Regulation of sulfate uptake and xylem loading of poplar roots (Populus tremula x P. alba)

Sulfate transport processes and its regulation were studied in roots of poplar trees (Populus tremula x P. alba). From the exponential increase in sulfate uptake with temperature an activation energy (E_a) of 9.0±0.8 kJ mol^–1 was calculated. In the concentration range 0.005–10 mM sulfate uptake showed biphasic Michaelis-Menten kinetics with a K_m of 3.2±3.4 M and a V_max of 49±11 nmol SO₄^2– g^–1 FW h^–1 for the high-affinity uptake system (phase 1) and a K_m of 1.33±0.41 mM and a V_max of 255±25 nmol SO₄^2– g^–1 FW h^–1 for the low-affinity system (phase 2). Xylem loading decreased linearly with temperature and remained unchanged within the sulfate concentration range studied. Regulation of sulfate uptake and xylem loading by O-acetyl serine (OAS), Cys, reduced glutathione (GSH), Met and S-methylmethionine (SMM) were tested by perfusion into the xylem sap with the pressure probe and by addition to the incubation medium. When added directly to the transport medium, Cys and GSH repressed, and OAS stimulated sulfate uptake; xylem loading was stimulated by Cys, repressed by GSH and only slightly affected by OAS. When perfused into the xylem, none of the compounds tested affected sulfate uptake of excised roots, but xylem loading was stimulated by SMM and OAS and repressed by Met. Apparently, the site of application strongly determined the effect of regulatory compounds of sulfate transport processes.     

Interaction of sulfate and glutathione transport in cultured tobacco cells

Photoheterotrophic and heterotrophic suspension cultures of tobacco (Nicotiana tabacum L.) were grown with 1 mM glutathione (reduced; GSH) as sole source of sulfur. Addition of sulfate to both cultures did not alter the rate of exponential growth, but affected the removal of GSH and sulfate in different ways. In photoheterotrophic suspensions, addition of sulfate caused a decline in the net uptake of GSH, whereas sulfate was taken up by the green cells immediately. In heterotrophic suspensions, however, addition of sulfate did not affect the net uptake of GSH and sulfate was only taken up by the cells after the GSH supply in the medium had been exhausted. Apparently, GSH uptake in photoheterotrophic cells is inhibited by sulfate, whereas sulfate uptake is inhibited by GSH in heterotrophic cells. The differences in the effect of GSH on sulfate uptake in photoheterotrophic and heterotrophic tobacco suspensions cannot be attributed to differences in the kinetic properties of sulfate carriers. In short-time transport experiments, both cultures took up sulfate almost entirely by an active-transport system as shown by experiments with metabolic inhibitors; sulfate transport of both cultures obeyed monophasic Michaelis-Menten kinetics with similar app. K_m (photoheterotrophic cells: 16.0±2.0 M; heterotrophic cells: 11.8±1.8 M) and V_max (photoheterotrophic cells: 323±50 nmol·min^-1·g^-1 dry weight; heterotrophic cells: 233±3 nmol·min^-1·g^-1 dry weight). Temperature- and pH-dependence of sulfate transport showed almost identical patterns. However, the cultures exhibited remarkable differences in the inhibition of sulfur influx by GSH in short-time transport experiments. Whereas 1 mM GSH inhibited sulfate transport into heterotrophic tobacco cells completely, sulfate transport into photoheterotrophic cells proceeded at more than two-thirds of its maximum velocity at this GSH concentration. The mode of action of GSH on sulfate transport in chloroplast-free tobacco cell does not appear to be direct: a 14-h exposure to 1 mM GSH was found to be necessary to completely block sulfate transport; a 4-h time of exposure did not affect this process. Consequently, glutathione does not seem to be a product of sulfur metabolism acting on sulfate-carrier entities by negative feedback control. When transferred to the whole plant, the observed differences in sulfate and glutathione influx into green and chloroplast-free cells may be interpreted as a regulatory device to prevent the uptake of excess sulfate by plants.Abbreviations DCCD N,N-dicyclohexylcarbodiimide - DNP dinitrophenol - DW dry weight - FW fresh weight - GSH reduced glutathione     

