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Heterogeneous Photochemistry of Agrochemicals at Leaf Surface: a Case Study of Plant Activator Acibenzolar-S-methyl Mohamad Sleiman, Pascal de Sainte Claire, and Claire Richard J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b02622 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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Journal of Agricultural and Food Chemistry

Heterogeneous Photochemistry of Agrochemicals at Leaf Surface: a Case Study of Plant Activator Acibenzolar-S-methyl

M. Sleiman1,2*, P. De Sainte Claire1,2, C Richard1,2

1

Equipe Photochimie CNRS, UMR 6296, ICCF, 63178 Aubière, France.

2

Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-

Ferrand, F-63000 Clermont–Ferrand, France.

Corresponding author: M Sleiman Email: [email protected] Tel: +33 (0)4 73 40 76 35

ACS Paragon Plus Environment

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Abstract.

2

The photoreactivity of plant activator benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl

3

ester (BTH) commonly named acibenzolar-S-methyl was studied on surfaces of glass,

4

paraffinic wax films and apple leaves. Experiments were carried out in a solar simulator using

5

pure and formulated BTH (BION®). Surface photoproducts were identified using liquid

6

chromatography coupled with electrospray ionization and high resolution Orbitrap mass

7

spectrometer, while volatile photoproducts were characterized using an online thermal

8

desorption system coupled to gas chromatography-mass spectrometry (GC-MS). Pure BTH

9

degraded quickly on wax surfaces with a half-life of 5.0 ± 0.5 h, whereas photolysis of

10

formulated BTH was seven times slower (t1/2 = 36 ± 14 h). On the other hand, formulated

11

BTH was found to photolyze quickly on detached apple leaves with a half-life of 2.8 h ± 0.4

12

h. This drastic difference in photoreactivity was attributed to the nature and spreading of BTH

13

deposit, as influenced by surfactant and surface characteristics. Abiotic stress of irradiated

14

apple leaf was also shown to produce OH radicals which might contribute to the enhanced

15

photodegradability. Eight surface photoproducts were identified whereas GC-MS analyses

16

revealed the formation of gaseous dimethyl disulfide and methanthiol. The yield of dimethyl

17

disulfide ranged between 1.5% and 12%, and a significant fraction of dimethyl disulfide

18

produced was found to be absorbed by the leaf. This is the first study to report on formation

19

of volatile chemicals and OH radicals during agrochemical photolysis on plant surfaces. The

20

developed experimental approach can provide valuable insights on the heterogeneous

21

photoreactivity of sprayed agrochemicals and could help improve dissipation models.

Keywords: benzothiadiazole, pesticides, apple, half-life, volatile, dimethyldisulfide.


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Introduction

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With a projected increase in the world population from 7.4 billion in 2016 to 9.7 billion in

24

2050, the pressure to achieve high crop yields will be ever growing.1 Thus, the use of

25

pesticides will likely continue to rise, despite concerns over environmental pollution and

26

associated health risks. In recent years, technological advances in biochemical and molecular

27

biological tools have improved our understanding of plant-pathogen interactions.2-4 This led

28

to the discovery of promising alternatives, in particular a new generation of plant protection

29

products, termed activators or synthetic elicitors, as illustrated in Table 1.5 Exogenous

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application of elicitors could trigger systemic acquired resistance (SAR)-like responses that

31

mimic pathogen-induced resistance in plants. One of the most frequently used elicitors is

32

benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) commonly named as

33

acibenzolar S-methyl, which was introduced by Syngenta.6-7 BTH is ananalog of salicylic acid

34

that is naturally produced by plants.5 The product is commercialized in the United States as

35

Actigard® 50WG, in Europe as BION®, and in other parts of the world as Blockade® and

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Boost® to control downey mildew on vegetables and to manage a range of fungal, bacterial,

37

and viral diseases of crops such as tomato, cucumber, broccoli, tobacco, melon, apple and

38

pear trees.8-11 BTH and other synthetic elicitors are typically applied by foliar spray. Upon

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spraying on crops, the active ingredients remain at the surface of leaves as solid deposit after

40

water evaporation. If they absorb solar radiations, photolysis can take place and accelerate the

41

dissipation of active ingredients, reducing the efficiency of the treatment and increasing

42

production costs due to the need for repeated applications. Thus, it is essential to study the

43

photoreactivty of such chemicals under environmentally relevant conditions, as an important

44

step to evaluate their fate and efficacy in the field but also to provide valuable input for

45

improving risk and impact assessment models. 12-13

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Although extensive data exist on photolysis of agrochemicals in water and soil media, a very

48

limited number of studies have assessed their photoreactivity at the surface of plant leaf

49

models such as cuticular waxes.14-18 Using model systems such as isolated cuticles or thin

50

paraffinic wax films, it was reported that photolysis rates of some pesticides at the surface of

51

plant waxes can be significantly faster or slower compared with corresponding photolysis

52

half-lives in water and soil.15,

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particularly surfactants and photosensitizing herbicides can promote photodegradation of

54

active ingredients when used in mixture.16 These studies have highlighted the need for

55

systematic and realistic assessment of the photodegradation of agrochemicals on the surface

56

of plants.

57

In the present work, we investigated the effect of key factors that can influence the photolysis

58

rate of BTH such as the formulation, the nature of the surface and the deposit characteristics

59

(e.g. crystalline vs. amorphous, shape and dispersal). Three surfaces were selected for this

60

study: glass for its ease of use and hydrophilicity, paraffin wax as surrogate for epicuticular

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wax of plants and apple leaf as one of the target plants treated with BTH and also one of the

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most treated crops with agrochemicals. In addition, efforts were made to fully characterize

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surface and potential gaseous photoproducts by developing a small scale chamber coupled

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with mass spectrometry analyses. Our main objective was to better understand the roles that

65

surface and formulation play in the kinetic and pathways of photodegradation. The results

66

were validated by comparing model systems using wax films with experiments conducted on

67

detached apple leaves. Moreover, the role of reactive oxygen species (ROS) produced by

68

abiotic stress and their contribution was assessed. Finally, the environmental relevance and

69

implications of this heterogeneous photochemistry under realistic conditions are discussed.

18-19

It has also been shown that some formulation agents


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MATERIALS AND METHODS

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Chemicals. BTH (Pestanal, 99.9%), paraffin wax (mp 70-80°C), terephthalic acid (TA, 98%),

72

hydroxyterephthalic acid (TAOH, 97%), dimethylsulfide (98%) were purchased from Sigma-

73

Aldrich (St. Louis, Missouri, USA). BION® 50WG water-dispersible granular formulation

74

containing 50% of active ingredient in weight and an ionic surfactant (Sodium

75

dibutylnaphthalenesulphonate) was supplied by Syngenta (Stein, Switzerland). Methanol

76

(99%, HPLC grade), acetonitrile (99%, HPLC grade) were provided by Riedel de Haën

77

(Saint-Quentin Fallavier, France). Water was purified using a Millipore Milli-Q system

78

(Millipore αQ, Darmstadt, Germany, resistivity 18 MΩ cm, DOC < 0.1 mg L-1). All other

79

chemicals were of the highest grade available.

