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Weed Res. (1966) 6, 233-242.

STRUCTURE-ACTIVITY RELATIONSHIPS OF ­ PHENOL OXIDE NON-IONIC SURFACTANTS AND THREE WATER-SOLUBLE HERBICIDES

L. W. SMITH, C. L. FOY AND D. E. BAYER

Department of Botany, University of California, Davis, California, U.S.A.

Summary. Several alkylphenol ethylene oxide ether non-ionic surfactants were tested in aqueous foliar sprays with dalapon, amitrole and paraquat for their enhancement of phytocidal activity against ,Zea mays L. With three of surfactants studied (octyl-, nonyl- and laurylphenol types), the herbicide, the surfactant concentration and the hydrophilic constitution (ethylene oxide content) of the surfactant molecule all markedly influenced maximum toxicity. Smaller apparent differences in effectiveness were also attributable to the hydrophobic (alkylphenol) portion of the surfactant. The results are discussed in relation to possible cuticle-spray solution interactions and their influence on herbicide penetration. Relations entre la structure et l'activite de produits tensio-actifs non ioniques, a base d'ether d'oxyde ethylenique et d' alkylphinol, en presence de trois herbicides hydrosolubles Resume. Plusieurs produits tensio-actifs non ioniques, a base d'ether d'oxyde ethylenique et d'alkylphenol, ont ete essayes en pulverisation foliaires avec du dalapon, de l'amitrole et du paraquat, en vue de renforcer leur activite phytocide sur Zea mays L. Avec Jes trois series homologues de produits tensio-actifs etudies (types octyl-, nonyl- et laurylphenol), !'herbicide, la concentration du produit tensio-actif et la partie hydrophile (oxyde ethylen­ ique) de la molecule du produit tensio-actif, influencerent la toxicite maximum de fai;on marquee. De plus faibles differences clans l'efficacite apparente etaient egalement attribuables a la partie hydrophobe de la molecule (alkylphenol) du produit tensio-actif. Les resultats sont discutes en relation avec des interactions possibles entre la cuticule et la solution pulverisee, ainsi que leur influence sur la penetration de !'herbicide. Beziehungen der Strukturaktivitiit nichtionischer oberjliichenaktiver Stoffe vom Typ Alkylphenol- iithylen-oxyd-iither und drei wasserliislichen Herbiziden Zusammenfassung. Verschiedene nichtionische oberflachenaktive Stoffe auf der Basis von Alkylphenol-athylen-oxyd-ather wurden in wassriger Liisung mit Dalapon, Amitrol und Paraquat auf ihre Phytotoxizitat gegen ,Zea mays L. bei Blattanwendung geprtift. Bei der Untersuchung von drei homologen Serien oberflachenaktiver Stoffe (Octyl-, Nonyl- und Laurylphenol-Typen), wurde die maximale Phytotoxizitat stark durcJ?. die Konzentration des oberflachenaktiven Stoffes und die hydrophyle Konstitution (Athy­ lenoxyd-Gehalt) des oberflachenaktiven Molektils beeinflusst. Weniger augenscheinliche Unterschiede in der Wirkungsstiirke waren ausserdem auf den hydrophoben Anteil (Alkylphenol) des oberflachenaktiven Stoffes zurtickzuftihren. Miigliche Interaktionen zwischen Kutikula und Spritzfltissigkeit sowie deren Einfluss auf das Eindringen von Herbiziden wird an Hand der Ergebnisse besprochen.

