Pesticide Formulations - ACS Publications - American Chemical Society

Chapter 2

Factors Affecting Foliar Penetration and Translocation of Pesticides

Downloaded by KTH ROYAL INST OF TECHNOLOGY on January 24, 2016 | http://pubs.acs.org Publication Date: June 24, 1988 | doi: 10.1021/bk-1988-0371.ch002

E. A. Baker and Grace M. Hunt Department of Agricultural Sciences, University of Bristol, Agricultural and Food Research Council, Institute of Arable Crops, Long Ashton Research Station, Long Ashton, Bristol, BS18 9AF, England

Foliar uptake of 12 radio-labelled agrochemicals, following microsprayer application, has been measured in maize, rape, strawberry, and sugar beet with and without a non-ionic surfactant. Uptake rates during the first 24 h after application were up to 50 times greater than those during the second and third 24 h periods and they were always greater in waxy (strawberry and rape) than non-waxy (sugar beet) leaves. Highest uptake rates were measured for highly lipophilic compounds [log P (partition coefficient) >3] applied with surfactant but rates of translocation within the treated leaves for these compounds were low. Lowest uptake rates were found for water soluble compounds. Greatest rates of uptake and translocation were observed for compounds of median lipophilicity [log P 1-2; log S (molar water solubility) -1.5 - -3.5] both with and without surfactant. Addition of surfactant enhanced uptake of active ingredient up to 27-fold but had little effect on translocation. For compounds of similar water solubility, log (% uptake) increased linearly with log P reaching a maximum at log P ~ 1 in strawberry, sugar beet and rape leaves. For compounds of similar log P, log (% uptake) into rape, strawberry and sugar beet in the presence of surfactant was linearly correlated with log S. Interest i n the e f f i c i e n t delivery of f o l i a r applied pesticides as a means of optimising b i o l o g i c a l performance has led to considerable advances i n spray technology with major improvements i n product formulation [1] and the development of new types of a p p l i c a t i o n equipment. Thus, sprays, formulated as emulsion concentrates, suspension concentrates, wettable powders, water d i s p e r s i b l e granules and o i l or water solutions, can be applied to leaves i n widely d i f f e r i n g volume rates (1-500 1 ha" ), droplet sizes (100-500 um) and a p p l i c a t i o n rates (0.05 - 10 Kg ha" ) [2J. However, r e l a t i v e l y l i t t l e attention has been given to systematic studies of the factors c o n t r o l l i n g the r e d i s t r i b u t i o n of pesticides and spray formulants into and through the outermost layers of the leaf and i n consequence these complex interactions are poorly understood. 1

0097-6156/88/0371-0008$06.00A) ° 1988 American Chemical Society

In Pesticide Formulations; Cross, Barrington, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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Foliar Penetration and Translocation

9

Published reports of pesticide uptake patterns have yielded c o n f l i c t i n g r e s u l t s , several authors [3-5] demonstrating r e l a t i o n s h i p s with l i p o p h i l i c ! t y and water s o l u b i l i t y of the active ingredient ( a . i . ) , others [6] f i n d i n g weak correlations with molecular properties. The l a t t e r may have resulted from the high v a r i a b i l i t y i n the c u t i c u l a r properties of field-grown plants. The factors governing the uptake of surfactants and other adjuvants seem equally complex involving multiple rate-determining processes [7]. Although r e l a t i v e l y l i t t l e of the parent surfactant i s translocated from the o r i g i n a l s i t e of a p p l i c a t i o n [8] the formation of metabolites by c e r t a i n plant species within a few hours of l e a f treatment [9] indicates that many adjuvants d i f f u s e r a p i d l y across the cuticle. Interpretation of the uptake patterns of f o l i a r applied pesticides i s often d i f f i c u l t due to the close c o r r e l a t i o n between p a r t i t i o n c o e f f i c i e n t (P) and water s o l u b i l i t y (S). In an attempt to resolve t h i s d i f f i c u l t y we have i d e n t i f i e d 26 chemicals ( i n c l u d i n g 22 pesticides) which can be arranged into 8 groups i n which one of these properties i s e s s e n t i a l l y held constant whilst the other varies over 2-3 orders of magnitude. In this paper we present an i n i t i a l report of the uptake and translocation p r o f i l e s of a representative range of these chemicals applied to the leaves of four plant species with widely d i f f e r i n g c u t i c u l a r properties [4]. We report the e f f e c t s of addition of a non-ionic surfactant on these p r o f i l e s and demonstrate correlations between uptake and the physicochemical properties of the a c t i v e ingredient. Materials and Methods Plant materials Plants of Zea mays cv. LG 11, rape Brassica napus cv. R a f a l , sugar beet Beta v u l g a r i s cv. Nomo and strawberry Fragaria ananassa cv. Red Gauntlet were grown i n pots (10 cm) of John Innes compost maintained i n c o n t r o l l e d environment cabinets. Environmental conditions during the day (16 h) were 20°C, rh 70%, radiant energy rate: 300 umol m" sec" and night: 16°C, rh 80%. Spray A p p l i c a t i o n and Analysis Sprays containing C-radio-labelled tracers were prepared as solutions (1 g 1" ) i n aqueous methanol ( l : l v / v ) with and without Ethylan TU (NP8) surfactant (nonylphenol ethoxylate; average 8 moles ethylene oxide). The l a b e l l e d tracers were r i n g l a b e l l e d with the exception of chlortoluron C=0, isoproturon C H . ( C H ) and NAA CH .C00H. For a preliminary formulation study, suspension concentrates (s.c.) were prepared by blending 2 mg of the C-labelled chemical with 2% NP8 surfactant (100 u l ) using a 1 ml glass tissue grinder. The s.c. ( p a r t i c l e s i z e 2-5 urn) was d i l u t e d with water (1.9 ml) immediately p r i o r to a p p l i c a t i o n to leaves of the four plant species. The use of the microsprayer [10] which dispensed monosized droplets(330-350 um diam.) at controlled rates (5-10 droplets sec" ) and v e l o c i t y (3-4 m sec" ) ensured that the leaf coverage, droplet spreading and droplet drying rates were comparable to those of f i e l d applied sprays. A water soluble tracer Uvitex 2B incorporated at low concentration (0.5 g 1" ) provided immediate confirmation that droplets had been applied uniformly to the target area [for rape, /