Uptake of iron byGeotrichum candidum,a non-siderophore producer

Summary Geotrichum candidum (isolate 1–9) pathogenic on citrus fruits, appears to lack siderophore production. Iron uptake byG. candidum is mediated by two distinct iron-regulated, energy-and temperature-dependent transport systems that require sulfhydryl groups. One system exhibits specificity for either ferric or ferrous iron, whereas the other exhibits specificity for ferrioxamine-B-mediated iron uptake and presumably other hydroxamate siderophores. Radioactive iron uptake from⁵⁹FeCl₃ showed an optimum at pH 6 and 35° C, and Michaelis-Menten kinetics (apparentK _m = 3 m,V _max = 0.054 nmol · mg^–1 · min^–1). The maximal rate of Fe²⁺ uptake was higher than Fe³⁺ (V _max = 0.25 nmol · mg^–1 · min^–1) but theK _m was identical. Reduction of ferric to ferrous iron prior to transport could not be detected. The ferrioxamine B system exhibits an optimum at pH 6 and 40° C and saturation kinetics (K _m = 2 M,V _max = 0.22 nmol · mg^–1 · min^–1). The two systems were distinguished as two separate entities by negative reciprocal competition, and on the basis of differential response to temperature and phenazine methosulfate. Mössbauer studies revealed that cells fed with either⁵⁷FeCl₃ or⁵⁷FeCl₂ accumulated unknown ferric and ferrous binding metabolites.     

Trehalase activity in mycorrhizal and nonmycorrhizal roots of leek and soybean

Root extracts of leek (Allium porrum L.) and soybean (Glycine max L. Merr.) showed trehalase activity which was inhibited by phloridzin and was several times higher than the activity of general -glucosidase. The activity had an acidic optimum. Trehalase activity in extracts of sporocarps and extraradical mycelium of the arbuscular mycorrhizal fungus Glomus mosseae Nicol. & Gerd. (Trappe & Gerd.) was higher than in root extracts and had an optimum at pH 7. Following inoculation with G. mosseae, trehalase activity increased in mycorrhizal roots above the levels observed in nonmycorrhizal roots. Irrespective of fungal colonization, root trehalase activity increased in the presence of Mg²⁺, decreased in the presence of Mn²⁺ and Zn²⁺, and was unaffected by Na₂EDTA.     

Interactive effects of mycorrhization and elevated atmospheric CO2 on sulphur nutrition of young pedunculate oak (Quercus robur L.) trees

Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO₂ concentration and 650 μmol mol^?1 CO₂ in the presence and absence of the ectomycorrhizal fungus Laccaria laccata for a total of 22 weeks under nonlimiting nutrient conditions. Sulphate uptake, xylem loading and exudation were analysed in excised roots. Despite a relatively high affinity for sulphate (K_M= 1.6 mmol m^?3), the rates of sulphate uptake by excised lateral roots of mycorrhizal oak trees were low as compared to herbaceous plants. Rates of sulphate uptake were similar in mycorrhizal and non-mycorrhizal roots and were not affected by growth of the trees at elevated CO₂. However, the total uptake of sulphate per plant was enhanced by elevated CO₂ and further enhanced by elevated CO₂ and mycorrhization. Sulphate uptake seemed to be closely correlated with biomass accumulation under the conditions applied. The percentage of the sulphate taken up by mycorrhizal oak roots that was loaded into the xylem was an order of magnitude lower than previously observed for herbaceous plants. The rate of xylem loading was enhanced by mycorrhization and, in roots of mycorrhizal trees only, by growth at elevated CO₂. On a whole-plant basis this increase in xylem loading could only partially be explained by the increased growth of the trees. Elevated CO₂ and mycorrhization appeared to increase greatly the sulphate supply of the shoot at the level of xylem loading. For all treatments, calculated rates of sulphate exudation were significantly lower than the corresponding rates of xylem loading of sulphate. Radiolabelled sulphate loaded into the xylem therefore seems to be readily diluted by unlabelled sulphate during xylem transport. Allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH to mature oak leaves. The rate of export of radioactivity from the fed leaves was 4–5 times higher in mycorrhizal oak trees grown at elevated CO₂ than in those grown at ambient CO₂. Export of radiolabel proceeded almost exclusively in a basipetal direction to the roots. From these experiments it can be concluded that, in mycorrhizal oak trees grown at elevated CO₂, the transport of sulphate to the shoot is increased at the level of xylem loading to enable increased sulphate reduction in the leaves. Increased sulphate reduction seems to be required for the enhanced allocation of reduced sulphur to the roots which is observed in trees grown at elevated CO₂. These changes in sulphate and reduced sulphur allocation may be a prerequisite for the positive effect of elevated CO₂ on growth of oak trees previously observed.     