INTRODUCTION Many workers over the past 20 years have shown that the toxicity of foliar­ applied herbicides is enhanced by the addition of surfactants to the spray solution (Zimmerman & Hitchcock, 1942; Blackman, 1950; Freed & Mont­ gomery, 1958; Leonard, 1958; Jansen, Gentner & Shaw, 1961; McWhorter, 233 234 L. W. Smith, C. L. Foy and D. E. Bayer 1963). Such additives have been used for many years in aqueous agricultural sprays for improving formulative properties and for their ability to wet plant surfaces. However, physical measurements such as contact angle, spreading coefficient, surface tension and interfacial tension of spray solutions and visual observations on ability to wet plant surfaces have shown little correlation with the enhancement of herbicidal activity. It is generally conceded that more subtle interactions and effects are involved (Freed & Montgomery, 1958; Jansen et al., 1961; Foy & Smith, 1965). There have been several suggestions in the literature that the interactions between surfactant, herbicide and plant surface are of more importance than the surface tension lowering or wetting ability of the herbicide-surfactant solu­ tion (Freed & Montgomery, 1958; Jansen et al., 1961; Foy & Smith, 1965). Very little is known about the mode of action or structure-activity relation­ ships of surfactants in herbicidal solutions. Furmidge ( 1959a, b) working with several series of surfactants concluded that, within a series of a similar chemical structure, the phytotoxicity of the surfactant was governed by the size of the molecule. The smaller molecules were more phytotoxic. Jansen ( 1964) was able to show that differences in the phytotoxicity of herbicide-surfactant mixtures were associated with variations in both the hydro­ philic and hydrophobic portions of the surfactant molecule. Maximal toxicity within a homologous group also varied with the herbicide and the plant species used. Nonionic surfactants of the type used in this study (alkylphenol-polyoxy­ ethylene glycol ) consist of an alkylphenol hydrophobe and an ethylene oxide (EO) hydrophile. The number of EO moieties in the molecule can be varied to give different properties to the surfactant which include modifications in solubility, molecular weight, surface tension and 'hydrophilic-lipophilic balance' (H.L.B.). · The reaction products formed during the manufacture of such materials are not pure compounds and the number of moles ofEO per molecule varies around a Poisson distribution. The average number is used to denote the EO content of the surfactant. The main purpose of this study was to evaluate a series of herbicide solutions containing alkylphenoloxyethylene glycol ether surfactants that varied in the average number of moles of EO and the length of the alkyl chain, thus allowing a study to be made of the influence of the EO content, the length of the hydro­ chain and other physical-chemical properties of these surfactants on the activity of certain herbicide solutions.

METHODS AND MATERIALS Suifactants and herbicides studied . A series oflaurylphenol, nonylphenol and octylphenol polyoxyethylene glycol ether type surfactants of the general structure shown in Fig. 1 were obtained to carry out these experiments. Three foliar-applied, water-soluble herbicides, dalapon (2,2-dichloropro­ pionic acid), amitrole (3-amino-1,2,4-triazole) and paraquat (1,1 '-dimethyl- Suifactants and Herbicides 235 4,4'-bipyridylium dichloride) were used to evaluate the surfactants. Previous workers have shown them to respond to the addition of surfactants (Jansen et al., 1961; Foy & Smith, 1965). The average content of the EO in the surfactant molecule varied from 6, 9, 12 to 15 moles for the laurylphenols (products 12-P-6, 12-P-9, 12-P-12 and 12-P-15, respectively); 4, 7, 9-10, 10·5, 13, 20 and 40 moles for the nonylphenols (NP-14, NP-27, N-101, NPX, NP-33, NP-40 and NP-44, respectively); 1, 5, 7-8, 9-10, 13, 16, 20 and 30 moles for the octylphenols (X-15, X-45, X-114, X-100, X-102,

C,,H,.,,Oo CCH 2CH2o>, H

Fm. 1. General structure of an alkylphenolpolyoxyethylene glycol ether surfactant.

X-165, X-205 and X-305, respectively). In all trials, the herbicides were used at rates which had been determined previously to give a 50 % reduction in fresh weight of corn when mixed with 1 g/1 of Tween 20 surfactant (containing polyoxyethylene sorbitan monolaurate). These rates were dalapon 10 lb/ac, amitrole 5 lb/ac, and paraquat 1/64 lb/ac. The first series of tests was carried out to compare 5 g/1 and 0·005 M solutions of surfactants alone and in combination with the herbicides. In the second series, the surfactants were used at concentrations of 1 and 10 g/1 which correspond to 0 ·33 lb/ac and 3 ·3 lb/ac, respectively. A third series of tests compared the effect of surfactant concentration of three nonylphenol surfactants on the toxicity of each of the herbicides.

