Foliar Retention Research Articles

Avaliação de danos de insetos e de severidade de oídio em genótipos de soja

Avaliou-se o desempenho de 24 linhagens de soja do programa de melhoramento do Instituto Agronômico (IAC) em experimentos de campo em relação ao ataque de insetos e à incidência de oídio. Três experimentos, um para cada ciclo (precoce, semiprecoce e médio) foram instalados nas unidades experimentais de Assis (anos agrícolas 2001/2002 e 2002/2003) e de Ribeirão Preto (2001/2002). Para os três experimentos, o delineamento utilizado foi o de blocos ao acaso, com dez tratamentos e seis repetições. As injúrias foliares causadas por lagartas de Anticarsia gemmatalis Hübner foram estimadas visualmente, atribuindo-se uma porcentagem de desfolha à parcela inteira. Os danos de percevejos [Piezodorus guildinii (West.) e Euschistus heros (F.)] foram avaliados através da porcentagem de retenção foliar atribuída à parcela inteira e por meio da massa de grãos comerciáveis. A severidade de oídio (Microsphaera diffusa Cke. Pk.) foi estimada mediante escala de notas de 1 a 5, de acordo com a intensidade dos sintomas. Considerando-se os resultados das duas localidades, entre o germoplasma de ciclo precoce destaca-se a linhagem IAC 98-4307, com baixa retenção foliar e maior produtividade em presença de percevejos. No grupo de ciclo semiprecoce, observa-se que na linhagem IAC 98-2814 não há retenção foliar, porém, maior produtividade, caracterizando-se como resistente a percevejos, embora seja o genótipo mais desfolhado por lagartas e com maiores sintomas de oídio. Entre os genótipos de ciclo médio, IAC 98-2856 tem produtividade superior em presença de altas ou baixas infestações de percevejos, indicando possuir resistência do tipo tolerância, embora apresente retenção foliar sob baixa pressão de percevejos.

Investigations into glyphosate-resistant horseweed (Conyza canadensis): retention, uptake, translocation, and metabolism

The mechanism of glyphosate resistance in horseweed was investigated. Eleven biotypes of putative sensitive (S) and resistant (R) horseweed were obtained from regions across the United States and examined for foliar retention, absorption, translocation, and metabolism of glyphosate. Initial studies used spray application of14C-glyphosate to simulate field application. When S and R biotypes were compared in the absence of toxicity at a sublethal dose, we observed comparable retention and absorption but reduced root translocation in the R biotypes. S and R biotypes from Delaware were further examined at field use rates and results confirmed similar retention and absorption but reduced root translocation in the R biotypes. Application of14C-glyphosate to a single leaf demonstrated reduced export out of the treated leaf and lower glyphosate import into other leaves, the roots, and the crown in R relative to S biotypes. Examination of the treated leaf by autoradiography showed that glyphosate loading into the apoplast and phloem was delayed and reduced in the R biotype. Our results consistently showed a strong correlation between impaired glyphosate translocation and resistance. Tissues from both S and R biotypes showed elevated levels of shikimate suggesting that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) remained sensitive to glyphosate. Analysis of tissue shikimate levels demonstrated reduced efficiency in EPSPS inhibition in the R biotypes. Our results suggest that resistance is likely due to altered cellular distribution that impaired phloem loading and plastidic import of glyphosate resulting in reduced overall translocation as well as inhibition of EPSPS.

The effect of different spray liquids on the foliar retention of agricultural sprays by wheat plants in a canopy.