Changes in the kinetics of phosphate and potassium absorption in nutrient-deficient barley roots measured by a solution-depletion technique

From measurements of the rates of depletion of labelled ions from solution in the low concentration range, we described the phosphate and potassium uptake characteristics of the roots of intact barley plants in terms of the kinetic parameters, K _m and I _max (the maximum rate of uptake). In relatively young (13 d) and older (42 d) plants, cessation of phosphate supply for 4 d or more caused a marked increase in I _max (up to four times), without concomitant change in K _m, which remained between 5 and 7 M. By contrast, 1 d of potassium starvation with 14-d plants caused a decline in the K _m (i.e. an increased apparent affinity for potassium) from 53 M to 11 M, without alteration to I _max. After longer periods of potassium starvation, I _max increased (about two times) while the K _m remained at the same low value. Growth of shoots and roots were unaffected by these treatments, so that concentrations of ions in the tissues declined after 1 d or more of nutrient starvation, but we could not identify a characteristic endogenous concentration for either nutrient at which changes in kinetic parameters were invariably induced. The possible mechanisms regulating carriermediated transport, and the importance of changes induced in kinetic parameters in ion uptake from solution and soil are discussed.Symbol I _max the maximum rate of absorption at saturating concentrations     

Nitrogen sink strength of ectomycorrhizal morphotypes of <Emphasis Type="Italic">Quercus douglasii</Emphasis>, <Emphasis Type="Italic">Q. garryana</Emphasis>, and <Emphasis Type="Italic">Q. agrifolia</Emphasis> seedlings grown in a northern California oak woodland

Little information is known on what the magnitude of nitrogen (N) processed by ectomycorrhizal (ECM) fungal species in the field. In a common garden experiment performed in a northern California oak woodland, we investigated transfer of nitrogen applied as ¹⁵NH₄ or ¹⁵NO₃ from leaves to ectomycorrhizal roots of three oak species, Quercus agrifolia, Q. douglasii, and Q. garryana. Oak seedlings formed five common ectomycorrhizal morphotypes on root tips. Mycorrhizal tips were more enriched in ¹⁵N than fine roots. N transfer was greater to the less common morphotypes than to the more common types. ¹⁵N transfer from leaves to roots was greater when , not , was supplied. ¹⁵N transfer to roots was greater in seedlings of Q. agrifolia than in Q. douglasii and Q. garryana. Differential N transfer to ectomycorrhizal root tips suggests that ectomycorrhizal morphotypes can influence flows of N from leaves to roots and that mycorrhizal diversity may influence the total N requirement of plants.     

The effect of endogenous and externally supplied nitrate on nitrate uptake and reduction in sugarbeet seedlings

The pericarp of the dormant sugarbeet fruit acts as a storage reservoir for nitrate, ammonium and -amino-N. These N-reserves enable an autonomous development of the seedling for 8–10 d after imbibition. The nitrate content of the seed (1% of the whole fruit) probably induces nitrate-reductase activity in the embryo enclosed in the pericarp. Nitrate that leaks out of the pericarp is reabsorbed by the emerging radicle. Seedlings germinated from seeds (pericarp was removed) without external N-supply are able to take up nitrate immediately upon exposure via a low-capacity uptake system (v_max = 0.8 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.12 mM). We assume that this uptake system is induced by the seed nitrate (10 nmol/seed) during germination. Induction of a high-capacity nitrate-uptake system (v_max = 3.4 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.08 mM) by externally supplied nitrate occurs after a 20-min lag and requires protein synthesis. Seedlings germinated from whole fruits absorb nitrate via a highcapacity uptake mechanism induced by the pericarp nitrate (748 nmol/pericarp) during germination. The uptake rates of the high-capacity system depend only on the actual nitrate concentration of the uptake medium and not on prior nitrate pretreatments. Nitrate deprivation results in a decline of the nitrate-uptake capacity (t_1/2 of v_max = 5 d) probably caused by the decay of carrier molecules. Small differences in K_s but significant differences in v_max indicate that the low- and high-capacity nitrate-uptake systems differ only in the number of identical carrier molecules.Abbreviations NR nitrate reductase - pFPA para-fluorophenylalanine This work was supported by a grant from Bundesministerium für Forschung und Technologie and by Kleinwanzlebener Saatzucht AG, Einbeck.     

