Residual thenylchlor concentration in soil water and its phytotoxic activity in soil

The phytotoxic activity of soil-applied thenylchlor (2-chloro- N -[3-methoxy-2-thenyl]-2',6'-dimethylacetanilide) on the growth of rice seedlings and its behavior in two different types of soil, Ryugasaki soil and Tsukuba soil, after application was investigated with emphasis on the concentration in the soil water. The greatest inhibition of thenylchlor on the growth of rice seedlings was found immediately after application to both the Ryugasaki and Tsukuba soils. The phytotoxic activity decreased with time in both soils. However, the rate of decrease in phytotoxic activity was slower in the Ryugasaki soil than in the Tsukuba soil. The concentration of thenylchlor in soil water, the amount adsorbed on the soil solid, and the amount in the total soil reduced with time after application to both soils. The amount of thenylchlor adsorbed on the soil solid phase was more persistent than that in the soil water in both soils and the concentration in the soil water was higher in the Ryugasaki soil than in the Tsukuba soil at any given time. The residual phytotoxic activity of thenylchlor on the growth of the rice seedlings in the soil was highly correlated with its concentration in the soil water but not with the amount in the total soil. These results suggested that the residual phytotoxic activity of thenylchlor in the soil is determined by its concentration in the soil water after application.     

Effects of the safeners CGA 154281, oxabetrinil and fenclorim on uptake and degradation of metolachlor in corn (Zea mays L.) seedlings

Summary. The influence of three crop safeners on the uptake and degradation of ¹⁴C-metolachlor was investigated in two corn varieties. Following application of herbicide and safener together to seedling shoots the concentrations of non-metabolized ¹⁴C-metolachlor in the tissues was found to be lower in the tolerant variety LG 9 than in the susceptible variety 211A. The difference between varieties was due to differences in both uptake and degradation of ¹⁴C-metolachlor.
Following shoot application most of the radioactivity was retained in the coleoptile and the mesocotyl. Two hours after application 95% of the herbicide had been degraded in coleoptiles and mesocotyls, whereas approximately 20% of non-metabolized ¹⁴C-metolachlor was present in the enclosed developing shoot leaves. In both corn varieties the safener CGA 154281 caused a substantial lowering of tissue levels of parent ¹⁴C-metolachlor. This was primarily due to an enhanced degradation. Glutalhione- S -transfer-ase (GST) enzyme activity in shoot tissues was found to be enhanced in both varieties by CGA 154281. Oxabetrinil and fenclorim were less effective than CGA 154281 both in reducing tissue levels of non-metabolized ¹⁴C-metolachlor and in enhancing GST activity in either variety.     

Red rice (Oryza sativa L.) and barnyardgrass (Echinochloa spp.) biotype susceptibility to postemergence-applied imazamox

Pot experiments were conducted to evaluate the level of imazamox tolerance in five red rice ( Oryza sativa L.) and four barnyardgrass (three Echinochloa crus-galli (L.) Beauv. and one Echinochloa oryzoides (Ard.) Fritch) morphologically distinct biotypes collected from rice fields in northern Greece. The susceptibility of barnyardgrass biotypes to propanil was also studied. Red rice biotypes were not controlled by imazamox applied at 40 g ha⁻¹. In contrast, 80 g imazamox ha⁻¹ provided 56–84% red rice control (averaged across shoot number and fresh weight reduction). Not all barnyardgrass biotypes were susceptible to imazamox applied postemergence. However, propanil applied at 2.6 kg ha ⁻¹ controlled the E. crus-galli biotypes well, but propanil applied at rates of 2.6 and 5.2 kg ha ⁻¹ was not effective in reducing the shoot number and fresh weight of the E. oryzoides biotype. Propanil applied at 10.4 kg ha ⁻¹ reduced the shoot number and fresh weight of this biotype by 78 and 85%, respectively. In most cases, a linear equation ( y  = % of control, x  = g ha⁻¹) provided the best fit for regressions between red rice or barnyardgrass shoot number or fresh weight and imazamox rates. The results of this study suggest that postemergence application of imazamox is not effective against all red rice and barnyardgrass biotypes found in the rice fields of Greece and that significant variability regarding herbicide efficacy among biotypes might exist.     

