Hygroscopic salts support the stomatal penetration of glyphosate and influence its biological efficacy

Abstract

The objective of this study was to better understand the role of chaotropic and kosmotropic salts in stomatal uptake, a process that occurs in parallel with cuticular penetration and affects the bioefficacy of glyphosate. In Trial 1, salt solutions of kosmotropic (NH4)2SO4 and chaotropic NaClO3, with or without the organosilicone surfactant, Break‐Thru® S233 (BT), and with or without glyphosate, were prepared. In Trial 2, sodium salts with a kosmotropic‐to‐chaotropic nature (Na2SO4, NaCl, NaNO3 or NaClO3) and BT were prepared with or without glyphosate. At very high concentrations, the salts of a kosmotropic nature induced a slight decrease in the surface tension, whereas those of a chaotropic nature triggered a significant decrease in the surface tension. Solution droplets were deposited onto the leaves of Viola arvensis and Chenopodium album. The dried deposits were analyzed with an environmental scanning electron microscope. The salts formed structures linking the exterior to the interior of the stomata in the crystalline ([NH4]2SO4) or amorphous (NaClO3) form. However, the low surface tension alone did not appear to be the unique driving force for stomatal uptake. With glyphosate, amorphous globular structures of glyphosate salt were formed in the Na2SO4 (kosmotropic) solution. Differently, the salts of a rather chaotropic nature (NaNO3 or NaClO3) formed a colloidal matrix. In general, salt residues were observed over the periclinal cell walls and inside the stomatal chamber. Neither (NH4)2SO4 nor Na2SO4 had a significant impact on glyphosate bioefficacy. However, the chaotropic salt, NaNO3, enhanced the bioefficacy of glyphosate the most, with a performance that was comparable to that of the solution glyphosate + organosilicone surfactant.

Heterogeneity of Stomatal Pore Area Is Suppressed by Ambient Aerosol in the Homobaric Species, Vicia faba

“…Hygroscopic, particularly chaotropic, aerosol enhances the formation of such films by deliquescence and facilitates their spread across the leaf surface and into stomatal pores by reducing the surface tension of the liquid on the leaf surface (Monteith, 1957;Eiden et al, 1994;Dutcher et al, 2010;Burkhardt and Hunsche, 2013;Burkhardt and Grantz, 2017;Fernandez et al, 2017). Electrical conductance measurements (Burkhardt and Eiden, 1994;Burkhardt et al, 1999;Burkhardt and Hunsche, 2013) and electron micrography (Grantz et al, 2018) demonstrate these films and their penetration into stomatal pores (Eichert et al, 1998(Eichert et al, , 2008Basi et al, 2014;Kaiser, 2014). The resulting hydraulic linkage of apoplast to the leaf boundary layer is associated with a reduction of mean pore area and skewing of areas toward the origin (Burkhardt et al, 2001a(Burkhardt et al, , 2012Burkhardt, 2010;Pariyar et al, 2013;Grantz et al, 2018).…”

“…Deposition of hygroscopic, particularly chaotropic, aerosol ( Tsigaridis et al, 2006 ; Pringle et al, 2010 ; Burkhardt and Grantz, 2017 ) reduces surface tension and results in development of thin liquid films on leaf surfaces ( Eiden et al, 1994 ; Dutcher et al, 2010 ; Burkhardt and Hunsche, 2013 ; Burkhardt and Grantz, 2017 ; Fernandez et al, 2017 ). The liquid films penetrate into stomatal pores ( Eichert et al, 1998 , 2008 ; Basi et al, 2014 ; Kaiser, 2014 ), providing a liquid phase linkage between the saturated leaf apoplast and the dry atmosphere. This pathway is not under diffusional (i.e., stomatal) control and thus water loss from this pathway through evaporation at the leaf surface increases as VPD increases, even in species such as V. faba that exhibit strong closing response to increasing VPD ( Pariyar et al, 2013 ; Grantz et al, 2018 ).…”

Foliar water and solute absorption: an update

“…Therefore, it was concluded that external processes may increase the wettability of the guard cell surface of individual stomata, 'activating' them for solute transport (Eichert and Burkhardt, 2001;Fern andez and Eichert, 2009;Burkhardt et al, 2012). Different processes were proposed to be involved in the activation of individual stomata for the diffusive exchange of matter across leaf surfaces, namely, presence of bacteria or fungal hyphae (Burgess and Dawson, 2004), epistomatal mucilage (Westhoff et al, 2009) and hygroscopic particles (Burkhardt et al, 2012;Basi et al, 2014).…”

Ambient aerosol increases minimum leaf conductance and alters the aperture–flux relationship as stomata respond to vapor pressure deficit (VPD)

“…Advance across the cuticularized leaf surface facilitates penetration into stomatal pores, linking apoplastic water with the leaf surface by thin films coating the walls of the stomatal pores. This has been demonstrated for dissolved solutes such as herbicides (Eichert et al ., ; Basi et al ., ), for nonsoluble nanoparticle suspensions (Eichert et al ., ), and even for coarse particles following application of surfactant (Kaiser, ). These solutions contrast with pure water which is prevented from entering stomatal pores by the interaction of pore geometry, cuticle hydrophobicity and surface tension (Schönherr & Bukovac, ).…”