Leaf Washing Research Articles

Disruption of N -Αcyl Homoserine Lactone-Mediated Cell Signaling and Iron Acquisition in Epiphytic Bacteria by Leaf Surface Compounds

Since N-acyl homoserine lactones (AHLs) are key mediators of cell density-dependent regulation of traits involved in virulence and epiphytic fitness in gram-negative bacteria such as Pseudomonas syringae, a variety of plant species were examined to determine their production of leaf surface compounds that could interact with these signaling systems. Leaf washings of 17 of 52 plant species tested stimulated or inhibited AHL-dependent traits in at least one of the bacterial reporter strains used. The active compounds from most plants could be distinguished from known AHLs due to different patterns of mobility during C8 and C18 reverse-phase thin-layer chromatography (TLC) and normal-phase TLC compared to the patterns for authentic bacterial AHLs. All plant extracts were also tested to determine their abilities to sequester iron and trigger bacterial siderophore synthesis on a medium containing abundant iron. Leaf washings from 16 of the 52 plant species, as well as tannic acid solutions, stimulated pyoverdine synthesis in P. syringae in a high-iron medium. These preparations also inhibited the growth of a P. syringae mutant unable to produce pyoverdine siderophores but not the growth of the wild-type bacterium. The stimulation of siderophore production and the growth inhibition by plant extracts and purified tannins were both reversed by addition of ferric chloride to culture media, indicating that iron was made unavailable by the compounds released onto the leaf surface.

Microbial, nutritional and sensory quality of rocket leaves as affected by different sanitizers

Leafy salad species are increasingly consumed in the human diet and there is increased concern about the levels of microbial organisms in these raw foods, and especially bacteria such as Salmonella that cause food poisoning. Various chemical sanitizers therefore are used to control microorganisms and fungi, but there is very little information on the effects of these chemicals on food composition. Wild rocket ( Diplotaxis tenuifolia L. DC) leaves were washed using tap water, chlorine (100 mg L −1), ozonated water (10 mg L −1), lactic acid (Purac ® 20 mL L −1), acidified sodium chlorite (Sanova ® 250 mg L −1) and peroxyacetic acid (Tsunami ® 300 mg L −1). The effects of sanitizers on the contents of Vitamin C, polyphenols and glucosinolates of rocket leaves were studied under air and low O 2 (1–3 kPa) + high CO 2 (11–13 kPa) for 15 days at 4 °C. All the sanitizers effectively reduced microbial growth on the day of processing, but only Purac, Tsunami and Sanova inhibited the microbial growth throughout the shelf life. The visual quality was acceptable for all treatments in air while it was poor under low O 2 + high CO 2. In addition, Purac was particularly detrimental for sensory quality. Both chlorophyll a and chlorophyll b contents were reduced throughout storage but were independent of washing treatments and storage conditions. The content of vitamin C was maintained for up to 8 days of storage under air and low O 2 + high CO 2, but Purac washes markedly reduced the vitamin C content. A clear decrease in ascorbic acid followed by an increase in dehydroascorbic acid was observed when samples were stored under low O 2 + high CO 2. The content of flavonoids was not affected by the washing solutions on the processing day and remained almost constant throughout the storage in air. However, marked reductions were observed when samples were stored under low O 2 + high CO 2. The glucosinolates were the most affected constituents of rocket leaves as the content was reduced from 4 to 33% when samples were stored in air while the decrease was between 60 and 100% in low O 2 + high CO 2. These data revealed that Sanova and Tsunami could be alternative sanitizers to chlorine for rocket leaf washes due to good retention of sensory quality with no detrimental reduction of the antioxidant constituents.