80

Deposit of BTH and BION®. BTH and BION® solutions were deposited using a micropipette

81

(Eppendorf®, Montesson, France) as fine droplets of 5 µL on the surface of glass petri dishes,

82

paraffin wax films and freshly cut apple leaves. The pure wax films were made by adding

83

directly 0.8 g of paraffin wax in polypropylene petri dishes and by heating it at 90° C to

84

achieve film formation according to the protocol previously described.18 BTH solution was

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prepared by dissolving pure BTH powder in acetonitrile at 0.14 g L-1. BION® suspension was

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diluted in acetonitrile to obtain a BTH final concentration of 0.29 g L-1. Deposit of BTH

87

solution was as follows: 144 droplets of 5 µl of a BTH solution were deposited on each glass

88

dish and wax film of 9 cm2 total surface. When BION® was used instead of pure BTH, 340 µl

89

of BION® suspension were deposited as droplets of 5 µL to obtain identical BTH total amount

90

per dish (100 µg). Three dishes were used as replicates for each experiment. Samples were

91

then allowed to dry overnight at room temperature in dark before use in irradiation tests.

92

Finally, apple leaves of equivalent surfaces (6-9 cm2) were cut just before the experiment,

93

then immediately treated with BION® solution following the same procedure. Using this

94

procedure, the leaves remained fresh after drying and only a fraction (30-50%) of the leaf


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surface was covered. After drying, the surface concentration of BTH on each sample was 11

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µg cm-2 corresponding to an application rate of 1100 g ha-1. This rate is higher than the

97

recommended application rates in the field (50 - 200 g ha-1) but it was chosen to allow better

98

monitoring of BTH decay and enhanced detection of BTH photoproducts. A set of

99

experiments was also carried out at an application rate of 140 g ha-1 for microscopic

100

measurements and also to compare the nature of photoproducts with the experiments carried

101

at high application rate (1100 g ha-1).

102

Irradiation. After drying, treated samples were irradiated inside a Suntest CPS

103

photosimulator (Atlas Material Testing Solutions, Élancourt, France) equipped with a xenon

104

lamp filtered below 290 nm (irradiance within the range of 290−800 nm). The intensity of the

105

lamp was set at 500 W m−2 to simulate the sunlight average intensity in summer in France. To

106

avoid overheating and potential thermal reactions, a 10° C cooled water flowed through the

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bottom of the sample holder to maintain the surface temperature as low as possible. Surface

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temperature of the samples was measured with an infrared thermometer gun and remained in

109

the range of 25° – 35° C during irradiation. Control samples were run in the Suntest using

110

dishes wrapped with an aluminium foil for protection from light.

111

Monitoring of BTH and formation of surface photoproducts. Three dishes were taken

112

after each selected irradiation time. A total of 2 mL of acetonitrile was then used to rinse each

113

dish and recover remaining BTH and its photoproducts. This same method was used with

114

BTH and BION® deposits, allowing satisfactory recovery yields of BTH prior to irradiation

115

(93-99%) based on performing three repetitions. However, 2 mL of methanol was also used in

116

some experiments for photoproducts identification as it was more efficient to extract specific

117

photoproducts such as photoproduct A. Extracts were analyzed by liquid chromatography

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coupled with diode array detector (HPLC-DAD, Waters, Saint-Quentin-en-Yvelines, France)

119

to monitor BTH decay and by ultrafast HPLC coupled with electrospray ionization and


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Orbitrap high resolution mass spectrometer (UHPLC-ESI-Orbitrap-HRMS, Thermo

121

Scientific, Waltham, Massachusetts, USA) for photoproducts identification. HPLC-DAD

122

separation was conducted using a reversed phase column, Macherey-Nagel C18, (150 mm

123

length, 4.6 mm i.d., 5 µm particle size) and 60:40 acetonitrile: water mobile phase, at a flow

124

rate of 1 ml min-1. Injection volume was 10 µL, flow cell volume was 12 µL and detection

125

wavelength was 330 nm. Mass spectrometry was performed on a UHPLC Ultimate 3000

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RSLC (Thermo Scientific, Waltham, Massachusetts, USA) equipped with electrospray

127

ionisation source and a high resolution Orbitrap Q-Exactive. Analyses were performed in both

128

negative and positive modes at a capillary voltage of 3.2 kV (ESI+) and 3.2 kV (ESI-) with a

129

detection of 80-1200 m/z range. UHPLC chromatographic separation was conducted using a

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Phenomenex (Le Peck, France) Kinetex™ Cl8 LC column (100 mm × 2.1 mm, 1.7 µm

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particle size). The binary solvent system used was composed of acetonitrile (ACN) and water

132

acidified with 0.5‰ v/v formic acid. The gradient program was 5% ACN for the 7 first min,

133

followed by a linear gradient to 99% in 7.5min and kept constant until 20 min. The flow rate

134

was set at 0.45 mL/min and injection volume was 5 µL. Identification of photproducts was

135

based on structural elucidation of mass spectra and the use of accurate mass determination

136

obtained with the Orbitrap high resolution. Quantification of dimethyldisulfide (DMDS) was

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based on calibration with dimethyl sulfide.

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UV-visible spectra were recorded using a Cary 3 (Varian, Palo Alto, California, USA)

139

spectrophotometer. Solid state spectra of BTH deposited on quartz plates were recorded using

140

a DRA-CA-30I integrating sphere accessory (Varian, Palo Alto, California, USA) and BaSO4

141

reflectance standard (Spectralon).

142

Volatile emissions. To explore the formation of volatile by-products during irradiation, BTH

143

samples described above were inserted into a stainless steel batch reactor (volume: 400 mL),

144

equipped with a quartz window (diameter 6.4 cm). The reactor was then placed inside the


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solar simulator for duration ranging between 0.5 and 6 hours. After each irradiation, the

146

reactor was connected via a heated transfer line (60° C) to a thermal desorption unit (SRA

147

instruments, Marcy-l'Étoile, France) equipped with a Tenax sorbent trap and coupled to a

148

Agilent 6890 gas chromatograph and Agilent 5973 mass-spectrometry detector . The reactor's

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gas phase was sampled for 3 min at a sampling rate of 100 mL min-1 into the Tenax trap that

150

was held at 5°C during sampling. After sampling, the trap was instantly heated to 200°C for

151

30 seconds. Separation of desorbed volatiles was carried out using a DB-5 ms column (25

152

m×0.25 mm×0.25 µm) operated initially at 50°C over 1 min, followed by a 20° C min-1 ramp

153

to reach 230° C. Mass spectra were scanned between m/z 35 and 300 with a source

154

temperature set at 230° C. Additional analyses of volatile products were conducted using a

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head space apparatus coupled with a GC-MS (HS-20, Shimadzu, Kyoto, Japan). This was

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performed by depositing 0.5 mL of BTH (10-3 M in acetonitrile) inside custom-made Quartz

157

20-mL vials. After drying, the HS vials were either heated at 60° C using the HS-20 system or

158

irradiated in the Suntest to verify if the volatile photoproducts are due to thermal or

159

photochemical reactions. GC analyses were performed using the same column and

160

temperature program described above.

