely packed clusters of ectomycorrhizal rootlets enclosed in a supplementary fungal peridium-like layer. Nitrogenase activity was primarily in the external layer and was greatly enhanced with added sucrose. The bacterium isolated, a nitrogen-fixing, spore-forming Bacillus sp., is an aerobe but requires anaerobic conditions for nitrogenase activity. Respiration in the tuberculate complex by the fungus, roots, and associated mycorrhizosphere microbes probably contributes to maintaining a microaerophilic niche where nitrogen fixation can take place. Water extracts of peridium or mycorrhizal root tips enhanced nitrogenase activity of this associative Bacillus sp., thereby indicating a close nutritional relationship between this bacterium and the tuberculate mycorrhizae. Thiamine more significantly enhanced bacterial nitrogenase activity than biotin; no activity was detected with p-aminobenzoic acid. Even though the levels ofnitrogenase activities in the tubercles in situ were low, as measured by the present methods, they may indicate a significant contribution to the nitrogen dynamics of these nitrogen-limited Douglas-fir forests over a long-term period.     

Root growth,aerenchyma development,and oxygen transport in rice genotypes subjected to drought and waterlogging

Soils under field conditions may experience fluctuating soil water regimes ranging from drought to waterlogging. The inability of roots to acclimate to such changes in soil water regimes may result in reduced growth and function thereby, dry matter production. This study compared the root and shoot growth, root aerenchyma development, and associated root oxygen transport of aerobic and irrigated lowland rice genotypes grown under well-watered (control), waterlogged, and droughted soil conditions for 30 days. The aerobic genotypes were as tolerant as the irrigated lowland genotypes under waterlogging because of their comparable abilities to enhance aerenchyma that effectively facilitated O₂ diffusion to the roots for maintaining root growth and dry matter production. Under drought, aerobic genotypes were more tolerant than the irrigated lowland genotypes due to their higher ability to maintain nodal root production, elongation, and branching, thus, less reduction in dry matter production. Aerenchyma was also formed in droughted roots regardless of genotypes, but was resistant to internal O₂ transport under O₂ deficiency. The ability of roots to resist temporal variations in drought and waterlogging stresses might have strong implications for the adaptation of rice growing in environments with fluctuating soil water regimes.     

Nitrogenase activity in the rhizosphere of sugar cane and some other tropical grasses

Summary Roots of sugar cane had considerable nitrogenase activity and produced up to 5 n moles ethylene/h/g root by the reduction of acetylene. The rhizosphere soil and soil mid-way between the cane rows also reduced acetylene.Beijerinckia indica was abundant on roots and in the soil. Nitrogenase activity was also associated with roots ofPanicum maximum,Pennisetum purpureum andCymbopogon citratus.