The aim of this work was to examine the effect of liquid properties on spray retention in a range of situations representative of practical pesticide application. Liquids with different physical properties were sprayed with conventional flat-fan and air-induction nozzles onto outdoor-grown wheat plants (Triticum aestivum L cv Axona) at growth stages between GS 22 and GS 35. Since total retention is affected by leaf surface as well as plant growth stage and canopy density, the experiments included spraying both single leaves and groups of plants with different canopy densities. Retention on whole plants was strongly influenced by plant growth and canopy properties. Individual leaf surfaces were not as water-repellent as was found previously for younger indoor-grown plants, reducing the effect of dynamic surface tension on retention. This, combined with the increased probability of droplets hitting foliage in a denser canopy, gave reduced differences in retention between liquids when compared with similar measurements made previously on younger plants. With a leaf area index greater than 7, retention on whole plants was almost independent of liquid. Air-induction nozzles gave lower levels of retention than flat-fan nozzles, with this difference greatest when spraying high dynamic surface tension liquids and at low crop densities.

Foliar interception and retention values after pesticide application: a proposal for standardised values for environmental risk assessment

In performing risk assessments for plant protection products by applicants or regulators in relation to the registration of the products, an important aspect to take into account is the foliar interception and retention of the active substance of the product on the plant. An overview is given of the approaches to this item in several parts of the world. The relevant circumstances and influencing variables, such as growth phase, planting density, and some physicochemical characteristics (e.g., vapor pressure and Henry’s coefficient) are dealt with. Finally, a proposal is presented for how to take into account the phenomenon of foliar interception and retention in the initial phase, first tier, of the risk assessment process.

Danos de percevejos e de lagartas em cultivares e linhagens de soja de ciclos médio e semi-tardio

Objetivou-se avaliar a resistência de genótipos de soja de dois grupos de maturação em relação à infestação de insetos. Instalaram-se dois experimentos, um envolvendo germoplasma de ciclo médio e outro, germoplasma de ciclo semi-tardio, na Estação Experimental de Ribeirão Preto (IAC), SP, nos anos agrícolas 1995/96 e 1996/97. Infestação de lagartas de Anticarsia gemmatalis Hüb. ocorreu apenas no primeiro ano. Os percevejos Piezodorus guildinii (West.) (predominante), Euschistus heros (Fabr.) e Edessa meditabunda (Fabr.) ocorreram nos dois anos agrícolas, com maior infestação no primeiro. Os danos de lagartas foram avaliados mediante estimativa da percentagem de área foliar cortada e, os de percevejos, através da percentagem de retenção foliar (PRF), índice percentual de dano na vagem (IPDV) e produção. Para o experimento com genótipos de ciclo médio, baseando-se principalmente no primeiro ano, pode-se considerar que a linhagem IAC 90-2971 apresentou resistência a lagartas e a percevejos, comparável à da linhagem IAC 78-2318, o controle resistente. Para o experimento de genótipos de ciclo semi-tardio, detectou-se suscetibilidade na linhagem IAC 93-2162 a lagartas e a percevejos, apesar de apresentar baixa PRF. Fato oposto ocorreu com a linhagem IAC 90-1970, com alta PRF mas com baixas injúrias foliares causadas por lagartas, baixo IPDV e boa produção. Além do bom comportamento de IAC 90-1970, devem ser destacados o desempenho da cultivar IAC 9 e da linhagem IAC 90-3052 em relação aos menores danos provocados por percevejos.

Deciduous conifers : High N deposition and O3 exposure effects on growth and biomass allocation in ponderosa pine

Ponderosa pines (Pinus ponderosa Dougl. ex. Laws) 21 to 60 yr old were used to assess the relative importance of environmental stressors (O3, drought) versus an enhancer (N deposition) on foliar retention, components of aboveground growth, and whole tree biomass allocation. Sites were chosen across a well-described gradient in ozone exposure (40 to 80 ppb per h, 24 h basis, 6 month growing season) and nitrogen deposition (5 to 40 kg ha-1 yr-1) in the San Bernardino Mountains east of Los Angeles, California. A high level of chlorotic mottle indicated high O3 injury at sites closest to the pollution source, despite potential for the mitigating effects of N deposition. At the least polluted site, foliar biomass was evenly distributed across three of the five needle-age classes retained. At the most polluted site, 95% of the foliar biomass was found in the current year's growth. High N deposition and O3 exposure combined to shift biomass allocation in pine to that of a deciduous tree with one overwintering needle age class. Based on whole tree harvests, root biomass was lowest at sites with the highest pollution exposure, confirming previous chamber exposure and field studies. Aboveground growth responses in the high-pollution sites were opposite to those expected for O3 injury. Needle and lateral branch elongation growth, and measures of wood production increased with increasing proximity to the pollution source. An enhancement of these growth attributes suggested that N deposition dominated the ponderosa pine response despite high O3 exposure.

