Comparative Distribution and Metabolism of Xylem-Borne Amino Compounds and Sucrose in Shoots of Populus deltoides

The transport and metabolism of xylem-borne amino compounds and sucrose were investigated in rapidly growing shoots of cottonwood (Populus deltoides Bartr. ex Marsh.). ¹⁴C-labeled glutamine, threonine, alanine, glutamic acid, aspartic acid, and sucrose were applied to the base of severed stems for transport in xylem. Distribution and metabolism of the compounds were followed with autoradiography, microautoradiography, and radioassay. Three utilization patterns were observed: (a) little alanine and sucrose was transported to the laminae of either mature leaves or developing leaves. These compounds were taken up from xylem free-space and utilized in adjacent tissue; (b) threonine also did not move into mature leaves but was translocated to developing leaves or utilized in the stem; (c) glutamic acid and aspartic acid were transported directly into the laminae of mature leaves via the xylem. Relatively less ¹⁴C was retained in stems compared to the other compounds.

Metabolism of the test compounds also differed considerably. ¹⁴C from amino acids moved primarily into organic acids and protein. The ¹⁴C from sucrose was widely distributed among the chemical fractions, with a high percentage found in structural carbohydrates. Clearly, cottonwood stems contain efficient uptake and transfer systems that differentiate among various compounds moving from root to shoot in xylem.

     

Uptake and Utilization of Xylem-borne Amino Compounds by Shoot Organs of a Legume

Amino compounds representative of the major N solutes of xylem sap were pulse-fed (10 to 20 minutes) singly in ¹⁴C-labeled form to cut transpiring shoots of white lupin (Lupinus albus L.). ¹⁴C distribution was studied by autoradiography and radioassays of phloem sap, leaflet tissues, and shoot parts harvested at intervals after labeling. Primary distribution of N by xylem was simulated using a 20-minute labeling pulse followed by a 30-minute chase in unlabeled xylem sap. Shoots fed ¹⁴C-labeled asparagine, glutamine, valine, serine, or arginine showed intense labeling of leaflet veins and marked retention (35 to 78%) of ¹⁴C by stem + petioles. Shoots fed ¹⁴C-labeled aspartic acid or glutamic acid showed heaviest ¹⁴C accumulation in interveinal regions of leaflets and low uptake (11 to 20%) of ¹⁴C by stem + petioles. Departing leaf traces were major sites of uptake of all amino compounds, and the implications of this were evaluated. Fruits acquired only 1 to 5% of the fed label directly from xylem, but more than doubled their intake during the period 30 to 160 minutes after feeding through receipt of ¹⁴C transferred from xylem to phloem in stem and leaves. ¹⁴C-Labeled asparagine and valine transferred directly from xylem to phloem, but the ¹⁴C of ¹⁴C-labeled aspartic acid and arginine appeared in phloem mainly as metabolic products of the fed compound. The labeling of the soluble pool of leaflets reflected these differences. The significance of heterogeneity in distribution and metabolism of xylem amino compounds in the shoot was discussed.     

Uptake and metabolism of purine nucleosides and nucleotides in isolated frog skeletal muscle.

When isolated frog skeletal muscles were incubated with ¹⁴C-labeled adenosine, the nucleoside was rapidly taken up by the cells and was either immediately incorporated into adenine nucleotides or deaminated to inosine. Incorporation was predominant at low (micromolar) concentrations whereas, deamination was the major route of metabolism at high (millimolar) concentrations. When muscles were incubated with ¹⁴C-labeled inosine the nucleoside, after entry into the cells, was metabolized to a lesser extent than adenosine. ATP and hypoxanthine were the major products of its metabolism. Intracellular concentrations were calculated using ³H-labeled sorbitol to measure the extracellular space.Because of its lower rate of intracellular metabolism inosine was used to investigate the characteristics of the nucleoside transport system. The uptake of inosine was saturable at high concentrations and was specifically inhibited by the presence of adenosine or uridine in the incubation media. Persantin, a well known specific inhibitor of nucleoside transport, also competitively inhibited inosine uptake, as did theophylline [1, Woo et al. Can J. Physiol. Pharmacol. $\text{52}$, 1063, 1974]. These data, along with the knowledge that in a well-oxygenated muscle, inosine entry follows a downhill chemical potential gradient, strongly support the view that the transport mechanism is facilitated diffusion.The muscle cell membrane does not appear to be permeable to ¹⁴C-labeled ATP under the conditions studied. Investigations of the permeability to the major extracellular degradation products of ATP suggest that AMP was the compound most likely to cross the cell membrane.     