THE EFFECT OF SIMAZINE ON THE PHOTOFIXATION OF CO2 AND ON TRANSLOCATION OF ASSIMILATES IN NORWAY SPRUCE (PICEA ABIES)

Summary. A method of exposing seedlings of Norway spruce (Picea abies (L.) Karst.) to ¹⁴CO₂ is described. Within the 1st hr alter ¹⁴CO₂ exposure, no translocation of the ¹⁴C out of the treated branch could be observed. After a 24-hr period, however, the ¹⁴C in dormant seedlings had been translocated basipetally to part of the root system only, with no lateral diffusion of the ¹⁴C-compounds in the stem. About a week after exposure, both symplastic and apoplastic patterns of translocation had caused a more uniform distribution of ¹⁴C. In seedlings at active internode elongation, the translocation patterns were fundamentally identical to those in dormant seedlings, but the active shoot growth had led to a more uniform distribution of the ¹⁴C.
Simazine at 20 ppm had apparently stimulated both the photofixation of ¹⁴CO₂ and the rate of translocation of the ¹⁴C-assimilates. At 30 ppm, however, simazine had blocked the translocation of nutrients to the roots. On the other hand, the ¹⁴CO₂ uptake was not influenced. The simazine incubation had apparently no influence on the synthesis of cationic photosynthate.     

Allelochemicals of barnyardgrass‐infested soil and their activities on crops and weeds

Barnyardgrass is one of the most noxious paddy weeds in the world and causes great trouble to many crops. In a bioassay, the aqueous extract of paddy soil infested with barnyardgrass showed phytotoxic action against the growth of the tested crops and paddy weeds, of which rice was the most suppressed among the crops. In contrast, barnyardgrass was the least affected paddy weed. By the use of a separation resin, 18 compounds belonging to terpenes, derivatives of cinnamic acid and ferulic acid, long‐chain fatty acids, and steroids that were potentially involved in the phytotoxic activities in barnyardgrass‐infested soil were isolated and identified by gas chromatography–mass spectrometry analysis. Of these, the quantities of linalool, 4‐terpinenol, coumaran, methyl phenethyl ketone, and methyl ester cinnamic acid were 1.42, 0.37, 0.02, 3.12, and 4.59 μg g^?1 of the wet soil, respectively. The herbicidal activity was varied among these qualified test compounds, in which methyl phenethyl ketone and methyl ester cinnamic acid were more herbicidal than coumaran, linalool, and 4‐terpinenol. A mixture of these compounds was also the least inhibitive against the growth of barnyardgrass, but exerted strong suppression against that of rice and monochoria. The present study demonstrates that barnyardgrass possesses strong phytotoxic properties and releases plant growth inhibitors into the soil to compete with rice and other paddy weeds in its vicinity by a chemical pathway.     

TRANSLOCATION OF 14C-ASSIMILATES AND 3-AMINO-1,2,4-TRIAZOLE AND ITS METABOLITES IN AGROPYRON REPENS

Summary. Translocation of 3-amino-1,2,4-triazole-5-¹⁴C (¹⁴C-aminotriazole) was compared to that of ¹⁴C-assimilates in couch grass ( Agropyron repens (L.) Beauv.) at three different growth stages.
Assimilates of ¹⁴CO₂ were translocated from the treated shoot to other shoots and rhizomes at the 2–3-leaf and 3–4-leaf stages of development. Much less labelled material was translocated into untreated shoots at the 5-leaf stage. More ¹⁴C-assimilates were translocated to the roots than to untreated shoots at all developmental stages. The translocation patterns of ¹⁴C-aminotriazole and ¹⁴C-assimilates were similar.
Two metabolites, A and B, were formed from ¹⁴C-aminotriazole, which chromatographed identically to previously described metabolites in Cirsium arvense (L.) Scop. A was further metabolized into B. Labelled aminotriazole and its two metabolites were translocated throughout the plants. Metabolite A was phytotoxic when concentrated and re-applied to couch grass, but its properties were not those of Unknown II from Cirsium arvense. They were the same as those of Unknown III. Whether or not metabolite A and Unknown III are identical was not established.
Migration des substances assimilables marquées au ¹⁴ C, du 3 amino-1,2,4-triazole et de ses métabolites chez Agropyron repens     

Residual phytotoxic activity of pethoxamid in soil and its concentration in soil water under different soil moisture conditions

