The interactions between surfactants and the epicuticular wax on soybean or weed leaves: Maximal crop protection with minimal wax solubilization

- The wax layer of soy leaves is solubilized by surfactants solutions.
- Surfactants with high critical micelle concentration present high adhesion to leaves.
- The chemical components of wax have no biocidal activity.
- The tridimensional wax platelets on soy leaves create a water repellent surface.
- The surface of weed is better wetted by water than soy leaves because it is flatter.

Surfactants are commonly added to aqueous herbicide formulations to reduce the surface tension, facilitating their application. In the present study the interaction between the surface of freshly harvested soybean leaves and pure water, solutions of cationic (CTAB), anionic (SDS) and uncharged (ethoxylated sorbitan monolaurate 20 EO - ESM20) surfactants and a commercial herbicide formulation (CF) was investigated by means of contact angle measurements and scanning electron microscopy (SEM). The wettability and the hysteresis in the contact angle values determined for soybean leaves were the largest for the surfactant with the highest critical micelle concentration. The epicuticular wax on soybean leaves presented rosette-like clusters of platelets, as observed by SEM. After interaction with the surfactants or CF, most of the wax platelet rosettes disappeared from the surface, indicating that the epicuticular wax was dissolved by the surfactant. The wax contained long hydrocarbon and long chain esters, as qualitatively analyzed by gas-chromatography coupled with mass spectroscopy. The wax extracted from soybean leaves presented no significant biocidal activity against Micrococcus luteus, Staphylococcus aureus or Escherichia coli. Its protective role against pathogens depends solely on the tridimensional arrangement on the surface, which impairs the physical attachment of microorganisms to the surface. For comparison, the wettability and morphology of Eleusine indica, a common weed, was investigated. The weed leaves presented smooth surfaces and a much lower contact angle values. Therefore, surfactants should be added to the herbicide formulations with the compromise to protect the crop adequately and to minimize the solubilization of protective epicuticular wax layer on soybean leaves.