The contribution of plant roots to CO2 fluxes from organic soils

The CO₂ released in soil respiration is formed from organic matter which differs in age and stability, ranging from soluble root exudates to more persistent plant remains. The contribution of roots, a relatively fast component of soil cycling, was studied in three experiments. (1) Willows were grown in a greenhouse and CO₂ fluxes from the substrate soil (milled peat) and from control peat were measured. (2) CO₂ fluxes from various peatland sites were measured at control points and points where the roots were severed from the plants. (3) CO₂ fluxes in cultivated grassland established on peatland were measured in grassy subsites and in subsites where the growth of grass was prevented by regular tilling. The root-derived respiration followed the typical annual phenology of the vegetation, being at its maximum in the middle and late summer. All the experiments gave similar results, root-derived respiration accounting for 35–45% of total soil respiration in the middle and late summer at sites with an abundant vegetation. The root-derived respiration from the virgin peatland sites correlated well with the tree biomass, and also partly with the understorey vegetation, but in the drained sites the root effect was greater, even in the presence of less understorey vegetation than at virgin subsites.     

Interaction between rhizosphere microorganisms and plant roots: 13C fluxes in the rhizosphere after pulse labeling

The input dynamics of labeled C into pools of soil organic matter and CO₂ fluxes from soil were studied in a pot experiment with the pulse labeling of oats and corn under a ¹³CO₂ atmosphere, and the contribution of the root and microbial respiration to the emission of CO₂ from the soil was determined from the fluxes of labeled C in the microbial biomass and the evolved carbon dioxide. A considerable amount of ¹³C (up to 96% of the total amount of the label found in the rhizosphere soil) was incorporated into the biomass of the rhizosphere microorganisms. The diurnal fluctuations of the labeled C pools in the microbial biomass, dissolved organic carbon, and CO₂ released in the rhizosphere of oats and corn were related to the day/night changes, i.e., to the on and off periods of the photosynthetic activity of the plants. The average contribution of the corn root respiration (70% of the total CO₂ emission from the soil surface) was higher than that of the oats roots (44%), which was related to the lower incorporation of rhizodeposit carbon into the microbial biomass in the soil under the corn plants than in the soil under the oats plants.     

Nitrogen transport in plants,with an emphasis on the regulation of fluxes to match plant demand

Physiological methods, especially the use of isotopes of N, have allowed for the detailed characterizations of the several putative transport systems for nitrate and ammonium in roots of higher plants. In the last decade, the cloning of genes that appear to encode both high- and low-affinity transporters represent major advances, as well as substantiating the inferences based on earlier physiological methods. Nevertheless, the unexpected plethora of genes that have been identified now presents even greater challenges, to resolve their individual functions and to attempt to place these functions in a whole plant/environmental context.     

底土胁迫效应与植物根系的适应性

系统分析了底土的物理、化学和生物特性及养分资源状况，阐述了底土对植物根系生长的直接和间接影响，探讨了植物根系生长对底土性质的影响，明确了底土改良方法主要包括常规方法和生物学改良方法，讨论了植物根系的生物穿孔效应对底土的改良作用及其影响因素，指出利用植物对底土环境胁迫的的适应性和抗逆能力，采用遗工程技术选育抗性品种，是持续改良底土的有效方法.研究底土胁迫效应及植物的适应性对合理开发底土资源，提高农业生产潜力，发展可持续农业具有重要意义。     

底土胁迫效应与植物根系的适应性

系统分析了底土的物理、化学和生物特性及养分资源状况，阐述了底土对植物根系生长的直接和间接影响，探讨了植物根系生长对底土性质的影响，明确了底土改良方法主要包括常规方法和生物学改良方法，讨论了植物根系的生物穿孔效应对底土的改良作用及其影响因素，指出利用植物对底土环境胁迫的适应性和抗逆能力，采用遗传工程技术选育抗性品种，是持续改良底土的有效方法。研究底土胁迫效应及植物的适应性对合理开发底土资源，提高农业生产潜力，发展可持续农业具有重要意义。     

