Mode of action of organosilicone surfactants in enhancing the performance of Triclopyr herbicide

Abstract

Triclopyr formulations in the presence or absence of added surfactants were evaluated in relation to optimum control of gorse (Ulex europaeus L.). From a wide range of non-ionic surfactants, superior formulations of triclopyr butoxyethyl ester were selected on the basis of radio labelled uptake and translocation studies. Five surfactants which significantly promoted uptake without major reductions in translocation of the triclopyr ester were identified; three organosilicone surfactants (Dow Corning 57, Dow Corning X25152 and Silwet L77), an N-alkyl pyrrolidone surfactant (Surfadone LP300) and an octylphenoxy surfactant (Triton X-45).

The inherently difficult nature of gorse as an experimental plant was identified as potentially limiting for a number of detailed physiological investigations. A model plant system, field bean (Vicia/aba L. cv. Maris Bead), was selected which could reproduce the behaviour of gorse with several triclopyr fonnulations in addition to being readily established and fast growing. Scanning electron microscopy of the plant surfaces revealed some similarities in morphological characteristics between gorse and field bean, such as percentage of the plant surface covered in stomatal pores. Gorse had a stomatal pore percent cover of 1.2 % while the adaxial leaf surface of field bean had a value of 0.8 %. The majority of investigations were conducted with the three organosilicoI.1e surfactants (Dow Corning 57, Dow Corning X25152 and Silwet L77) and Surfadone LP300.

In both gorse and field bean, uptake of ¹⁴C-triclopyr butoxyethyl ester was increased as surfactant rate was increased, up to an optimum (between 0.2 and 0.5 %v/v for all surfactants), above which uptake tended to decline or plateau, particularly for the three organosilicones. Translocation expressed as a percent of applied herbicide tended to increase as surfactant rate increased although this was not always significant and the highest rate (0.5 %v/v) sometimes caused a reduction. In contrast, translocation as a percent of net uptake was reduced when any of the surfactants were included. Optimising herbicide efficacy depends on maximising uptake and translation processes. By considering translocation as a percent of applied herbicide, an index encompassing formulation effects on both uptake and translocation was obtained from which an optimum rate could be selected. Optimum rates for the organosilicone surfactants were identified between 0.1 and 0.35 %v /v while for Surfadone LP300 it was more difficult to define; between 0.1 and 0.5 %v/v. Surfactant rate studies with field bean showed that uptake was most comparable with gorse when triclopyr formulations included organosilicone surfactants.

A spray application trial for gorse control in the field was established to evaluate the four chosen surfactants with the triclopyr ester formulations. Two rates (0.2 and 0.5 %v/v) of each surfactant were formulated with a 2 kg a.i. ha⁻¹ rate of triclopyr butoxyethyl ester and compared with 2 or 6 kg a.i. ha⁻¹ triclopyr alone treatments. Applications were made in late spring, Objective and subjective measurements of the trial were conducted over a 12 month period to assess formulation efficacy. Chlorosis and regrowth within plots were visually estimated at 1-2 month intervals. Within each plot, individual branches were assessed regularly. Chlorophyll concentration in young shoots on bush tops was determined at 2, 4 and 6 weeks after spraying to evaluate the rate of chlorophyll loss from sprayed gorse and to see whether this objective measurement was representative of formulation efficacy after longer periods (ie: up to 12 months). Regrowth from each plot was collected after 12 months. Optimum control of gorse was achieved with the highest rate of triclopyr active ingredient (6 kg a.i. ha⁻¹) used without surfactant while the poorest control was observed with the 2 kg a.i. ha⁻¹ rate of triclopyr without surfactant. It was concluded that the 0.5 %v/v rate of organosilicone surfactants provided greater long term control compared to the 0.2 %v/v rate while the opposite was true for Surfadone LP300.

Correlations between the various measurement techniques were made. Chlorophyll concentrations after 2 weeks were highly and negatively correlated to 12 month shoot regrowth but by 6 weeks, this had become a highly positive correlation. Visual estimates of chlorosis were closely and inversely related to visual shoot regrowth ratings at 5, 6, 8, 10 and 12 months. Measured shoot regrowth was highly positively related to visual regrowth ratings and negatively related to chlorosis after 5, 6, 8, 10 and 12 months.

A controlled environment experiment was conducted to compare uptake of ¹⁴C-triclopyr triethylamine fonnulations with and without one of the four surfactants (Dow Coming 57, Dow Coming X25152, Silwet L77 and Surfadone LP300) by gorse and field bean after 6 and 24 h. All surfactants significantly enhanced triclopyr uptake compared to control in both species at both times. The inclusion of Dow Coming X25152 or Silwet L77 provided the greatest increases in uptake by gorse while Dow Corning X25152, Silwet L77 or Surfadone LP300 were the best surfactants with field bean. Results supported the use of field bean as a model plant system for gorse with triclopyr triethylamine plus organosilicone surfactants.