Factors affecting foliar retention of some model adjuvant oil‐in‐water emulsions

The foliar retention of dilute oil-in-water emulsions containing three chemical types of adjuvant oil formulated either as ECs or EWs was compared in track-sprayer experiments using the water-soluble tracer, fluorescein. Although both types of emulsion enhanced tracer deposition onto water-repellant pea and barley foliage, compared with sprays containing no adjuvant, consistently better retention was achieved using the EW emulsions, irrespective of oil composition. On young plants, fluorescein retention using the EC emulsifier alone was similar to that from the corresponding emulsions. However, on older plants, retention from EC emul-sions was superior to that from the EC emulsifier. Compar-able effects were not observed using the EW stabiliser, which had little influence on the efficiency of spray deposition. The retention behaviour of the various oil-based adjuvants is discussed in relation to their effects on spray quality and their formulation ingredients. © 1998 Society of Chemical Industry

Factors affecting foliar retention of some model adjuvant oil-in-water emulsions

The foliar retention of dilute oil-in-water emulsions containing three chemical types of adjuvant oil formulated either as ECs or EWs was compared in track-sprayer experiments using the water-soluble tracer, fluorescein. Although both types of emulsion enhanced tracer deposition onto water-repellant pea and barley foliage, compared with sprays containing no adjuvant, consistently better retention was achieved using the EW emulsions, irrespective of oil composition. On young plants, fluorescein retention using the EC emulsifier alone was similar to that from the corresponding emulsions. However, on older plants, retention from EC emul-sions was superior to that from the EC emulsifier. Compar-able effects were not observed using the EW stabiliser, which had little influence on the efficiency of spray deposition. The retention behaviour of the various oil-based adjuvants is discussed in relation to their effects on spray quality and their formulation ingredients. © 1998 Society of Chemical Industry

Foliar retention of 15N tracers: implications for net canopy exchange in low- and high-elevation forest ecosystems

Nitrogen-15 ( 15N) labelled ammonium and nitrate were applied to understory saplings in a low-elevation deciduous forest and a high-elevation coniferous forest during the 1993 growing season to estimate the minimum retention of inorganic N from wet deposition. Tissue samples were collected at both sites before and after the tracer applications to determine the initial and the final 15N content. The fraction of the tracer application retained by deciduous leaves or by current-year leaves and stems on conifers was calculated from measurements of 15N and total N content. At both study sites, tracer applications significantly increased the 15N content of sapling tissues, although there was no significant difference between the fractional retention of ammonium and nitrate. The results support earlier studies indicating that the foliar retention of 15N applied in simulated wet deposition is small. Our estimates of annual N uptake from wet deposition by the forest canopy at the low- and high-elevation study sites were only 14 and 7%, respectively. This study, along with prior studies of the different chemical forms of N deposition, indicates that canopy uptake of dry N deposition is a major contributor to the net canopy exchange of N in both forests.