Effect of Concentration of Organic Chemicals on Their Biodegradation by Natural Microbial Communities

The effect of concentration on the biodegradation of synthetic organic chemicals by natural microbial communities was investigated by adding individual ¹⁴C-labeled organic compounds to stream water at various initial concentrations and measuring the formation of ¹⁴CO₂. The rate of degradation of p-chlorobenzoate and chloroacetate at initial concentrations of 47 pg/ml to 47 μg/ml fell markedly with lower initial concentrations, although half or more of the compound was converted to CO₂ in 8 days or less. On the other hand, little mineralization of 2,4-dichlorophenoxyacetate and 1-naphthyl-N-methylcarbamate, or the naphthol formed from the latter, occurred when these compounds were present at initial concentrations of 2 to 3 ng/ml or less, although 60% or more of the chemical initially present at higher concentrations was converted to CO₂ in 6 days. It is concluded that laboratory tests of biodegradation involving chemical concentrations greater than those in nature may not correctly assess the rate of biodegradation in natural ecosystems and that low substrate concentration may be important in limiting biodegradation in natural waters.     

Partitioning of 14C-labeled photosynthate to allelochemicals and primary metabolites in source and sink leaves of aspen: evidence for secondary metabolite turnover

Theories on allelochemical concentrations in plants are often based upon the relative carbon costs and benefits of multiple metabolic fractions. Tests of these theories often rely on measuring metabolite concentrations, but frequently overlook priorities in carbon partitioning. We conducted a pulse-labeling experiment to follow the partitioning of ¹⁴CO₂-labeled photosynthate into ten metabolic pools representing growth and maintenance (amino acids, organic acids, lipids plus pigments, protein, residue), defense (phenolic glycosides, methanol:water and acetone-soluble tannins/phenolics), and transport and storage (sugars and starch) in source and importing sink leaves of quaking aspen (Populus tremuloides). The peak period of ¹⁴C incorporation into sink leaves occurred at 24 h. Within 48 h of labeling, the specific radioactivity (dpm/mg dry leaf weight) of phenolic glycosides declined by over one-third in source and sink leaves. In addition, the specific radioactivity in the tannin/phenolic fraction decreased by 53% and 28% in source and sink leaves, respectively. On a percent recovery basis, sink leaves partitioned 1.7 times as much labeled photosynthate into phenolic glycosides as source leaves at peak ¹⁴C incorporation. In contrast, source leaves partitioned 1.8 times as much ¹⁴C-labeled photosynthate into tannins/phenolics as importing sink leaves. At the end of the 7-day chase period, sink leaves retained 18%, 52%, and 30% of imported ¹⁴C photosynthate, and labeled source leaves retained 15%, 66%, and 19% of in situ photosynthate in metabolic fractions representing transport and storage, growth and maintenance, and defense, respectively. Analyses of the phenolic fractions showed that total phenolics were twice as great and condensed tannins were 1.7 times greater in sink than in source leaves. The concentration of total phenolics and condensed tannins did not change in source and sink leaves during the 7-day chase period. Received: 31 July 1998 / Accepted: 8 February 1999     

Decomposition of 14C-labeled cellulose substrates in litter and soil from a beechwood on limestone

The decomposition of three different ¹⁴C-labeled cellulose substrates (plant holocellulose, plant cellulose prepared from ¹⁴C-labeled beech wood (Fagus sylvatica) and bacterial cellulose produced by Acetobacter xylinum) in samples from the litter and mineral soil layer of a beechwood on limestone was studied. In a long-term (154 day) experiment, mineralization of cellulose materials, production of ¹⁴C-labeled water-soluble compounds, and incorporation of ¹⁴C in microbial biomass was in the order Acetobacter cellulose > holocellulose > plant cellulose in both litter and soil. In general, mineralization of cellulose, production of ¹⁴C-labeled water-soluble compounds, and incorporation of ¹⁴C in microbial biomass were more pronounced, but microbial biomass ¹⁴C declined more rapidly in litter than in soil. In short-term (14 day) incubations, mineralization of cellulose substrates generally corresponded with cellulase and xylanase activities in litter and soil. Pre-incubation with trace amounts of unlabeled holocellulose significantly increased the decomposition of ¹⁴C-labeled cellulose substrates and increased cellulase activity later in the experiment but did not affect xylanase activity. The sum of ¹⁴CO₂ production, ¹⁴C in microbial biomass, and ¹⁴C in water-soluble compounds is considered to be a sensitive parameter by which to measure cellulolytic activity in soil and litter samples in short-term incubations. Shorter periods than 14 days are preferable in assays using Acetobacter cellulose, because the decomposition of this substrate is more variable than that of holocellulose and plant cellulose.Offprint requests to: S. Scheu.     

