Pestic. Sci. 1991, 33, 305-317

Cuticular Penetration of Foliar-applied Diflufenican in Galium aparine L.

Heather Knight* & Ralph C. Kirkwood

Department of Bioscience and Biotechnology, University of Strathclyde, Todd Centre, 31 Taylor Street, Glasgow G4 ONR, UK

(Revised manuscript received 26 March 1991; accepted 28 May 1991)

ABSTRACT

The herbicide dijlufenican is an effective pre-emergence compound used for the control of ‘di&ult’ dicotyledonous weeds in winter cereals. It is less eficient when appliedpost-emergence, activity being found to depend on the dose reaching the apical meristems. Contact with dijlufenican had no significant effect on other parts of the plant and translocation was negligible. In the absence of surfactants, the uptake of dijlufenican was minimal and complete approximately 2 h after treatment. Surfactant incorporation increased the time over which uptake occurred, eficiency depending on surfactant type and lipophilicity. ‘Silwet L-77’ (1.09 litre-’) was the only surfactant tested which caused suficient damage to the apex to prevent subsequent recovery of the plant. The effect of nonylphenol ethoxylate surfactants at concentrations above the critical micelle concentration (c 1.0 g litre- ‘1 appeared to be partly attributable to leaf damage caused by the surfactant, possibly facilitating uptake. Increasing leaf coverage by surfactants did not necessarily improve uptake and activity.

1 INTRODUCTION

Diflufenican is a broad-spectrum herbicide, developed for pre-emergence or early post-emergence application against a variety of weeds, both broad-leaved and grasses,’,2 particularly in autumn-sown cereal^.^ Interest in diflufenican as a foliar treatment has been initiated in order to determine the efficiency of early post-emergence foliar applications. Initial studies have shown diflufenican to be less effective when applied to leaves than when applied to the The present * Present address: Division of Biological Sciences, Institute of Cell and Molecular Biology, University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh EH9 3JH, UK

305

festic. Sci. 0031-613X/91/$03.50 #$ 1991 SCI. Printed in Great Britain

306 H. Knighl, R. C . Kirknood

study was carried out in order to elucidate some of the factors limiting the herbicidal activity under post-emergence conditions, with particular reference to the role of surfactants and their effect on the uptake and activity of diflufenican.

Diflufenican acts as an inhibitor of the enzyme phytoene desaturase which catalyses the dehydrogenation of phytoene in the carotenoid biosynthetic pathway.6 The herbicide has no activity on pre-existing carotenoids and to be effective it must reach the sites of production of new tissue in which carotenoid biosynthesis is occurring.

Surfactants have varying effects on herbicide activity, including e n h a n ~ e m e n t ~ . ~ and reduction9-” as well as lack of effect. Generally the enhancement of herbicidal activity by surfactants is achieved through increasing the amount of active ingredient entering the plant, although this may occur due to one or more of a variety of factors. Many surfactants reduce the surface tension of spray solutions and thus increase surface coverage;’ ’ increased herbicidal activity may or may not be correlated with these factor^.'^.'^ Most surfactants cause an effect at concentrations well above those required to effect changes in surface tension, suggesting the involvement of other, more complex, processes.’ Other causes of surfactant effect include dissolution of leaf surface wax,’ solubilisation of the active ingredient,l7-’ inherent phytotoxicity of the surfactant,” increased droplet drying time” and effects on membrane permeability.21

2 MATERIALS AND METHODS

2.1 Materials

Seeds of the weed species Galium aparine L. were produced from plants grown at RhGne-Poulenc Agrochemicals, Ongar, Essex. Diflufenican was supplied by Rh6ne- Poulenc as an aqueous suspension concentrate (FR1078/3) equivalent to the commercially available formulation. [ 14C]diflufenican labelled in the pyridine ring was supplied in a formulated form (specific activity 189kBqmg-I). The surfactants ‘Tween 20’ (ethoxylated sorbitan monolaurate; 20 moles ethylene oxide) and ‘Tween 61’ (ethoxylated sorbitan monostearate; 4 moles ethylene oxide) were gifts of Koch-Light Ltd and ‘Silwet L-77’ (organosilicone surfactant) of Union Carbide UK Ltd. A series of nonylphenol ethyoxylate surfactants containing 4.0, 5.5, 65 , 10, 12 and 14 moles ethylene oxide were used. These are referred to as NPE 4, NPE 5.5 , etc, and were obtained from Lankro Chemicals, with the exception of NPE 10 (‘Synperonic NP10’; ICI) and NPE 14 (‘Lutensol A P 14’; BASF).