Resistência de soja a insetos: X. Comportamento de cultivares e linhagens em relação a percevejos e desfolhadores

Avaliou-se o comportamento de linhagens e cultivares de soja de ciclos precoce, semi-precoce, médio e semi-tardio em relação à infestação de percevejos e de insetos desfolhadores, em condições de campo, nas localidades paulistas de Campinas (ano agrícola 1993/94) e de Ribeirão Preto (1994/95). Dentro do germoplasma de ciclo precoce e semi-precoce (IAC 100, IAC 17, IAS 5, IAC Holambra Stuart-2 e IAC 83-311) não se observou comportamento diferenciado em relação a insetos desfolhadores (principalmente Anticarsia gemmatalis Hübner e, em menor proporção, coleópteros crisomelídeos) nas duas localidades, porém, detectaram-se diferenças entre os genótipos em relação ao ataque dos percevejos Piezodorus guildinii (West.) (predominante), Nezara viridula (L.) e Euschistus heros (Fabr.). Considerando-se os critérios porcentagem de dano nas vagens e retenção foliar, IAC 83-311 e IAS-5 foram os de pior desempenho, ao passo que, tomando-se o fator produção, apenas IAS-5 diferiu dos demais, sendo o mais suscetível. O germoplasma de ciclo médio e semi-tardio abrangeu onze linhagens do programa de melhoramento de soja do Instituto Agronômico e três cultivares (IAC 14, IAC 8 e IAC PL-1). Quanto aos insetos desfolhadores, houve diferenças entre os tratamentos: a linhagem IAC 78-2318 confirmou sua resistência enquanto IAC PL-1 sofreu as maiores injúrias foliares, exibindo alta suscetibilidade. Os critérios de avaliação de danos de percevejos permitiram também discriminação desses genótipos de ciclo médio e semi-tardio; considerando-se a produção, a maioria das linhagens teve bom comportamento, com médias acima das das três cultivares, a despeito das altas populações de percevejos.

Dicamba Antagonizes Grass Weed Control with Imazethapyr by Reducing Foliar Absorption

Greenhouse and laboratory studies were conducted to determine the effects of tank-mixing the sodium salt of dicamba (Na-dicamba) with imazethapyr on the efficacy and foliar absorption of imazethapyr, applied with non-ionic surfactant (NIS) or methylated seed oil (MSO), by shattercane, giant foxtail, and large crabgrass. The effects of various salt formulations of dicamba and the addition of ammonium sulfate on efficacy,14C-absorption and on foliar spray retention by the same species were also evaluated. Na-dicamba antagonized imazethapyr efficacy by reducing14C-absorption. Using MSO instead of NIS prevented antagonism when Na-dicamba was applied at 70 and 140 g/ha and reduced the severity of the antagonism at greater application rates by greatly increasing14C-absorption compared to NIS. Reductions in14C-absorption and spray retention were due to the salt formulations of dicamba rather than the parent acid. The addition of ammonium sulfate prevented dicamba antagonism of imazethapyr toxicity to grassy weeds by maintaining14C foliar absorption and spray retention at normal levels.

Foliar ozone injury and radial growth of ponderosa pine

Foliar injury symptoms and stem diameter growth were measured on well-watered and drought-stressed ponderosa pine (Pinusponderosa Dougl. ex Laws.) seedlings at the end of each of 3 years of exposure to three levels of ozone: charcoal-filtered air, nonfiltered air, and nonfiltered air plus 1.5 times ambient ozone. Ozone-injury indices were constructed by adding percent chlorotic mottle and percent necrosis for each needle age-class. Percent necrosis was weighted from 1 to 5 times to construct different indices. Seedlings grown in nonfiltered air plus 1.5 times ambient ozone developed severe foliar injury after 2 years of exposure and were the only seedlings with significant reductions in radial growth after three seasons of exposure to a mean seasonal ozone concentration of 88 ppb. Foliage that had developed >30% chlorotic mottle by September of the 2nd year had abscised by September of the following year. Reduction in radial growth was significantly correlated with amount of foliar injury in well-watered trees, and the best-fit regression equation occurred when percent necrosis was weighted by a factor of 4. Regressions between foliar-injury indices and radial growth in drought-stressed trees were not significant, nor were regressions between radial growth and foliar injury among well-watered trees with only 1 year of premature needle abscission. The low R2 (0.30) between foliar injury and radial growth in well-watered trees and the lack of a significant regression in drought-stressed trees suggest that detection of reductions in stem diameter growth of ponderosa pine in the field in response to ozone injury could be difficult, except for severely injured trees with fewer than 2 years of foliar retention.