Determination of low levels of 14C radioactivity in cell exudates by means of ultraviolet photooxidation and a rapid 14CO2-collection technique

The low levels of ¹⁴C radioactivity found in many biological samples, such as cell exudates of marine algae, can be determined conveniently by uv photooxidation of the ¹⁴C-labeled compounds. The acid distillation of the resultant ¹⁴CO₂ and its rapid absorption (cf. 15 min) by a quaternary amine in a closed recirculating system prior to liquid scintillation counting provides a means of concentrating the ¹⁴C activity by up to 10 times. Quench problems are thus reduced to that of one compound, namely ¹⁴CO₂. The kinetics of photooxidation of various ¹⁴C-labeled compounds in seawater are complex but similar in form for the several different classes of compounds tested. The role of the nature and concentration of the oxidant, sample pH, and the source of uv irradiation during photooxidation are discussed.     

Distribution and metabolism of xylem-borne ureido and amino compounds in developing soybean shoots

Pulse-chase feeding (30-120 minutes) of ¹⁴C-labeled nitrogenous compounds to cut transpiring shoots was used to investigate the early fate of the major xylem-borne solutes in N₂-fixing soybean (Glycine max) plants at the V₄ growth stage. By comparison with the foliar distribution of [¹⁴C]inulin (a xylem marker), it was determined that the phloem supply of allantoin, allantoic acid, asparagine, glutamine, aspartate, and arginine, respectively, provided about 20, 10, three, two, five, and 20 times the ¹⁴C delivered to the developing trifoliolate in the xylem stream. Recovery of unmetabolized asparagine, aspartate, and arginine in this indicator trifoliolate, and significant declines in the percentage of ¹⁴C from allantoic acid and allantoin recovered in the first trifoliolate, provided some support for the direct xylem-to-phloem transfer of these compounds, but did not preclude the involvement of indirect transfer. Data on stem retention and foliar distribution, expressed as a function of the relative xylem sap composition, indicated that ureides provide the major sources of nitrogen to all plant parts. There was no consistent distinction in distribution patterns between pairs of similar anionic and neutral compounds. The extent of xylem-to-phloem transfer among the ureido or the amino compounds was inversely related to its prominence in xylem sap.     

Microbial Transformation of Polycyclic Aromatic Hydrocarbons in Pristine and Petroleum-Contaminated Sediments

To determine rates of microbial transformation of polycyclic aromatic hydrocarbons (PAH) in freshwater sediments, ¹⁴C-labeled PAH were incubated with samples from both pristine and petroleum-contaminated streams. Evolved ¹⁴CO₂ was trapped in KOH, unaltered PAH and polar metabolic intermediate fractions were quantitated after sediment extraction and column chromatography, and bound cellular ¹⁴C was measured in sediment residues. Large fractions of ¹⁴C were incorporated into microbial cellular material; therefore, measurement of rates of ¹⁴CO₂ evolution alone would seriously underestimate transformation rates of [¹⁴C]naphthalene and [¹⁴C]anthracene. PAH compound turnover times in petroleum-contaminated sediment increased from 7.1 h for naphthalene to 400 h for anthracene, 10,000 h for benz(a)anthracene, and more than 30,000 h for benz(a)pyrene. Turnover times in uncontaminated stream sediment were 10 to 400 times greater than in contaminated samples, while absolute rates of PAH transformation (micrograms of PAH per gram of sediment per hour) were 3,000 to 125,000 times greater in contaminated sediment. The data indicate that four- and five-ring PAH compounds, several of which are carcinogenic, may persist even in sediments that have received chronic PAH inputs and that support microbial populations capable of transforming two- and three-ring PAH compounds.     

