Production,standing biomass and natural abundance of <Superscript>15</Superscript>N and <Superscript>13</Superscript>C in ectomycorrhizal mycelia collected at different soil depths in two forest types

Nutrient uptake by forest trees is dependent on ectomycorrhizal (EM) mycelia that grow out into the soil from the mycorrhizal root tips. We estimated the production of EM mycelia in root free samples of pure spruce and mixed spruce-oak stands in southern Sweden as mycelia grown into sand-filled mesh bags placed at three different soil depths (0–10, 10–20 and 20–30 cm). The mesh bags were collected after 12 months and we found that 590±70 kg ha^–1 year^–1 of pure mycelia was produced in spruce stands and 420±160 kg ha^–1 year^–1 in mixed stands. The production of EM mycelia in the mesh bags decreased with soil depth in both stand types but tended to be more concentrated in the top soil in the mixed stands compared to the spruce stands. The fungal biomass was also determined in soil samples taken from different depths by using phospholipid fatty acids as markers for fungal biomass. Subsamples were incubated at 20°C for 5 months and the amount of fungal biomass that degraded during the incubation period was used as an estimate of EM fungal biomass. The EM biomass in the soil profile decreased with soil depth and did not differ significantly between the two stand types. The total EM biomass in the pure spruce stands was estimated to be 4.8±0.9×10³ kg ha^–1 and in the mixed stands 5.8±1.1×10³ kg ha^–1 down to 70 cm depth. The biomass and production estimates of EM mycelia suggest a very long turnover time or that necromass has been included in the biomass estimates. The amount of N present in EM mycelia was estimated to be 121 kg N ha^–1 in spruce stands and 187 kg N ha^–1 in mixed stands. The ¹³C value for mycelia in mesh bags was not influenced by soil depth, indicating that the fungi obtained all their carbon from the tree roots. The ¹³C values in mycelia collected from mixed stands were intermediate to values from pure spruce and pure oak stands suggesting that the EM mycelia received carbon from both spruce and oak trees in the mixed stands. The ¹⁵N value for the EM mycelia and the surrounding soil increased with soil depth suggesting that they obtained their entire N from the surrounding soil.     

Old-growth red spruce communities in the mid-Appalachians

Old-growth stands of red spruce (Picea rubens) were sampled at the only four localities in the mountains of southwestern Virginia and central West Virginia where examples of such are known to exist. Based upon mean ages (±SE) of cored trees, sampled stands ranged in age from 164±18 to 201±10 yr. Dendro-ecological (tree-ring) analysis showed a marked decline in growth of trees at three localities during the late 1800s, followed by recovery to previous levels of growth within two decades. This growth-trend decline generally coincides with a period of extensive mortality of red spruce reported to have occurred in central West Virginia. Basal area of trees 2.5 cm DBH ranged from 35.4 to 46.1 m²/ha. These figures are considerably lower than those recorded at earlier dates for similar old-growth stands in the Appalachians, which suggests that a general decline has occurred over the last half-century.     

Effects of GABA antagonists,SR 95531 and bicuculline,on GABAA receptor-regulated chloride flux in rat cortical synaptoneurosomes

Synaptoneurosomes isolated from cerebral cortices of male Sprague-Dawley rats were used for studying GABA_A receptor-regulated chloride influx. The in vitro effects of GABA antagonists, SR 95531 (a pyridazinyl GABA derivative) and bicuculline, on pentobarbital-stimulated, muscimol-stimulated or flunitrazepam-enhanced, muscimol-stimulated chloride uptake were studied. The chloride uptake was determined at 30°C, for 5 sec. Pentobarbital and muscimol produced a maximal stimulation of chloride uptake in cortical synaptoneurosomes at 500 M and 50M, respectively. SR 95531 as well as bicuculline had no effect on the basal uptake of chloride. Whereas, SR 95531 (0.3–30 M) and bicuculline (0.1–100 M), when added 5 min before muscimol (50 M), produced a significant concentration-dependent inhibition of muscimol (50 M)-stimulated chloride uptake (IC₅₀ s of 0.89±0.11 M and 13.45±2.10M, respectively). In studies of the inhibitory effects of SR 95531 and bicuculline on pentobarbital (500 M)-stimulated chloride uptake, the IC₅₀ s were 0.81±0.12 M and 3.86±1.14 M, respectively. SR 95531 exhibited a more potent inhibitory effect than bicuculline on flunitrazepam-enhanced, muscimol-stimulated chloride uptake. The results revealed that SR 95531 has a more potent antagonistic effect than bicuculline on GABA_A-regulated chloride flux.     