Influence of wetting agents on the foliar uptake and herbicidal activity of glufosinate

AbstractThe influence of four different wetting agents on the foliar retention, uptake and herbicidal activity of the glutamine synthetase inhibitor, glufosinate, was examined in growth‐chamber experiments on barley (Hordeum vulgare L. cv Roland) and barnyard grass (Echinochloa crus‐galli (L.) P.B.) as test species.The non‐formulated monoammomum salt, glufosinate‐ammonium, was applied as a spray, either alone or mixed with a wetting agent. The dose rates of herbicide and wetting agent were 0.5 g a.i. litre−1 and 2.0 g litre−1, respectively, on barnyard grass, and 2.0 g a.i. litre−1 and 60 g litre−1, respectively, on barley. Herbicide damage, rated 10 days after spraying, was greatest when glufosinate was used with a sodium C12/C14‐alcohol‐diglycolether sulfate (FAEO‐sulfate) and least with polyoxyethylene (POE)(8) tridecyl ether; intermediate effectiveness was obtained with a combination of herbicide and a POE(15) tridecyl ether or POE(15)‐tallow amine.The activity of the target enzyme, glutamine synthetase, measured 2 h after spraying, was reduced most when FAEO‐sulfate was present and least with POE(8) tridecyl ether.The behaviour of the glufosinate wetting agent solutions on plant foliage was analysed by measurements of spray retention, droplet contact angles and foliar uptake of [14C]glufosinate. The results led, for both grass species, to the conclusion that differential ability of the wetting agents to enhance the permeation of glufosinate from the leaf surface deposit into the leaf tissue was the main factor responsible for the differences in herbicidal effectiveness of the glufosinate/wetting agent combinations used in this study.

Influence of surfactant concentration on foliar retention of pesticides used in forestry

Abstract Aqueous tank mixes of permethrin, fenitrothion, Bacillus thuringiensis (B.t.), diflubenzuron (DFB), and glyphosate containing different amounts of Triton® X‐114, a nonionic surfactant, were prepared. Glyphosate formed clear solutions, permethrin and fenitrothion formed emulsions, DFB and B.t provided suspensions. Emulsion stability of permethrin and fenitrothion increased with increasing surfactant level, while the emulsion drop size decreased. Foliage of white oak, trembling aspen, white spruce and balsam fir were dipped in tank mixes of pesticides (except B.t.) labelled with 14carbon. The amount of pesticide retained on foliage was determined by liquid scintillation counting. Foliage was also dipped in non‐radioactive B.t. tank mixes, and the protein retained was determined colorimetrically. With all tank mixes, a direct relationship was observed between the mass of liquids retained on foliage and liquid viscosity. In contrast, the amount of pesticide retained was unaffected by viscosity, but was influenced by emulsion drop size. Initially, the amount of pesticide retained on foliage increased with increasing surfactant concentration. Beyond an optimum surfactant level, the emulsion drop sizes were too small and the emulsions became too stable to allow maximum retention of pesticides on foliage. With the glyphosate solutions, however, no optimum surfactant level was indicated because foliar concentrations continued to increase with increasing surfactant levels.