2.2 General methods

2.2.1 Plant culture All plants were grown in a glasshouse at 20( & 5)OC with supplementary illumination of 14 h per 24 h (1 kW MB mercury vapour lamps). Seeds were planted in John Innes no. 2 potting compost at a depth of 1 cm in pots of 13cm diameter lined with polyethylene to prevent carry-over of herbicide between experiments. The pots were placed for one week in a cold frame for cold treatment before being trans- ferred to the glasshouse. Plants used in radiotracer studies were transferred after

Aclion of diyufenican in selecied crop and weed species 307

treatment to a growth cabinet and maintained at 22( t-0.5)"C and 75( t- 5)% r.h. A daylength of 14 h was maintained using cool white fluorescent lamps (27ooO lux). The night temperature was 15( &O.S)OC.

2.2.2 Herbicide activity studies Plants were sprayed with diflufenican (2.5 x M) at a volume rate equivalent to the field rate of 200litreha-', using a Camlab chromatography sprayer. After the appropriate time period, the individual whorls of plants were scored on a scale of &5 based on the degree of chlorosis; 0: unaffected leaves, 1: slight bleaching, 2: c. one quarter of tissue bleached, 3: c. one half of tissue bleached, 4: c. three quarters of tissue bleached, and 5: complete chlorosis of all leaves in the whorl.

2.2.3 Radiotracer studies Formulated radiolabelled diflufenican was applied as 2 x 0.5-pl droplets, using a Hamilton microsyringe, to the middle lamina region of the adaxial surface of leaves of plants at the two-whorl stage. Plants were harvested 48 h later unless otherwise stated. An equal number of droplets was applied directly to plastic minivials containing scintillant (Safefluor, Rh8ne-Poulenc) to give a measure of the applied dose. At harvest the treated tissue was washed in acetone (1 ml, 60s) to remove any surface residues of [14C]diflufenican and thereafter in chloroform (1 ml, 30s) to remove the epicuticular waxes and any [14C]diflufenican contained therein. Scintillation fluid (5 ml) was added to each of the minivials which contained the residues from the acetone and chloroform washes. All vials were radioassayed using a Packard Tricarb Scintillation Spectrometer (model 300 C) and the distribution of ['4C]diflufenican throughout the tissues calculated as percentages of the applied dose. The untreated and treated tissues were pelletised on Whatman no. 1 filter paper and combusted using a Packard TriCarb sample oxidiser (model B306). The [14C]carbon dioxide thus formed from each sample was trapped in reactive organic amine (Lumasorb; 8 ml) to which the appropriate scintillation fluid (Carboluma; 10 ml) was automatically added; the samples were radioassayed as before.

2.3 Experimental descriptions

2.3.1 Experiment I . Eflect of growth stage and position on activity of dgufenican Seedlings were treated at two growth stages (cotyledon and first true leaf) with FR1078/3 at the recommended field rate (2.5 x M, 200litre ha-'). Two x 0.5-pl droplets were applied using a Hamilton microsyringe to the seedlings at either the apex, the cotyledons or the first leaf. After 2, 6 and 14 days each whorl of leaves or pair of cotyledons was scored for chlorosis on a scale of &5 as described above.

2.3.2 Experiment 2. Eflect of 'Silwet L- 77', 'Tween 20'and 'Tween 61 'on the activity of dijlufenican The effect of three nonionic surfactants of different hydrophilic-lipophilic balance (HLB), 'Silwet L-77' (13.0), 'Tween 20' (16.7) and 'Tween 61' (9.6) on the activity of FR1078/3 was compared at a range of concentrations (0.1, 1.0, and 10g litre- '). Treatments were applied to plants at the two-whorl stage. There were two replicate

308 H . Knight, R. C . Kirkwood

pots per treatment and three plants per pot. Shoot height measurements and a visual score for the apical leaf whorl were made 3,7,14 and 28 days after treatment.