Variable adhesion and diurnal population patterns of epiphytic yeasts on creeping bentgrass

Irrigation and an in vitro agitation assay were used to determine the percentage of the epiphytic yeast community (Cryptococcus, Pseudozyma, Rhodotorula, and Sporobolomyces) adhering to the phylloplane of creeping bentgrass (Agrostis palustris (Huds.) Pers.). Colony-forming units (cfu) of total epiphytic yeast populations (adherent and nonadherent cells) and of adherent populations (cells not removed by agitation) were determined by leaf washing and dilution plating. In an in vitro assay, 40.0% and 57.1% of the yeast adhered to the leaves, whereas, in initial field trials the percentage of adherent yeasts ranged from 40.0% to 71.9% of the total population. Adherent yeast cfu on leaves in the morning were significantly lower on bentgrass (8.0 x 103 to 3.1 x 104 cfu.cm(-2)) compared with total yeast cfu (1.4 x 104 to 4.7 x 104 cfu.cm(-2)) on the nonirrigated control. No differences in yeast populations were observed between irrigated and nonirrigated plots 2 h after the 0900 treatments. Yeast populations followed a diurnal pattern, with larger cfu recovered from bentgrass leaves in the morning and significantly lower populations recovered in the afternoon. At 1400 the adherent yeast were 83.1%-100% of the total yeast population recovered from the leaves. The relative adhesiveness of the epiphytic yeast community on bentgrass leaves is dynamic with nonadherent cells making up a larger percentage of the population in the mornings than the afternoons.

Long- and Short-Chain Plant-Produced Bacterial N-Acyl-Homoserine Lactones Become Components of Phyllosphere, Rhizosphere, and Soil

Two N-acyl-homoserine lactone (acyl-HSL) synthase genes, lasI from Pseudomonas aeruginosa and yenI from Yersinia enterocolitica, were introduced into tobacco, individually and in combination. Liquid chromatograph-tandem mass spectrometry and thin-layer chromatography confirmed products of lasI and yenI activity in single and cotransformants. Cotransformants expressing plastid-localized LasI and YenI synthases produced the major acyl-HSLs for each synthase in all tissues tested. Total acyl-HSL signals accumulated in leaf tissue up to 3 pmol/mg of fresh weight, half as much in stem tissue, and approximately 10-fold less in root tissues. Acyl-HSLs were present in aqueous leaf washes from greenhouse-grown transgenic plants. Transgenic lines grown for 14 days under axenic conditions produced detectable levels of acyl-HSLs in root exudates. Ethyl acetate extractions of rhizosphere and nonrhizosphere soil from transgenically grown plants contained active acyl-HSLs, whereas plant-free soil or rhizosphere and nonrhizosphere soil from wild-type plants lacked detectable amounts of acyl-HSLs. This work shows that bioactive acyl-HSLs are exuded from leaves and roots and accumulate in the phytosphere of plants engineered to produce acyl-HSLs. These data further suggest that plants that are bioengineered to synthesize acyl-HSLs can foster beneficial plant-bacteria communications or deter deleterious interactions. Therefore, it is feasible to use bioengineered plants to supplement soils with specific acyl-HSLs to modulate bacterial phenotypes and plant-associated bacterial community structures.

Linking bacteriophage infection to quorum sensing signalling and bioluminescent bioreporter monitoring for direct detection of bacterial agents.

To incorporate into the lambda phage genome, a luxI-based acyl-homoserine lactone (AHL) synthase genetic construct and exploit the autoamplified power of quorum sensing to translate a phage infection event into a chemical signature detectable by a lux-based bioluminescent bioreporter, with focus towards facile detection of microbial pathogens. The luxI gene from Vibrio fischeri was inserted into the lambda phage genome to construct a model phage-based biosensor system for the general detection of Escherichia coli. The AHL signalling molecules synthesized upon phage infection are detected by an AHL-specific bioluminescent bioreporter based on the luxCDABE gene cassette of V. fischeri. The assay generates target-specific visible light signals with no requisite addition of extraneous substrate. This binary reporter system was able to autonomously respond to lambda phage infection events at target E. coli concentrations ranging from 1 x 10(8) to 1 CFU ml(-1) within 1.5-10.3 h, respectively, in pure culture. When assayed against artificially contaminated lettuce leaf washings, detection within an E. coli inoculum range from 1 x 10(8) to 130 CFU ml(-1) was achieved within 2.6-22.4 h, respectively. The initial feasibility of binary phage-based reporter assays indicates that quorum sensing can be used to translate a phage infection event into an autoamplified chemical signature. With further modification, binary phage-based reporter assays may be capable of rapidly and cost effectively detecting pathogenic agents at very low population densities.