An Investigation of the Ability of a 14C-Labeled Hydrocarbon Mineralization Test to Predict Bioremediation of Soils Contaminated with Petroleum Hydrocarbons

Bioremediation is a widely accepted technology for the remediation of hydrocarbon-contaminated soil. Treatability studies are usually carried out to assess the biodegradation potential of the contaminants and to design optimal treatments. Laboratory studies measuring soil respiration are often used. One method consists of monitoring the mineralization of a ¹⁴C-labeled hydrocarbon surrogate added to the contaminated soil. This study investigates the ability of this method to properly predict the removal of the hydrocarbon contaminants initially found in soils. Mineralization of ¹⁴C-labeled hexadecane was monitored in seven soils contaminated with various hydrocarbon mixtures, both fresh and weathered, in microcosm experiments. Reduction of total petroleum hydrocarbon (TPH) concentrations was measured simultaneously in separate microcosms. Both types of microcosms were subjected to the same amendment regimes. For all soils, poor correlation was observed between the mineralization and TPH reduction data sets. Mineralization data supported contaminants removal data in only one soil. Findings indicate that the radioactive surrogate method does not reliably predict the extent of, and the effect of amendments on, the removal of the hydrocarbons initially present in soil, and may therefore predict suboptimal treatment regimes. Recommendations for soil treatability protocols are provided.     

Light stimulation of cold acclimation: production of a translocatable promoter

The light stimulation of cold acclimation of Hedera helix L. var. Thorndale has been shown to result in the production of translocatable promoters of hardiness. Movement of the promoters from an illuminated donor portion to a darkened receptor portion was demonstrated. The majority of transport was acropetal through the phloem and to a much lesser degree basipetal through the xylem. In the early stages of hardening, transport was strictly acropetal. It is suggested that acropetal transport is under the influence of a mobilizing center located in the apex of the plant. Mobilization of the promoters of hardiness was induced by applications of ⁶N-benzyladenine. Attempts to characterize the light-generated promoters through fixation of ¹⁴CO₂ and subsequent translocation of ¹⁴C-labeled compounds from the illuminated donor to the darkened receptor indicated that the translocatable promoting material was either some component of the Dowex 1 fraction or sucrose. Furthermore, the hardiness of leaves was significantly increased by sucrose solutions but not by equi-molar solutions of glucose, galactose or mannitol.     

Itaconic acid: An inhibitor of isocitrate lyase in Tetrahymena pyriformis in vitro and in vivo

The succinate analog itaconic acid was observed to be a competitive inhibitor of the glyoxylate cycle specific enzyme isocitrate lyase (EC 4.1.3.1) in cell-free extracts of Tetrahymena pyriformis. Itaconic acid also inhibited net in vivo glycogen synthesis from glyoxylate cycle-dependent precursors such as acetate but not from glyoxylate cycle-independent precursors such as fructose. The effect of itaconic acid on the incorporation of ¹⁴C into glycogen from various ¹⁴C-labeled precursors was also consistent with inhibition of isocitrate lyase by this compound. Another analog of succinate which shares a common metabolic fate with itaconic acid, mesaconic acid, had no effect on isocitrate lyase activity in vitro or on ¹⁴C-labeled precursor incorporation into glycogen in vivo. In addition, itaconic acid did not affect gluconeogenesis from lactate in isolated perfused rat livers, a system lacking the enzyme isocitrate lyase. These results are taken as evidence that itaconic acid is an inhibitor of glyoxylate cycle-dependent glyconeogenesis Tetrahymena pyriformis via specific competitive inhibition of isocitrate lyase activity.     

Loading and transport of assimilates in different maize leaf bundles

The loading and transport functions of vascular bundles in maize (Zea mays L.) leaf strips were investigated by microautoradiography after application of ¹⁴CO₂. The concentrations of ¹⁴C-contents in thin-walled sieve tubes of individual bundles in the loading and transport regions were determined by digital image analysis of silver-grain density over the sieve tubes and compared. In the loading region, relatively high concentrations of ¹⁴C-contents were found in the thin-walled sieve tubes of small bundles and in the small, thin-walled sieve tubes of the intermediate bundles; the concentration of ¹⁴C-label in large bundles was very low. In the transport region, at a transport distance of 2 cm, all of the small bundles contained ¹⁴C-assimilates, but generally less than the same bundles did in the loading region; by comparison, at that distance intermediate and large bundles contained two-to threefold more ¹⁴C-assimilates than the same bundles in the loading region. The lateral transfer of assimilates from smaller to larger bundles via transverse veins could be demonstrated directly in microautoradiographs. A reverse transport from larger to smaller bundles was not found. At a transport distance of 4 cm, all large and intermediate bundles were ¹⁴C-labeled, but many of the small bundles were not. Although all longitudinal bundles were able to transport ¹⁴C-asimilates longitudinally down the blade, it was the large bundles that were primarily involved with longitudinal transport and the small bundles that were primarily involved with loading.     