2.3.3 Experiment 3 . Phytotoxicity of four surfactants ‘Silwet L-77’, NPE 4, NPE 6.5 and NPE 14 were applied at 0.1, 1-0 and 10g litre- ’ in the presence and absence of formulated diflufenican (2.5 x M). Three 0 . 5 ~ 1 droplets were applied to leaves of the second whorl and the visible damage recorded after 48 h.

2.3.4 Experiment 4 . Effect of NPE surfactants singly or in combination on uptake of [ 4qd$ufenican NPE 4 and NPE 6 5 were incorporated into radiolabelled diflufenican (2.5 x M), singly at loglitre-’ and in combination, each at a concentration of 5 glitre-’. Plants were harvested after 2 h or 48h and the uptake of [‘4C]diflufenican measured.

2.3.5 Experiment 5 . Effect of ‘Silwet L-77’ on the uptake of [‘4Cldijlufenican The uptake of [‘4C]diflufenican was determined at 1, 2, 4, 8 and 24h after the application of 2 x 0 5 p I droplets of radiolabelled diflufenican (2.5 x M) containing ‘Silwet L-77’ (10 g litre- ’).

2.3.6 Experiment 6 . The effect of prespraying plants with ‘Silwet L-77’ on uptake of [‘4qdiJrufenican Plants at the three-whorl stage were sprayed with ‘Silwet L-77’ (loglitre- ’) at a volume rate equal to 200 litre ha- ’, and 2 x 05-pl droplets of radiolabelled diflufenican (2.5 x M) were applied 0, 5 and 7 days after spraying. At each application date, radiolabelled diflufenican was applied also to unsprayed plants. The experiment was repeated with plants in which spraying and [ 14C]diflufenican treatments were separated by 0, 1 ,2 ,4 and 8 h; all plants including the unsprayed control were treated with radiolabelled diflufenican at the same time.

2.3.7 Experiment 7 . Effect of position of droplet application on uptake of [ ’ q d@u fen ican Plants at the three-whorl stage were treated with 2 x 0 . 5 ~ 1 droplets of radiolabelled diflufenican (2.5 x M) applied to the cotyledons, the second leaf whorl or apical bud. Plants were harvested after 4 h in order to be certain of recovering the treated apical bud and not new tissue present at the apex and the uptake of [ 14C]diflufenican measured.

2.3.8 Experiment 8. Measurement of surface tension and contact angles Surface tensions of herbicide/surfactant mixtures were measured using the Wilhelmy plate method” in which the torsion required to remove a glass plate from the solution was measured. Formulation FR1078/3 at 2.5 x M was centrifuged (10 min, 3000 rev min- ’ ) and filtered (Millipore filter) before addition of NPE6.5, NPE8, NPE 10, NPE 12, NPE 14. ‘Tween 20’ or ‘Tween 61’, all at 1.0 g litre-

Action of dflufenican in selected crop and weed species 309

Contact angles were measured by observing a droplet on a plane surface using a telescopic eyepiece with rotating protractor. Contact angles of droplets were measured as follows: (1) formulated diflufenican (2-5 x M; centrifuged and filtered) on parafilm, with and without ‘Tween 20’, ‘Tween 61’ and NPE surfactants (all at l.Oglitre-’), (2) water on fully expanded leaves, leaves of medium age and newly emerged leaves, (3) formulation FR1078/3 on medium-aged tissue with and without ‘Tween’, NPE and ‘Silwet L-77’ (all at 1.Og litre-’).

2.4 Statistical treatment

Where appropriate, analysis of variance (ANOVA) was performed on experimental data according to the procedure of Snedecor and C~chran . ’~ For analyses involving a single variable, the differences between means were tested using Duncan’s Multiple Range Test.24

3 RESULTS

3.1 Effect of growth stage and position on activity of diflufenican

Visible damage for each whorl was scored and a mean score calculated for each plant. Damage to the apex was also recorded (Table 1). The most effective treatment position was the apex, especially when application was made at the earlier stage. The overall effect of each treatment decreased between 6 and 14 days. Measurement of the apical score indicated that, initially, treatments to the apex were most effective but that after 14 days new, unbleached tissue was present at the apex. Treatment of the leaf or cotyledons was ineffective.