The relationship between the residual phytotoxic activity of pethoxamid (2-chloro- N -[2-ethoxyethyl]- N- [2-methyl-1-phenyl-1-propenyl] acetamide) on rice ( Oryza sativa cv. Kiyohatamochi) seedlings and its behavior in soil was investigated under different moisture conditions. The phytotoxic activity of pethoxamid on the shoot growth of rice seedlings in soil was higher in 80% soil moisture content than in 70% and 60% soil moisture contents. The phytotoxic activity in soil in 70% and 80% soil moisture conditions decreased with the increasing time after application, but the phytotoxic activity was slight in 50% soil moisture conditions at any given time after application. The residues of pethoxamid in soil water, the amount adsorbed on soil solid, and the amount in total soil was reduced with the time after application in a similar manner among these soil moisture conditions. The residual phytotoxic activity of pethoxamid on the shoot growth of rice seedlings in soil was more highly correlated with the concentration in soil water than with the amounts adsorbed on soil solid and in total soil. The partition coefficients between the amounts of pethoxamid adsorbed on soil solid and its concentration in soil water were similar among the soils with different moisture conditions at each day, and the partition coefficient increased with the time after application. These results suggested that the residual phytotoxic activity of pethoxamid in soil depends on the decreasing concentration of pethoxamid in soil water with time, except in low soil-moisture conditions, which were insufficient for seedling growth.     

Phytotoxic activity of pethoxamid in soil under different moisture conditions

The phytotoxic activity of soil-applied pethoxamid [2-chloro- N -(2-ethoxyethyl)- N -(2-methyl-1-phenyl-1-propanyl) acetamide], (TKC-94), on the plant growth of rice ( Oryza sativa cv. Kiyohatamochi ) seedlings as an assay plant in soil was investigated under different soil moisture conditions. The phytotoxic activity of pethoxamid mixed with soil on the shoot and root growth of rice seedlings was uppermost under the highest soil moisture condition and it decreased with declining soil moisture content, while the inhibition was greater on the root growth than the shoot growth. The amount of pethoxamid adsorbed on soil solid and the concentration of pethoxamid in soil water from soil applied with this herbicide were not influenced by the soil moisture content. In addition, the phytotoxic activity on the growth of rice seedlings in sea sand culture applied with the soil water from the herbicide-applied soil was not influenced by the soil moisture content. In the sea sand culture, the phytotoxic activity of pethoxamid was significantly reduced in negative water potential as the concentration of polyethylene glycol-6000 added to the water increased. It is suggested that the phytotoxic activity of pethoxamid in the soil primarily depends on the concentration in soil water, but the phytotoxic activity was affected by soil moisture through the effect on absorption of this herbicide by rice seedlings.     

Uptake and translocation of imazethapyr in peas as affected by parasitism of Orobanche crenata and herbicide application methods

[¹⁴C]-Imazethapyr was applied as a seed treatment and at plant pre-emergence and post-emergence to peas ( Pisum sativum L.) parasitized by Orobanche crenata Forsk. Herbicide uptake increased with time regardless of the application method. Uptake reached about 98%, 89%, 81% and 94% of the total herbicide applied for the seed coating, seed soaking, pre-emergence and post-emergence treatments respectively. Herbicide translocation within the host plants consistently differed between O. crenata -infected and non-infected plants. High levels of ¹⁴C activity were accumulated by parasitic plants from the host. In non-infected pea plants, pods were stronger sinks for imazethapyr than the other parts of the plant, regardless of the application method. The herbicide distribution in the pea plant: O. crenata complex showed the same pattern regardless of the application methods. However, accumulation of radioactivity in the parasite was lower with pre-emergence and post-emergence application than with the seed treatments. In addition, radioactivity concentration in O. crenata plants was slightly higher when [¹⁴C]-imazethapyr was applied to pea seeds by coating than by soaking.     

SHOOT ZONE UPTAKE OF SOIL-APPLIED HERBICIDES IN PISUM SATIVUM L.