Influence of Adjuvants on the Efficacy, Absorption, and Spray Retention of Primisulfuron

The effect of methylated seed oil (MSO), the organosilicone adjuvant DC-X2-5394, and ammonium sulfate on the efficacy, absorption, and spray retention of primisulfuron applied alone or with atrazine, dicamba, and bentazon to shattercane and giant foxtail was evaluated. Primisulfuron efficacy on both species was reduced by the three tank-mix combinations. Atrazine antagonism was not explained by decreases in foliar absorption and/or spray retention. Reductions in primisulfuron absorption and/or foliar spray retention appeared to cause bentazon antagonism on both weeds and dicamba antagonism on shattercane. MSO, DC-X2-5394, and ammonium sulfate completely reversed dicamba and bentazon antagonism on shattercane and partially reversed bentazon antagonism on giant foxtail by increasing foliar absorption and/or spray retention. Compared with non-ionic surfactant, MSO and DC-X2-5394 consistently increased giant foxtail control with primisulfuron by increasing foliar absorption and/or spray retention.

UPTAKE AND PHLOEM MOBILITY OF FOLIAR-APPLIED, LABELLED BORON IN VARIOUS TREE FRUIT SPECIES

Factors affecting the phloem mobility of foliar-applied B have received little study. The purpose of this experiment was to evaluate foliar retention of B solutions, foliar uptake kinetics, and phloem mobility of foliar-applied B among four tree fruit species. Leaves on current-year shoots of nonbearing 'Red Delicious' apple, 'Bartlett' pear, 'French' prune, and 'Bing' cherry were immersed in 1000 mg/liter B solutions (supplied as 10B-enriched boric acid) in midsummer. Export of the applied label from leaves was monitored between 0 and 24 h, and throughout the following 20 days by ICP-mass spectrometry. Uptake by leaves increased steadily in all species between 0 and 24 h, and reached 80% to 95% of the applied quantity within 24 h. By 24 h, 62% to 75% of the applied label, depending on species, had been exported from the treated leaves. Apple leaves retained, absorbed, and exported over twice the amount of labelled B as prune and pear leaves, and nearly four times the amount of cherry leaves. Foliar retention largely controlled the capacity for uptake and export.

Seasonal patterns of biomass accumulation and carbon allocation in Pinustaeda seedlings exposed to ozone, acidic precipitation, and reduced soil Mg

Seasonal patterns of biomass accumulation and carbon allocation were determined for 1-year-old loblolly pine (Pinustaeda L.) seedlings grown in soil having either 15 or 35 μg Mg•g−1 and exposed to subambient, ambient, or twice ambient levels of ozone (O3) and simulated rainfall at pH 3.8 or 5.2 in open-top chambers. Seedlings were harvested during the 1988 dormancy period, immediately prior to bud break in 1989, during the first and third foliar growth flushes of 1989, and at the end of the 1989 growing season. At each harvest, biomass of individual seedling components was determined for all seedlings, and 14C allocation among components was determined on a subset of seedlings. Seedlings exposed to twice ambient O3 exhibited a delay in fine-root production at the beginning of the 1989 growing season and reduced coarse-root biomass at the end of the 1989 growing season. Acidic precipitation and soil Mg treatments did not significantly affect biomass at any harvest date, nor were any treatment interactions observed. Biomass accumulation and carbon allocation were strongly affected by plant phenological stage, with maximum root allocation immediately prior to bud break and maximum shoot allocation at the end of the growing season. Seedlings exposed to twice ambient levels of O3 exhibited increased total respirational losses, decreased foliar retention, and slightly reduced export of initial 14C label to stem and roots compared with seedlings exposed to subambient O3; however, these trends were not statistically significant.