Characteristics of ammonium and nitrate fluxes along the roots of Picea asperata

Nitrogen (N), ammonium (NH₄⁺) and nitrate (NO₃^?), is one of the key determinants for plant growth. The interaction of both ions displays a significant effect on their uptake in some species. In the current study, net fluxes of NH₄⁺ and NO₃^? along the roots of Picea asperata were determined using a Non-invasive Micro-test Technology (NMT). Besides, we examined the interaction of NH₄⁺ and NO₃^? on the fluxes of both ions, and the plasma membrane (PM) H⁺-ATPases and nitrate reductase (NR) were taken into account as well. The results demonstrated that the maximal net NH₄⁺ and NO₃^? influxes were detected at 13–15?mm and 8–10.5?mm from the root apex, respectively. Net NH₄⁺ influx was significantly stimulated with the presence of NO₃^?, whereas NH₄⁺ exhibited a markedly negative effect on NO₃^? uptake in the roots of P. asperata. Also, our results indicated that PM H⁺-ATPases and NR play a key role in the control of N uptake.     

Absorption kinetics of Ca,Mg, Na and P by intact corn and onion roots

The influence of Ca on the influx kinetics of Mg and Na, and Mg effects on Ca influx kinetics were evaluated in intact corn roots. Increasing levels of Mg (25 to 75 μM) reduced the Imax for Ca. However, in absence of Mg, increasing levels of Ca (10 to 30 μM) increased the Imax and Km for Ca. Higher levels of Mg interfered with Ca influx. Increasing levels of Ca promoted the Mg influx rates and increased Km as well. Absence of Ca reduced the magnitude of Imax and Km for Mg. At an initial level of 50 μM Ca, increasing Na levels from 50 to 100 μM enhanced the Imax and Km for Na. In the absence of Ca, higher Imax were recorded for Na. Calcium reduced the Na influx rates.

Ion influx kinetics for Ca, Mg, Na and P were determined with the two morphologically differing intact root systems of corn and onion. The rate of influx for Ca, Mg and Na in corn were lower than onion roots. The Imax for P in corn were slightly higher than in onion. The Km vaues for Ca, Mg and Na in corn were also higher than onion indicating a lower affinity by the carrier sites for these ions in corn roots.     

Tea plant (Camellia sinensis L.) roots secrete oxalic acid and caffeine into medium containing aluminum

We examined the response of the tea plant (Camellia sinensis L.) to aluminum (Al) exposure under sterile conditions, focusing specifically on the secretion of low molecular mass organic compounds from roots. After germination in agar medium, tea seedlings together with medium were placed on agar containing 0.4?mM Al with 0.2% hematoxyline (hematoxylin-Al medium). The purple color of the hematoxylin-Al medium was observed to fade gradually, until none of the color remained 6 days later. The tea seedlings were then treated with simple calcium solution (0.2?mM, at pH 4.2) containing AlCl₃, which ranged in concentration from 0 to 0.8?mM, for 24?hrs. The amount of oxalate secreted into the medium increased as the external Al concentration increased, while the concentrations of malate and citrate in the medium remained unchanged. Oxalate secretion started within 30?min after Al exposure and increased linearly thereafter. The findings demonstrated that oxalate was a key compound in the Al-tolerance mechanism employed by the tea plant, which detoxifies Al³⁺ externally in the rhizosphere. In addition to oxalate, caffeine was also secreted by tea roots in response to Al exposure. It is possible that caffeine excretion from the roots of tea plants may stimulate root growth through the inhibition of callose deposition in root tips.     