Summary. The effects of localized herbicide placement at different internodes of pea ( Pisum sativum L. cv. Alaska) shoots below the soil surface after plant emergence were studied by removing the soil from around the shoots and replacing with herbicide-treated soil. The first internode proved insensitive to linuron, diuron, atrazine and simazine at 4·5 kg/ha, while treatment of the second and third internodes markedly reduced plant growth 4–6 days after treatment. Separate exposure of the first internode alone, and the second and part of the third together to ¹⁴C-labelled atrazine indicated no difference in herbicide metabolism. However, a two- to threefold increase in ¹⁴C uptake and movement to the foliar parts occurred when the second and part of the third internode was treated, as compared to first internode treatment. Thus the differential sensitivity of the internodes to atrazine, and possibly to the other herbicides, may be because the more mature first internode allows less uptake and subsequent movement to the foliar parts.
Absorption dans la zone des tiges des herbicides appliqués dans le sol chez Pisum sativum L.     

Studies on herbicidal carbamoyltetrazolinone derivatives: Selection of fentrazamide

1-Phenyl-5(4 H )-4-dialkylcarbamoyltetrazolinones represent a new class of herbicides that are active against barnyardgrass ( Echinochloa spp.) in paddy fields. To select the most favorable compound as a paddy herbicide, herbicidal activity and selectivity of the carbamoyltetrazolinone compounds to transplanted rice were determined in greenhouse tests. Higher herbicidal activities were associated with the compounds having a 2-Cl- or 2-Cl-6-Me-substituted or an unsubstituted phenyl ring, and a combination of C_2–3 linear alkyl group and a C₄ branched-chain alkyl or a C_5–6 cycloalkyl group on the nitrogen atom in the carbamoyl structure. In addition, the compounds carrying a total of seven to nine carbon atoms in the substituents on the carbamoyl nitrogen exhibited a good selectivity between barnyardgrass and transplanted rice plants. From the results of soil thick-layer chromatography, the good crop compatibility of carbamoyltetrazolinones was correlated to the mobility of the compounds in paddy soil. The investigation resulted in the selection of an excellent compound, 4-(2-chlorophenyl)- N -cyclohexyl- N -ethyl-4,5-dihydro-5-oxo-1 H -tetrazole-1-carboxamide (common name: fentrazamide), as a rice paddy herbicide for controlling barnyardgrass.     

TRANSLOCATION OF AMINOTRIAZOLE AND DALAPON IN AG ROPY RON REPENS (L.) BEAUV.

Summary. Tracer studies using single drops of solutions containing 3–amino-1,2,4–triazole-5–¹⁴C (aminotriazole-¹⁴C) or 2,2–dichloropropionic acid-2–¹⁴C (datapon-¹⁴C) revealed that in couch plants (Agropyron repens (L.) Beauv.) growing under field conditions in the autumn and at the stage where the aerial shoots were 40–50 cm long, both compounds moved in both symplast and apoplast. Dalapon was less mobile in the symplast than aminotriazole and only negligible amounts of dalapon were translocated to the rhizomes. The nodes of the treated shoots appeared to act as barriers to translocation, a phenomenon more pronounced for dalapon than for aminotriazole.
Application to a basal green leaf led to a more uniform distribution of the compounds within plants and rhizomes than when the application was made to the youngest fully-expanded leaf.
In couch plants with aerial shoots 10–15 cm long treated in the stubble, the distribution of both aminotriazole and dalapon was mainly restricted to the treated shoots. Even 15 days after application only trace amounts of radioactivity could be found in the rhizomes and untreated shoots.     

Synthesis and herbicidal activity of new 3-(3-bromothiophen-2-yl)-5-(2,6-difluorobenzyl)oxymethyl-5-methyl-4,5-dihydroisoxazole as a paddy field herbicide

A new compound 3-(3-bromothiophen-2-yl)-5-(2,6-difluorobenzyl)oxymethyl-5-methyl-4,5-dihydroisoxazole was synthesized and the herbicidal activity was assessed under glasshouse and flooded paddy field conditions. This compound demonstrated good rice selectivity and potent herbicidal activity against annual weeds at 125 g ai ha⁻¹ under greenhouse conditions. Soil application of this compound showed the complete control of barnyardgrass to the 4th leaf stage at 250 g ai ha⁻¹. Field trials indicated that this compound, combined with pyrazosulfuron-ethyl, controlled annual and perennial weeds rapidly with good tolerance by transplanted rice seedlings from postemergence and soil application. This compound showed a low mammalian and environmental toxicity in various toxicological tests.     