Invasive Frogs’ Influence on Lowland Forest Arthropod Communities and Ecological Processes in Hawaii

Author(s): Sin, Hans; Beard, Karen H.; Pitt, William C. | Abstract: We conducted an enclosure experiment at two low­land sites on the eastern side of the Island of Hawaii for 6 months (September 2004 to February 2005) to determine the effect of Eleutherodactylus coqui on 1) three invertebrate communi­ties (aerial, herbivorous, and leaf litter), 2) nutrient cycling rates and leaf litter decomposition rates, and 3) native and non-native plant growth rates. E. coqui was found to reduce all invertebrate communities at one study site, while the other site did not have any difference with treatment. At both sites, herbivory rates were lower in the presence of E. coqui than in their absence. At both sites, E. coqui was associated with increased NH4+ in leaf washes, increased K, Mg, N, and P concentrations in the leaf litter, and increased decomposition of leaf litter. P. cattleianum had more new leaves in presence of the E. coqui than in the absence of E. coqui across sites. Non-native plants had higher survivorship than native plants. In summary, invertebrate community responses to E. coqui differed by site but ecological processes responded similarly across sites. Path analyses suggest that the E. coqui increases leaf litter decomposition and non-native plant growth rates by making nutrients more available to microbes and plants. These findings are of concern in Hawaii, where E. coqui can potentially affect the 44 endangered invertebrates and potentially increase non-native plant growth.

Effect of Nitrogen Fertilization and Seed Contamination on Epiphytic Populations of Xanthomonas axonopodis pv. allii and Development of Xanthomonas Leaf Blight of Onion

Xanthomonas leaf blight of onion, caused by Xanthomonas axonopodis pv. allii, is a yield-limiting disease in Colorado, for which few effective management strategies are available. The effects of rates of nitrogen fertilization and levels of seed contamination by X. axonopodis pv. allii on epiphytic populations of the pathogen and subsequent disease development were determined in field studies over two years. Epiphytic populations of a rifampicin-resistant mutant were quantified from bulked, asymptomatic leaf tissue using a leaf wash assay. There was no significant relationship between nitrogen fertilization treatment and epiphytic population development, but excessive nitrogen (> 200 kg/ha) fertilization increased disease severity 27 to 50% compared with non-fertilized and low (112 kg/ha) nitrogen fertilizer treatments in 2004. Seed inoculated with various levels of X. axonopodis pv. allii did not result in persistent epiphytic populations of the bacterium or development of symptoms, even at 107 CFU/g of seed. The results of these studies suggest moderate rates of nitrogen fertilization may reduce severity of Xanthomonas leaf blight compared to higher rates, and that seed contamination by X. axonopodis pv. allii may be of limited importance for disease development under the semi-arid conditions of furrow-irrigated onion production in Colorado. Accepted for publication 13 March 2005. Published 31 March 2005.

Effect of foliar disease on the epiphytic yeast communities of creeping bentgrass and tall fescue

The effect of mechanical wounding or foliar diseases caused by Sclerotinia homoeocarpa or Rhizoctonia solani on the epiphytic yeast communities on creeping bentgrass and tall fescue were determined by leaf washing and dilution plating. Total yeast communities on healthy bentgrass and tall fescue leaves ranged from 7.9 x 103 to 1.4 x 105 CFU.cm-2 and from 2.4 x 103 to 1.6 x 104 CFU.cm-2, respectively. Mechanically wounded leaves (1 of 2 trials) and leaves with disease lesions (11 of 12 trials) supported significantly larger communities of phylloplane yeasts. Total yeast communities on S. homoeocarpa infected or R. solani infected bentgrass leaves were 3.6-10.2 times and 6.2-6.4 times larger, respectively, than the communities on healthy leaves. In general, healthy and diseased bentgrass leaves supported larger yeast communities than healthy or diseased tall fescue leaves. We categorized the majority of yeasts as white-pigmented species, including Cryptococcus laurentii, Cryptococcus flavus, Pseudozyma antarctica, Pseudozyma aphidis, and Pseudozyma parantarctica. The percentage of pink yeasts in the total yeast community ranged from 2.6% to 9.9% on healthy leaves and increased to 32.0%-44.7% on S. homoeocarpa infected leaves. Pink-pigmented yeasts included Rhodotorula glutinis, Rhodotorula mucilaginosa, Sakaguchia dacryoidea, and Sporidiobolus pararoseus. Foliar disease significantly affected community size and composition of epiphytic yeasts on bentgrass and tall fescue.