Inward rectifier K channels in renal epithelioid cells (MDCK) activated by serotonin

Summary The present study has been performed to test for the effect of intracellular calcium and of serotonin on the channel activity in patches from subconfluent MDCK-cells. In inside-out patches, inwardly rectifying potassium-selective channels are observed with open probabilities of 0.01±0.01, 0.24±0.03 and 0.39±0.07, at 100 nmol/liter, 1 mol/liter or 10 mol/liter calcium activity, respectively. The single-channel slope conductance is 34±2 pS, if the potential difference across the patch (V ) is zero, and approaches 59±1 pS, ifV is –50 mV, cell negative. In the cell-attached mode, little channel activity is observed prior to application of serotonin (open probability=0.03±0.03). If 1 mol/liter serotonin is added to the bath perfusate, the open probability increases rapidly to a peak value of 0.34±0.04 within 8 sec. In continued presence of the hormone, the open probability declines to approach 0.06±0.02 within 30 sec. At zero potential difference between pipette and reference in the bath (i.e., the potential difference across the patch is equal to the potential difference across the cell membrane), the single-channel conductance is 59±4 pS. In conclusion, inwardly rectifying potassium channels have been identified in the cell membrane of subconfluent MDCK-cells, which are activated to a similar extent by increase of intracellular calcium activity to 1 mol/liter and by extracellular application of 1 mol/liter serotonin.     

Changes in phosphorus concentrations and pH in the rhizosphere of some agroforestry and crop species

The aim of this work was to assess whether agroforestry species have the ability to acquire P from pools unavailable to maize. Tithonia diversifolia(Hemsley) A. Gray, Tephrosia vogelii Hook f., Zea mays and Lupinus albusL. were grown in rhizopots and pH change and depletion of inorganic and organic P pools measured in the rhizosphere. Plants were harvested at the same growth stage, after 56 days for maize and white lupin and 70 days for tithonia and tephrosia, and the rhizosphere sampled. The rhizosphere was acidified by tithonia (pH change –0.3 units to pH 4.8) and lupins (–0.2 units to 4.9), alkalinised by tephrosia (+0.4 units to pH 5.4), and remained unchanged with maize growth. Concurrent with acidification in the rhizosphere of tithonia there was a decline in resin-P (0.8 g P g^–1). However, there was also a decline in NaOH extractable inorganic P (NaOH-P_i) (5.6 g P g^–1 at the root surface) and organic P pools (NaOH-P_o) (15.4 g P g^–1 at 1.5 mm from the root), which would not be expected without specific P acquisition mechanisms. Alkalinisation of tephrosia rhizosphere was accompanied by changes in all measured pools, although the large depletion of organic P (21.6 g P g^–1 at 5 mm from the root) suggests that mineralisation, as well as desorption of organic P, was stimulated. The size of changes of both pH and P pools varied with distance away from the rhizoplane. Decline of more recalcitrant P pools with the growth of the agroforestry species contrasted with the effect of maize growth, which was negligible on resin-P and NaOH-P_i, but led to an accumulation of P as NaOH-P_o (14.2 g P g^–1 at 5 mm from the root). Overall the depletion of recalcitrant P pools, particularly P_o, suggests that the growth of tithonia and tephrosia enhance desorption and dissolution of P, while also enhancing organic P mineralisation. Both species appear to have potential for agroforestry technologies designed to enhance the availability of P to crops, at least in the short term.     