Biological evaluation of suifactant-herbicide solutions Corn (Zea mays L., var. De Kalb 414) was grown under standard glasshouse conditions in 4-in. diameter pots. The original seeding rate was six seeds per pot, and thinning of plants to four per pot was carried out 10-12 days after planting. Growth was maintained at a vigorous level by fertilizing with nutrient solution (Hoagland & Amon, 1950) and a nitrogen-phosphorus fertilizer. Before spraying, the plants were ranked for uniform growth according to replica­ tion; thus a uniform plant population was provided for each replication. The corn was usually between 35 and 40 cm tall at this time. The herbicidal solutions were applied using an experimental spraying table as described by Shaw & Swanson ( 1952). All treatments were applied in water in a total volume of 40 gal/ac. The soil of the pots was covered with vermiculite which was removed after the spray deposit had dried. This procedure prevented any herbicide or surfactant from reaching the soil. Growth reduction and toxic effects are thus solely attributable to foliar absorption. Plant response or toxicity of the herbicide-surfactant solutions was assessed " by visual observations of symptoms and by weighing the fresh green growth of the above-ground portions of the plant. The phytotoxicity of the surfactant­ herbicide mixtures is reported in terms of a toxicity index value, which was calculated by expressing the fresh weight of the plant as a percentage of the un­ treated control and subtracting this value from 100. B* 236 L. W. Smith, C. L. Foy and D. E. Bayer RESULTS Fig. 2 shows the effect on herbicide toxicity of the number of moles of EO when the surfactants were added at an equimolar rate of O·005 M to the various herbicides. Since there were only minor differences among the octyl-, nonyl­ and laurylphenol hydrophobes of the surfactants in these trials, the scatter area of points has been delimited by two dotted lines to represent the general effects of the surfactants. However, the actual points for each type of surfactant are shown. The results were similar with the three herbicides, although the peak of maximum activity was reached sooner with amitrole and dalapon than with paraquat. If EO is plotted as a percentage of the total molecular weight of the surfactant molecule, similar graphs to those shown in Fig. 2 are obtained. Fig. 3 shows the results of adding the octyl- and nonylphenol surfactants at 1 and 10 g/1 to paraquat and dalapon; the results with amitrole were similar to those for dalapon, and are not included. For clarity, a single line has been fitted by eye to represent the effects of both the octylphenol and nonylphenol sur­ factants. From these results, the most marked differences occurred between the surfactant concentrations, the herbicides and the moles of EO contained in the surfactant molecule ..An interesting interaction which was noticed with the 1, 5 and 10 g/1 concentration series was that the number of moles of EO in the surfactant molecule correspo'nding to the maximal toxicity of the herbicide decreased as the surfactant concentration was increased. For paraquat, the maximal toxicity was reached when the surfactant contained 15-20 moles of EO for 1 g/1 solutions, 10-15 moles for 5 g/1 solutions and 7-10 moles for 10 g/1 solutions. This same effect was evident with the dalapon and amitrole-surfact­ ant solutions. In this case, I g/1 solutions showed maximum toxicity with 10-15 moles of EO, 5 g/1 solutions with 5-10 moles and 10 g/1 solutions with 1-5 moles. Despite the generalization made in Fig. 2, where the effects of the octyl-, nonyl- and laurylphenol hydrophobes were all represented by a single area, there was nevertheless an indication that the hydrophobic portion of the molecule also influenced the maximum toxicity of the herbicide-surfactant mixtures, at least slightly. The hydrophobe effect was never as pronounced as the concentra­ tion effect noted earlier. However, it did appear that the octyl- and nonyl­ phenol hydrophobes gave their maximum enhancement of herbicidal activity at a lower EO content than did the laurylphenol hydrophobes. This subtle in­ fluence of the hydrophobe portion of the surfactant in enhancing herbicidal activity requires further study. The enhancement of herbicidal activity was very spectacular when surfact­ ants were added to paraquat solutions; there was much less overall enhancement with amitrole solutions, and an intermediate response was observed with dalapon. The results of the series of experiments with three nonylphenol surfactants comparing the effect of surfactant concentration on the toxicity of the herbicide spray are shown in Fig. 4. They show the typical dramatic increase in herbicide toxicity when the surfactant concentration is increased above I g/1. The maximum increase occurs between I and IO g/1; above these values the enhance­ ment effects diminish, probably due partly to the phytotoxicity of the surfactant Surfactants and Herbicides 237 50 .. ···ti'""'•,······ • 50 D.. ... ········• ...... : 40 • .•. .... • 30 ' • q·

20 :• o: 10 ... . j (a) 0

50 ·········· D ... .. D ····· . 40 • • ..>< • 'C ' C • ... ··•··· 30 ... .. ········· >, ····· ···'··· D .•· .... :~ o: >< ... ,2 20 ; 10 • ( b) 0 70 ...... • a .•.. 50 • D • • ..... , ...... 50 C!·" ...... 40 o; • 30 ..' (c) 20 5 10 15 20 25 30 35 40 Moles of ethylene oxide Fm. 2. Relationship between the number of moles of ethylene oxide in octylphenol (•), nonylphenol ( •) or laurylphenol (o) polyoxyethylene glycol ether surfactant molecules and the toxicity index of these surfactants in mixtures with (a) paraquat, (b) dalapon, and (c) amitrole on corn plants. Herbicides applied at 1/64, 10 and 5 lb/ac, respectively; surfactant concentration was 0·005 Min all cases. Toxicity index calculated by expressing fresh weight for each treatment as percentage of untreated control and subtracting this value from 100.