Properties of the phosphate and phosphite transport systems of Phytophthora palmivora

Germlings of Phytophthora palmivora possess at least two systems for the uptake of inorganic phosphate (P_i). The first is synthesized on germination in medium containing 50 M P_i and has a K_m of approx. 30 M (V_max=7–9 nmol P_i/h·10⁶ cells). The second is synthesized under conditions of P_i-deprivation and has a higher affinity for P_i (K_m=1–2 M), but a lower V_max (0.5–2 nmol P_i/h·10⁶ cells). The fungicide phosphite likewise enters the germlings via two different transport systems, the synthesis of which also depends on the concentration of P_i in the medium. The K_m of the lower affinity system is 3 mM (V_max=20 nmol phosphite/h·10⁶ cells) and that of the higher affinity system is 0.6 mM (V_max=12 nmol/h·10⁶ cells). P_i and phosphite are competitive inhibitors for each other's transport in both systems. However, whereas mM concentrations of phosphite are necessary to inhibit P_i transport, only M concentrations of P_i are required to inhibit phosphite transport. A third system of uptake for P_i also exists, since when phosphate-deprived cells are presented with mM concentrations of P_i, they transport the anion at a very high rate (around 100 nmol/h·10⁶ cells). High rates of transport of phosphite are also observed when these cells are presented with mM concentrations of this anion.     

Ferric chelate reduction by suspension culture cells and roots of soybean: A kinetic comparison

The abilities of suspension cultures and intact roots of soybean (Glycine max L. cv. Hawkeye) to reduce ferric chelate were compared. Ferric chelate was supplied as ferric hydroxyethylethylenediaminetriacetic acid (FeHEDTA) and reduction was measured spectrophotometrically using bathophenan-throlinedisulfonic acid (BPDS) as the ferrous scavenger. Ferric chelate reduction by cell suspension cultures showed typical saturation kinetics; however, no difference was observed between cells that had been continuously grown with Fe (+Fe) and those that had been grown for four days without added Fe (–Fe). Values for K_m and V_max, determined from a Lineweaver-Burk plot, were 57 M and nmoles mg^-1 dry weight for the +Fe cells and 50 M and 22 nmoles mg^-1 dry weight for the -Fe cells, respectively. Ferric chelate reduction by Fe-deficient roots also exhibited saturation kinetics, while roots grown with adequate Fe did not reduce ferric chelate. The K_m and V_max values for Fe-deficient roots were 45 M and 20 nmoles mg^-1 dry weight, respectively, and did not differ from values obtained for cells in culture. This study offers strong evidence that the mechanism responsible for the reduction of ferric chelate is the same for cultured cells and roots and that the process is controlled at the cellular level. We propose that suspension cultures can be used as an alternative to intact roots in the study of ferric chelate reduction.     