Summer variation in the concentration of steroidal sapogenins in and the degree of fungal infection on Narthecium ossifragum plants from Møre og Romsdal County, Norway.

Thirty-nine leaf samples of Narthecium ossifragum collected from eight sites in Møre og Romsdal County, Norway, during June-September 1997 and 41 leaf samples collected at five sites in the same county during June-August 1998 were analysed for the concentrations of steroidal sapogenins using GC-MS. The 1998 samples were also examined for fungal elements (conidia and hyphae) after incubation in a moist chamber for 10-14 days. The highest 1997 and 1998 leaf sapogenin concentrations (4881 and 7115 mg/kg dry matter, respectively) were 13-14 times greater than the lowest sapogenin concentrations found (344 and 531 mg/kg dry matter, respectively). The results did not reveal systematic differences in sapogenin concentrations between the two seasons, or between samples harvested early or late in the same seasons, or between sapogenin concentrations in plants harvested at different sites. Cladosporium magnusianum was the predominant fungus found in the samples. The degree of fungal infection on the samples was in generally low, but the number of C. magnusianum colonies in the moist chamber preparations and fungal elements (conidia and hyphae) in leaf washings and on leaves tended to increase with time. Factor analysis and multiple regression analysis performed on the chemical and fungal results suggest that sporulation may have occurred in the fungi in response to increase in sapogenin concentrations.

Assessing the source of mercury in foliar tissue of quaking aspen

Foliar accumulation of mercury has been demonstrated to occur as plants leaf out, yet the primary source of this mercury is not known. Using closed-system growth chambers, uptake of mercury by quaking aspen (Populus tremuloides) foliage was measured over time as a function of soil mercury concentrations (0.01, 6.2, and 25.6 microg/g) and atmospheric mercury exposure concentrations (1.4, 14.9, and 68.5 ng/m3). Foliar mercury concentrations increased as a function of time for all exposures. Twice during the experiment, leaf washes were analyzed for mercury to assess surface deposition, and little mercury was removed (0.02-0.04 ng/m2), suggesting that direct deposition to the leaf surface was not significant during this experiment. At the end of the four-month experiment, whole-plant mercury concentrations were determined. It was found that whereas mercury in the atmosphere primarily influenced foliar uptake, root concentrations were related to the soil mercury concentration. The implication of this study is that litterfall may serve as a pathway for new, atmospherically derived mercury to be deposited to forest soils. This has significant implications for watershed management of ecosystems where mercury is of concern.

ASSESSING THE SOURCE OF MERCURY IN FOLIAR TISSUE OF QUAKING ASPEN

Foliar accumulation of mercury has been demonstrated to occur as plants leaf out, yet the primary source of this mercury is not known. Using closed-system growth chambers, uptake of mercury by quaking aspen (Populus tremuloides) foliage was measured over time as a function of soil mercury concentrations (0.01, 6.2, and 25.6 μg/g) and atmospheric mercury exposure concentrations (1.4, 14.9, and 68.5 ng/m3). Foliar mercury concentrations increased as a function of time for all exposures. Twice during the experiment, leaf washes were analyzed for mercury to assess surface deposition, and little mercury was removed (0.02-0.04 ng/m2), suggesting that direct deposition to the leaf surface was not significant during this experiment. At the end of the four-month experiment, whole-plant mercury concentrations were determined. It was found that whereas mercury in the atmosphere primarily influenced foliar uptake, root concentrations were related to the soil mercury concentration. The implication of this study is that litterfall may serve as a pathway for new, atmospherically derived mercury to be deposited to forest soils. This has significant implications for watershed management of ecosystems where mercury is of concern.