Does soil acidity reduce subsoil rooting in Norway spruce (Picea abies)?

Increasing evidence suggests that forest soils in central and northern Europe as well as in North America have been significantly acidified by acid deposition during the last decades. The present investigation was undertaken to examine the effect of soil acidity on rooting patterns of 40-year-old Norway spruce trees by comparing fine and coarse roots among four stands which differed in soil acidity and Mg (and Ca) nutrition. The coarse root systems of four to five 40-year-old Norway spruce trees per stand were manually excavated. The sum of cross sectional area (CSA) at 60 cm soil depth and below of all vertical coarse roots, as a measure of vertical rooting intensity, was strongly reduced with increasing subsoil acidity of the stands. This pattern was confirmed when 5 additional acidic sites were included in the analysis. Fine root biomass in the mineral soil estimated by repeated soil coring was strongly reduced in the heavily acidified stands, but increased in the humic layer. Using ingrowth cores and a screen technique, we showed that the higher root biomass in the humic layer of the more acidic stands was a result of higher root production. Thus, reduced fine root biomass and coarse root CSA in deeper soil layers coincided with increased root growth in the humic layer. Root mineral analysis showed Ca/Al ratios decreased with decreasing base saturation in the deeper mineral soil (20–40 cm). In the top mineral soil, only minor differences were observed among stands. In general, low Ca/Al ratios coincided with low fine root biomass. Calcium/aluminum ratios determined in cortical cell walls using X-ray microanalysis showed a similar pattern as Ca/Al ratios based on analysis of whole fine roots, although the amplitude of changes among the stands was much greater. Aluminum concentrations and Ca/Al ratios in cortical cell walls were at levels found to inhibit root growth of spruce seedlings in laboratory experiments. The data support the idea that Al (or Ca/Al ratios) and acid deposition-induced Mg (and possibly Ca) deficiency are important factors influencing root growth and distribution in acidic forest soils. Changes in carbon partitioning within the root system may contribute to a reduction in deep root growth.     

Aerenchyma tissue development and gas-pathway structure in root of Avicennia marina (Forsk.) Vierh.

The aerenchyma differentiation in cable roots, pneumatophores, anchor roots, and feeding roots of the mangrove plant, Avicennia marina (Verbenaceae) was analyzed using a light microscope and scanning electron microscope. In all types, cortex cells were arranged in longitudinal columns extending from the endodermis to the epidermis. No cells in the cortex had intercellular spaces at the root tip (0–150 m), and aerenchyma started developing at 200 m from the root apex. The aerenchyma formation was due to cell separation (schizogeny) rather than cell lysis. The cell separation occurred between the longitudinal cell columns, forming long intercellular spaces along the root axis. During aerenchyma formation, the cortex cells enlarged longitudinally by 1.8–3.9 times and widened horizontally by 2.2–2.9 times. As a result, the aerenchyma had a pronounced tubular structure that was radially long, elliptical or oval in cross section and that ran parallel to the root axis. The tube had tapering ends, as did vessel elements, although there were no perforated plates. The interconnection between neighboring tubes was made by abundant small pores or canals that were schizogenous intercellular spaces between the wall cells. All aerenchyma tubes in the root were interconnected by these small pores serving as a gas pathway.     

Mycorrhizal infection and ageing affect element localization in short roots of Norway spruce [Picea abies (L) Karst.]

Norway spruce [Picea abies (L.) Karst.] seedlings, nonmycorrhizal of mycorrhizal with Laccaria laccata or Paxillus involutus were grown in a quartz sand-nutrient solution system for 6 months and then treated with 5 M Pb for 4 days. Element contents of cortex cell wall of young, medium and old short roots were determined by X-ray microanalysis of longitudinal thin sections. The Pb content was influenced neither by age nor by the distance from the root tip (up to 1.7 mm) but was significantly lower in the P. involutus mycorrhizae than in the L. laccata mycorrhizae or in nonmycorrhizal short roots. In the P. involutus mycorrhizae, the P content of the cortex cell walls was twice as high in young mycorrhizae than in old mycorrhizae. In the nonmycorrhizal short roots and the L. laccata mycorrhizae, P content was influenced neither by age nor by distance from the root tip. The Ca and Fe contents of the cortex cell walls increased with age in the nonmycorrhizal short roots and the mycorrhizae. It is concluded that the element content of the cortex cell walls of short roots is strongly influenced by age, while the distance from the root tip seems to be of minor importance.     