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OH Radical Formation. 100 µL of TA solution prepared in water buffered at pH 8.9 at 7 x

162

10-3 M were deposited as fine droplets of 5 µL using a manual micropipette adjustable from 2

163

to 20 µL (Eppendorf®, Montesson, France) on the surface of an apple leaf of 4 cm2. As a

164

control experiment, 100 µL of TA (7 10-3 M) was deposited on a glass petri dish and on apple

165

leaves. After drying, samples were irradiated in the Suntest for 3h. TA and its fluorescent

166

photoproduct TAOH were recovered from the surface using 2 mL of acetonitrile. Recovery

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efficiency ranged between 94 and 98% based on TA. The detection of OH radicals at the

168

surface of apple leaves was based on their reaction with TA and on the generation of TAOH.

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The photogenerated TAOH was measured by HPLC equipped with fluorescence detection


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(λexc= 330 nm and λem= 430 nm) after calibration with TAOH standard solutions. Due to the

171

high TA concentration used, we assumed that TA traps all the OH radicals formed and we

172

postulated that the initial rate of •OH formation was equal to the rate of TAOH formation

173

divided by the chemical yield of TAOH (η), which is around 0.2, as previously reported.20

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Less than 0.1% of TA was oxidized into TAOH in dark after several hours of storage

175

suggesting that no significant oxidation occurs without light or via potential metabolism of

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TA. Moreover, TAOH was shown to be stable when deposited on leaf surface since no

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significant degradation occurred during storage or under light exposure for 2 hours.

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Microscopic analysis. Several 5 µL droplets of BTH solution, and BION® suspension were

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deposited on glass dishes, paraffin wax films and apple leaves. After drying overnight at room

180

temperature, the deposits were then imaged with a Shimadzu micro hardness tester HMV-2E,

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using a 40x objective lens. For each sample, an average of 3 deposit spots was measured.

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Measurement of contact angle. Static contact angle measurements were performed using an

183

Attension Theta Lite optical tensiometer (Biolin Scientific, Västra Frölunda, Sweden)

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associated with an imaging source camera. The contact angle was measured using the sessile

185

drop method: a 5 µL droplet of ultrapure water was gently deposit on the surface of each

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surface (glass, wax and apple leaf) using an adjustable micropipette (2-20 µL, Eppendorf®,

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Montesson, France). Images captured to measure the angle formed between liquid and support

188

were treated using imageJ, an open source java-based software developed for the digital

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processing of scientific image. A contact angle is the angle, conventionally measured through

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the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of

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a solid surface by a liquid via the Young equation.

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RESULTS AND DISCUSSION


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Kinetics of BTH Photolysis on model supports. Figure 1 presents the first-order kinetic

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curves and corresponding rate constants (k) for BTH and BION® photolysis on glass and wax

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surfaces. As seen, the photolysis rate constant (k) can vary by up to a factor of 20 depending

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on the combination of solution/surface used. This finding highlights the importance of

197

experimental conditions in the evaluation of surface photochemistry of agrochemicals. The

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half-life of BTH on glass is 1.70 ± 0.05 compared to 5.0 ± 0.7 on wax. On the other hand,

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half-life of BION® is 16 ± 4 on glass vs. 36 ± 14 on wax. When the same surface is used, it

200

appears that the photolysis half-life can be up to 10 times faster when using pure BTH

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solution than when uisng BION®. In contrast, when the same solution is used, half-life is 2-3

202

times shorter on glass than on wax. Analysis of UV spectra of pure BTH and BION®

203

dissolved in water didn't reveal the presence of any UV absorber able to reduce light

204

absorption of BTH above 290 nm (see Figure S1, supporting info). Furthermore, no band shift

205

or broad broadening of BTH spectrum was observed when BTH is deposited on glass or

206

paraffinic wax, suggesting that BTH does not bind or interact strongly with the substrate. On

207

the other hand, contact angle measurements revealed that a water droplet deposited on glass

208

forms an angle of 70°-80° whereas on paraffin wax the angle is in the range of 90°-100°. This

209

translates into smaller coverage (spreading) area after water evaporation on wax surface

210

leading to a higher concentration per surface and thus a larger light screening effect.

211

Similarly, when a droplet of acetonitrile is deposited on glass, the droplet spreads better than

212

water and forms a small contact angle in the range 35°-40°. Since BTH solution was prepared

213

in acetonitrile whereas BION® contained water, the higher dispersal of BTH deposit due to

214

the smaller contact angle, could be responsible for reducing the light screening effect and

215

resulting in a more effective and faster photolysis. Hence, the composition of the solution in

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the case of wet deposition is a key factor that impact the rate of photolysis.

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Given the moderate vapor pressure of BTH (4.6 10-4 Pa), it is also possible that a small

219

volatilization could have occurred during long irradiation periods as suggested by the reported

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volatilization data for other agrochemicals with similar vapor pressure.21 Nevertheless, no

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significant volatilization was found during overnight drying as measured by the very high

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recoveries and no BTH was detected in the gas phase samples taken during irradiation. This

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can be due to the fact that BTH was irradiated in solid form and as it is well known

224

volatilization from the solid phase (sublimation) is of lesser importance.

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Effect of Surface and Adjuvants on BTH Photodegradability. The characteristics of BTH

226

deposits on the surface may have an impact on the photodegradability. To better understand

227

this effect, micrographs of BTH and BION® dry deposits on glass, parrafin wax and on an

228

apple leaf were recorded, as illustrated in Figure 2. BTH deposition pattern and form were

229

very different depending on the substrate but also the presence of formulants. On glass, BTH

230

residues were uniformly distributed with random shapes of small size particles (1-2 µm)

231

which are mostly amorphous. In contrast BTH on paraffin wax formed crystalline deposits of

232

needle shape (10-30 µm). The growth of crystalline form of BTH on wax could be due to its

233

surface roughness and hydrophobicity causing a delay in droplet evaporation and favoring the

234

crystallization.22-23 These factors resulted also in a highly localized surface coverage of BTH

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leading to a stronger light screening effect which can explain the slower photolysis of BTH on

236

paraffin wax. However, one cannot rule out a possible higher stability of crystallized BTH

237

that can be due to intermolecular interactions or an auto-deactivation in the crystal lattice.