Propanil activity, uptake and metabolism in resistant Echinochloa spp. biotypes

Field resistance of Echinochloa spp. to propanil has been previously reported in Costa Rica, Colombia and Arkansas (USA). In this study, the mechanism of resistance was investigated in three resistant (R) and three susceptible (S) biotypes. The shoot fresh weight reduction in pot-grown plants from a post-emergence spray of propanil at 2.44 kg a.i. ha⁻¹ on biotypes R/S from Costa Rica, Colombia and Arkansas was 35/98%, 25/79% and 20/82% respectively. In vitro chlorophyll fluorescence data from leaf tissue incubated in propanil showed that photosynthesis was inhibited in all biotypes, indicating that the propanil-binding site and enzyme were not altered. After transfer to herbicide-free solution, photosynthesis recovered only in resistant biotypes, indicating that the mechanism of resistance was caused by enhanced metabolism of the herbicide. Simultaneous treatment with fenitrothion, an aryl acylamidase inhibitor, prevented the recovery of photosynthesis in leaf tissue in two resistant biotypes. In contrast, the cytochrome P450 mono-oxygenase inhibitor, 1-aminobenzotriazole, did not prevent recovery from propanil in leaf tissue. Application of ¹⁴C-propanil to the second leaf of intact Echinochloa plants showed that c . 90% of the radioactivity remained in the treated leaf for up to 72 h after application. No major differences in translocation between R and S biotype plants were found. TLC analysis of tissue extracts from the treated leaves showed substantially less radioactivity associated with propanil, present after 72 h in rice or in the three R biotypes, compared with S biotypes.     

AN AUTORADIOGRAPHIC STUDY OF THE DISTRIBUTION OF 14C LABELLED ASSIMILATES AT DIFFERENT STAGES OF DEVELOPMENT OF OXALIS LATIFOLIA H.B.K. AND O. PES-CAPRAE L.

Summary. An autoradiographic study of the distribution of ¹⁴C-labelled assimilates, derived from ¹⁴CO₂ fed to the aerial parts, was made at ten different developmental stages of Oxalis latifolia H.B.K. and Oxalis pes-caprae L. Changing patterns of distribution of labelled assimilates to the underground parts of both species were revealed. These patterns are discussed in the context of the timing of control by defoliation and by foliageapplied herbicides.
Etude autoradiographique de la distribution des métabolites marqués an ¹⁴C à différents stades de développement de l'Oxalis latifolia H.B.K. et de l' O. pes-caprae L.     

Allelopathic potential of itchgrass (Rottboellia exaltata L. f.) powder incorporated into soil

Itchgrass ( Rottboellia exaltata L. f.) is a widespread weed in northern Thailand. The farmers in this area have been using itchgrass as a mulching material in order to control other weeds in vegetable fields. Laboratory experiments were undertaken to investigate the phytotoxic activity of itchgrass powder incorporated into soil in order to evaluate the allelopathic activity in the field. The phytotoxic activity on the growth of radish seedlings ( Raphanus sativa L. var. radicula ), used as a test plant, was more pronounced in the root than in the shoot growth. The phytotoxic activity was found to be similar for the soils incorporated with the shoot or the root powder of itchgrass. The growth of the radish seedlings grown in sea sand and watered with soil water obtained from the soil previously incorporated with itchgrass powder showed a similar inhibition to those planted in the treated soil. The phytotoxic activity on the growth of the radish seedlings in the soil incorporated with the powder decreased over time. It is suggested that itchgrass releases phytotoxic compound(s) into soil water and the concentration of the active compound(s) in the soil water decreases over time.     

CHLORAMBEN (AMIBEN) DEGRADATION JN SOIL

Summary. When columns of three soil types were perfused with a microbial medium containing 5·7 and 26·9 ppm ¹⁴COOH-labelled chloramben, there was a rapid decrease in the ¹⁴C activity remaining in solution, attributable to soil adsorption. Soil pH and organic matter content were the primary factors controlling chloramben adsorption. Soil pH may enhance adsorption through its effect on the number of soil anion exchange sites, and sites for polyvalent cation bridging and H bonding. Following adsorption, liver decreases in solution ¹⁴C-chloramben activity were attributed to degradation; ¹⁴CO2 evolved during perfusion amounted to 25–30% of the total decrease in ¹⁴C-chloramben from solution. It was concluded that more than 90% of the ¹⁴C-activity remaining in solution could be attributed to intact chloramben. Thus, decarboxylation appeared to be the primary mechanism of chloramben degradation in soils. Lag phases observed during degradation and subsequent investigations with re-amended solutions implicated the soil microflora; even at chloramben concentrations of 26.9 ppm no toxic effect on the chloramben-degrading micro-organisms was apparent.
Dégradation du chlorambène [amibène) dans le sol     