and partly because the maximum potential for enhancement has been reached. The surfactants with the lower EO contents are the most phytotoxic when used alone, but phytotoxicity was not usually noticed until the EO content dropped below 10 moles and then only at concentrations of 10 and 50 g/1. No phyto­ toxicity was evident with any surfactant when used at 0·005 M or 5 g/1 and 238 L. W. Smith, C. L. Foy and D. E. Bayer surfactant NP-44 (40 moles EO) showed no phytotoxicity even when used at 50 g/1. The results of plotting herbicide toxicity versus the molar concentration of the surfactant are also shown in Fig. 4. These results show that at low molar concentrations NP-44, or molecules with a high EO content, are more efficient for herbicide enhancement but as the concentration or number of molecules present is increased, the surfactant molecules with lower EO content become more efficient at increasing herbicide penetration. These results parallel the observations already made from Fig. 3.

100 (a) (b)

80 )( xo x·.x~a .. 0 ---x C 0~ " 60 '?:' • u / )( 40 __.:1o·--~-----·····.···········- ...... 0 . .·: ·- ... 20 ..·· .. ···...... 0 • 5 10 15 20 25 30 5 10 15 20 25 30 Moles of et hy Lene oxide Fm. 3. Relationship between the number of moles of ethylene oxide in octyl-, or nonyl­ phenol polyoxyethylene glycol ether surfactant molecules and the toxicity index of these surfactants in mixtures with (a) paraquat and (b) dalapon on corn plants. Herbicides applied at 1/64 and 10 lb/ac, respectively; surfactant concentrations were l ·O and 10·0 g/1. Toxicity index as defined for Fig. 2 and in the text. e, 0·1% nonylphenol; •, 0·1% octylphenol; D, 1·0% octylphenol; x, 1·0% nonylphenol.

DISCUSSION The results obtained in these experiments confirm, in general, the previous work of Furmidge (1959a) and Jansen (1964) who showed that the phyto­ toxicity of surfactant solutions, whether used alone or in mixtures with herbi­ cides, varies as structural changes are made in the hydrophilic and hydrophobic portions of the surfactant molecule. It would appear that with the alkylphenol­ type ethylene oxide condensate surfactants tested, the hydrophilic portion of the molecule played a more significant role than the hydrophobic part in determining enhancement of phytotoxicity. However, one must consider that only a range of alkyl chains of 8-12 carbon atoms in length was tested against 1-40 moles of ethylene oxide. The results also show that the maximum toxicity of these herbicide-surfactant mixtures can be influenced by the herbicide used and the concentration of the surfactant. Thus, within these three homologous series of surfactants, the herbicide, the surfactant concentration and the hydrophilic portion or EO content all influenced markedly the maximum toxicity of the herbicide. The EO content of the surfactant molecule giving maximum toxicity to the three herbicides occurs in the range 10-20 moles when the surfactant is mixed Suifactants and Herbicides 239

(a) 40 50

40 30 30 20 20

10 10 ,,

0 0

,\ ,,.~ (bl ". : ... I" so _-..:.-- 40 I X I "Cl"' I 40 .!: I >,30 I I u 30 I )( ~ 20 I I 20 I I 10 10

0 0

( C) ,, BO ,.,, ,,.~ .. - 90 I ,/.· .,._,.... - /; BO ,, ...... 70 ,. .. ,, ,, . ,. I i 70 I ., I 60 ,1 .. ··, 60 .··,, ,,,,." 50 .....: .· ,,,,, ,, ..- __ 50 ,.