Dry deposition washoff and dew on the surfaces of pine foliage on the urban- and mountain-facing sides of Mt. Gokurakuji, western Japan

Dry deposition and dew components on pine foliage were measured from 1998 to 2000 on the urban- and mountain-facing sides of Mt. Gokurakuji in order to estimate the effect of anthropogenic activities to dry deposition and dew concentration on the surfaces of pine foliage. A leaf wash experiment was employed to determine the dry deposition rates on the pine foliage. The NO 3 − and SO 4 2− dry deposition rates per unit surface area of pine foliage were 1.47 and 0.28 μmol m −2 h −1 respectively on the urban-facing side and 0.32 and 0.09 μmol m −2 h −1 on the mountain-facing side. Dry deposition fluxes of N (NO 3 −+NH 4 +) and S (SO 4 2−) to the forest floors were 8.4 kg N ha −1 yr −1 and 2.8 kg S ha −1 yr −1 on the urban-facing, and 3.3 kg N ha −1 yr −1 and 1.8 kg S ha −1 yr −1 on the mountain-facing side, respectively. The higher dry deposition fluxes of N and S on the urban-facing side could be attributed to its proximity to traffic roads and the urban area. The concentrations of most ions in the dew were higher on the urban-facing side (U130) than on the mountain-facing side (M430). NO 3 − and SO 4 2− concentrations in dew at U130 were 802 and 428 μeq l −1, respectively, while at M430 they were 199 and 222 μeq l −1, respectively, suggesting that higher dry deposition rates on the urban-facing side enhanced their concentrations in the dew on this side. The role of dry deposits and subsequently dissolved ones in dew on the needle surfaces is discussed in terms of pine tree damage by atmospheric depositions.

Top-down effects of a terrestrial frog on forest nutrient dynamics.

Many studies have found top-down effects of predators on prey, but few studies have linked top-down effects of vertebrate predators to nutrient cycling rates in terrestrial systems. In this study, large and significant effects of a terrestrial frog, Eleutherodactylus coqui (coquí), were recorded on nutrient concentrations and fluxes in a subtropical wet forest. In a manipulative experiment, coquís at natural densities were contained in or excluded from 1m3 enclosures for 4months. Chemistry of leaf wash (throughfall), foliage, and decomposed leaf litter in the enclosures were measured as indicators of coquí effects on nutrient cycling. Coquí exclusion decreased elemental concentrations in leaf washes by 83% for dissolved organic C, 71% for NH4+, 33% for NO3-, 60% for dissolved organic N, and between 60 and 100% for Ca, Fe, Mg, Mn, P, K, and Zn. Coquí exclusion had no effect on foliar chemistry of plants transplanted into the enclosures. However, coquí exclusion decreased nutrient availability in decomposing mixed leaf litter by 12% and 14% for K and P, respectively, and increased C:N ratios by 13%. Changes in nutrient concentrations that occurred with coquí exclusion appear to be due to concentrations of nutrients in coquí waste products and population turnover. The results supported our hypothesis that coquís have an observable effect on nutrient dynamics in this forest. We suggest that the primary mechanism through which they have this effect is through the conversion of insects into nutrient forms that are more readily available for microbes and plants. The potential for higher trophic level species to affect nutrient cycling through this mechanism should not be overlooked.

Phylloplane fungi in Hong Kong mangroves: evaluation of study methods

Many methods have been used to study phylloplane fungi, most of which have constraints and may result in biased results. This study used light microscopy and scanning electron microscopy (SEM) to investigate fungal abundance on the leaves of the most common mangrove trees in Hong Kong, Kandelia candel and Aegiceras corniculatum. Species richness was investigated using light microscopy and a leaf washing method. Methods to study phylloplane fungi are discussed and the performances of these three investigation methods are evaluated. Seven mitosporic fungal taxa were found by light microscopy, while 30 sporulating taxa and 18 Mycelia sterilia were isolated using the leaf washing method. Fungal abundance in terms of percentage cover investigated with light microscopy was similar using the SEM method, and was significantly higher on Aegiceras corniculatum than on Kandelia candel. Fungal abundance peaked in the summer and was lowest in the winter. This study indicates that light microscopy reveals the most typical phylloplane fungi and is more efficient than SEM, while the leaf washing method reveals many casual species and is not quantitative.