TABLE 1 Effect of Treatment Position and Growth Stage on Diflufenican

Activity

Growth Position Visual score“ stage of

treatment 2 days 6 days 14 days

1 Cotyledons 0.0 (0.0)

1 Apex 1.5 (1.5)

2 Cotyledons 0.0 (0.0)

2 Leaf 0.4 (0.0)

2 Apex 1.3 (2.5)

a Figures in parentheses indicate score for apex.

whorls. Indicates some whitening of the axillary buds but no bleaching of

H . Knight, R. C . Kirkwood 310

TABLE 2 Effect of ‘Silwet L-77’, ‘Tween 2 0 and ‘Tween 61’ on the Activity of Diflufenican

(a) Shoot height

Surfactant Concentration ( g Iitre- ‘)

Shoot height (cm)

3 days 7 days 14 days 28 days

Control -

Diflufenican -

‘Tween 20’ 0.1 1 .o

10.0 ‘Tween 61’ 0.1

1 .o 10.0

‘Silwet L-77’ 0.1 1 .o

10.0

4.7 5.8 6.4 6.6 5.5 5.1 5.2 4.7 5.8 5.5 4.4

6.1 7.1 6.9 7.0 5.8 6.3 5.8 5.2 6.6 5.8 5.5

11.5 10.1 8 .O 8.9 6.5 8.0 7.1 5.8 8.5 5.8 5 .O

29.4 17.3 21.6 18.6 10.0 21.3 19.5 14.0 24.9 6.2 4.1

(b) Apex score

Surfactant Concentration Score ( g l i tre-’)

3 days 7 days 14 days 28 days

Control - Diflufenican - ‘Tween 20’ 0.1

1 .o 10.0

‘Tween 61’ 0.1 1 .o

10.0 ‘Silwet L-77’ 0.1

1 .o 10.0

0 0 2.0 3.3 3.7 1.7 2.3 3.3 2.0 3.0 4.0 2.3

0.0 0.0 0.0 1.7 2.7 0.0 1.3 3.0 0.0 4.3 5.0

0.0 0.0 0.0 0.0 2.0 0.0 0.0 0.7 0.0 4.0 5.0

0.0 0.0 0.0 0.0 1 .o 0.0 0.0 0.0 0.0 3.3 5.0

3.2 Effect of the nonionic surfactants ‘Silwet L-77’, ‘Tween 20’ and ‘Tween 61’ on the activity of diflufenican

Plants at the two- to three-whorl stage were sprayed with three concentrations of surfactant (0.1, 1-0 and 10g litre-’) in combination with diflufenican and assessment of apex bleaching made to assess ‘recovery’ (Tables 2(a) and 2(b)).

Shoot height increased with time (P>O.OOl) but after 28 days, was reduced by diflufenican increasingly with surfactant concentration (concentration/time inter- action P > 0.001, Table 2(a)). In combination with diflufenican, ‘Silwet L-77’ caused greatest damage to the apex ( P > O . O O l ) especially at 1.0 glitre- ’ (Table 2(b)) at 3 days and at later dates. The effect of the ‘Tweens’ decreased dramatically with time but ‘Silwet L-77’ remained effective, especially at 10 g litre- (surfactant/time interaction P=0*009). At 3 days after treatment the most effective surfactant

Action of diflufenican in selected crop and need species 31 1

251

DFF*NFE4 DFF+NPE65 DFF+ NPE4+ NPE 6.5

Fig. 1.Effect ofethoxylatesurfactantson the uptakeof ['4C]diflufenican after (.)2 h and (H)48 h.

concentration was 1.0 g litre-' but at later times 10g litre-' was the most effective (concentration/time interaction P > 0.001). Plants treated with diflufenican alone or in combination with 0.1 glitre-' surfactant exhibited no bleaching in the apical region after 7 days whereas damage was still evident after 28 days with 1.0 and toglitre-' surfactant.

3.3 The effect of ethylene oxide content and concentration on surfactant phytotoxicity

The NPE surfactants (loglitre-') all caused damage to leaves in the presence and absence of diflufenican NPE 4 > 6.5 > 14) but did not cause visible damage at 0.1 or 1.0glitre-'. No damage was visible in plants treated with 'Silwet L-77' (data not presented).