238

When looking at the deposit of water dispersible granules of BION® containing BTH, the

239

difference is even more striking with the formation of large and highly dense coverage.

240 241

This

behavior is

not surprising given

the

presence

242

dibutylnaphthalenesulphonate) in the formulation, identified by HR-MS. In fact, surfactants


of a

surfactant (Sodium

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are generally designed to improve the dispersing, spreading, and sticking properties of active

244

ingredients. Besides, it is relatively well known that the physical type of formulation

245

influences deposit characteristics and distribution patterns on leaf surfaces.24-26 Pesticides

246

formulated as wettable powders or granules as in the case of BION® yield deposits with

247

greater median diameter than liquid formulations due to the low solubility and presence of

248

particle agglomerates.27 Interestingly, the nature of BION® deposit on wax is not crystalline

249

as in the case of BTH on wax. This can also be due to the presence of surfactant and other

250

adjuvants such as the silica particles that are used in the formulation as flow aid and

251

anticaking agent. The melting point also influences the physical form of an active ingredient

252

(a.i.) within a dried deposit. Baker et al. have reported in an extensive study with 26 a.i.’s that

253

chemicals with a melting point of greater than 200°C formed crystalline deposits whether

254

formulated with or without surfactant.28 Chemicals with lower melting point between 135°

255

and 200°C similar to BTH (m.p. 135 °C) formed crystalline deposits in the absence of

256

surfactant but amorphous deposits in the presence of surfactant. The formation of amorphous

257

and highly dense spreading of BTH can likely explain the large increase in half-life (36 h) of

258

BION® due to light screening effect. Finally, BION® deposit on an apple leaf appears more

259

uniform and well spread than that on wax surface which could lead to a more efficient light

260

absorption and photodegradability, as shown in the next section. This improvement in BTH

261

spreading can be attributed to combined effects of leaf shape, surface roughness and presence

262

of stomata and trichomes which can influence BTH mobility but cannot be mimicked in the

263

model paraffinic wax.

264

BTH photolysis on apple leaves. To verify if BTH photolysis occurs similarly on apple

265

leaves and paraffinic wax, dried deposits of BION® aqueous suspension on wax and leaves

266

cut from an apple seedling (10 to 15 leaves) just before depositing BION® were irradiated in

267

the stainless steel chamber using the solar simulator. To reduce leaf dehydration, a small


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volume of water (0.5 mL) was added underneath the unexposed side of the leaf. Identical

269

volume of water was also deposited on a clean corner of the wax surface to reproduce the

270

same condition. Initial Relative humidity inside the chamber was around 55-60% at 21° C as

271

measured with a hygrometer (Testo 174H, France). Extraction of BTH residues using 2 mL

272

acetonitrile from both samples showed very high recovery (94 ± 5%). No significant

273

transformation of BTH occurred on apple leaf during the night in the absence of light,

274

indicating that no uptake and metabolism occurred in dark. Monitoring of BTH decay during

275

irradiation revealed that it is more rapidly degraded on apple leaf than on wax, with a half-life

276

of 2.8 ± 0.4 hours. About 75% of BTH was transformed within 4 hours on apple leaf whereas

277

only 10% of BTH disappeared on wax for the same duration. Our initial hypothesis was that

278

uptake and metabolism in the apple leaf promoted by the irradiation could be responsible for a

279

fraction of the observed fast BTH disappearance. LC-ESI-MS analysis of the residue revealed

280

the presence of Benzo-1,2,3-thiadiazole-5-carboxylic acid (product G), a known major

281

metabolite of BTH, suggesting that a small fraction of BTH does indeed undergo metabolism

282

during irradiation. Nevertheless, other factors may be involved such as the nature and form of

283

deposit as discussed above but also the chemistry of apple leaf cuticle and the possibility of

284

formation of reactive oxygen species (ROS) induced by abiotic stress.14,

285

microscopic view of the the deposit on apple leaf indicates clearly a good spreading of BTH

286

which can enhance photodegradability, as discussed above. While these leaf physical

287

attributes can influence photodeagradability, it remains difficult to confirm if they can be

288

solely responsible for the 7-fold increase in photolysis rate. Other studies have also reported

289

enhancement of photolysis of fungicides when plant wax was used as support. As an example,

290

on bean leaf, more S-oxidation of carboxin proceeded as compared with that on a glass

291

surface.30-31 Accelerated photolysis was also reported for 2,4-D on Zea mays leaves.32

292

Moreover, photoinduced oxidation was reported for guazatine on dwarf apple trees.33 It was


29

Moreover,

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293

proposed that the induced photodegradation proceeds via reaction with the hydroxyl radical or

294

sensitizers in wax components. To explore if such mechanism can be involved in our

295

conditions, an attempt was made to measure OH radicals on the surface of an apple leaf.

296

Using TA (7 10-3 M) as a molecular probe deposited as drops like BTH, we found that

297

irradiation of an apple leaf for 3 hours in the solar simulator results in the formation of 3 ± 0.4

298

10-6 M of fluorescent TA-OH in the final extract (3mL). This value corresponds to an •OH

299

produced surface concentration of 2.2 ± 0.3 10-9 moles cm-2. It is important to note that this

300

value was obtained after subtracting the signal of TA-OH formed when TA is deposited on

301

leaf surface for 3 hours in dark conditions. On the other hand, TAOH was also formed when

302

TA was irradiated on glass surface, however only 0.3 ± 0.2 10-6 M was measured. Thus, OH

303

radical production on leaf surface was about 10 times higher. Moreover, our method for

304

measuring OH radicals might underestimate the real concentration of •OH produced since

305

only a fraction of OH radicals produced by the leaf are effectively scavenged by TA, given

306

their short life-time and the absence of solvent in our conditions. If we assume a TA-OH yield

307

from •OH reaction with TA of 20% as reported by Charbouillot et al. in aqueous solution, the

308

actual OH radical cumulative production can reach 1.50 ± 0.04 10-5 M in solution

309

corresponding to a surface concentration of 1.13 ± 0.03 10-8 moles cm-2.20 This gives 4.50 ±

310

0.12 10-8 moles of OH radicals produced on the surface of the leaf assuming a leaf surface of

311

4 cm2. During the same period of irradiation, 1.0 ± 0.1 10-7 moles of BTH in BION® was

312

phototransformed. Hence, the levels of OH radicals produced by the leaf are relatively

313

comparable to that of converted BTH in BION®. Moreover, the detection of hydroxylated

314

photoproducts such as H, and E (see Table 2) on the surface of leaf supports a potential

315

implication of OH radicals in the phototransformation of BTH. The mechanism by which OH

316

radicals are produced is currently unclear and requires further investigation. One possible

317

source could be oxidative burst as well as direct photolysis of leave chemical constituents in


14

Page 15 of 30


318

particular phenolic compounds such as rutin and phloridzin.34-35 This is the first time that OH

319

radicals are detected using the terephthalic method on the surface of plant leaves. Despite the

320

relatively high uncertainty of the method (0.3 10-9 moles cm-2), it can provide useful insights

321

on the contribution of OH radicals in the degradation of deposited agrochemicals.