Population biology ofAureobasidium pullulanson apple leaf surfaces

Colonization of apple leaves by the yeast-like fungus Aureobasidium pullulans was studied in the field on eight dates over 2 years. Population densities from adaxial leaf surfaces were approximately log10 0.5-2.6 U higher when enumerated directly along line transects as microscopic counts of A. pullulans cells specifically identified by fluorescence in situ hybridization (FISH) than indirectly as CFU obtained by plating leaf washings from comparable surfaces onto nutrient media. Site-specific mapping of the leaf landscape colonized by A. pullulans was facilitated by use of geographic information system (GIS) software. Colonization was plotted and analyzed both qualitatively (as occupancy) and quantitatively (as density). Overall, when expressed as mean occupancy per date, 22-42% of the microscope fields (each 0.196 mm2) contained > or = 1 A. pullulans cell. Occupancy on a microscope field basis was greater over the midvein (47-89%) or smaller veins (49-65%) than over interveinal regions (11-21%). Intensity of colonization, whether expressed as percentage of total A. pullulans cells associated with a particular leaf feature or as cell density per unit area, was also significantly greater (P < 0.05) over the veinal areas compared with the interveinal areas. The primary fungal morphotypes involved in colonization were blastospores, swollen cells, and chlamydospores; only infrequently were hyphae or pseudohyphae seen. Numbers of microcolonies (> or = 10 clustered cells) and percentage of total A. pullulans cells that occurred as microcolonies increased over the growing season and were significantly greater (P < 0.05) over veinal regions compared with interveinal regions. Locally high concentrations of A. pullulans were associated with naturally occurring micro-wounds in interveinal areas. We conclude that A. pullulans colonizes the phylloplane predominantly as single cells and groups thereof in a highly heterogeneous fashion and that sites exist that are relatively conducive (veins; wounds) or nonconducive (unwounded interveinal areas) for epiphytic fungal growth.

Evaluation of Methods to Quantify Herbicide Penetration in Leaves1

Epicuticular wax is a major barrier to the movement of foliar-applied herbicides into plant leaves. Many researchers have evaluated the penetration of herbicides in plants using similar methods. However, the methods used to remove unabsorbed herbicide from the leaf surface have been inconsistent among scientists. Laboratory experiments were conducted to evaluate the effectiveness of various compounds as a leaf wash for removing unabsorbed 14C-labeled herbicides from shattercane and velvetleaf leaves. Product B and Product C provided poor results as a leaf wash compared with water, chloroform, methanol, and Product A. Product A increased the amount of 14C-bentazon removed from velvetleaf by 26% compared with the amount removed by chloroform, methanol, or water. Removal of 14C-nicosulfuron was not different between chloroform, Product A, methanol, or water. Although water was the best treatment for the two salts of 14C-glyphosate, Product A was as good or better than chloroform and methanol.Nomenclature: Bentazon; glyphosate; nicosulfuron; shattercane, Sorghum bicolor L. #3 SORBI; velvetleaf, Abutilon theophrasti L. # ABUTH.Additional index words: Epicuticular wax, leaf wash, radiolabeled herbicides.Abbreviations: IPA, isopropylamine; LSS, liquid scintillation spectrometry; TMS, trimethylsulfonium.

Microbial Ecology of the Mango Phylloplane.

Bacteria, filamentous fungi, and yeasts were enumerated on the mango phylloplane by indirect leaf impression and washing- and dilution plating. The phylloplane microbial community was qualitatively and quantitatively related to leaf age, position in the tree canopy, seasonality, and chemical spraying. Filamentous fungi and yeasts were more abundant during winter and spring, whereas bacterial population densities increased during autumn. Community density and diversity increased progressively with leaf age. The western tree aspect sustained the least diverse bacterial, filamentous fungal, and yeast communities. Chemical sprays reduced bacterial, filamentous fungal, and yeast community diversities. Gram-positive bacteria (Bacillus spp. and coryneform spp.) exceeded gram-negative bacteria. The most common fungal genera isolated were Cladosporium and Alternaria. Yeasts prevalent in the mango phylloplane were of the genera Aureobasidium, Cryptococcus, and Sporobolomyces.