3.4 Effect of NPE 4 and NPE 6.5 singly or in combination on the uptake of ['4C]diflufenican

The surfactants NPE4 and NPE6.5 (loglitre-') had little effect on uptake of ['4C]diflufenican after 2 h but markedly increased uptake after 48 h (Fig. 1); this was reflected in the reduction of the surface residues of ['4C]diflufenican with time (P=0-005). The amount of ['4C]diflufenican in the chloroform wash varied with time and surfactant treatment (treatment/time interaction P =0.038). Uptake into the tissues increased with time particularly in the presence of surfactant (treatment/time interaction P =- 0.001). A similar effect was obtained for overall uptake (treatment/time interaction P = 0-001 ); at 48 h, NPE 4 caused a significantly greater uptake of ['4C]diflufenican than the combined surfactant treatment.

3.5 Effect of 'Silwet L-77' on the uptake of ['4C]diflufenican over a 24-h period

The effect of 'Silwet L-77 on uptake of ['4C]diflufenican over 24h is seen in Fig. 2. The incorporation of surfactant reduced the amount of ['4C]diflufenican present in the acetone wash (P=O.OOl) and increased the amount recovered from the treated tissue (P = 0.05). Surfactant incorporation increased the amount of ['4C]diflufenican recovered from treated tissue after 1,2,4,8 and 24 h; this response

312

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H . Knight, R. C. Kirkwood

a- - r I

was absent without surfactant (surfactant/time interaction P = 0.001). Movement of [ 14C]diflufenican into the untreated tissue was similarly enhanced especially after 4h (P=O.o03). In the presence of surfactant the uptake of ['4C]diflufenican was increased approximately four times after 1 h and it continued to increase up to 24 h when it reached a level three times that attained at 1 h (surfactant/time interaction P>O-OOl). At 24 h, the level of uptake obtained with 'Silwet' was eight times that without surfactant.

3.6 The effect of prespraying plants with 'Silwet L77' on uptake of ['4C]diflufenican over a 7day period

The effect of prespraying plants with 'Silwet L-77' (log litre-') on the subsequent uptake of [14C]diflufenican was examined after 7 days (Table 3(a)). The amount of [ 14C]diflufenican in the acetone wash decreased with time (P = 0006) while treated tissue levels and overall uptake tended to increase between 5 and 7 days (P=O.O13 and P=O.OI 1 respectively). There was no difference in uptake between the presprayed and unsprayed plants.

The experiment was repeated over a 1-8h time scale. The amount of [ 4C]diflufenican present in the untreated tissue was higher in plants sprayed 15min before treatment (0 h) (P=OWl); total uptake was also greater (P=O.OOl, Table 3(b)).

3.7 Effect of position of droplet application on uptake of ['4C]diflufenican

Only apex-treated plants contained significantly more ['4C]diflufenican in the chloroform wash (P>O-001) and the treated tissue (P>O.OOl) than the other plants. Total uptake was significantly greater for apex-treated plants (P > 0.001, Table 4).

3.8 Measurement of surface tension and contact angles

The surface tension of formulated diflufenican (FR 1078/3) was significantly (P >0-001) lower than that of water. All of the surfactants tested significantly reduced the surface tension of the formulation. The greatest reduction within the

Acrion of d8ufenican in selected crop and weed species 313

TABLE 3 Effect of Prespraying with 'Silwet L-77' on Uptake of [ '4C]Diflufenican

(a) Prespraying CL7 days before treatment

Time Surfactant [14Cl-distribution" (d) (YO of applied)

Ab cb P ub Total Uptake

0 - 93.1 1 *o 1.6 0.8 96.5 2.4 + 93.0 2.0 2.9 4.3 99.3 7.2

5 - 73.6 0.6 2.5 1.8 78.6 4.3 + 65.7 1.4 2.6 2.3 72.1 4.9

7 - 85.0 2.5 8.3 2.6 98.5 10.9 + 83.2 2.6 6.2 5.0 95.9 11.2

(b) Prespraying &8 h before treatment

Time [ ''Cl-distribution' ( h ) (YO of applied)

A b Cb 'Ib U b Total Uptake

0 93.2 0.5 1 -3 3.0 98.0 4.3 1 94.3 0.9 0.5 a0.7 96.3 1.2 2 95.7 0.8 0.7 a0-2 97.3 a1.2 4 93.1 0 5 0.7 a0.2 97.3 al.0 8 97.2 0 6 0.6 a02 98.7 a04

Control 106.1 1.4 0.5 al.0 107.8 a0.7

P NS NS NS 0.001 NS 0001

" D test-within a column, figures with the same letter prefix do not differ significantly at P = 0.05.