The heterotrophic uptake of dissolved organic carbon by eelgrass (Zostera marina L.) and its epiphytes

The uptake of ¹⁴C-labeled organic compounds by Zostera marina L. (eelgrass) and its epiphytes was examined in an eelgrass community near Beaufort, North Carolina. Assimilation and respiration by new leaf growth (few epiphytes), heavily colonized Zostera, and an epiphytized artificial substratum were determined. Glutamic acid was removed from the medium most rapidly, followed by acetate, glucose, and glycine, which were removed at approximately equal rates. The compound with the highest assimilation efficiency was glucose (90.4%), but all compounds were incorporated with efficiencies of > 75%. Incorporation by epiphytes on the artificial substratum was greater than uptake by epiphytes on eelgrass. The leaves of Zostera also accumulated radioactive material, but at low rates, and when combined with uptake by the epiphytic community, resulted in average turnover times for the tested compounds of < 7 h. Artificial epiphyte communities had similar turnover times. Kinetic analysis showed no saturation effect, with uptake being linear for the concentrations of substrata tested. We hypothesize that heterotrophic epiphytes are potentially a significant source of new particulate matter in estuarine food webs, and that the microbial communities play a significant role in seagrass carbon cycles.     

Long distance translocation of sucrose, serine, leucine, lysine, and carbon dioxide assimilates: I. Soybean

To determine the selectivity of movement of amino acids from source leaves to sink tissues in soybeans (Glycine max [L.] Merr. `Wells'), ¹⁴C-labeled serine, leucine, or lysine was applied to an abraded spot on a fully expanded trifoliolate leaflet, and an immature sink leaf three nodes above was monitored with a GM tube for arrival of radioactivity. Comparisons were made with ¹⁴C-sucrose and ¹⁴CO₂ assimilates. Radioactivity was detected in the sink leaf for all compounds applied to the source leaflet. A heat girdle at the source leaf petiole essentially blocked movement of applied compounds, suggesting phloem transport. Transport velocities were similar (ranged from 0.75 to 1.06 cm/min), but mass transfer rates for sucrose were much higher than those for amino acids. Hence, the quantity of amino acids entering the phloem was much smaller than that of sucrose. Extraction of source, path, and sink tissues at the conclusion of the experiments revealed that 80 to 90% of the radioactivity remained in the source leaflet. Serine was partially metabolized in the transport path, whereas lysine and leucine were not. Although serine is found in greater quantities than leucine and lysine in the source leaf and path of soybeans, applied leucine and lysine were transported at comparable velocities and in only slightly lower quantities than was applied serine. Thus, no selective barrier against entry of these amino acids into the phloem exists.     

Glutamine Transfer from Xylem to Phloem and Translocation to Developing Leaves of Populus deltoides

The distribution of ¹⁴C from xylem-borne [¹⁴C]glutamine, the major nitrogen compound moving in xylem sap of cottonwood (Populus deltoides Bartr. ex Marsh), was followed in rapidly growing shoots with a combination of autoradiographic, microautoradiographic, and radioassay techniques. Autoradiography and ¹⁴C analyses of tissues showed that xylem-borne glutamine did not move with the transpiration stream into mature leaves. Instead, most of it was transferred from xylem to phloem in the upper stem and then translocated to young developing tissues. Microautoradiography showed that metaxylem parenchyma, secondary xylem parenchyma, and rays were the major areas of uptake from xylem vessels in the stem. Accumulation in phloem (high ¹⁴C concentrations in sieve tubes) took place in internodes subtending recently mature leaves. Little ¹⁴C from xylem-borne glutamine was found in phloem of mature leaves, which indicates restricted retransport of glutamine that did enter the leaf. In the primary tissues of the upper stem, most ¹⁴C was found in the phloem. Cottonwood stems have an efficient uptake and transfer system that enhances glutamine movement to developing tissues of the upper stem.     