Linking root morphology, longevity and function to root branch order: a case study in three shrubs

Root branching order supports a powerful approach to understanding complex root systems; however, how the pattern of root morphological characteristics, tissue carbon (C) and nitrogen (N) concentrations, and root lifespan are related to anatomical features of variable root orders for mature shrubs (～19 years old) in sandy habitats is still unclear. In this study, these relationships were investigated for three typical shrubs in Horqin Sand Land, Northeast China. Root diameter, individual root length, tissue carbon concentration, C:N ratio, root lifespan, root cross-sectional area (CSA), stele CSA, proportion of stele in root CSA, mean xylem vessel CSA and the number of xylem vessels all increased with root order for the three shrubs, while specific root length and nitrogen concentration decreased with root order. The combined root biomass of the first two orders accounted for more than 63% of the first–fourth order root biomass for all the three shrubs. Proportion of stele to root CSA and number of xylem vessels of third-order root segments were significantly higher than that of the first two orders, and third-order roots showed secondary development with a continuous cork layer. All first-order and most second-order roots exhibited primary development, had an intact cortex, a lower proportion of stele to root CSA, and a smaller number of vessels. Our research suggests that the first two order roots of shrubs in sandy habitats are responsible mainly for absorption, and that they play a major role in root turnover and C and N flux in the soil organic matter pool due to their high proportion of biomass and N concentration, as well as their short lifespan.     

Chemical composition of soil solutions extracted from New Zealand beech forests and West German beech and spruce forests

Concentrations of ions were measured in soil solutions from beech (Nothofagus) forests in remote areas of New Zealand and in solutions from beech (Fagus sylvatica) and Norway spruce (Picea abies) forests in North-East Bavaria, West Germany, to compare the chemistry of soil solutions which are unaffected by acid deposition (New Zealand) with those that are affected (West Germany). In New Zealand, soil solution SO₄ ^2– concentrations ranged between <2 and 58 mol L^–1, and NO₃ ^– concentrations ranged between <1 and 3 mol L^–1. In West Germany, SO₄ ^2– concentrations ranged between 80 and 700 mol L^–1, and NO₃ ^– concentrations at three of six sites ranged between 39 and 3750 mol L^–1, but was not detected at the remaining three sites. At all sites in New Zealand, and at sites where the soil base status was moderately high in West Germany, pH levels increased, and total Al (Al_t) and inorganic monomeric Al (Al_i) levels decreased rapidly with increasing soil depth. In contrast, at sites on soils of low base status in West Germany, pH levels increased only slightly, and Al levels did not decline with increasing soil depth.Under a high-elevation Norway spruce stand showing severe Mg deficiency and dieback symptoms in West Germany, soil solution Mg²⁺ levels ranged between 20 and 60 mol L^–, and were only half those under a healthy stand. Al_t and Al_i levels were substantially higher the healthy stand than under the unhealthy stand, indicating that Al toxicity was not the main cause of spruce decline.     