Aluminum effects on the kinetics of calcium uptake into cells of the wheat root apex

The effects of aluminum on the concentration-dependent kinetics of Ca²⁺ uptake were studied in two winter wheat (Triticum aestivum L.) cultivars, Al-tolerant Atlas 66 and Al-sensitive Scout 66. Seedlings were grown in 100 M CaCl₂ solution (pH 4.5) for 3 d. Subsequently, net Ca²⁺ fluxes in intact roots were measured using a highly sensitive technique, employing a vibrating Ca²⁺-selective microelectrode. The kinetics of Ca²⁺ uptake into cells of the root apex, for external Ca²⁺ concentrations from 20 to 300 M, were found to be quite similar for both cultivars in the absence of external Al; Ca²⁺ transport could be described by Michaelis-Menten kinetics. When roots were exposed to solutions containing levels of Al that were toxic to Al-sensitive Scout 66 but not to Atlas 66 (5 to 20 M total Al), a strong correlation was observed between Al toxicity and Al-induced inhibition of Ca²⁺ absorption by root apices. For Scout 66, exposure to Al immediately and dramatically inhibited Ca²⁺ uptake over the entire Ca²⁺ concentration range used for these experiments. Kinetic analyses of the Al-Ca interactions in Scout 66 roots were consistent with competitive inhibition of Ca²⁺ uptake by Al. For example, exposure of Scout 66 roots to increasing Al levels (from 0 to 10 M) caused the K _m for Ca²⁺ uptake to increase with each rise in Al concentration, from approx. 100 M in the absence of Al to approx. 300 M in the presence of 10 M Al, while having no effect on the V _max. The same Al exposures had little effect on the kinetics of Ca²⁺ uptake into roots of Atlas 66. The results of this study indicate that Al disruption of Ca²⁺ transport at the root apex may play an important role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars, and that differential Al tolerance may be associated with the ability of Ca²⁺-transport systems in cells of the root apex to resist disruption by potentially toxic levels of Al in the soil solution.We would like to thank Dr. Lionel F. Jaffe, Director of the National Vibrating Probe Facility, Marine Biological Laboratory, Woods Hole, Mass., USA, for making his calcium-selective vibrating-mi-croelectrode system available for a portion of this work. The research presented here was supported in part by USDA/NRI Competitive Grant number 91-37100-6630 to Leon Kochian. Contribution from the USDA-ARS, U.S. Plant, Soil and Nutrition Laboratory, Cornell University, Ithaca, N.Y. This research was part of the program of the Center for Root-Soil Research, Cornell University, Ithaca, N.Y. Department of Soil, Crop and Atmosphere Science, paper No. 1741.     

Nitrogen limitation in mycorrhizal Norway spruce (Picea abies) seedlings induced mycelial foraging for ammonium: implications for Ca and Mg uptake

Although many studies support the importance of the external mycelium for nutrient acquisition of ectomycorrhizal plants, direct evidence for a significant contribution to host nitrogen nutrition is still scarce. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus (Batsch) Fr. in a sand culture system with two compartments separated by a 45-m Nylon mesh. Hyphae, but not roots, can penetrate this net. Nutrient solutions were designed to limit seedling growth by nitrogen. Hyphal density in the hyphal compartment, host N status and shoot growth of mycorrhizal seedlings significantly increased in response to NH₄ ⁺ addition to the hyphal compartment. Labeling the compartment only accessible to hyphae with ¹⁵NH₄ ⁺ showed that the increase in N uptake in the mycorrhizal seedlings was a result of hyphal N acquisition from the hyphal compartment. These results indicate that hyphae of P. involutus may actively forage into N-rich patches and improve host N status and growth. In the mycorrhizal seedlings, which received additional NH₄ ⁺ via their external mycelium, the increase in NH₄ ⁺ supply less negatively affected Ca and Mg uptake than in nonmycorrhizal seedlings, where the additional NH₄ ⁺ was directly supplied to the roots. This was most likely due to the close link of NH₄ ⁺ uptake and H⁺ extrusion, which, in the nonmycorrhizal seedlings, lead to a strong acidification in the root compartment, and subsequently reduced Ca and Mg uptake, whereas in the mycorrhizal seedlings the site of intensive NH₄ ⁺ uptake and acidification was in the hyphal and not in the root compartment. Our data support the idea that the ectomycorrhizal mycelium connected to an N-deficient host may actively forage for N. The mycelium may also be important as a biological buffer system ameliorating negative influence of high NH₄ ⁺ supply on cation uptake.     