322

Characterization of surface photoproducts. Table 2 summarizes the identified surface

323

photoproducts obtained by high resolution mass spectrometry (HR-ESI-Orbitrap-MS)

324

analyses at around 55-70% conversion of irradiated BTH and BION® on glass, wax and apple

325

leaf surfaces.

326 327

The same photoproducts identified on apple leaves were detected at BION® application rate

328

of 140 g ha-1 and at 1100 g ha-1. This indicates that concentration of BTH did not influence

329

the nature of photoproducts. Given the lack of reference standards, quantification was not

330

possible. Instead, the peak area of pseudo-molecular ion (m/z) for each product was compared

331

between the three samples. Overall, eight major photoproducts were identified. Seven of the

332

photorpducts have lost the N2 fragment which indicates that N2 elimination is the primary

333

photolytic step. This is consistent with previous studies reporting that the absorbing moiety of

334

BTH (1,2,3-benzothiadiazole) functionality can undergo photodissociation by cleavage of the

335

heteroatomic ring and N2 loss that yields a diradical from which a variety of photoproducts

336

can be formed, in particular dimeric species.36-37 Photoproduct A is produced by cyclization

337

of the diradical generated after N2 elimination and photocleavage of the C-S bond followed by

338

oxidation of the acyl radical. The photochemical scission of thioesters is a well-known

339

reaction as well as the oxidation of the acyl radical into a carboxylic acid. 38-39 Interestingly,

340

product A was only observed when methanol was used for extraction, suggesting that either

341

acetonitrile is not effective for recovering A or that it might be unstable in acetonitrile.

342

Photoproduct B seems to undergo similar oxidation of the acyl group with the difference that


15


Page 16 of 30

343

the S-atom is oxidized into sulfonic acid. The formation of this compound appears to be more

344

pronounced on wax than on glass surface. Similarly, products C, D and E, are formed through

345

oxidation of S-atom into sulfinic (C) or sulfonic acid (D) but these products conserve the

346

COSCH3 group, whereas compounds E and H likely arise from ring hydroxylation of

347

intermediate D. In contrast with product B, higher concentrations of compounds D and E were

348

found on glass and apple leaf than on wax surface. These findings indicate that the surface can

349

have an impact on the nature and distribution of the photoproducts. For example,

350

hydroxylation of product D into E on glass surface can be rationalized by the presence of OH

351

groups on glass as silanols or adsorbed molecular water and presence of water or OH radicals

352

produced on leaf surface, whereas this is not possible on wax surface which is highly

353

hydrophobic. Besides, the detection of photoproduct F (CH3SO3H) support the hypothesis of

354

C-S photocleavage as it can only be formed via oxidation of the •SCH3 radical released.

355

Finally, it is noteworthy that many of the photoproducts recently reported in solution in

356

particular coupling products are not detected here, and similarly some of the photoproducts

357

found on glass and wax (A-C) were not detected on leaf surface.40 This provides an additional

358

reason for carrying out heterogeneous photolysis experiments on plant surfaces, for a better

359

evaluation of agrochemical fate and risk assessment of by-products.

360

Analysis of volatile photoproducts. In addition to characterizing surface composition during

361

photolysis of BTH, we attempted to measure volatile photoproducts. Hence, we carried out

362

irradiations of BTH and BION® samples deposited on glass,wax and apple leaf surfaces inside

363

an air-tight stainless steel small chamber. Periodic measurements of gas phase composition

364

every 30 min to 1h was performed. Two products were successfully detected in all

365

experiments: DMDS [CH3S-SCH3] predominantly and traces of methanthiol [CH3S-H].

366

These two compounds can arise from recombination of two SCH3 radicals and H-abstraction

367

respectively. The detection of these compounds confirms the cleavage of the CO-SCH3 bond


16

Page 17 of 30


368

and the release of the SCH3 radical. This is in accordance with the observation of products B

369

and F on the surface after irradiation and in line with our recently published work on the

370

photolysis of BTH in solution.40 Density Functional Theory calculations performed at the

371

B3LYP/6–311+ +G(d,p) level indicated that the electronic dissociation energy of the S-N

372

bond is 27.9 kcal/mol, therefore much lower than that of the C-SMe bond (69.0 kcal/mol).

373

The release of Me-S-S-Me is however still possible because the N2 elimination from the

374

diradical involves a significant activation barrier (27.9 kcal/mol). This makes possible the

375

back reaction through the process of ring closure reaction (the respective activation barrier is

376

less than 1 kcal/mol in that case) with regeneration of BTH and thus increasing the probability

377

of the C-SMe bond photocleavage to occur.40 On the contrary, the C-S homolytic dissociation

378

leads to separated fragments (radicals). In that case, recombination is prevented. Thus, in the

379

absence of O2, C-SMe bond dissociation is favored. This oxygen effect was confirmed

380

experimentally where the concentration of DMDS increased by a factor of 2.8 when solid

381

BTH was irradiated in an oxygen purged quartz headspace vial compared with saturated air

382

condition.

383

It is worthy to note that the identified volatiles were only formed during the photochemical

384

reaction and not as thermal products during thermal desorption. This was confirmed by

385

headspace analysis of a sealed glass vial containing deposited dry residues of BTH irradiated

386

for 2h in the solar simulator. The vial was incubated at 60°C for 2 min followed by GC-MS

387

analysis. The same volatiles observed with the thermal desorption system were also found,

388

and DMDS was in both cases the major volatile released. Figure 3 shows the yields of

389

DMDS after 2 hours of irradiation of BTH and BION® on glass and wax surfaces as well as

390

on apple leaves.

391


17


Page 18 of 30

392

The first observation is that DMDS yield was about six times smaller on glass surface (2%)

393

than on wax (12%). On the other hand, the yield was identical for pure BTH and BION®

394

formulation deposited on wax. It is unclear why DMDS yield is smaller on glass but one

395

possible explanation is that the higher density and stacking of BTH molecules on the wax

396

surface enhances the probability of recombination of two released SCH3 radicals to yield

397

DMDS. Considering that 12% of converted BTH yield DMDS, this is a relatively important

398

degradation pathway. Thus, more systematic analysis of volatile products during

399

agrochemicals photodegradation should be included in future studies for better understanding

400

of their environmental fate on plants, soils and other surfaces.