14

14

3

2

2

* aqueous methanol (7:3 v/v) used for l i p o p h i l i c compounds with log S 98%) was recovered from the target area using 1 ml aqueous methanol (1:1 v/v) followed by 1 ml methanol. Accordingly, these solvents were used for the recovery of surface residues from a d i s c (2.4 cm diam.) including the target area ( f o r maize a 24 x 8 mm segment), removed from the leaf 24, 48 and 72 h a f t e r droplet a p p l i c a t i o n . After removal of the surface deposits, compounds retained i n the e p i c u t i c u l a r wax layer were recovered by washing the d i s c with chloroform (1 ml). The annulus of tissue (1 cm wide) surrounding the target area was also removed for radio-analysis from rape, strawberry and sugar beet leaves ( f o r maize the area included a 2 cm length of leaf above, below and adjacent to the treated surface). R a d i o a c t i v i t y i n the surface wash, wax extract, washed treated tissue and the area surrounding the treated tissue were determined by radiocombustion analysis and l i q u i d s c i n t i l l a t i o n counting [4]. Each experiment was replicated 4 times. Uptake i s defined as the proportion of applied chemical not recovered i n the methanol wash. Translocation i s defined as the proportion of applied chemical recovered i n the area surrounding the treated tissue. In separate experiments, compounds which showed s i g n i f i c a n t rates of uptake were applied to leaves, and l a b e l l e d products were recovered i n a methanol surface wash and methanol homogenate 72 h a f t e r a p p l i c a t i o n . These extracts were analysed by radio t . l . c . using a Raytest R i t a 68000 analyser• Published octanol/water p a r t i t i o n c o e f f i c e i n t s were confirmed by the shake f l a s k method and by HPLC using aqueous methanol (4:1 v/v) as mobile solvent and a C^g reverse phase column [11]. Water s o l u b i l i t i e s are expressed i n molar units (Table 1). Procedures used to study dried spray deposits by fluorescence microscopy, scanning electron microscopy (SEM) and autoradiography have been described i n d e t a i l elsewhere [12].


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Results


Preliminary Formulation Studies For pesticide evaluation, chemicals are often applied as solutions i n aqueous methanol or acetone but i t has been suggested that the use of these solvents may affect uptake behaviour. Therefore, at the s t a r t of t h i s i n v e s t i g a t i o n ve compared uptake rates of two compounds naphthalene a c e t i c acid (NAA) and atrazine, which have markedly d i f f e r e n t water s o l u b i l i t i e s , formulated with surfactant both as an s.c. and a s o l u t i o n i n aqueous methanol. Uptake of the two chemicals into rape leaves over a 72 h post-treatment period followed an almost i d e n t i c a l pattern (Table 2). Spread factors and i n t e r f a c i a l areas f o r both formulations were also s i m i l a r , with a . i . and surfactant being l a r g e l y concentrated within a broad annulus at the periphery of the dried deposit. The uptake patterns were s i m i l a r also when the two formulations were applied to maize and strawberry leaves. Uptake of e i t h e r chemical into sugar beet leaves was consistently greater from aqueous methanol s o l u t i o n . Since the droplets dried within 20-30 seconds following application t h i s increase can be attributed to differences i n the spreading properties of the formulations rather than to solvent-mediated c u t i c u l a r penetration. Aqueous methanol was used therefore, to apply compounds i n solution both with and without surfactant. In separate experiments to determine the effect of increasing surfactant concentration on uptake no increase i n NAA uptake into rape or sugar beet leaves was observed at concentrations above 1 g 1" (Table 3). On the other hand, droplet shatter and reduced uptake were noted f o r rape leaves using the lower concentration (0.3 g 1" ). 14