The influence of salinity on the kinetics of NH inf4 sup+ uptake in Spartina alterniflora

Summary The effects of short- and long-term exposure to a range in concentration of sea salts on the kinetics of NH _inf4 ^sup+ uptake by Spartina alterniflora were examined in a laboratory culture experiment. Long-term exposure to increasing salinity up to 50 g/L resulted in a progressive increase in the apparent K_m but did not significantly affect V_max (mean V_max=4.23±1.97 mole·g^–1·h^–1). The apparent K_m increased in a nonlinear fashion from a mean of 2.66±1.10 mole/L at a salinity of 5 g/L to a mean of 17.56±4.10 mole/L at a salinity of 50 g/L. These results suggest that the long-term effect of exposure to total salt concentrations within the range 5–50 g/L was a competitive inhibition of NH _inf4 ^sup+ uptake in S. alterniflora. No significant NH _inf4 ^sup+ uptake was observed in S. alterniflora exposed to 65 g/L sea salts. Short-term exposure to rapid changes in salinity significantly affected both V_max and K_m. Reduction of solution salinity from 35 to 5 g/L did not change V_max but reduced K_m by 71%. However, exposing plants grown at 5 g/L salinity to 35 resulted in an decrease in V_max of approximately 50%. Exposure of plants grown at 35 g/L to a total sea salt concentration of 50 g/L for 48h completely inhibited uptake of NH _inf4 ^sup+ . For both experiments, increasing salinity led to an increase in the apparent K_m similar to that found in response to long-term exposure. Our data are consistent with a conceptual model of changes in the productivity of S. alterniflora in the salt marsh as a function of environmental modification of NH _inf4 ^sup+ uptake kinetics.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

Modulation of glyceraldehyde-3-phosphate dehydrogenase inAnacystis nidulans by glutathione

Glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13; GAPDH) from the cyanobacteriumAnacystis nidulans was activated up to five-fold by reduced glutathione (GSH) in the physiological concentration range (0.1–2 mM GSH). Non-physiological reductants, like dithiothreitol (DTT) and -mercaptoethanol, also activated the enzyme. Oxidized glutathione (GSSG) had no effect on the cyanobacterial GAPDH but treatment with H₂O₂ led to a rapid, reversible deactivation of both untreated and GSH-treated enzyme preparations. GSH reversed the inhibition induced by H₂O₂. An oligomeric form of the enzyme (apparentM _r440,000) was dissociated by GSH into a lower-M _r, more active enzyme form (M _r200,000). The enzyme was shown to obey regular Michaelis-Menten kinetics. The activation of GAPDH by GSH was associated with a decrease inK _m and an increase inV _max values of the enzyme for 3-phosphoglycerate. GSH had virtually no effect on a GAPDH preparation isolated from corn chloroplasts and studied for comparison.Abbreviations GAPDH glyceraldehyde-3-phosphate dehydrogenase - GSH reduced glutathione - GSSG oxidized glutathione - DTT dithiothreitol     

Uptake and transport of organic and inorganic nitrogen by arbuscular mycorrhizal fungi

New information on N uptake and transport of inorganic and organic N in arbuscular mycorrhizal fungi is reviewed here. Hyphae of the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (BEG 107) were shown to transport N supplied as ¹⁵N-Gly to wheat plants after a 48 h labelling period in semi-hydroponic (Perlite), non-sterile, compartmentalised pot cultures. Of the ¹⁵N supplied to hyphae in pot cultures over 48 h, 0.2 and 6% was transported to plants supplied with insufficient N or sufficient N, respectively. The increased ¹⁵N uptake at the higher N supply was related to the higher hyphal length density at the higher N supply. These findings were supported by results from in vitro and monoxenic studies. Excised hyphae from four Glomus isolates (BEG 84, 107, 108 and 110) acquired N from both inorganic (¹⁵NH₄ ¹⁵NO₃, ¹⁵NO₃ ⁻ or ¹⁵NH₄ ⁺) and organic (¹⁵N-Gly and ¹⁵N-Glu, except in BEG 84 where amino acid uptake was not tested) sources in vitro during short-term experiments. Confirming these studies under sterile conditions where no bacterial mineralisation of organic N occurred, monoxenic cultures of Glomus intraradices Schenk and Smith were shown to transport N from organic sources (¹⁵N-Gly and ¹⁵N-Glu) to Ri T-DNA transformed, AM-colonised carrot roots in a long-term experiment. The higher N uptake (also from organic N) by isolates from nutrient poor sites (BEG 108 and 110) compared to that from a conventional agricultural field implied that ecotypic differences occur. Although the arbuscular mycorrhizal isolates used contributed to the acquisition of N from both inorganic and organic sources by the host plants/roots used, this was not enough to increase the N nutritional status of the mycorrhizal compared to non-mycorrhizal hosts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Differences in Anatomical Structure and Lignin Content of Roots of pedunculate Oak and Wild Cherry-Tree Plantlets During Acclimation