401

Analysis of volatile products during photolysis of BION® deposited on an apple leaf revealed

402

that only 1.5% of BTH transformed yield DMDS, compared to 12% on wax surface. While

403

the difference in deposit characteristics might have played a role, we investigated if a fraction

404

of DMDS produced was absorbed by the leaf itself. Thus, we repeated the experiment with

405

BION® deposited on wax and we added 2 apple leaves surrounding the internal walls of the

406

reactor. The measurement of DMDS showed that the yield was reduced from 12% in absence

407

of leaves to 3% in presence of leaves suggesting that plant leaves are indeed capturing

408

DMDS. Plants are known to absorb and metabolize gaseous organic compounds such as

409

formaldehyde and carbonyl sulfide (COS) which can enter plant leaves through stomata.41-42

410

Data on DMDS uptake by plant leaves or its impact is however lacking despite its use as soil

411

fumigant.43-44 In addition, mass spectrometry analyses of BION® formulation before and after

412

irradiation on the leaf revealed the formation of adducts resulting from the addition of CH3S•

413

radical on structures of the surfactant used (Sodium dibutylnaphthalenesulphonate), as shown

414

in SI, Table S1. Based on an application rate of 150 g ha-1 of BTH on apples and on an

415

estimated chemical yield of DMDS of 1.5%, a maximum emission level of 2.2 g ha-1 of

416

DMDS could be generated via BTH photolysis.


18

Page 19 of 30


417

Recent studies have identified DMDS as an elicitor of induced systemic resistance in N.

418

Benthamiana against Botrytiscinerea.45 Others have also found that DMDS promotes growth

419

of plants by enhancing sulfur nutrition and shows antimicrobial and nematicidal activity.46-48

420

It can also help plants to fight against non-specialist herbivores feeding and functions as both

421

an oviposition repellent and as an attractant to different natural enemies of some plants.49

422

Giving the role of DMDS in plant signaling and growth, further studies on the effect of

423

gaseous DMDS on plants are needed to evaluate its potential impact. These findings along

424

with our observation imply that DMDS and potentially volatile degradation by-products of

425

other agrochemicals should be characterized and their impacts on plant physiology and air

426

quality merit further attention.

427

In summary, our study shows that a better understanding of the nature and form of

428

agrochemicals deposit is key to evaluate their photochemical reactivity on plant surfaces. The

429

discrepancy between half-life of BTH on paraffin wax and apple leaves suggests that paraffin

430

wax is not an adequate model to mimic the photolysis on apple leaf surface. A simple

431

comparison between irradiance of the Suntest photosimulator (46 W m-2 between 300 and

432

400 nm) and sunlight irradiance in central Europe in summertime (22 W m-2) over 24h of a

433

day, allows us to estimate that half-lifetime of BTH on apple leaves would not exceed 6h.

434

This very short time scale demonstrates that photolysis on crops is a major dissipation process

435

for BTH and it raises the question of the efficiency of BTH treatment and whether

436

photoproducts formed on surface and in gas phase can contribute to the observed elicitor

437

activity. Due to experimental constraints, our validation experiments were carried out using

438

freshly cut dead leaves and not attached live leaves. We acknowledge that the results might

439

differ when using live leaves because other factors such as metabolism, transport, antioxidant

440

activity, microbe load could contribute to the observed transformation of BTH. Therefore, a

441

systematic investigation of formulated pesticides photolysis on dead and live leaves should be


19


Page 20 of 30

442

performed to test the validity of results using wax surrogates or extrapolation based on soil

443

studies. This will also improve the assessment of photolysis contribution to overall pesticide

444

dissipation from plants and will be very valuable for risk and impact assessment models.

445

Moreover, our study demonstrates that formation of volatile degradation products can

446

possibly occur during agrochemicals photolysis, however this process is overlooked. Our

447

experimental methodology presented here can help to assess the role of heterogeneous

448

photochemistry in the production of volatile pollutants that may be toxic. For instance,

449

farmers could be exposed to released volatile photoproducts during and after field application

450

of pesticides, especially in confined atmosphere like in agricultural greenhouses.

451

AKNOWLEDGMENTS. This work was partially supported by the European Regional

452

Development Fund and a new investigator award from the regional of Auvergne-Rhône-

453

Alpes. The authors gratefully acknowledge Malgorzata Stawinoga, who prepared the

454

paraffinic wax films, Dr. Loïc Della Puppa for the contact angle measurements and Dr. Pascal

455

Goupil for providing the apple seedlings and for her valuable comments and suggestions.

456 457 458 459

ASSOCIATED CONTENT Supporting information Absorption spectrum of BTH in water, BION® and their difference (Figure S1), UV spectrum

460

of BTH deposit on a quartz plate (Figure S2), and structures of formulants detected in BION®

461

before and after irradiation on wax and on detached leave, identified by high resolution

462

electrospray mass spectrometry (Table S1).