A second experiment compared the uptake of triethylamine and butoxyethyl ester formulations of ¹⁴C-triclopyr with or without an organosilicone surfactant, Silwet L77. Uptake of the ester formulation was not significantly increased by Silwet L77 in either species while the, uptake of triclopyr amine was dramatically increased by surfactant (by 513 % in field bean and by 1555 % in gorse).

Detailed investigations into the mode of action of the four surfactants were carried out. A fluorescence indicator was included in the triclopyr-surfactant formulations to visualise predominant absorption pathways into field bean leaves for various herbicide mixtures. Surfactants made up in water alone and in triclopyr triethylamine formulations were found to best demonstrate the differences between surfactant effects on absorption. Additional emulsifying components of ester formulations were found to obscure the absorption trends induced by the different surfactants. Organosilicone surfactant formulations primarily promoted uptake via stomatal pore regions while Surfadone LP300 appeared to have a phytotoxic effect of its own, with major leaf surface perturbations. Surfactants reduced surface tension to the greatest extent when made up in water alone (down to 21 mN m⁻¹) and amine (down to 22 mN m⁻¹). In all formulations, Dow Coming X25152 and Silwet L77 reduced surface tension the most with Dow Coming 57 reducing it the least. Contact angle determinations showed similar trends to those observed with surface tension. Transmission electron microscopy found stomatal pore wall angles were such that stomatal infiltration would be expected through converging sections of pores for all formulations and through diverging sections for several formulations; particularly the amine or water alone solutions containing Dow Corning X25152 or Silwet L77 at or above 0.1%v/v.

Uptake of ¹⁴C-triclopyr triethylamine by field bean as the rate of Silwet L77 was increased from 0 up to 0.5 %v/v was compared to the extent of absorption shown by fluorescence microscopy, changes in surface tension, droplet spread and droplet drying rates for each formulation. Radiotracer and fluorescence studies found optimum penetration of the amine occurred at 0.25 and 0.5 % v/v in spite of the 0.1 %v/v Silwet L77 formulation having the same surface tension and droplet spread as the higher surfactant rates. An innovative interpretation of the findings using a physical chemistry approach explained the discrepancy in physical measurements and herbicide uptake for a 0.1 %v/v Silwet L77 in triclopyr amine solution. The tendency for surfactant molecules to aggregate to the surface of the droplet was termed an - adsorptive dilution effect which effectively reduced the concentration of a 0.1 %v/v Silwet L77 solution within the bulk solution of the droplet to a 0.048 %v/v Silwet L77 solution. The corresponding surface tension of a 0.048 %v/v Silwet L77 solution would be higher and was found to occur at a sensitive part of the surface tension-surfactant concentration curve, below the inflexion point.

Radiotracer studies of triclopyr uptake by field bean over time in the absence or presence of Silwet L77 (0.25 %v/v) identified the extent to which stomatal uptake mechanisms contributed to the uptake process and added to the understanding of the mechanisms involved. A Michaelis-Menten model was found to be a good description of the foliar uptake data over 24 h. The inclusion of Silwet L77 significantly reduced k (the time at half the maximum uptake of herbicide) but did not significantly change Vmax (the maximum amount of herbicide taken into the plant). This illustrated how Silwet L77 could be used to enhance the rapid movement of active ingredient into the plant. Michaelis-Menten kinetics were believed to apply in this situation because of a facilitated diffusion process occurring either in the uptake of triclopyr through the cuticle and/or in the loading of cells across membranes, such as into the phloem.

A closer examination of uptake during the first 2 h suggested a two phase model was operating when Silwet L77 was included in the formulation. The initial phase was believed to involve a very rapid or instantaneous mass flow of herbicide into the stomatal pores and/or substomatal cavities, taking place in the first 20 minutes after application. After 20 and before 45-60 minutes, uptake began to slow which represented the saturation of substomatal cavities. Uptake increased again after 45-60 minutes with both Silwet L77 and non-Silwet L77 treatments having equivalent rates of uptake. Evidence supported that this second phase was the result of the triclopyr active ingredient having had sufficient time to break down the epicuticular waxes and/or underlying cells to allow a more rapid diffusion of material into the plant. Further support for the promotion of stomatal uptake by Silwet L77 was provided when abscisic acid pretreatments closed field bean stomata, significantly reducing the uptake of triclopyr.