A =acetone wash; C =chloroform wash; T= treated tissue; U = untreated tissue.

TABLE 4 Effect of Position of Droplet Application on Uptake of [14C]diflufenican

Treated area

[ ''CJdistributiono (YO of applied)

A b Cb 7 b ub Total Uptake

Apex al l l .2 5.7 9.1 b0.4 126.4 b9.5 Leaf 101.9 a06 a05 aO.0 103.0 a0.5 Cotyledon a112.0 ale3 a05 abO. 1 113.9 a0.6

P 0001 > o m 1 >0.001 0.047 NS >OW1

a D test-within a column, figures with the same letter prefix do not differ significantly at P=0*05. See footnote to Table 3.

314 H . Knight, R. C . Kirkwood

‘Tween’ group was caused by ‘Tween 61’ and within the NPE group by NPE6.5 (Table 5(a)).

The measurement of contact angles on parafilm showed negligible differences between water and the formulation. Reductions in contact angle were obtained with ‘Silwet L-77’> ‘Tween 61’> ‘Tween 20’; in the NPE series, contact angles increased with ethoxylate group number (Table 5(b)). The contact angle of water on leaf tissue was significantly lower on fully expanded leaves than on medium-aged or young tissue.

The contact angle for ‘Tween 61’ was significantly lower than that for ‘Tween 20’. ‘Silwet L-77’ caused complete spreading on the leaves and no measureable contact angle. In general, among the NPE surfactants, contact angles increased with increasing ethoxylate group number.

4 DISCUSSION

The three surfactant groups used in this study (‘Tweens’, nonyl phenol ethoxylates and ‘Silwet’) were all of the non-ionic class which is most commonly employed for the application of agrochemicals. Preliminary studies showed that it was important for diflufenican to reach the apex, presumably as the apical meristems are the prime target for its inhibitory action. Delivery of a sub-lethal dose to this area resulted in regrowth, similar to the situation observed with nitrodiphenyl ether herbicide^.^^ Apical uptake appears to be particularly important since there is evidence that very little or no transport of diflufenican occurs from other regions of the plant to the apex. Damage to the apex was considered to be a relevant measure of the herbicidal efficiency of various diflufenican/surfactant combinations and the ‘recovery’ of plants treated with them. Experiments with ‘Silwet’ and ‘Tween’ surfactants showed that a high degree of initial damage to the apex was necessary for continued control up to four weeks after application. This was demonstrated by a good correlation between high apical damage scores and reduced growth rates. The effectiveness of control over this time period depended on both surfactant type and concentration.

Generally the level of uptake of diflufenican was low, consistent with that expected for a compound of high log K O , (4.9); such highly lipophilic herbicides tend to partition into the plant cuticle w a x e ~ . ~ ~ ” ~ The apparent lack of transport of diflufenican is also consistent with its high logK,,.28

The uptake of [14C]diflufenican was greater at the apex than at areas of older tissue, probably due to the different distribution and thickness of leaf cuticle waxes at these areas. The greater wettability of older leaves may have been due to changes in surface waxes, possibly related to the casting off of wax structures by older leaves rendering them smoother and more wettable.29

Although ‘Silwet L-77’ was the most effective surfactant, it had no noticeable phytotoxic effect, as also found by Coupland et ~ 2 1 . ; ~ ’ this contrasts with the visible damage caused by the less effective NPE surfactants. Its effectiveness may be due to its dramatic reduction of surface t e n ~ i o n , ~ ’ although further enhancement of effect was obtained at concentrations above typical critical micelle concentration

Action of dijlufenican in selected crop and weed species 315

TABLE 5 (a) Surface Tensions of Diflufenican Form- ulation and Surfactants

Formulation" Surjiace tensionb (dyne em-')

Water DFF (FR1078/3) DFF + 'Tween 20' DFF+'Tween 61' DFF + NPE 6.5 DFF + NPE 8 DFF + NPE 10 DFF + NPE 12 DFF + NPE 14