463


20

Page 21 of 30

464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511


References 1. United Nations 2017 Revision of World Population Prospects. https://esa.un.org/unpd/wpp/ (accessed August 03, 2017). 2. Barakate, A.; Stephens, J., An Overview of CRISPR-Based Tools and Their Improvements: New Opportunities in Understanding Plant-Pathogen Interactions for Better Crop Protection. Front. Plant Sci. 2016, 7. 3. Rojas, C. M.; Senthil-Kumar, M.; Tzin, V.; Mysore, K. S., Regulation of primary plant metabolism during plant-pathogen interactions and its contribution to plant defense. Front. Plant Sci. 2014, 5. 4. Welling, L. L., Induced resistance: from the basic to the applied. Trends Plant Sci. 2001, 6, 445-7. 5. Bektas, Y.; Eulgem, T., Synthetic plant defense elicitors. Front. Plant Sci. 2015, 5. 6. Cole, D. L., The efficacy of acibenzolar-S-methyl, an inducer of systemic acquired resistance, against bacterial and fungal diseases of tobacco. Crop Prot. 1999, 18, 267-273. 7. Friedrich, L.; Lawton, K.; Ruess, W.; Masner, P.; Specker, N.; Rella, M. G.; Meier, B.; Dincher, S.; Staub, T.; Uknes, S.; Metraux, J. P.; Kessmann, H.; Ryals, J., A benzothiadiazole derivative induces systemic acquired resistance in tobacco. Plant J. 1996, 10, 61-70. 8. Jiang, S.; Park, P.; Ishii, H., Ultrastructural study on acibenzolar-S-methyl-induced scab resistance in epidermal pectin layers of Japanese pear leaves. Phytopathology 2008, 98 (5), 585-91. 9. Pajot, E.; Silue, D., Evidence that DL-3-aminobutyric acid and acibenzolar-S-methyl induce resistance against bacterial head rot disease of broccoli. Pest Manag. Sci. 2005, 61, 1110-4. 10. Scarponi, L.; Buonaurio, R.; Martinetti, L., Persistence and translocation of a benzothiadiazole derivative in tomato plants in relation to systemic acquired resistance against Pseudomonas syringae pv tomato. Pest Manag. Sci. 2001, 57, 262-8. 11. Zavareh, A. H.; Tehrani, A. S.; Mohammadi, M., Effects of Acibenzolar-S-methyl on the specific activities of peroxidase, chitinase and phenylalanine ammonia-lyase and phenolic content of host leaves in cucumber-powdery mildew interaction. Commun. Agric. Appl. Biol. Sci. 2004, 69, 555-63. 12. Fantke, P.; Gillespie, B. W.; Juraske, R.; Jolliet, O., Estimating half-lives for pesticide dissipation from plants. Environ. Sci. Technol. 2014, 48, 8588-602. 13. Jacobsen, R. E.; Fantke, P.; Trapp, S., Analysing half-lives for pesticide dissipation in plants. SAR QSAR Environ. Res. 2015, 26, 325-42. 14. Angioni, A.; Cabizza, M.; Cabras, M.; Melis, M.; Tuberoso, C.; Cabras, P., Effect of the epicuticular waxes of fruits and vegetables on the photodegradation of rotenone. J. Agric. Food Chem. 2004, 52, 3451-5. 15. Monadjemi, S.; El Roz, M.; Richard, C.; Ter Halle, A., Photoreduction of Chlorothalonil Fungicide on Plant Leaf Models. Environ. Sci. Technol. 2011, 45, 9582-9589. 16. Monadjemi, S.; ter Halle, A.; Richard, C., Accelerated dissipation of the herbicide cycloxydim on wax films in the presence of the fungicide chlorothalonil and under the action of solar light. J. Agric. Food Chem. 2014, 62, 4846-51. 17. Sinderhauf, K.; Schwack, W., Photodegradation chemistry of the insecticide phosmet in lipid models and in the presence of wool wax, employing a 15N-labeled compound. J. Agric. Food Chem. 2004, 52, 8046-52. 18. Ter Halle, A.; Drncova, D.; Richard, C., Phototransformation of the herbicide sulcotrione on maize cuticular wax. Environ. Sci. Technol. 2006, 40, 2989-95.


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19. Lavieille, D.; Ter Halle, A.; Bussiere, P. O.; Richard, C., Effect of a Spreading Adjuvant on Mesotrione Photolysis on Wax Films. J. Agric. Food Chem. 2009, 57, 96249628. 20. Charbouillot, T.; Brigante, M.; Mailhot, G.; Maddigapu, P. R.; Minero, C.; Vione, D., Performance and selectivity of the terephthalic acid probe for (OH)-O-center dot as a function of temperature, pH and composition of atmospherically relevant aqueous media. J. Photochem. Photobiol., A 2011, 222, 70-76. 21. European Commission, health & consumer protection directorate-general, pesticides in air: considerations for exposure asssessment. http://esdac.jrc.ec.europa.eu/public_path/projects_data/focus/air/docs/FOCUS_AIR_GROUP _REPORT-FINAL.pdf (accessed on August 07, 2017). 22. Bukovac, M. J.; Whitmoyer, R. E.; Reichard, D. R., Spray Droplet - Leaf Surface Interaction - Droplet Drying Characteristics and Nature of Growth-Regulator Deposits as Revealed by Dispersive-X-Ray Analysis. Hortscience 1984, 19, 577-577. 23. Krämer, T., Deposit characteristics, penetration and biological efficacy of selected agrochemicals as affected by surfactants and plant micromorphology. Cuvillier Verlag: Göttingen, Germany, 2009. 24. Lownds, N. K.; Bukovac, M. J., Foliar Penetration of Growth-Regulators from Droplets - Physical Parameters and Their Modification by Surfactants. Hortscience 1982, 17, 496-496. 25. Stevens, P. J. G.; Baker, E. A., Factors Affecting the Foliar Absorption and Redistribution of Pesticides .1. Properties of Leaf Surfaces and Their Interactions with Spray Droplets. Pestic. Sci. 1987, 19, 265-281. 26. Stevens, P. J. G.; Baker, E. A.; Anderson, N. H., Factors Affecting the Foliar Absorption and Redistribution of Pesticides .2. Physicochemical Properties of the Active Ingredient and the Role of Surfactant. Pestic. Sci. 1988, 24, 31-53. 27. Kudsk, P.; Mathiassen, S. K.; Kirknel, E., Influence of Formulations and Adjuvants on the Rainfastness of Maneb and Mancozeb on Pea and Potato. Pestic. Sci. 1991, 33, 57-71. 28. Baker, E. A.; Hayes, A. L.; Butler, R. C., Physicochemical Properties of Agrochemicals - Their Effects on Foliar Penetration. Pestic. Sci. 1992, 34, 167-182. 29. Katagi, T., Photodegradation of pesticides on plant and soil surfaces. Rev. Environ. Contam. Toxicol. 2004, 182, 1-189. 30. Buchenauer, H., Inactivation of Triforine by Uv and Sunlight on Glass and on Leaves of Bean-Plants. Pestic. Sci. 1975, 6, 553-559. 31. Buchenauer, H., Differences in Light Stability of Some Carboxylic-Acid Anilide Fungicides in Relation to Their Applicability for Seed and Foliar Treatment. Pestic. Sci. 1975, 6, 525-535. 32. Venkatesh, R.; Harrison, S. K., Photolytic degradation of 2,4-D on Zea mays leaves. Weed Sci. 1999, 47, 262-269. 33. Sato, K.; Kato, Y.; Maki, S.; Matano, O.; Goto, S., Penetration, Translocation and Metabolism of Fungicide Guazatine in Dwarf Apple-Trees. J. Pestic. Sci. 1985, 10, 81-90. 34. Bhattacharjee, S., Reactive oxygen species and oxidative burst: Roles in stress, senescence and signal transduction in plants. Curr. Sci. 2005, 89, 1113-1121. 35. Shao, X.; Bai, N. S.; He, K.; Ho, C. T.; Yang, C. S.; Sang, S. M., Apple Polyphenols, Phloretin and Phloridzin: New Trapping Agents of Reactive Dicarbonyl Species. Chem. Res. Toxicol. 2008, 21, 2042-2050. 36. European Food Safety Authority., Conclusion on the peer review of the pesticide risk assessment of the active substance acibenzolar-S-methyl. EFSA J. 2014, 12(8), 3691-3764. 37. Krantz, A.; Laureni, J., Matrix Photolysis of 1,2,3-Thiadiazole - on Possible Involvement of Thiirene. J. Am. Chem. Soc. 1974, 96, 6768-6770.