STUDIES ON THE TRANSLOCATION IN AGROPYRON REPENS OF 14C-LABELLED 2,2-DICHLOROPROPIONIC ACID

Summary. Previous findings suggested that the translocation in Agropyron repens of 2,2-dichloropropionic acid (dalapon) may be considerably affected by the transpiration rate. This relationship has been studied further in the present investigation using ¹⁴C-labelled material and autoradiography.
It was found that when dalapon was applied to the leaves and the treated plants were placed in darkness a reduction in the transpiration rate of ca. 90% was associated with a marked increase in the amount of dalapon translocation into the roots and tillers. The evidence provided by the autoradiographs was confirmed by a quantitative assay of the ¹⁴C present in the roots. It was also found, however, that translocation was not appreciably affected either by a 50% reduction in the light intensity or when the transpiration rate was reduced by placing the plaints under conditions of high humidity in the light or by the application of white petroleum jelly to the leaves. An alternative hypothesis, namely that the effect of darkness on the movement of the herbicide might be due to changes induced in the normal pattern of assimilate translocation was also investigated but was not supported by the results obtained. Further experimentss, in which various parts of the treated shoot were placed in the dark, suggested that the effect of darkness on the translocation of dalapon is exerted primarily on the treated leaf itself.
Etudes sur to migration dans Agropyron repens de l'acide 2,2-dichloropropionique marqué avec ¹⁴C     

Allelopathy of buckwheat: Assessment of allelopathic potential of extract of aerial parts of buckwheat and identification of fagomine and other related alkaloids as allelochemicals

The allelopathic potential of buckwheat ( Fagopyrum esculentum ) was investigated. Field study showed that living buckwheat reduced weed biomass compared with plots without buckwheat. Laboratory study revealed that root exudates suppressed root and shoot growth of weeds and reduced weed dry weight. Sequential partitioning of the aqueous ethanolic extract of the aerial parts of buckwheat showed that the chloroform extract caused an 80% reduction in radicle elongation of lettuce seedlings at a concentration of less than 100 p.p.m. Fagomine, 4-piperidone and 2-piperidinemethanol were isolated and identified by ¹H and ¹³C nuclear magnetic resonance from the chloroform extract. These compounds caused a 50% inhibition of radicle elongation in lettuce seedlings at concentrations of less than 100 p.p.m. These results suggest that compounds 1, 2, and 3 might act as allelochemicals and affect the growth or germination of different plant species.     

Physiological basis of differential phytotoxic activity between fenoxaprop-P-ethyl and cyhalofop-butyl-treated barnyardgrass

This study investigated the physiological causes of differences in phytotoxic symptoms shown in barnyardgrass from foliar applications of the herbicides fenoxaprop-P-ethyl and cyhalofop-butyl. When these were applied to the third leaves of the whole plant, the chlorosis and desiccation in the third leaf was greater in fenoxaprop-P-ethyl than cyhalofop-butyl. However, initial growth inhibition of the fourth leaf was greater when using cyhalofop-butyl than when using fenoxaprop-P-ethyl. In the shoot regrowth test, regrowth at five days after treatment (DAT) was smaller in cyhalofop-butyl than in fenoxaprop-P-ethyl; the regrowth at 10 DAT exhibited the reverse trend. The chlorosis (decrease of chlorophylls: carotenoids ratio) in barnyardgrass leaf segments that were floated on herbicide solution was greater in the fenoxaprop-P-ethyl treatment. These results indicate that different herbicidal responses induced by the two herbicides are likely to be related to differential translocation and metabolism. The relatively light chlorosis and desiccation in treated leaves, severe cessation of initial growth (but a lower final herbicidal efficacy in the cyhalofop-butyl treatment) are probably related to its rapid translocation to the meristem region from the treated leaf, followed by faster metabolism. In contrast, the relatively greater chlorosis and desiccation compared to inhibition of initial growth in the fenoxaprop-P-ethyl treatment is likely to be related to its relatively slower translocation and metabolism in the treated leaf.