Response of oilseed rape plant to low root temperature and nitrate: Ammonium ratios

Oilseed rape (Brassica napus L.) response to root temperature regimes (20/20, 16/8 and 12/12°C day/night) at constant 20°C air temperature was studied. At each regime, three NO₃ ^‐:NH₄ ⁺ ratios (10:0, 8:2, or 6:4), at constant 10 mM N, in the irrigation solution were tested. Plant growth, transpiration, ionic composition and level of cytokinins and gibberellins in the xylem exudate were monitored. The two low root temperature regimes, 12/12 and 16/8°C, reduced rape shoot growth by 28 and 22%, and increased the accumulation of soluble carbohydrates by 42 and 26% in the roots, respectively, as compared to the 20/20°C regime. Low root temperatures reduced plants transpiration. The NO₃ ^‐:NH₄ ⁺ ratios had no effect on rape growth. At low root temperatures NO₃ ^‐contents increased in the shoot and decreased in the roots. The sum of cations and that of anions at 12/12 and 16/8°C root temperatures decreased significantly as compared to 20/20°C. The presence of NH₄ ⁺ in the irrigation solution decreased the concentrations of Ca²⁺ and Mg²⁺ in the shoots and roots and increased that of Cl^‐ in the shoots and of H₂PO₄ ^‐ in the roots at all root temperatures. Cytokinins and gibberellins contents in the xylem exúdate decreased at the low root temperature regimes. Low root temperature reduced total upward transport of the mineral nutrients and phytohormones, most probably because of reduced water flow through the plant.     

Carbon routes from decomposing plant residues and living roots into soil food webs assessed with 13C labelling

This field experiment investigated how C from fresh organic amendments and from a growing leek crop was allocated into different soil microbial and faunal groups in an arable field. A ¹³C-enriched red clover green manure was incorporated in one treatment, while the growing leek crop was pulse labelled with ¹³CO₂ in another. Incorporation of ¹³C into microbial fatty acids, micro- and macroarthropods, enchytraeids and earthworms was determined on several occasions during the growing season in order to determine whether different groups or species of microorganisms and fauna were specialised on either the decomposing green manure material or root-derived C. Compound-specific stable isotope ratio analysis showed fatty acid markers of actinomycetes and Gram-positive bacteria to be more strongly linked to C originating from the decomposing green manure material, whereas the marker for arbuscular mycorrhizal fungi was more linked to C from the growing leek crop. In contrast, several markers for Gram-negative bacteria were the most ¹³C-enriched and had incorporated more ¹³C than the other phospholipid fatty acids in both treatments, indicating a general dominance irrespective of C source. Most soil fauna seemed to derive their C directly or indirectly from the decomposing plant material, while C from the growing crop appeared to be of secondary importance in this agroecosystem.     

Quantifying the effects of the plant canopy,plant roots,and biological soil crust on soil detachment by overland flow

Journal of Soils and Sediments - The plant canopy, plant roots, and biological soil crusts play important roles in soil detachment by overland flow. This study aims to quantify and analyze the...     

Bound phenolic compounds in water extracts of soils,plant roots and leaf litter

Seven soils were examined for their contents of p-hydroxybenzoic, vanillic, p-coumaric and ferulic acids, p-hydroxybenzaldehyde and vanillin. Water-soluble forms, both “free” and “bound” of the phenolic compounds accounted for less than 0.7% of the total amount of each acid or aldehyde as determined by extraction of the soil with 2 M NaOH. In most instances, more than 50% of the water-soluble compounds were in the bound form, which was estimated after conversion to the free form by treatment of the water extract with NaOH. Water-soluble forms, both free and bound, of each compound also occurred in roots associated with six of the soils, and in beech litter associated with the seventh.     

Species differences in calcium and potassium distributions within plant roots

Abstract

The Ca and K distributions in the roots of soybean, kidneybean and corn were investigated in relation to their lateral movement within the roots using an electronprobe X-ray microanalyzer. In the corn roots, the accumulation site of Ca was the wall of the cortical cells and little Ca was detected in the stele, suggesting that apoplasmic movement of Ca was blocked by the endodermis. In the legume roots, Ca was detected at the wall of the endodermal and stelar cells as well as the cortical cells, suggesting that the endodermis of the legume roots was permeable to the apoplasmic movement of Ca. K was mainly found within the cells of the roots, suggesting symplasmic movement of K. Neither accumulation nor depletion of the cations at the soil-root interface was observed, but the concentrations of the cations at the root surface appeared to be reduced when the roots were grown in the soil.     