Spruce–fir decline and gap dynamics on Camels Hump, Vermont

Widespread and conspicuous dieback of red spruce (Picearubens Sarg.) accompanied by asymptomatic declines in balsam fir (Abiesbalsamea (L.) Mill.) and white birch (Betulapapyrifera var. cordifolia (Regel.) Fern.) trees on Camels Hump, Vermont, prompted an investigation into the role of spruce–fir decline on gap expansion, tree health, and regeneration dynamics. Forty percent of the study area on Camels Hump was occupied by recent forest gaps in the summer of 1989. Gaps were growing northwesterly (direction 322°) at a rate of 0.8–1.2 m•year−1. Prevailing winds in the study area are from the south-southeast. Gap expansion appears to play a dominant role in continuing forest disturbance. Red spruce vigor was low in all dominance classes (53% foliar retention), while vigor of balsam fir and birch (yellow (Betulaalleghaniensis Britt.) and white) was somewhat higher. Exposure and wind flagging of different tree species within the same crown dominance classes were similar, but apical foliar damage was greater in spruce than in fir or birch. Red spruce and birch trees were older than balsam fir within comparable crown classes. Wind flagging was significantly correlated with crown exposure in red spruce and balsam fir, while diameter was the most important factor related to flagging in birch. All three species tended to die while standing. Dead balsam fir trees were twice as likely as red spruce to have been windthrown or diseased, whereas dead red spruce showed a much higher incidence of past and present insect infestation. Live birch had low levels of biotic damage. Density of red spruce seedlings was low within gaps and adjacent canopy sites. Balsam fir seedling density was significantly higher in the canopied areas, whereas birch density was marginally higher in gap areas. There were no significant differences in ages of spruce or fir seedlings growing in gaps and closed canopy areas. Birch seedlings in gaps were older and taller than those under the canopy, indicating low survivorship of birch under the forest canopy. It is likely that this forest will be predominantly composed of balsam fir and birch in the near future.

Diclofop Resistance in Wild Oat (Avena fatua)

The differences in tolerance, morphology, and physiological response of diclofop-resistant and -susceptible wild oat biotypes collected from fields in Saskatchewan, Canada, were investigated under growth room and field conditions. Under herbicide-free conditions the resistant biotype had more upright leaves with about 12% less leaf area and 50% less leaf width than the susceptible biotype. A marked difference in the level of tolerance to diclofop was observed. Photosynthesis was initially significantly reduced in both biotypes after treatment with diclofop at the rate of 0.70 kg ai ha−1, but the resistant biotype was able to recover. Injury to the susceptible biotype was reduced by coating the seeds with 1,8-naphthalic anhydride. Differential foliar retention was not an important factor in selectivity of diclofop among the two biotypes.

Foliar retention of 15N-nitrate and 15N-ammonium by red maple ( Acer rubrum) and white oak ( Quercus alba) leaves from simulated rain

Studies of nitrogen cycling in forests indicate that trees assimilate atmospheric nitrate and ammonium and that differences between atmospheric deposition to the forest canopy and deposition measured in forest throughfall can be attributed to the removal of these ions from rain by tree leaves. Red maple and white oak leaves were exposed to artificial rain solutions (pH 4.1) containing 15N-labeled nitrate (3.5 μg N/ml) or ammonium (2.2 μg N/ml). At two time intervals after exposure (2 hr and 2 days) an exposed leaf and a control (non-exposed) leaf were removed from replicate seedlings. Based on results from 15N analysis, most of the nitrate applied to tree leaves was removed by washing with water; the mean per cent removal (± standard error, N = 4) was 87 ± 1 and 73 ± 4% of the 15NO-N Applied to red maple and white oak leaves, respectively. Relative retention of 15NH 4-N by the leaves was greater than that observed for 15NO 3-N. In red maple and white oak leaves, 58 ± 9 and 84 ± 7% (mean ± standard error, N = 4), respectively, of the applied ammonium was not removed by washing treatments. Our results show that the foliar uptake of 15NH 4 + from simulated rain by deciduous tree leaves is greater than that for 15NO 3 −. Greater retention of NH 4 + than NO 3 − ions by red maple and white oak leaves from simulated rainfall is consistent with field observations showing a preferential retention of ammonium from rainfall by forest canopies. As nitrogen chemistry and the relative importance of nitrogen compounds in the atmosphere change in response to proposed emission reductions (and possibly climate change), an improved understanding of the fate of airborne nitrogen compounds in forest biogeochemical cycles will be necessary.