The lignin contents and anatomical structure of roots of wild cherry (Prunus avium L.) and pedunculate oak (Quercus robur L.) plantlets were compared to explain differences in response during transfer from in vitro to ex vitro conditions. Lignification of cell walls increased significantly in both oak and cherry roots during the period of acclimation and finally lignin content of root tissues of in vitro propagated plantlets reached the levels not significantly different from seedlings grown in soil. Later on when secondary tissues appeared, lignified secondary xylem constituted most of the tissues of both species. The most conspicuous interspecific difference in root structure was the presence of phi-thickenings in cortical layers just outer to endodermis in cherry roots cultivated ex vitro. Formation of phi-thickenings was avoided in vitro and their presence thus seems to be under environmental control. Suberised well established exodermis was present in roots of oak but not detected in those of cherry. Very early development of exodermis in oak roots, preceding suberisation of endodermis, was recorded in vitro but not in well aerated soil. While multilayered and well-developed cork occurred in oak, only thin walled and less suberised secondary dermal tissues were found in cherry.     

Influence of soil pH on the soybean-endomycorrhiza symbiosis

Summary Soybean (Glycine max {L.} Merr.) cultivars were inoculated withGigaspora gigantea andGlomus mosseae to determine mycorrhizal: cultivar relationships as affected by soil pH. The specific cultivarfungal response was dependent on soil pH. Overall cultivar responses in unlimed soil (pH 5.1) were greater forG. gigantea thanG. mosseae. The Bossier —G. gigantea combination was particularly responsive in unlimed soil and showed an increase of 10% in shoot length, 35% in shoot dry weight. 75% in root dry weight, and 397% in nodule dry weight over uninoculated controls. Little cultivar response was observed withG. mosseae inoculation in unlimed soil. In limed soil (pH 6.2), the larger responses were obtained withG. mosseae inoculated plants, although inoculation with eitherG. mosseae orG. gigantea appeared effective. In general, nodulation was greater on mycorrhizal roots than on control roots.     

Kinetics of ammonium, nitrate and phosphorus uptake by Canna indica and Schoenoplectus validus

Canna indica L. is an upright perennial rhizomatous herb, and Schoenoplectus validus (Vahl) A. Löve and D. Löve is a tall, perennial, herbaceous sedge. The nutrient uptake kinetics of C. indica and S. validus were investigated using the modified depletion method after plants were grown for 4 weeks in simulated secondary-treated wastewater. The maximum uptake rate (I_max) and Michaelis–Menten constant (K_m) were estimated by iterative curve fitting. The I_max for NH₄N (623 μmol g⁻¹ dry root weight h⁻¹) was significantly higher than that for NO₃N (338 μmol g⁻¹ dry root weight h⁻¹) in S. validus. In contrast, no difference was observed in C. indica. The I_max values for NO₃N and NH₄N were higher in S. validus than in C. indica. A significantly lower K_m was detected for NO₃N uptake in C. indica (385 μmol L⁻¹) compared to that in S. validus (1908 μmol L⁻¹). The I_max for PO₄P did not differ between the plant species. The K_m for PO₄P was significantly higher in C. indica (157 μmol L⁻¹) than in S. validus (60 μmol L⁻¹). In conclusion, we found that S. validus preferred NH₄N over NO₃N, had greater capacity for N uptake and higher affinity for PO₄P, but C. indica had greater affinity for NO₃N. Nutrient uptake capacity is likely related to habitat preference, and is influenced by the structure of roots and rhizomes.