Effects of Cd on the fluxes of K+ and H+ in excised roots

As a result of Cd treatment, K concentrations decreased in Cd sensItive maize and kidney bean calli (Obata et al. 1994) and in intact roots of kidney bean plants (Obata et al. unpublished). Potassium may be extruded from the roots or the absorption of K may be depressed by the Cd treatment in these Cd sensitive plants. Obata et al. (1996) observed that Cd inhibited both the efflux of H⁺ and influx of K⁺ following K⁺ addition in intact roots of bean. Thus Cd may affect the activity of proteins essential to ion movement., i.e. ioncarriers, channels and ATPase embedded in the membranes and/or may affect the permeability of the lipids of the membrane.     

Interference from plant roots in the estimation of soil microbial ATP,C, N and P

Excised, solution-grown roots of maize or ryegrass added to two pasture soils at the rate of 6.0mg g^?1 and 13.8 mg g^?, respectively, increased the flush (fumigated minus control values) of CO₂-C by up to 1.89-fold, KCl extractable N by up to 1.88-fold, and NaHCO₃ extractable P by 3.28-fold. The ATP content of the soil was increased by up to 1.42-fold. Because of high variability the effect of the roots on the C and N flushes was not significant at P < 0.05.Incubation of the root-amended soils for 7 days at 25°C prior to fumigation much decreased the contribution from the roots to the C and N flush, and to the ATP content. There was, however, still a large significant effect of the roots on the P-flush, this being up to 3 times greater than the equivalent soil without roots.In soil samples with a high viable root density (> 6mg g^?1) such as may occur in dense pastures, greenhouse pot experiments or rhizosphere soil samples, it is recommended that they be incubated for 7 days prior to fumigation and analyses. Without such prior incubation there is the risk that root material may be included in the “microbial” biomass estimations.     

Extractant ph and the release of phenolic compounds from soils,plant roots and leaf litter

The influence of pH was examined, over the range from 6 to 14, on the amounts of p-hydroxybenzoic, vanillic, p-coumaric, ferulic and syringic acids, p-hydroxybenzaldehyde and vanillin, extracted from four soils and associated roots or leaf litter. Adjustment of pH was obtained by the addition of graded amounts of Ca(OH)₂ to water or by 2 m NaOH. The roots associated with three of the soils were from permanent pasture, perennial ryegrass and red clover, while the leaf litter associated with the fourth soil was from beech.The amounts of each phenolic compound extracted increased continuously with increasing pH, from a “threshold” value which varied between pH 7.5 and 10.5. The amounts extracted by water alone from the soil under permanent pasture, at pH 5.8, were equivalent to concentrations in the soil solution ranging from 1.4 μm for p-hydroxybenzoic acid to < 10 nm for ferulie acid. Amounts of up to 2000 times greater than these were extracted by 2 m NaOH. Similar effects of extractant pH were found with the other soils.Comparisons of the amounts of the phenolic compounds extracted from the soils, with the amounts extracted from the associated roots or leaf litter, suggested that substantial proportions of the soil phenolic compounds were either derived from organic residues more than 4 yr old or were the result of microbial synthesis.     

Carbon partitioning in plant and soil,carbon dioxide fluxes and enzyme activities as affected by cutting ryegrass