001 0·1 1·0 10 100 0·001 0·01 0·1 g/l moles/l Surfactant concentration (log scale) Fm. 4. Effect of concentration (left, g/1; right, moles/1) of three nonylphenol polyoxy­ ethylene glycol ether surfactants on the toxicity of (a) paraquat, (b) dalapon, and (c) amitrole solutions sprayed on corn plants. Herbicides applied at 1/64, 10 and 5 lb/ac, respectively. Surfactants NP-14 (· · · ·), NP-33 (- -- -) and NP--44 (-) contained 4, 13 and 40 moles of ethylene oxide per molecule, respectively. Toxicity index as defined for Fig. 2 and in the text.

at a concentration of 0·005 M (Fig. 2). The same result occurs when the sur­ factants are mixed at 1 g/1 but at 10 g/1 the number of moles of EO giving maximum toxicity is shifted to the left into the lower EO content range. At these high concentrations of 1-10 g/1, the surfactant molecules exist in 240 L. W. Smith, C. L. Foy and D. E. Bayer solution as micellar complexes. Micelles are colloidal aggregates that form by the association of surfactant molecules or ions into distinct units within the solution. The formation of these micelles with regard to number and size is dependent upon the length of the chain and the presence of aromatic rings in the surfactant molecule as well as the presence of other ions and solutes in the solution (Durham, 1961). Probably there is also minor dependence of micelle size on surfactant concentration; however the impor­ tance of this factor is unknown. Shinoda, Nakagawa, Tamamushi & Isemura (1963) also showed that the smaller molecular weight surfactants (low in EO) have a higher micelle weight than the higher molecular weight compounds (high in EO). It has also been shown that the molecules or ions of other solutes present in the surfactant solu­ tion can be adsorbed or incorporated into a micelle in several different ways. If they are non-polar or lipophilic, they may be incorporated into the centre portion of the micelle (this incorporation is often called solubilization) which, in the case of non-ionic surfactants, consists of the hydrophobic portions of the surfactant. If the solutes are polar or water soluble, as with the herbicides studied, they may be adsorbed by ionic attractions or bonds. Also, they may be enclosed around the micelle by the long hydrophilic chains of the high molecular weight surfactants. Adsorption is probably the most logical process in this case and would result in an accumulation of herbicide molecules around the outside of the micelle. When a spray droplet falls on a leaf it will spread and wet the leaf surface to an extent depending on the leaf surface, the surface tension and interfacial tension of the spray solution. Evaporation of the water occurs and the concentra­ tions of the surfactant and herbicide increase until an equilibrium is reached between the air on the outside and the leaf surface on the inside of the spray deposit. Presumably there exists in this deposit a layer of material on the leaf surface which consists of herbicide, surfactant and some water molecules (probably in the minority). This situation, at first glance, does not appear to bear any relationship to the spray solution before impinging on a leaf surface. The leaf surface contains many imperfections, i.e. cracks, insect punctures and possibly hydrophilic and/or hydrophobic areas (perhaps not pores in the usual sense), through which transpiration water escapes and precursors move to the leaf surface. It seems reasonable that the surfactant molecules should diffuse from the liquid spray droplet into these areas along the surfaces of the lipophilic cuticle and cutin, the molecules perhaps aligning themselves in monolayers. If true, this would result in the lipophilic end being in or on the cuticle waxes and thus creating a hydrophilic layer or layers in these imperfec­ tions. Water molecules would then be attracted to these hydrophilic regions and channels would be formed, conceivably bringing about a slight swelling of the cuticle. These areas will be called 'hydrophilic channels'. As the herbicide molecules are highly water-soluble they would be free to diffuse through these 'hydrophilic channels' into the cell wall region of the plant cell and thence either through the ectoplast into the cytoplasm or via the apoplast into the transpiration stream. Surfactants. and Herbicides 241 The results of these experiments would indicate that a certain balance between hydrophilicity-lipophilicity and molecular size is necessary to give maximum enhancement to the herbicide which would parallel production of these 'hydrophilic channels'. Surfactant molecules with more than 20 EO moieties are apparently too large and unwieldy to form these channels as efficiently as those with 10~20 EO moieties. Surfactants with less than 5 EO moieties are apparently too lipophilic and the hydrophilic channels too small for the maximum diffusion of the herbicide. This may not explain satisfactorily the concentration effect except that more channels can be formed and maybe multilayers of surfactants are formed with larger channels. Dalapon and amitrole, being smaller molecules than paraquat, will diffuse faster through the smaller channels produced by the lower molecular weight surfactants. Because of the wide differences in ionic nature of the three herbicides (dalapon, anionic; amitrole, slightly cationic; paraquat, strongly cationic), however, specific charge effects may be equally or more jmportant. If one plots toxicity index versus molar concentration of the surfactant for each of the three herbicides, the graphs shown in Fig. 4 are obtained. These results show that at low molar concentrations NP-44 (40 moles EO) > NP-33 (13 moles EO) > NP-14 (4 moles EO) for enhancing the toxicity of the herbi­ cides, but as the concentration is increased NP-33 becomes the most effective and finally, at high concentration and with amitrole and dalapon, NP-14 gives the best results. These results correspond to Fig. 3 where it was shown that as the concentration of surfactant is increased, the EO content of the surfactant giving maximum toxicity is shifted from a higher to a lower value. Thus it appears that the actual number of surfactant molecules present cannot be cor­ related directly with increased toxicity. But since the surfactant in solution exists in micelles which contain a certain number of molecules of surfactant and since the lower molecular weight surfactants have both a larger micelle and more surfactant molecules per micelle (Shinoda et al., 1963) there will be less micelles in solution with a lower EO number than a high one. Another important factor appears to be the stability of the surfactant solution. It is readily observed that with the lower molecular weight surfactants, which are more hydrophobic than the higher molecular weight materials, as the con­ centration is raised the solution becomes more stable and will not partition. It would appear that at high surfactant concentrations (5 g/1 and above) interactions occur between the herbicide and the surfactant micelles, probably causing greater solubilization or incorporation of the herbicide within or adsorption upon the micelle. This solubilization or production of a better colloidal dispersion has been observed by McWhorter (1963) with diuron and in our experiments with dalapon and amitrole solutions when cloudy spray solutions clear appreciably on adding high concentrations of surfactants.

