Penetration into leaf tissues and physical properties of fungicides

​Knowing the physicochemical characteristics of fungicides and their penetration into leaf tissues is essential to obtain better effectiveness and prevent losses in the field

31.08.2020 | 20:59 (UTC -3)

Knowing the physicochemical characteristics of fungicides and their penetration into leaf tissues is essential to obtain better effectiveness and prevent losses in the field. Whatever the classification, one cannot lose sight of the fact that their movement over long distances in the plant is limited. A factor that demands a lot of attention to application technology because if the product only reaches the top sheets, it will never reach the bottom sheets.

The penetration of substances into leaves is a passive process driven by concentration gradients. According to Fick's law, the concentration gradient is the driving force for diffusion. The diffusion penetration rates of any solution applied externally to the leaf surface depend on both its concentration on the surface and the concentration within the leaf.

The concentration of a given solution inside the leaf depends on the nature of the compound and physiological factors of the plants, such as mobility and rate of penetration into the epidermis and mesophyll cells (GRIGNON et al., 1999; EWERT et al., 2000). The concentration of the product within the leaf tissue, just below the droplet, is supposedly much lower, especially when the droplet arrives. Therefore, it can be said that diffusion rates into the leaf are mainly governed by the external concentration of solutes (Figure 1).

Figure 1 - Diffusion rates for the interim of plants.
Figure 1 - Diffusion rates for the interim of plants.

Leaf diffusion is determined by many characteristics inherent to the active ingredients and their formulation, such as plant-environment interaction, which will ultimately have a direct influence on factors such as leaf morphology, structure, position, sun exposure and process rate. physiological in the plant. When coming into contact with the plant, the first barrier that the chemical encounters is the cuticle, a thin superficial film composed of soluble and polymeric lipids (JEFFREE, 1996). The most important function of the cuticle is the protection of living plant tissues against water loss (SCHÖNHERR, 1982), but it also constitutes a barrier to the penetration of chemical products applied via the foliar route.

The movement of agrochemicals through plant cuticles has been extensively modeled and reviewed (BRIGGS; BROMILOW, 1994; WANG; LIU, 2007; SATCHIVI et al., 2006), and is considerably more complex than the movement predicted by simple laws of mass transfer (RIEDERER; FRIEDMANN, 2006). Adjuvants, formulation type, active ingredient (AI) to adjuvant ratios, and AI concentration in the droplet spray are among the many application parameters that influence foliar absorption (ZABKIEWICZ, 2007; STOCK, 1996; FORSTER et al ., 2006; STOCK et al., 1993). Among the physical properties of IA that influence foliar uptake, the octanol-water partition (logKow) is often considered the fundamental parameter for penetration through the cuticle and redistribution in the plant (WANG; LIU, 2007; KIRKWOOD, 1999). In fact, Bromilow and Chamberlain (1989) stated that the systemicity of compounds can be predicted by lipophilicity and that compounds with LogKow values ​​greater than 3 cease to move in plants. Molar volume (MV) has also been considered a key predictor of the movement of compounds across the cuticular membrane (SATCHIVI et al., 2006; SCHÖNHERR; BAUR, 2004). Briggs and Bromilow (1994) consider that the melting point (MF) can be a key property to control the solubilization of a compound on the leaf surface, being the first step towards penetration and redistribution in the plant. Sauter (2007) also proposed that PF is an important parameter, with low PF driving strong translaminar activity of pyraclostrobin, positively affecting the spectrum of activity of this product, crop safety, and yield improvements.

Fungicides can be classified regarding mobility in the plant, from remaining on the surface of the plant after deposition, or from absorption and translocation by the conductive system to locations distant from deposition. Thus, fungicides can be classified as:

(i) Topics or properties: are fungicides that, when applied to aerial organs, are neither absorbed nor translocated, remaining on the surface of the plant (from the Greek topykos = place), in the place where they were deposited (Figure 2). They are also called non-systemic. Example: multisite fungicides (Table 1). 

Figure 2 - Fungicides that remain on the surface of the plant.
Figure 2 - Fungicides that remain on the surface of the plant.

(ii) Systemic or mobile: Substances that, when absorbed by the roots and leaves, are translocated by the plant's conductive system via the xylem (vast majority) or phloem (Figure 3).

Once inside the plant, these fungicides provide a prolonged protective action. They are not exposed to leaching by rain and photodecomposition and, therefore, do not require frequent applications.

Systemic fungicides applied to seeds are neither absorbed nor translocated, as the seeds do not have a conductive system. In this way, the fungicides remain on the surface, being only absorbed via the soil (via the radicle) and translocated (via the xylem) to the aerial organs of the seedling when germination occurs. Example: triazoles, some strobilurins and carboxamides (Table 1).

Figure 3 - Substances that are absorbed by the roots and leaves are translocated via the xylem or phloem
Figure 3 - Substances that are absorbed by the roots and leaves are translocated via the xylem or phloem

(iii) Mesostemic: Mesostemic fungicides consist of the union of two concepts: translaminar systemicity and episystemicity. The action translaminar occurs when the fungicide is applied to the adaxial surface of the leaf (top) and is translocated to the abaxial surface (bottom). Fungicide episystemic It presents distribution in the wax layers of the leaves through its vapor phase. If the vapor pressure of the substance is high enough, migration can begin from deposition on the surface of the sheet and transport as vapor.

Mesostemic fungicides have lipophilic characteristics, whose deposits adhere strongly to the cuticular wax layer. Therefore, they are highly resistant to removal by rainwater or irrigation. The main examples are: strobilurins and carboxamides (Table 1).

Figure 4 - Mesostemic fungicides consist of the union of the concepts of translaminar systemicity and episystemicity
Figure 4 - Mesostemic fungicides consist of the union of the concepts of translaminar systemicity and episystemicity

There is also a term in the literature called True systemic ou Amfimobile (from Greek lecture hall = around). This terminology is defined as the ability of the fungicide to move through both the xylem and the phloem, reaching all parts of the plant, regardless of where it was deposited. There are no known examples of large crop fungicides with this characteristic. The only practical example is Fosetyl-Aluminum, an active ingredient from the chemical group of ethyl phosphonates, recommended for most chromists (oomycetes) in fruits and vegetables.

Figure 5 - Ability of the fungicide to move through both the xylem and the phloem, reaching all parts of the plant
Figure 5 - Ability of the fungicide to move through both the xylem and the phloem, reaching all parts of the plant

Knowledge of the physical-chemical characteristics of the fungicide to be used is essential to improve effectiveness and reduce losses in the field. An example is multisite fungicides, which are immobile and require good droplet coverage per leaf unit and, therefore, are very susceptible to being washed away by rain. On the other hand, fungicides that penetrate the tissue have advantages that are inversely proportional to those mentioned above. However, it is important to emphasize that the movement of any fungicide over long distances within the plant is limited. Therefore, significant attention should not be given to application technology, since if the product only reaches the top leaves of the plant, it will never reach the bottom leaves.



Marlon Tagliapietra Stefanello, Leandro Nascimento Marques, Marcelo Gripa Madalosso, Ricardo Silveiro Balardin, UFSM/Instituto Phytus


Article published in issue 213 of Cultivar Grandes Culturas.

Mosaic Biosciences March 2024