71.5 52.6 37.9 32.0 30.9

a31.5 a31.5 34.1 365

(b) Contact Angles on Parafilm and Cuticle Tissue

Formulation" Contact angle ("1

Parafilm Tissue

Water FR 1078/3 FR 1078/3 + 'Tween 2 0 FR1078/3 + 'Tween 61' FR1078/3 + 'Silwet L-77' FR 1078/3 + NPE 4 FR1078/3+NPE5.5 FR 1078/3 +NPE 6.5 FR 1078/3 + NPE 8 FR 1078/3 +NPE 9 FR1078/3+NPE 10 FR1078/3+NPE 12 FR 1078/3 + NPE 14

98 97 63 52

ND ND ND 34

ND ND 43 52 56

51 46 39 33 0

20 22 26 29 29 27 35 37

Effect of age of tissue on contact angle of water

Age of tissue Contact angleb ("1

Old Medium Young

36-0 a46.5 a51.5

"All surfactants at 1.0glitreC'. D test-within a column, figures with the same

letter prefix do not differ significantly at P = 005.

H. Knight, R . C. Kirkwood 316

(CMC) values. Draves wetting times for ‘Silwet L-77’, however, do indicate an increase in wetting between 1.0 and 10glitre-’.32

Surfactants increased the amount and duration of [ 14C]diflufenican uptake. In the absence of surfactants, uptake was mostly complete within 2 h after application, whereas the effects of NPE surfactants and ‘Silwet L-77’ were greatest at times later than 2 h after application. This may have been due to the hygroscopicity of these surfactants causing an increase in droplet drying time; alternatively it may indicate the involvement of other, slower, processes caused by the direct effect of surfactants on the plant tissues. Lownds et observed that the effect of a surfactant on penetration of NAA was most pronounced during the droplet drying phase but that it continued to influence uptake from the dried down deposit. ‘Silwet L-77’ was only effective when applied with diflufenican and not when applied as a pretreatment, suggesting that its effect is through increasing droplet drying time and/or surface coverage rather than by altering the plant surface or tissues.

The effects of NPE4 and NPE6.5 on the uptake of [14C]diflufenican were compared. NPE4 had the lowest HLB of any of the surfactants tested, causing the greatest reduction in surface tension and a slightly greater level of phytotoxicity, whereas NPE6.5 was the member of the series in which diflufenican was found to be most soluble (Rh6ne Poulenc, unpublished data). Combination of the two surfactants did not appear to optimise droplet spread or water solubility of diflufenican, as no increase in uptake was observed; in fact the combination was less effective than NPE 4 alone. This agrees with the results of Wyrill and B ~ r n s i d e ~ ~ who postulated that, in such circumstances, a surfactant mixture is produced, having properties intermediate between those of the two surfactants, as it has a mean ethoxylate chain length equal to the mean of the two chain lengths.

Overall, the effect of the surfactants tested was not always related to their effect on wettability; the greatest effects were usually observed at concentrations of loglitre-’, which is well above the CMC (which is usually in the range of 0.1-1.0glitre- 134) and therefore indicates the involvement of other processes. It seems possible that the visible damage observed in the case of the NPE surfactants may be related to increases in membrane permeability, allowing the entry of more diflufenican into the underlying cells. There are no indications from the present study that this is the case with ‘Silwet L-77’, although there is evidence to suggest that this surfactant does have such effects in other system^.^'

ACKNOWLEDGEMENTS

The authors express appreciation to Rh6ne-Poulenc for the studentship to H.K. and for the gift of [14C]diflufenican and other materials. Thanks are expressed to Dr T. L. Whateley (Department of Pharmacy, University of Strathclyde) and Dr A. H. Catchpole (Rh6ne-Poulenc) for helpful discussions and to Mrs I. M. McKay for technical assistance.

Action of diflufenican in selected crop and weed species 317

REFERENCES

1. Cramp, M. C., Gilmour, J., Hatton, L. R., Hewett, R. H., Nolan, C. J. & Parnell, E.

2. Wightman, P. & Haynes, C., Brit. Crop Protect. Con$ - Weeds, 1 (1985) 171-8. 3. Cramp, M. C., Gilmour, J., Hatton, L. R., Hewett, R. H., Nolan, C. J. & Parnell, E.