The effect of a single cut (simulated grazing) and regrowth of Lolium perenne on CO₂ efflux from soil (loamy Haplic Luvisol), on below-ground C translocation and on the distribution of plant C among different soil particle size fractions was investigated under controlled conditions with and without N fertilization by pulse labelling with ¹⁴C 7 times (four before and three after the cutting). The amount of ¹⁴C respired from the rhizosphere of Lolium decreased by a factor of about 3 during 1 month of growth. At the same time the amount of ¹⁴C stored in soil increased. Cut and non-fertilized plants respired less C in the rhizosphere compared to the uncut plants and cut fertilized plants. About 80% of the root-derived CO₂ efflux originated from the C assimilated after defoliation, and 20% originated from the C assimilated before cutting. N fertilization decreased the below-ground C losses (root respiration and exudation) during regrowth. The shoot is the main sink of assimilated C before and after the defoliation. N fertilization led to higher C incorporation into the shoot parts growing after defoliation compared to unfertilized plants. A lower incorporation of ¹⁴C was observed in the roots of N fertilized plants. The relative growth rates (expressed as ¹⁴C specific activity) of roots and stubble were minimal and that of shoot parts growing after defoliation was maximal. Twelve percent of ¹⁴C was found in the newly grown leaves after regrowth; nevertheless, 4.7% and 2.4% of ¹⁴C in the new shoot parts were translocated from the root and shoot reserves of unfertilized and fertilized plants, respectively. Most of the C retranslocated into the new Lolium leaves originates from the stubble and not from the roots. Between 0.5% and 1.7% of ¹⁴C recovered in shoots and below-ground C pools was found in the soil microbial biomass. Cutting and fertilization did not change ¹⁴C incorporation into the microbial biomass and did not affect xylanase, invertase, and protease activities. Tracing the assimilated C in particle size fractions revealed maximal incorporation for the sand and clay fraction.     

Organic acid excretion from roots: a plant mechanism for enhancing phosphorus acquisition,enhancing aluminum tolerance,and recruiting beneficial rhizobacteria

The beneficial effects of organic acids (OAs) excretion from plant roots were first proposed as being associated with the mechanism of superior phosphorus utilization by the cluster roots of white lupin (Lupinus albus L.), and these effects are now widely accepted as pleiotropic effects associated with stress tolerance of plants. Excreted OAs detoxify rhizotoxic aluminum, recruit beneficial bacterium for induced systemic resistance, and modify root architecture to enhance phosphorus starvation. OA excretion is probably optimized in the carbon economy and is coordinately regulated with other traits that additively confer each stress factor. Here we present an overview of the molecular physiology of OA excretion from roots, how plants activate OA excretion, and how this excretion can be managed as a specific response.     

Effect of methionine sulphoximine on13 N-ammonium fluxes in the roots of barley and squash seedlings

¹³ N-Iabelled experiments were carried out with barley and squash seedlings to analyse the effect of methionine sulphoximine (MSO), as an inhibitor of the ammonium assimilation, on NH₄⁺ fluxes in the roots at the level (150 μM NH₄⁺) of the high affinity uptake system (HATS). MSO pretreatment in the root medium significantly accelerated NH₄⁺ influx and efflux in NH₄⁺ -fed plants of both species. Concomitantly, the level of NH₄⁺ extracted from the root and shoot tissues was strongly elevated whereas the translocation of¹³ N-Iabelled organic substances (presumably amino products) into the shoots was markedly reduced. The onset of the increased NH₄⁺ influx did not occur immediately but was indicated about 1 h after the application of MSO to the nutrient solution. Also, MSO did not directly affect the uptake of¹³ NO₃^?. The reason why MSO treatment leads to an acceleration of the NH₄⁺ influx against an increasing level in the cell tissue remains obscure. In squash, an intermediate of NO₃^? metabolism is thought to suppress NH₄⁺ uptake since NH₄⁺ influx was high in N-free- and NH₄⁺ -grown seedlings but was reduced about threefold in NO₃^? -fed squash plants; the suppression was not eliminated by MSO treatment.     

植物根系与叶片吸收硒的关键过程及影响因素

硒元素具有抗癌、抗衰老、提高人体免疫力等功能,经植物吸收转化后,可以更加安全高效地被机体吸收利用.富硒植物产品是时下功能性农业的主要产品之一,经济器官富集较高的硒是该产品的核心.然而,目前市面上富硒植物产品的硒含量却参差不齐且普遍偏低.因此,系统研究植物吸收硒元素的途径、关键过程与影响因素对提高植物的富硒效率具有十分重...