Acclimation of Two Distinct Plant Species,Spring Barley and Norway Spruce,to Combined Effect of Various Irradiance and CO2 Concentration During Cultivation in Controlled Environment

The short-term acclimation (10-d) of Norway spruce [Picea abies (L.) Karst] to elevated CO₂ concentration (EC) in combination with low irradiance (100 mol m^–2 s^–1) resulted in stimulation of CO₂ assimilation (by 61 %), increased total chlorophyll (Chl) content (by 17 %), significantly higher photosystem 2 (PS2) photochemical efficiency (F_v/F_m; by 4 %), and reduced demand on non-radiative dissipation of absorbed excitation energy corresponding with enhanced capacity of photon utilisation within PS2. On the other hand, at high cultivation irradiance (1 200 mol m^–2 s^–1) both Norway spruce and spring barley (Hordeum vulgare L. cv. Akcent) responded to EC by reduced photosynthetic capacity and prolonged inhibition of F_v/F_m accompanied with enhanced non-radiative dissipation of absorbed photon energy. Norway spruce needles revealed the expressive retention of zeaxanthin and antheraxanthin (Z+A) in darkness and higher violaxanthin (V) convertibility (yielding even 95 %) under all cultivation regimes in comparison with barley plants. In addition, the non-photochemical quenching of minimum Chl a fluorescence (SV₀), expressing the extent of non-radiative dissipation of absorbed photon energy within light-harvesting complexes (LHCs), linearly correlated with V conversion to Z+A very well in spruce, but not in barley plants. Finally, a key role of the Z+A-mediated non-radiative dissipation within LHCs in acclimation of spruce photosynthetic apparatus to high irradiance alone and in combination with EC was documented by extremely high SV₀ values, fast induction of non-radiative dissipation of absorbed photon energy, and its stability in darkness.     

Biosorption and solubilization of copper oxychloride fungicide by Aspergillus niger and the influence of calcium

The biosorption of copper oxychloride fungicide particulates(1 m diameter), at concentrations ranging from 25 to 500 ppm active ingredient (ai), by pelleted mycelium of Aspergillus niger grown on Czapek Dox medium was evaluated. The concentration of the fungicide adsorbed to the mycelium, remaining suspended or solubilized in the medium, was determined by analysis of its copper content (CuF)using atomic absorption spectrophotometry (AAS). 2-day-old pellets exhibited highbiosorption efficiency ranging from 97 ± 1.0 to 88 ± 1.2% of the initially added fungicide concentrations, respectively, within 10 min. However, underthe same conditions, amounts of the removed fungicide by 6-day-old mycelial pellets were significantly lower and ranged from 0.5 ± 0.03 to 0.15 ± 0.01%. Scanning electron microscopy studies of 2-day-old pellets supplemented with thefungicide revealed predominant aggregations of clumps and dense particulates on the hyphal tips. The adsorbed CuF of 125 ppm ai fungicide subsequently decreased from 7.5 ± 0.5 to 2.1 ± 0.1 mol Cu (mg dry wt)^-1 after 12 h incubation. Simultaneously, the soluble portion of CuF remaining in the medium increased from 0.9 ± 0.6 to4.9 ± 0.2 mol Cu ml^-1. The presence of 50 mM CaCl₂ resulted in a decrease of the adsorbed CuF to 3.5 ± 0.5 mol Cu (mg dry wt)^-1 and solubilizedcopper in the medium increased to 5.9 ± 0.8 mol Cu ml^-1. Additionally, the cellular copper contents attained after 2 h were 0.08 ± 0.01 and 0.16 ± 0.007 mol Cu (mg dry wt)^-1 in absence and presence of calcium, respectively. The addition of calcium to glucose-starved pellets greatly increased the medium [H⁺] which was conclusively discussed in relation to Ca²⁺/H⁺ exchangecapacity of the fungal cells. These results are of potential environmental,biotechnological and agricultural importance.     