4. Richardson, W. G. & West, T. M., Technical Report, Agricultural and Food Research

5. Wightman, P. S., PhD Thesis, University of Strathclyde, Glasgow, UK (1988). 6. Britton, G., Barry, P. & Young, A. J., Brit. Crop Protect. Confi ~ Weeds, 3 (1987)

7. Babiker, A. G. T. & Duncan, H. J., Weed Res., 15 (1975) 123-7. 8. Lownds, N. K., Leon, J. M. & Bukovac, M. J., J. Amer. Soc. Hort. Sci., 112 (1975)

9. Babiker, A. G. T. & Duncan, H. J., Pestic. Sci., 6 (1975) 655-64.

W., Brit. Crop Protect. Con$ - Weeds, l(1985) 23-8.

W., Pestic. Sci., 18 (1987) 15-28.

Council, Long Ashton Research Station, Weed Research Division, 89 (1985) 3&9.

1015-22.

55460.

10. O’Donovan, J. T., O’Sullivan, P. A. & Caldwell, C. D., Weed Res., 25 (1985) 81-6. 1 1 . Ayres, P., Weed Res., 27 (1987) 195-205. 12. Hull, H. M., Davis, D. G. & Stolzenberg, G. E. In Adjuvants for Herbicides, ed. R. H.

Hodgson, Weed Science Society of America Publications, Illinois, 1982, pp. 26-67. 13. Smith, L. W., Foy, C. L. & Bayer, D. E., Weed Res., 6 (1966) 23342. 14. Falk, R. H., Bayer, D. E. & Hess, F. D., Pestic Sci., 27 (1989) 243-51. 15. Morrison, I. N. & Cohen, A. S. In The Handbook of Environmental Chemistry, Vol.

2/Part A, ed. 0. Hutzinger. Springer-Verlag, Berlin, Heidelberg, 1980, pp. 193-219. 16. Furmidge, C. G. L., J. Sci. Food Agric., 10 (1959) 27482. 17. Attwood, D. & Florence, A. T., Surjactant Systems: Their Chemistry, Pharmacy and

18. Stevens, P. J. G. & Bukovac, M. J., Pestic. Sci., 20 (1987) 19-35. 19. Lownds, N. K. & Bukovac, M. J., J . Amer. SOC. Hort. Sci., 113 (1988) 205-10. 20. Cook, G. T., Stephen, N. H. & Duncan, H. J., Proc. Roy. Soc. Edinb. B., 81B (1982)97-109. 21. Schonherr, J. & Bauer, H. 2nd International Symposium on Adjuvants for Agrochemicals.

22. Burri, J. & Hartland, S., Coll. Polymer. Sci., 255 (1977) 675-81. 23. Snedecor, G. W. & Cochran, W. G., Statistical Methods. 6th edn. Iowa State University

24. Duncan, D. B., Biometries., 11 (1965) 1 4 2 . 25. Whitehouse, P. & Harries, W. N. Proc. EWRS Symp. Factors affecting Herbicidal

Activity and Selectivity, 1988, pp. 23944. 26. Kirkwood, R. C., McKay, I. & Livingstone, R. In The PIant Cuticle, ed. D. F. Cutler,

K. L. Alvin & C. E. Price. Academic Press, New York, 1982, pp. 253-66. 27. Kerler, F. &. Schonherr, J., Arch. Environ. Contam. Toxicol., 17 (1988) 1-6. 28. Chamberlain, K., Briggs, G. G., Bromilow, R. H., Evans, A. A. & Fang, C. Q., Asp.

29. Rentschler, I., Planta, 96 (1971) 119-35. 30. Coupland, D., Zabkiewicz, J. A. & Ede, F. J., Ann. Appl. Biol., in press. 31. Stevens, P. J . G. & Zabkiewicz, J. A. In Proc. EWRS Symp. Factors affecting Herbicidal

32. Anonymous, Union Carbide Corporation, 1988. 33. Wyrill, J . B., 111 & Burnside, 0. C., Weed Sci., 25 (1977) 275-87. 34. Parr, J. F. & Norman, A. G., Botan. Gaz., 126 (1965) 86-96. 35. Gaskin, R. E. & Kirkwood, R. C. In Adjuvants and Agrochemicals Vol. 1 . Mode of

Action and Physiological Activity, ed. P. N . P. Chow, C. A. Grant, A. M. Hinshalwood & E. Simunsson. CRC Press, Boca Raton, 1989, 129-39.

Biology. Chapman and Hall, New York, 1983.

Blacksburg, Virginia (in press).

Press, Iowa, 1967.

Appl. Biol., 14 (1987) 293-304.

Activity and Selectivity, 1988, pp. 145-50.