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38. Chatgilialoglu, C.; Crich, D.; Komatsu, M.; Ryu, I., Chemistry of acyl radicals. Chem. Rev. 1999, 99, 1991-2069. 39. Grunwell, J. R.; Marron, N. A.; Hanhan, S. I., Photochemistry of Aromatic Thiol Esters. J. Org. Chem. 1973, 38, 1559-1562. 40. Sleiman, M.; Stawinoga, M.; Wang, S.; de Sainte-Claire, P.; Goupil, P.; Richard, C., Photochemical transformation of the plant activator Acibenzolar-S-methyl in solution. J. Photochem. Photobiol., A 2017, 333, 79-86. 41. ProtoschillKrebs, G.; Wilhelm, C.; Kesselmeier, J., Consumption of carbonyl sulphide (COS) by higher plant carbonic anhydrase (CA). Atmos. Environ. 1996, 30, 3151-3156. 42. Stimler, K.; Nelson, D.; Yakir, D., High precision measurements of atmospheric concentrations and plant exchange rates of carbonyl sulfide using mid-IR quantum cascade laser. Glob. Change Biol. 2010, 16, 2496-2503. 43. Le Bechec, M.; Costarramone, N.; Fouillet, T.; Charles, P.; Pigot, T.; Begue, D.; Lacombe, S., Photocatalytic films for soil fumigation: Control of dimethyl disulfide concentration after fumigation. Appl. Catal., B 2015, 178, 192-200. 44. McAvoy, T.; Freeman, J. H.; Reiter, M., Soil Persistence of Dimethyl Disulfide Fumigant due to Application Rate, Chemical Formulation, and Plastic Mulch Type. Hortscience 2010, 45, 502-503. 45. Huang, C. J.; Tsay, J. F.; Chang, S. Y.; Yang, H. P.; Wu, W. S.; Chen, C. Y., Dimethyl disulfide is an induced systemic resistance elicitor produced by Bacillus cereus C1L. Pest Manag. Sci. 2012, 68, 1306-1310. 46. Dandurishvili, N.; Toklikishvili, N.; Ovadis, M.; Eliashvili, P.; Giorgobiani, N.; Keshelava, R.; Tediashvili, M.; Vainstein, A.; Khmel, I.; Szegedi, E.; Chernin, L., Broadrange antagonistic rhizobacteria Pseudomonas fluorescens and Serratia plymuthica suppress Agrobacterium crown gall tumours on tomato plants. J. Appl. Microbiol. 2011, 110, 341-352. 47. Kai, M.; Haustein, M.; Molina, F.; Petri, A.; Scholz, B.; Piechulla, B., Bacterial volatiles and their action potential. Appl. Microbiol. Biotechnol. 2009, 81 (6), 1001-1012. 48. Meldau, D. G.; Meldau, S.; Hoang, L. H.; Underberg, S.; Wunsche, H.; Baldwin, I. T., Dimethyl Disulfide Produced by the Naturally Associated Bacterium Bacillus sp B55 Promotes Nicotiana attenuata Growth by Enhancing Sulfur Nutrition. Plant Cell 2013, 25, 2731-2747. 49. Ferry, A.; Le Tron, S.; Dugravot, S.; Cortesero, A. M., Field evaluation of the combined deterrent and attractive effects of dimethyl disulfide on Delia radicum and its natural enemies. Biol. Control 2009, 49, 219-226.


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Page 24 of 30

Figure Captions Figure 1. Time course of acibenzolar-S-methyl photolysis on glass and wax surfaces, in a solar simulator at an irradiance of 500 W m-2. BTH application rate was 1100 g ha-1. BION® is the commercial formulation of BTH. Error bars represent uncertainty obtained for triplicate samples. Figure 2. Microscopic view of surface deposit of pure BTH (140 g ha-1) on glass surface (A), on wax film (B), and of formulated BTH (BION®) on wax film (C) and on an apple leaf (D Figure 3. Yields of gaseous dimethyl disulfide (DMDS) produced after 2h of irradiation of pure BTH and BION® on glass, paraffin wax film and apple leaf, applied at the rate of 1100 g ha-1 in a solar simulator at 500 W m-2. The far right data bar (light grey) was obtained by irradiating BTH on wax surface in the presence of two surrounding apple leaves inside the reactor.


24

Page 25 of 30


Figure 1

1 0.9

k = 0.40 ± 0.01 R² = 0.989

0.8

h-1 k = 0.14 ± 0.02 h-1 R² = 0.988

BTH/glass BION®/glass BTH/wax BION®/wax

Ln [BTH]0/[BTH]

0.7 k = 0.044 ± 0.010 h-1 R² = 0.989

0.6 0.5

k = 0.019 ± 0.008 h-1 R² = 0.976

0.4 0.3 0.2 0.1 0 0

2

4

6

8 10 12 14 16 18 20 22 24 Irradiation time (hours)


25


Page 26 of 30

Figure 2


26

Page 27 of 30


Figure 3

16

DMDS yield (%)

14 12 10 8 6 4 2 0


27


Page 28 of 30

Table 1: Synthetic plant defense elicitors, commercialized (*) or emerging Tiadinil Benzo(1,2,3)thi N N H adiazole-7N N N carbothioacid SS S methyl ester O BTH *

O

SCH3

2,6-dichloroisonicotinic acid

Cl

N

Cl

Isotianil*

N

Cl

Cl

Cl

H N

CO2H 3,5dichoroanthra nilic acid

Cl

N S O

Cl

Sulfamethoxazole

NH2 CO2H


N N O O

S

O

N H NH2

28

Page 29 of 30


Table 2. Proposed structures and relative abundance of BTH surface photoproducts, identified by high resolution Orbitrap electrospray mass spectrometry. Gaseous dimethyldisulfide (DMDS) and methyl mercaptan were identified by gas chromatoghraphy coupled to mass spectrometry. The abundance on different supports is illustrated by “minor” and “major”; n.d.: not detected.

Structure

m/z in negative mode

glass

Paraffin wax

Apple leaf

S

A

150.9852

major

major

n.d.

minor

minor

n.d.

minor

minor

n.d.

230.9795

major

minor

major

246.9742

minor

minor

major

94.9721

major

major

n.d.

178.9911

n.d.

n.d.

minor

262.9692

n.d.

n.d.

minor

CO2H

200.9857

B

SO3H CO2H

214.9812

C

SO2H COSCH3

D

SO 3H COSCH3

HO

E

SO3H COSCH3

F

CH3SO3H N N

G

S CO2H OH

H

HO SO 3H COSCH3

I

CH3S-SCH3

94 (100), 79

minor

major

minor

J

CH3SH

48

minor

minor

minor


29


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For Table of Contents Only

TOC graphic


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Heterogeneous Photochemistry of Agrochemicals ... - ACS Publications

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