Tissue culture ofKarwinskia humboldtiana—a plant producing toxins with antitumoural effects

Isolated embryos ofKarwinskia humboldtiana were cultured in vitro. The growth of embryos and development to plantlets on woody plant medium supplemented with indole-3-acetic acid 6.10^-2 mol l^–1, gibberellic acid (GA₃) 3.10^-2 mol l^–1, and 6-benzylaminopurine (BA) 2 mol l^–1 was obtained. Multiplication of shoots and rooting of excised shoots has been achieved. Callus formation on modified Murashige-Skoog medium supplemented with 1-naphthaleneacetic acid 10 mol l^–1, GA₃ 14 mol l^–1, and kinetin 5 mol l^–1 on hypocotyls, or on root cultures on medium supplemented with 2.4-dichlorophenoxyacetic acid 10 mol l^–1 and BA 10 mol l^–1 was induced.Abbreviations BA 6-benzylaminopurine - 2,4-d 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid - IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - TEM transmission electron microscopy     

Biomass, morphology and nutrient contents of fine roots in four Norway spruce stands

Fine root systems may respond to soil chemical conditions, but contrasting results have been obtained from field studies in non-manipulated forests with distinct soil chemical properties. We investigated biomass, necromass, live/dead ratios, morphology and nutrient concentrations of fine roots (<2 mm) in four mature Norway spruce (Picea abies [L.] Karst.) stands of south-east Germany, encompassing variations in soil chemical properties and climate. All stands were established on acidic soils (pH (CaCl₂) range 2.8–3.8 in the humus layer), two of the four stands had molar ratios in soil solution below 1 and one of the four stands had received a liming treatment 22 years before the study. Soil cores down to 40 cm mineral soil depth were taken in autumn and separated into four fractions: humus layer, 0–10 cm, 10–20 cm and 20–40 cm. We found no indications of negative effects of N availability on fine root properties despite large variations in inorganic N seepage fluxes (4–34 kg N ha⁻¹ yr⁻¹), suggesting that the variation in N deposition between 17 and 26 kg N ha⁻¹ yr⁻¹ does not affect the fine root system of Norway spruce. Fine root biomass was largest in the humus layer and increased with the amount of organic matter stored in the humus layer, indicating that the vertical pattern of fine roots is largely affected by the thickness of this horizon. Only two stands showed significant differences in fine root biomass of the mineral soil which can be explained by differences in soil chemical conditions. The stand with the lowest total biomass had the lowest Ca/Al ratio of 0.1 in seepage, however, Al, Ca, Mg and K concentrations of fine roots were not different among the stands. The Ca/Al ratio in seepage might be a less reliable stress parameter because another stand also had Ca/Al ratios in seepage far below the critical value of 1.0 without any signs of fine root damages. Large differences in the live/dead ratio were positively correlated with the Mn concentration of live fine roots from the mineral soil. This relationship was attributed to faster decay of dead fine roots because Mn is known as an essential element of lignin degrading enzymes. It is questionable if the live/dead ratio can be used as a vitality parameter of fine roots since both longevity of fine roots and decay of root litter may affect this parameter. Morphological properties were different in the humus layer of one stand that was limed in 1983, indicating that a single lime dose of 3–4 Mg ha⁻¹ has a long-lasting effect on fine root architecture of Norway spruce. Almost no differences were found in morphological properties in the mineral soil among the stands, but vertical patterns were apparently different. Two stands with high base saturation in the subsoil showed a vertical decrease in specific root length and specific root tip density whereas the other two stands showed an opposite pattern or no effect. Our results suggest that proliferation of fine roots increased with decreasing base saturation in the subsoil of Norway spruce stands.     

Physiological energetics of a midge,Chironomus riparius Meigen (Insecta,Diptera): normoxic heat output over the whole life cycle and response of larva to hypoxia and anoxia