Eucalyptus urograndis and Pinus taeda enhance removal of chlorobenzene and benzene in sand culture: A greenhouse study

Contamination of soils and groundwater by chlorobenzene and benzene is a common problem at industrial sites worldwide. Since chemical remediation techniques are rarely completely effective, remnants of these contaminants often persist at levels that can still influence ecosystem health. We evaluated the potential of Pinus taeda and Eucalyptus urograndis to accelerate the removal of these compounds from sand/water systems using a completely randomized block greenhouse experiment with a no-plant control. At 2-day intervals, we added a solution containing both chlorobenzene and benzene with the same concentration of 50 mg L^?1 (25 mg pot^?1), and we monitored leachate concentrations daily. The planted treatments showed greater decrease of contaminants over time. In the absence of plants, the contaminant mass decreased 50–60% during each 2-day cycle; whereas, in the planted treatments the contaminant mass decreased 91–98%. At the end of the experiment the plant roots, leaves, and the sand-substrate each contained less than 1 mg kg^?1 of contaminants, which is ～1% of the total contaminant mass added. In addition, we observed no tree mortality even at concentrations exceeding the aqueous solubility limit of both compounds. Our results suggest both trees are good candidates for remediating chlorobenzene and benzene in soils and groundwater.     

The accumulation and metabolism of [C]hypoxanthine by slices of rabbit renal medulla

The uptake of hypoxanthine by rabbit renal medulla has been studied with in vitro conditions. Unlike the uptake by renal cortex slices reported earlier, no evidence was found for involvement of an organic cation transport system. Medullary accumulation of the ¹⁴C-labeled material occurred in the absence of O₂ if glucose was present as substrate. Uptake of the ¹⁴C label was not supported by other sugars or metabolic intermediates, however. Metabolic inhibitors reduced both aerobic and anaerobic uptake. Tissue extracts were subjected to high-voltage electrophoresis and gel filtration for identification of possible metabolites. These studies indicated that most of the uptake of ¹⁴C-labeled material was accounted for by its conversion to inosine-like and/or inosinic acid-like compounds. That is, when experimental conditions were designed to retard slice metabolism of hypoxanthine, tissue to medium ratios for ¹⁴C approximated 1.0. A third metabolite was found occasionally, but remains unidentified.     

The effect of wilting on proline metabolism in excised bean leaves in the dark

The effects of wilting on the fate of proline and on the rates of nonprotein proline formation and utilization have been determined in excised bean leaves. Wilting did not alter the fate of exogenously added ¹⁴C-l-proline (2 mm) in either non-starved leaves (from plants previously in the light) or starved leaves (from plants previously in the dark). The fate of proline in nonstarved leaves was protein synthesis and in starved leaves was protein synthesis and oxidation to other compounds.     

Fungal Growth, Production, and Sporulation during Leaf Decomposition in Two Streams

I examined the activity of fungi associated with yellow poplar (Liriodendron tulipifera) and white oak (Quercus alba) leaves in two streams that differed in pH and alkalinity (a hardwater stream [pH 8.0] and a softwater stream [pH 6.7]) and contained low concentrations of dissolved nitrogen (<35 μg liter⁻¹) and phosphorus (<3 μg liter⁻¹). The leaves of each species decomposed faster in the hardwater stream (decomposition rates, 0.010 and 0.007 day⁻¹ for yellow poplar and oak, respectively) than in the softwater stream (decomposition rates, 0.005 and 0.004 day⁻¹ for yellow poplar and oak, respectively). However, within each stream, the rates of decomposition of the leaves of the two species were not significantly different. During the decomposition of leaves, the fungal biomasses determined from ergosterol concentrations, the production rates determined from rates of incorporation of [¹⁴C]acetate into ergosterol, and the sporulation rates associated with leaves were dynamic, typically increasing to maxima and then declining. The maximum rates of fungal production and sporulation associated with yellow poplar leaves were greater than the corresponding rates associated with white oak leaves in the hardwater stream but not in the softwater stream. The maximum rates of fungal production associated with the leaves of the two species were higher in the hardwater stream (5.8 mg g⁻¹ day⁻¹ on yellow poplar leaves and 3.1 mg g⁻¹ day⁻¹ on oak leaves) than in the softwater stream (1.6 mg g⁻¹ day⁻¹ on yellow poplar leaves and 0.9 mg g⁻¹ day⁻¹ on oak leaves), suggesting that effects of water chemistry other than the N and P concentrations, such as pH or alkalinity, may be important in regulating fungal activity in streams. In contrast, the amount of fungal biomass (as determined from ergosterol concentrations) on yellow poplar leaves was greater in the softwater stream (12.8% of detrital mass) than in the hardwater stream (9.6% of detrital mass). This appeared to be due to the decreased amount of fungal biomass that was converted to conidia and released from the leaf detritus in the softwater stream.