ACKNOWLEDGMENTS The work was supported in part by allotments under the Regional Research Fund Hatch Act (as amended August 11th, 1955), W-63 and W-77 Projects, 242 L. W. Smith, C. L. Foy and D. E. Bayer California Agricultural Experiment Station Projects 1874 and 2239, respectively. The surfactants and herbicides used in this study were generously provided by industry as follows: (a) nonyl- and laurylphenol polyoxyethylene glycol ether series-Union Carbide Corporation, New York, N.Y.; (b) octylphenol polyoxyethylene glycol ether series-Rohm and Haas Company, Philadelphia, Pennsylvania; (c) paraquat-Chevron Chemical Company (Ortho Division), Richmond, California; (d) dalapon-Dow Chemical Company, Midland, Michigan; and (e) amitrole-American Cyanamid Company, Princeton, New Jersey.

REFERENCES BLACKMAN, G. E. (1950) The principles of selective toxicity and the action of selective herbicides. Sci. Prag., Land., 150, 637-651. DURHAM, K. (1961) Surface Activiry and Detergency, pp. 37-50. Macmillan, London. FoY, C. L. & SMITH, L. W. ( 1965) Surface tension lowering, wettability of paraffin and corn leaf surfaces, and herbicidal enhancement of dalapon by seven surfactants. Weeds, 13, 15-18. FREED, V. H. & MONTGOMERY, M. ( 1958) The effects of surfactants on foliar absorption of 3-amino- 1,2,4-triazole. Weeds, 6, 386-389. FuRMIDGE, C. G. L. (1959a) Physico-chemical studies on agricultural sprays. Part II. J. Sci. Fd Agric., 10, 274-284. FuRMIDGE, C. G. L. (1959b) Physico-chemical studies on agricultural sprays. Part III. J. Sci. Fd Agric., 10, 419-425. HoAGLAND, D. R. & ARNON, D. I. (1950) The water culture method for growing plants without soil. Gire. Calif. agric. Exp. Stn No. 347. JANSEN, L. L. (1964) Relation of structure of ethylene oxide ether-type nonionic surfactants to herbicidal activity of water-soluble herbicides. J. agric. Fd Chem., 12, 223-227. JANSEN, L. L., GENTNER, W. A. & SHAW, W. C. (1961) Effects of surfactants on the herbicidal activity of several herbicides in aqueous spray systems. Weeds, 9, 381-405. LEONARD, 0. A. (1958) Studies on the absorption and translocation of2,4-D in bean plants. Hilgardia, 28, 115-160. McWHORTER, C. G. (1963) Effects of surfactants on the herbicidal activity of foliar sprays of diuron. Weeds, 11, 265-269. SHAW, W. C. & SWANSON, C. R. (1952) Techniques and equipment used in evaluating chemicals for their herbicidal properties. Weeds, 1, 352-365. SHINODA, K., NAKAGAWA, T., TAMAMUSHI, B. & lsEMURA, T. (1963) Colloidal Surfactants, pp. 97-178. Academic Press, New York. ZIMMERMAN, P. W. & HITCHCOCK, A. E. ( 1942) Substituted phenoxy and benzoic acid growth substances and the relation of structure to physiological activity. Contr. Boyce Thompson Inst. Pl. Res., 12, 321-343.

(Received 10th December 1965)