The rate of metabolism of laboratory reared Chironomus riparius was monitored by direct calorimetry over the entire life cycle from egg to adult stage. The metabolic response of the fourth instar larva to decreasing oxygen concentrations and anoxia was also measured. Normoxic measurements were carried out at 20°C and the hypoxic-anoxic experiments at 10°C. In larvae with body sizes ranging from 0.0028 to 0.645 mg ash-free dry mass (afdm), the rate of heat dissipation was related to body mass by a power function, with a mass exponent of 0.71±0.02 corresponding to an exponent of -0.29 for the relationship between mass-specific metabolic rate and body mass. However, the allometric equations applicable to larvae would not predict the metabolic rates of eggs, pupae and adults. Single egg batches used in the experiments consisted of 354±90 eggs, the individual egg with a mass of 0.99±0.01 g (mean±SD). The mass-specific rate of heat dissipation of the egg (13.7±1.8 W mg^-1 afdm) was considerably lower than that of the first and second instar larvae (44–53 W mg^-1) but equal to that of fourth instar larvae (13.1±3.9 W mg^-1). Heat dissipation by a pupa shortly before adult emergence was high (14.8±1.8 W mg^-1), probably due to high metabolism during metamorphosis. Emergence of the adult in the calorimeter was indicated by a short but intense burst of heat. The newly emerged imago had a ca. 20–35% higher metabolic rate than the pupa. In response to reduced O₂ partial pressure the fourth instar larva of C. riparius displayed metabolic regulation. In continuously declining oxygen partial pressure, the fourth instar larva maintained its aerobic energy metabolism (4.2 W mg^-1) with only a small decrease down to 0.8 kPa, corresponding to an oxygen concentration of 0.42 mg O₂l^-1 H₂O. Below this critical oxygen concentration (P_c), the rate of heat dissipation decreased rapidly down to the anoxic level which was only 14–17% of the normoxic level. The high relative reduction of metabolic rate under anoxia gives a wrong impression of short-term tolerance of C. riparius to anoxia. The absolute energetic costs of C. riparius associated with anaerobic energy metabolism (0.64±0.11 W mg^-1) are almost 6 times higher than those of more anoxia tolerant invertebrates such as sphaeriid bivalves.     

The extent and differences in mitotic activity of the root tip ofVicia faba L.

Mitotic activity does not stop for different meristematic cells of the root apex at the same distance from the initials. The differences are connected with the functional heterogeneity of the apical meristem of the root. The arrangement of vascular bundles,i.e. the alternation of independent xylem and phloem groups, is of major importance. In broad bean roots, the protophloem sieve elements stop dividing first. The centre of the stelei. e. late metaxylem elements stop dividing next. Division in the stele gradually ceases centrifugally, while it ceases centripetally in the peripheral part of the root. The cylindrical region with prolonged cell division includes internal layers of the cortex including endodermis, pericycle and adjoining cells of the stele. Proximally apical meristem is reduced to isolated strands of cells adjacent to the protoxylem poles. Pericycle cells stop dividing last at a distance of approx. 9–10 mm from the initials. The number of the division cycles is limited and is specific for individual cell types. Epidermal and cortical cells divide in broad bean roots transversely approximately seven times, cells of late metaxylem approximately five times. Root apical meristem is an asynchronous cell population with a different duration of the mitotic cycle. We determined local variations in the duration of the mitotic cycle in the apical meristem of broad bean root by means of colchicine-induced polyploidy. The cells of the quiescent centre had the longest mitotic cycle after colchicine treatment. The region of the proper root adjacent to the quiescent centre was mixoploid (2n and 4n). Isolated cells with a long cycle occurred also in the cortex and in the central cylinder. Cells with a division cycle of 18h were found in the root cap, in the epidermis, in the cortex and in the central cylinder. Relatively numerous cells with the shortest division cycle, approx. 12 h, occurred farther of the quiescent centre in the epidermis, in the cortex, in the pericycle, and in adjacent layers of the stele through-out the entire meristematic region. The results derived from the analysis of the apical meristem are discussed in connection with the ontogenesis of different types of cells taking part in the primary structure of the root.     

Liming induced stimulation of the amino acid metabolism in mycorrhizal roots of Norway spruce (Picea abies [L.] Karst.)