Table 2 Uptake {% applied C ) of atrazine and NAA, formulated as solutions and suspension concentrates, into rape and sugar beet leaves (means and s.e. of means) Species and chemical

Time a f t e r application (h) 24 48

72

Rape Atrazine solution " suspension' NAA solution " suspension*

34 35 94 79

(2.6) (0.9) (0.8) (4.5)

44 38 95 73

(5.6) (5.0) (1.6) (3.1)

41 49 96 86

(4.0) (4.2) (0.5) (1.1)

Sugar beet Atrazine solution " suspension' NAA solution " suspension*

25 U 31 29

(4.6) (1.3) (3.0) (4.1)

27 13 52 36

(1.1) (0.9) (5.7) (7.0)

27 15 53 45

(1.0) (1.3) (1.0) (4.5)

water s o l u b i l i t y 30 mg l"

1

* water s o l u b i l i t y 420 mg 1


12

PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS


Table 3 E f f e c t of surfactant concentration on the uptake (% applied C) of NAA into rape and sugar beet leaves (means and s.e. of means) Species

Surfactant concentration (g 1"- ) 3 0.3 1

Rape Sugar beet

60*(10.3) 25 (6.8)

1

94 (0.8) 22 (2.8)

93 (3.8) 25 (2.6)

* some droplet shatter Factors A f f e c t i n g F o l i a r Uptake V a r i a b i l i t y between species The uptake patterns shown i n Figure 1 f o r the polar maleic hydrazide and l i p o p h i l i c diclofop-methyl, applied to leaves i n the presence of surfactant, were t y p i c a l of the species-to-species v a r i a t i o n found f o r most compounds tested. Uptake rates f o r a l l compounds were consistently greater f o r the waxy leaves of strawberry and rape than f o r maize and sugar beet. These species differences were evident even though uptake rates d i f f e r e d as much as 20-fold between the d i f f e r e n t chemicals. The o v e r a l l patterns were

100

80

I 5"

60

1

*

maize

•

rape

A

strawberry

o

sugar beet

1« 20

0 24

48

72

Time after application (h.)

Figure 1 Uptake of maleic hydrazide ( ) and diclofop-methyl ( — — ) into leaves of selected species. V e r t i c a l bars are s.e. of treatment means.


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BAKER AND HUNT

13



also s i m i l a r , with uptake rate d e c l i n i n g r a p i d l y within 24 h following droplet a p p l i c a t i o n . For instance, the uptake of maleic hydrazide i n t o strawberry leaves was completed within 24 h of a p p l i c a t i o n whilst the average uptake rate f o r diclofop-methyl during t h i s i n i t i a l period was 6- and 30-fold greater respectively than during each of the succeeding 24 h periods. One of the few exceptions to this uptake pattern was found with diclofop-methyl uptake into maize leaves, i n which average rates during successive 24 h periods were 1.2, 1.8 and 0.5% applied dose/h. Effect of Surfactant The uptake p r o f i l e s with and without surfactant measured 72 h a f t e r droplet a p p l i c a t i o n f o r compounds ranging from polar maleic hydrazide to the highly l i p o p h i l i c b i t e r t a n o l (Table 1) are shown i n Figure 2a and b. Uptake was enhanced i n most cases by the addition of surfactant although there was some v a r i a t i o n with the physico-chemical properties of the compound. For example, uptake was generally very low (4-11%) for water soluble chemicals ( l o g P 94% of the variance i n the data f o r these species, equations for the l i n e s being: 2

log (% uptake) = 1.7 + 0.6 l o g P - 0.26 l o g P (rape) log (% uptake) = 1.8 + 0.51 l o g P - 0.28 l o g P (strawberry) and log (% uptake) = 1.5 + 0.6 l o g P - 0.30 l o g P (sugar beet) In the absence of surfactant uptake for these three species increased l i n e a r l y with log P f o r the four compounds examined; a quadratic curve accounted f o r

Pesticide Formulations - ACS Publications - American Chemical Society

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