Development of a novel method to measure NH3 fluxes from grass swards in a controlled laboratory environment (a mini-tunnel system)

A description of the development of a mini wind-tunnel system for the measurement of ammonia volatilisation from grass swards is given. Tunnel design was based on that of Lockyer (1984), but scaled in order to enable controlled laboratory studies to be carried out and allow investigations of the pathways and mechanisms that control NH3 emission and deposition from grassland swards. The potential of the system was examined by using 15N as a tracer in a study measuring NH3 volatilisation from a source of artificial urine. Deposition along the sward was examined up to 70 cm from the source. Deposition onto the leaf surface and uptake into the leaf via stomata were examined with leaf washing experiments and a compensation point in the range of 0.5–1.9 μg m−3 was calculated.

Assessment of Dry Deposition and Foliar Leaching of Mercury and Selected Trace Elements Based on Washed Foliar and Surrogate Surfaces

Throughfall and litterfall are the primary deposition pathways of Hg to forested ecosystems, yet the source of this Hg (soil or atmospheric) remains unclear. Leaf samples were harvested from live trees and used in leaf washing experiments to assess whether the recovered Hg originated from surface deposits (wash-off of dry deposition) or internal sources (foliar leaching). The behavior of Hg was compared to eight other elements also found in leaf wash with known external (Cd, La, Ce, Pb) or internal (Mn, Rb, Sr, Ba) sources. These experiments indicated that wash-off of dry deposition occurred rapidly, while foliar leaching occurred continuously during the entire wash period (up to 12 h). To isolate dry deposition, surrogate Teflon surfaces exposed for a 98 h dry period were washed and compared to washed foliar surfaces. Computed dry deposition velocities for both surfaces were within a factor of 2 for Hg, Cd, La, Ce, and Pb, suggesting that dry deposition was the primary source of these elements from the washed foliage. Deposition velocities varied widely (3−18 times higher from the foliar surfaces) for Mn, Rb, Sr, and Ba, most likely due to the leaching behavior exhibited by these elements. These experiments show that dry deposition of atmospheric Hg is likely the largest source of Hg in throughfall.

Resistance to glyphosate inLolium rigidum.II. Uptake, translocation, and metabolism

Experiments were conducted to determine potential mechanisms of glyphosate resistance inLolium rigidumfrom Australia.14C-Glyphosate uptake, translocation, and metabolism were compared between resistant (R) and sensitive (S) biotypes. The R seed (48118a) represented the F1progenies of plants having survived a 1.73-kg ae ha−1(4.8 L ha−1) application of a Roundup®formulation. The S seed was a sensitive biotype ofL. rigidumfrom Australia. Plants (one to four tillers, 2 to 4 wk old) were presprayed with a high (1.26 kg ae ha−1) or a low (0.28 kg ae ha−1) dose of formulated glyphosate. The first leaf of the first tiller, which was shielded from the spray, was immediately treated with a14C-glyphosate solution via manual application to the adaxial surface. Harvest was made 6 days after treatment (DAT), and glyphosate residues in the leaf wash, treated leaf, roots, and shoots were quantified based on radioactivity as percentage of applied dose. The overall radioactivity recoveries were very good (90.2 to 97.3% of applied dose). R and S plants showed comparable uptake at the high (79.2 vs. 78.0%) or the low (64.0 vs. 64.7%) doses of glyphosate. About one-half of the absorbed glyphosate in both R and S (32.9 to 38.3% appl.) was translocated into the plant and distributed almost equally into roots (13.6 to 16.0% appl.) and shoots (18.1 to 22.6% appl.). Autoradiography studies demonstrated no difference in tissue localization of glyphosate between the R and S plants. For metabolism studies, tissues from individual plants were homogenized in water, and extracts were analyzed by anion exchange high-pressure liquid chromatography (HPLC) with radioactivity detection. There was little to no metabolism of glyphosate in extracts from various tissues of either R or S plants. Based on these results, we conclude that neither uptake, translocation, nor metabolism play a major role in glyphosate resistance inL. rigidum.