Localization and activity of three enzymes involved in the amino acid metabolism of ectomycorrhizas were investigated within an interdisciplinary experiment performed in a mature Norway spruce stand in Southern Germany (Höglwald). The enzymes NAD-glutamate dehydrogenase and aspartate aminotransferase were present in root cells, whereas aminopeptidase was found in mycorrhizas of Norway spruce such as Piceirhiza nigra and those with the fungi Cenococcum geophilum, Elaphomyces sp., Russula ochroleuca and Tylospora sp. Mycorrhizas growing in the humus layer contained about double the amount of protein found in those taken from the upper mineral soil (0–5 cm).Acid irrigation of the soil had no effect on the activity of any of the investigated enzymes, soluble protein or total N-contents irrespective of whether roots were taken from the organic layer or from the upper mineral soil. Liming, however, stimulated the activity of the three enzymes in mycorrhizas of the organic layer (O_f+O_h) whereas it had no effect on the activity of the investigated enzymes of mycorrhizas in the upper mineral soil. This effect is attributed to increased contents of soluble organic nitrogen compounds in the soil of the limed plots as compared to the unlimed plots.     

Roots growing in rock fissures: Their morphological adaptation

On sites with shallow soil in semi-arid climate conditions, whiteleaf manzanita (Arctostaphyllos viscida) and Pacific madrone (Arbutus menziesii) utilize water from the bedrock. Roots of these plants occupy rock fissures as small as 100 m. Although the root stele remains cylindrical in shape without visible mechanical stress, the cortex may become flat, creating wing-like structures on the sides of the stele. Fine particles of soil and rock that fill the space between root cortex and rock matrices create good contact for water flow.     

Aluminum-induced secretion of both citrate and malate in rye

Aluminum (Al)-resistant mechanisms responsible for Al-induced secretion of organic acids are poorly understood. In this study, we characterized the Al-induced secretion of both citrate and malate from rye (Secale cereale L. cv. King). Secretion of organic acids increased with increasing concentration (10, 30 and 50 M) and duration of Al treatments. Neither phosphorous (P) deficiency up to 15 days nor addition of 50M lanthanum, 50 M lead, 10 M cadmium, or 200 M manganese caused secretion of organic acids, suggesting that this secretion was a specific response to Al stress. Aluminum activated citrate synthase, the main enzyme for the synthesis of citrate, but its activation occurred only in the root tip. The elongation of roots of an Al-sensitive cultivar of wheat (Tritium aestivum L. cv. Scout 66) was not inhibited by 50 M Al in the presence of externally applied 50 M citrate or 400 M malate. The secretion of citrate and malate from intact rye roots exposed to 50 M Al corresponded to 31.3 ± 1.7 M and 11.5 ± 2.5 M, respectively, in the rhizosphere based on an assumption of a 2 mm thick unstirred layer around root tips. This result indicated that Al-resistance in rye was achieved by the Al-induced synthesis of citrate in root apices followed by Al-induced specific secretion of citrate from root tips.     

Purification and characterization of membrane-bound hydrogenase from Methanosarcina barkeri MS

Hydrogenase was solubilized from the membrane of acetate-grown Methanosarcina barkeri MS and purification was carried out under aerobic conditions. The enzyme was reactivated under reducing conditions in the presence of H₂. The enzyme showed a maximal activity of 120±40 mol H₂ oxidized · min^–1 · min^–1 with methyl viologen as an electron acceptor, a maximal hydrogen production rate of 45±4 mol H₂ · min^–1 · mg^–1 with methyl viologen as electron donor, and an apparent K _m for hydrogen oxidation of 5.6±1.7 M. The molecular weight estimated by gel filtration was 98,000. SDS-PAGE showed the enzyme to consist of two polypeptides of 57,000 and 35,000 present in a 1:1 ratio. The native protein contained 8±2 mol Fe, 8±2 mol S^2–, and 0.5 mol Ni/mol enzyme. Cytochrome b was reduced by hydrogen in a solubilized membrane preparation. The hydrogenase did not couple with autologous F₄₂₀ or ferredoxin, nor with FAD, FMN, or NAD(P)⁺. The physiological function of the membrane-bound hydrogenase in hydrogen consumption is discussed.Abbreviation CoM-S-S-HTP the heterodisulfide of 7-mercaptoheptanoylthrconine phosphate and coenzyme M (mercaptoethanesulfonic acid)